So you give someone an inch and they want a yard. Given them a rocket ship and suddenly they want a star ship. SF writers want to use exotic settings on alien planets, but the real estate in our solar system mostly looks like a bunch of rocks. "That's OK," the writer thinks, "There are a million-jillion other solar systems in the galaxy, surely they are not all a bunch of rocks (I know they are there, I've got a map). I know that those spoil-sports at NASA have ruined our solar system for SF writers since their nosy space probes failed to find dinosaur-infested jungles of Venus and scantily-clad Martian princesses. But they haven't sent probes to other stars yet! Why not turn my rocket ship into a star ship?"

Unfortunately it isn't that easy. The basic problem is that interstellar distances are freaking huge.

The introduction begins like this: "Space," it says, "is big. Really big. You just won't believe how vastly hugely mindboggingly big it is. I mean you may think it's a long way down the road to the chemist, but that's just peanuts to space. Listen ..." and so on.


Consider: a single light-year is an inconceivable abyss. Denumerable but inconceivable. At an ordinary speed — say, a reasonable pace for a car in a megalopolitan traffic, two kilometers per minute — you would consume almost nine million years in crossing it. And in Sol's neighborhood, the stars averaged some nine light-years apart. Beta Virginis was thirty-two distant.

From TAU ZERO by Poul Anderson (1970)

Let's make a mental model. Say the scale is such that one astronomical unit is equal to one millimeter (1/25th inch). There is a glowing dot for the Sun, and one millimeter away is a microscopic speck representing the Earth. The edge of the solar system is about at Pluto's orbit, which varies from 30 mm to 50 mm from the Sun (about 1 and 3/16 inch to almost 2 inches). Imagine this ten-centimeter model floating above your palm.

This would put Proxima Centauri, the closest star to the Sun, at about 272 meters away. That's 892 feet, the length of about two and a half football fields or four and a half New York city blocks! Glance at the ten-centimeter solar system in your hand, then contemplate the nearest solar system four and a half city blocks away.

And the center of the galaxy would be about 1600 kilometers away (about 990 miles), which is a bit more than the distance from Chicago, Illinois to Houston, Texas.

"All right, all right!" the SF author grumbles, "So the distance is outrageous. What of it?"

This of it. How long do you think it is going to take to travel such distances? As an example, the Voyager 1 space probe is currently the fastest human made object with a rest mass, zipping along at a blazing 17.46 km/s. This means that in the space of an eyeblink the little speed demon travels a whopping eleven miles! That's smokin'. What if it was aimed at Proxima Centauri (it isn't), how long would it take to reach it?

About 74,000 years! Which means that if Neanderthal men had launched something as fast as Voyager 1 to Proxima, it would just barely be arriving right now. And the joke's on them. Neanderthals are extinct so not even their descendants would reap the benefit of any scientific broadcasts from the Proxima probe. A similar argument could be used against any interstellar probes we could launch.

This leaves us with two alternatives: deal with the fact that average human lifespan is 74 years, not 74,000; or make the starship go faster.

Well, three, if you count "faster than light", but that will be covered later.

As Gordon Woodcock put it, the three methods of travelling to other stars are "go slow", "go fast", and "go tricky." This means "deal with short human lifespan", "use relativistic speeds", and "go faster than light".


Pineconez' first law:

A society capable of building a successful interstellar generation ship will also be capable of building an interstellar relativistic ship simply by virtue of its tech level.

First Corollary:

Building a perfect, failsafe biosphere (as required for a generation ship) is not necessarily simpler than building an antimatter-fuelled torch drive, and (unlike the latter) can't be solved by throwing more power at it. And this is not even discussing cryogenic sleep.

Second Corollary:

It is not necessarily simpler to build a successful, interstellar generation ship than it is to build a successful, interstellar relativistic ship, and the latter is preferable for almost any use case. (The one major advantage of generation ships is probably payload.)

From Tobias Pfennings (2015)

Go Slow

The first of Gordon Woodcock's methods of interstellar travel is "go slow".

Distance between stars is huge, traveling said distance slower-than-light will take a huge amount of time, human beings have a very limited lifespan. And it is much easier to travel at 10% the speed of light than it is to travel at 99.99999% the speed of light

"Go Slow" means to focus on the limited human lifespan problem, and be content to travel slowly ato 10% c or so.


There are several ways of dealing with the lifespan issue. Go to the Tough Guide to SF and read the entry "Slowboat".

Digital Crew

Since every atom of mass is a penalty, the logical starship would just carry a master computer and no human crew. This avoids the payload mass of the crew, the habitat module, the life support system, food, water, and everything. The starship might be under a meter long, which would make this concept the lowest mass of all the slowboat starships.

However, nobody wants wants to read about the adventures of a computer (yes, I know there have been a couple of SF stories on this theme, but it requires extraordinary skill on the part of the author, and the stories are not wildly popular. With the exception of the Bolo stories by Keith Laumer et al.).

Enter the "digital crew" concept. You postulate technology capable of "uploading" human brain patterns into a computer. In essence, the ship's computer is running incredibly advanced simulations of the crew, creating a virtual reality much like that found in the movie The Matrix. This also allows the author to pontificate upon the nature of reality, ask if we are actually unaware virtual people in a virtual reality, and stuff like that. Authors who have used this concept include Sean Williams, Shane Dix, and Greg Egan.

The point is the author is allowed to write stories about human beings, but the digital humans and their digital environment take up zero mass.

One could add equipmment capable of manufacturing artificial bodies for the crew from local materials upon arrival at the destination. However, the advantage of a digital crew ship over a seed ship is the lower ship mass due to the absence of frozen embryos, artificial wombs, and robot mommies. Adding artifical body manufacturing facilites would reduce or remove the advantage. The only remaining advantage is that the new bodies inhabited by adults instead of babies.

You could regain the advantage if the manufacturing equipment is really tiny. Say a couple of grams worth of nanotechnology self-replicating machines, intended to work on handy asteroids or other free materials lying around the destination solar system. The nanotechnology bootstraps itself by replicating using in-situ resources as feedstocks until it has mass of a few tons, then shifts gears to start manufacturing artificial bodies.


But there was a catch: Living humans could not be sent. Even with the Earth’s vastly expanded resources—cheap fusion power and the new tools of nanotechnology seeming to exponentially expand the horizons every year—there was simply no way to send thousands of people light-years away from Earth in every direction. Quite aside from the colossal cost, there was also the issue of lost time as well as the physical and mental well-being of the individuals undertaking such voyages. Instead, the first wave of survey vessels would represent humanity in the best way possible but would carry no actual live specimens.

At first it was hoped that sophisticated artificial intelligences would fill the pilot seats, but AI research took longer to deliver than its engineering counterpart. While vast orbital shipbuilding facilities evolved new generations of drives, power supplies, and protective magnetic bubbles, programmers explored dead end after dead end, never quite succeeding in creating the right sort of mind to ensure even one mission’s success, let alone thousands. UNESSPRO could not afford to throw away trillions of dollars on ships that might die or go AWOL at any moment. With 5 percent of the Earth’s gross product being channeled into the project, there had to be some sort of guarantee of returns. So they were forced to explore other options.

By 2048, it was clear that only one of these options promised anything like the sort of reliability required, and that was to send out electronic facsimiles of humans to the stars, as opposed to flesh and blood. Consciousness research had not yet managed to re-create an entire person’s mind in an electronic environment, except by inefficient neuron-by-neuron simulation, but they could decipher a great deal that had once been thought a mystery. The processes underlying consciousness could be emulated, as could the way emotions and other impulses ebbed and flowed throughout the body. Memory alone had proven elusive under such reduced conditions, defying all attempts to record it indirectly. The only efficient way it could be captured and simulated was secondhand, by interviewing the original at length about his or her past and using physical records to supply the images. Emotions could be attached later, during the fine-tuning phase, to color the recollection correctly, even though the details might still be slightly askew. Preawakening memory in such a mind was, at best, a patchwork quilt pieced together from a million isolated fragments.

But that was enough. So-called “engrams” behaved more or less the same as their template minds, the flesh-and-blood originals who had devoted six months of their lives to the task of being effectively taken apart and rebuilt inside a computer. When left to run for long periods, the engrams displayed no greater tendency toward unreliability than those same originals, neither failing at familiar tasks nor unable to learn. They were, in fact, ideal candidates for any space-faring crew: They did not eat, breathe, excrete, sleep, or grow sick; they took up very little space—less than a cubic decimeter (as measured in the new Adjusted Planck units created for the international venture)—and weighed less than half a kilogram; they could adjust easily to the long stretches of time during which nothing happened on an interstellar mission; and they could be trained as easily as a real person. In fact, it proved no great difficulty to train sixty real astronauts, then copy them as many times as was required to fill the crew registers of 1,000 survey vessels.

It was the latter detail that aroused the greatest ire among those still concerned about matters of the soul. Each survey vessel had a crew of thirty; there were one thousand ships; that meant a total survey crew of 30,000 individuals had been selected from that initial pool of just sixty. Roles on each mission were allocated randomly—while Caryl Hatzis might be the civilian survey manager on the Frank Tipler, on another ship she might have a junior role—but that didn’t remove the fact that there were in total over 500 Caryl Hatzises in the bubble of surveyed space surrounding the Earth. Were they really all the same person?

From ECHOES OF EARTH by Sean Williams and Shane Dix (2002)

Seed Ship

The next higher mass class of slowboat is the Seed ship aka Embryo Space Colonization via an embryo-carrying interstellar spaceship (EIS). It will tend to have more mass than a Digital Crew ship and less than a Sleeper Ship.

The starship is tiny, containing a payload of millions of frozen fertilized eggs, artificial wombs, robot factory, and a master computer. No mass is needed for life-support, habitat modules, or any human crew.

After traveling for thousands of years, the ship lands in a good spot for a colony. The robot factory starts cranking out robots. Robots build the settlement buildings and start growing food (if the planet is really nasty they might have to spend a few centuries terraforming the planet first). Then the master computer thaws out enough eggs for the available artificial wombs, brings the babies to term, then tries to convince the babies that the robots are mommy and daddy.

I don't know about you but I suspect that the first generation is going to grow up a little bit emotionally stunted.

The most straightforward method is to cryogenically preserve human embryos. The more difficult but more flexible method is to carry frozen sperm and egg cells, and do in vitro fertilization at the destination. The most unobtanium method is to carry genetic information in computer files, then synthesize the required genetic sequences at the destination.

As with all interstellar colonization proposals, there are quite a few technological challenges to solve:

Artificial Intelligence
The ship's computer has to be smart enough to not only pilot the ship, plan the settlement, and coordinate the building; but also be smart enough to perform parenting duties for all the children. This includes teaching the children survival skills, cultural heritage, and healthy psychological functioning. Its hands are going to be real full when the children become teenagers.

The ship robots will have to be advanced enough to raise and nurture the children, as well as building the settlement and growing crops. They could be teleoperated drones controlled by the ship's computer.

The side problem is they will have to be manufactured at the destination using in situ resources. The whole idea behind the Seed Ship is to minimize the payload mass, carrying an army of robots negates this.

Artificial Wombs
An artificial uterus is way beyond our current technology, but it is being worked on. Brave New World is just around the corner. The techno-wombs will be working overtime until the first generation is old enough to make babies the old fashioned way.
Long-duration Hardware
As with other starship proposals, but with the ship's Artificial Intelligence in particular, the equipment will have to reliably operate for how ever many thousands of years the journey will take.
The frozen-embryos/frozen-gametes/genetic-computer-files will need to be protected from cosmic rays and other damaging influences. In addition, the human gut microbiota is a critical part of the body. On Terra it is obtained from the mother and/or the general environment. At the Seed Ship destination, neither will be available. The microbiota will have to be recreated along with the babies.
Unsurprisingly this entire process opens a can of worms with several sticky moral questions. For one, you are deliberately creating children who will grow up without (human) parents. Should the children be taught/programmed behaviour biased to colony success, or biased towards freedom? Should their records of Terra's history be censored? If so, who decides what gets cut? Some of these issues are mentioned in Clarke's The Songs of Distant Earth.

Examples of Seed Ships in science fiction include The Songs of Distant Earth by Sir. Arthur C. Clarke, 2001 Nights chapter Night 4 by Yukinobu Hoshino, Long Shot by Vernor Vinge, Voyage from Yesteryear by James Hogan and the movie Interstellar.


“I want to talk about matters that are of global significance and which affect every individual alive on this planet, and indeed the generations yet to be born—assuming there will be future generations.” He paused. “I want to talk about survival—the survival of the human species.”

Congreve went on. “We have already come once to the brink of a third world war and hung precariously over the edge. Today, in 2015, twenty-three years have passed since U.S. and Soviet forces clashed in Baluchistan with tactical nuclear weapons, and although the rapid spread of a fusion-based economy at last promises to solve the energy problems that brought about that confrontation, the jealousies, mistrusts, and suspicions which brought us to the point of war then and which have persistently plagued our race throughout its history are as much in evidence as ever.

“Today the sustenance that our industries crave is not oil, but minerals. Fifty years from now our understanding of controlled-fusion processes will probably have eliminated that source of shortages too, but in the meantime shorter-sighted political considerations are recreating the climate of tension and rivalry that hinged around the oil issue at the close of the last century. Obviously, South Africa’s importance in this context is shaping the current pattern of power maneuvering, and the probable flashpoint for another East-West collision will again be the Iran-Pakistan border region, which our strategists expect the Soviets to contest to gain access to the Indian Ocean in preparation for the support of at war of so-called black African liberation against the South.”

Congreve paused, swept his eyes from one side of the room to the other, and raised his hands in resignation. “It seems that as individuals we can only stand by as helpless observers and watch the events that are sweeping us onward collectively. The situation is complicated further by the emergence and rapid economic and military growth of the Chinese-Japanese Co-Prosperity Sphere, which threatens to confront Moscow with an unassailable power bloc should it come to align with ourselves and the Europeans. More than a few Kremlin analysts must see their least risky gamble as a final resolution with the West now, before such an alliance has time to consolidate. In other words, it would not be untrue to say that the future of the human race has never been at greater risk than it is at this moment.”

Congreve pushed himself back from the podium with his arms and straightened. When he resumed speaking, his tone had lightened slightly. “In the area that concerns all of us here in our day-to-day lives, the accelerating pace of the space program has brought a lot of excitement in the last two decades. Some inspiring achievements have helped offset the less encouraging news from other quarters: We have established permanent bases on the Moon and Mars; colonies are being built in space; a manned mission has reached the moons of Jupiter; and robots are out exploring the farthest reaches of the Solar System and beyond. But”—he extended his arms in an animated sigh—“these operations have been national, not international. Despite the hopes and the words of years gone by, militarization has followed everywhere close on the heels of exploration, and we are led to the inescapable conclusion that a war, if it comes, would soon spread beyond the confines of the surface and jeopardize our species everywhere. We must face up to the fact that the danger now threatening us in the years ahead is nothing less than that.”

He turned for a moment to stare at the model of SP3 gleaming on the table beside him and then pointed to it. “Five years from now, that automated probe will leave the Sun and tour the nearby stars to search for habitable worlds … away from Earth, and away from all of Earth’s troubles, problems, and perils. Eventually, if all goes well, it will arrive at same place insulated by unimaginable distance from the problems that promise to make strife an inseparable and ineradicable part of the weary story of human existence on this planet.” Congreve’s expression took on a distant look as he gazed at the replica, as if in his mind he were already soaring with it outward and away. “It will be a new place,” he said in a faraway voice. “A new, fresh, vibrant world, unscarred by Man’s struggle to elevate himself from the beasts, a place that presents what might be the only opportunity for our race to preserve an extension of itself where it would survive, and if necessary begin again, but this time with the lessons of the past to guide it.”

An undercurrent of murmuring rippled quickly around the hall. Congreve nodded, indicating his anticipation of the objections he knew would come. He raised a hand for attention and gradually the noise abated.

“No, I am not saying that SP3 could be modified from a robot craft to carry a human crew. The design could not feasibly be modified at this late stage. Too many things would have to be thought out again from the beginning, and such a task would require decades. And yet, nothing comparable to SP3 is anywhere near as advanced a stage of design at the present time; let alone near being constructed. The opportunity is unique and cannot, surely, be allowed to pass by. But at the same time we cannot afford the delay that would be needed to take advantage of that opportunity. Is there a solution to this dilemma?” He looked around as if inviting responses. None came.

“We have been studying this problem for some time now, and we believe there is a solution. It would not be feasible to send a contingent of adult humans, either as a functioning community or in some suspended state, with the ship; it is in too advanced a stage of construction to change its primary design parameters. But then, why send adult humans at all?” He spread his arms appealingly. “After all, the objective is simply to establish an extension of our race where it would be safe from any calamity that might befall us here, and such a location would be found only at the end of the voyage. The people would not be required either during the voyage or in the survey phase, since machines are perfectly capable of handling everything connected with those operations. People become relevant only when those phases have been successfully completed. Therefore. we can avoid all the difficulties inherent in the idea of sending people along by dispensing with the conventional notions of interstellar travel and adopting a totally new approach: by having the ship create the people after it gets there!”

Congreve paused again, but this time not so much as a whisper disturbed the silence.

Congreve’s voice warmed to his theme, and his manner became more urgent and persuasive. “Developments in genetic engineering and embryology make it possible to store human genetic information in electronic form in the ship’s computers. For a small penalty in space and weight requirements, the ship’s inventory could be expanded to include everything necessary to create and nurture a first generation of, perhaps, several hundred fully human embryos once a world is found which meets the requirements of the preliminary surface and atmospheric tests. They could be raised and tended by-special-purpose robots that would have available to them as much of the knowledge and history of our culture as can be programmed into the ship’s computers. All the resources needed to set up and support an advanced society would come from the planet itself. Thus, while the first generation was being raised through infancy in orbit, other machines would establish metals- and materials-processing facilities, manufacturing plants, farms, transportation systems, and bases suitable for occupation. Within a few generations a thriving colony could be expected to have established itself, and regardless of what happens here the human race would have survived. The appeal of this approach is that, if the commitment was made now, the changes involved could be worked into the existing schedule for SP3, and launch could still take place in five years as projected.”

By this time life was flowing slowly back into his listeners. Although many of them were still too astonished by his proposal to react visibly, heads were nodding, and the murmurs running around the room seemed positive. Congreve nodded and smiled faintly as if savoring the thought of having kept the best part until last.

“The second thing I have to announce tonight is that such a commitment has now been made. As I mentioned a moment ago, this subject has been under study for a considerable period of time. I can now inform you that, three days ago, the President of the United States and the Chairman of the Eastern Co-Prosperity Sphere signed an agreement for the project which I have briefly outlined to be pursued on a joint basis, effective immediately. The activities of the various national and private research institutions and other organizations that will be involved in the venture will be coordinated with those of the North American Space Development Organization and with those of our Chinese and Japanese partners under a project designation of Starhaven.”

Congreve’s face split into a broad smile. “My third announcement is that tonight does not mark my retirement from professional life after all. I have accepted an invitation from the President to take charge of the Starhaven project on behalf of the United States as the senior member nation, and I am relinquishing my position with NASDO purely in order to give undivided attention to my new responsibilities. For those who might believe that I’ve given them some hard times in the past, I have to say with insincere apologies that I’m going to be around for some time longer yet, and that before this project is through the times are going to get a lot harder.”

Several people at the back stood up and started clapping. The applause spread and turned into a standing ova- tion. Congreve grinned unabashedly to acknowledge the enthusiasm, stood for a while as the applause continued, and then grasped the sides of the podium again.

“We had our first formal meeting with the Chinese yesterday, and we’ve already made our first official decision.” He glanced at the replica of the star-robot probe again. “SP3 now has a name. It has been named after a goddess of Chinese mythology whom we have adopted as a fitting patroness: Kuan-yin—the goddess who brings children. Let us hop/e that she watches over her children well in the years to come.”

From VOYAGE FROM YESTERYEAR by James Hogan (1982)

Sleeper Ship

Sleeper ship tend to have more mass than a Seed Ship and less than a Generation Ship.

The crew is frozen into suspended animation, so they do not age nor require food and oxygen during the thousand year journey. Or spacious living accomodations. The Sleeper Ship does require the mass of the crew, enough mass for a spartan habitat module, and only enough consumables for the time the crew will be awake.

Poul Anderson warned that frozen crew have a limited shelf life. Naturally-occurring radioactive atoms in the human body will cause damage. Normally the body will repair such damage, but one in suspended animation cannot. After a few hundred years, enough damage will accumulate so that a corpse instead of a living person is thawed out at journey's end. This may force one to thaw each crew member every fifty years or so to allow them to heal the damage, then freezing them again.

Generation Ship

The highest mass type of slowboat tends to be the Generation ship. This is because it has to carry the mass of an entire community as crew, a habitat module at the minimum the size of a small town, and enough life support for the people for however many hundreds of years the journey takes. As the ship crawls to its destination, generations of people are born, have children, and die of old age.

Problems include the later generations refusing to cooperate with their forefather's vision, civil wars that wreck the ship, failure of the closed ecological life support system, and the later generations forgetting where they came from, forgetting where they are going, and indeed forgetting the fact that they are in a starship. The classic "forgetting you are on a ship" stories are Robert Heinlein's Orphans of the Sky (1941) and Brian Aldiss' Non-Stop (1958).

In Larry Niven and Jerry Pournelle's FOOTFALL, the aliens deal with the "forgetful generation" problem by including a group of original crew frozen in suspended animation. Members of the original crew are periodically woken so they can ensure that the generational crew keeps the faith. The concept is sort of a combination of sleeper starship and generation starship.

The concept was sort of touched on in Don Wilcox's The Voyage that Lasted 600 Years (1940), though in that story only the captain was frozen. Since he was only a single person he had a limited influence on the generational tribes.

If the generation ship is escaping from some Terra-destroying catastrophe; carrying Terra's scientific and cultural heritage, a representative sample of animal species, colony equipment and supplies, and a fertile representative sample of humanity, the craft is termed an Interstellar Ark.


Well, indeed. We sit warm, at ease, breathing sweet air, smoking, drinking, snacking as we feel like it. The artificial gravity is a solid one g underfoot, its vector so aligned that we cannot detect the slight pressure of our acceleration. Nor do we sense the monstrous outpouring of engine energy by which this mass is driven starward. Modern technology is subtle as well as powerful.

We are not yet at Bussard velocity, where we can begin scooping up interstellar hydrogen to burn in the fusion reactors. But we have enough fuel of our own to reach that condition, and afterward to brake at interplanetary speeds as we back down on Alpha Centauri. We have a closed biocycle—every'thing essential to life can be reclaimed and reused indefinitely, for millions of years if need be—which at the same time is expansible. Boats, machines, robots, computers, instruments, and in the microfiles virtually all the knowledge of all the human civilizations that ever were, lie waiting for us like Aladdin’s genie.

“Very well.” Amspaugh turns to me. “I suppose you know that our single agendum today is educational policy.”

“I'd heard mention of that, " I reply. “ But, uh, what's the rush? The first babies are scarcely born.”

“They'll keep that up, though,” McVeagh reminds me.

Missy Blades murmurs: "‘And thick and fast they came at last, and more, and more, and more.’ Right up to the legal limit of population, whatever that may be at any given time. It's still the favorite human amusement. ”

Amspaugh takes pipe and tobacco pouch from various pockets and fumbles with them. “The children will grow," he points out earnestly. "They will require schools, teachers, and texts. The non-controversial basics pose no problem, I imagine—literacy, science, math, et cetera. But even while the pupils are small, they'll also be studying history and civics. Presently they'll be adolescent, and start inquiring into the value of what they've been taught. A few years after that, they'll be franchised adults. And a few years after that, they'll be running the society.

“This isn't a planet, or even an asteroid, where people simply live. The voyage is the ship's entire raison d ’étre. Let the ideal be lost, and the future will be one of utter isolation, stagnation, retrogression, probably eventual extinction. To avoid that, we're uniquely dependent on education.

“We'll only have a thread of maser contact with Sol, years passing between question and answer. We'll only have each other for interaction and inspiration; no fruitful contacts with different countries, different ways of living and thinking. Don't you see how vital it is, Mr. Sanders, that our children be raised right? They must have a proper understanding, not simply of the technology they need, but of the long-range purpose and significance."

Having stuffed his pipe, he pauses to light it. Orloff talks into the silence: “Basically, what we must decide is what the history courses should include. Once we know that, we have writers who can put it into textbooks, actors who can put it on tape, and so forth for every level from kindergarten through college. In the absence of outside influences, those teachings are likely to be accepted, unchallenged, for generations if not forever. So what ought they to be?”

“The truth," I blurt.

“What is truth?”

“Why the facts what really happened—”

"Impossible," Amspaugh says gently. “First, there are too many facts for any human skull to hold, every recorded detail of everybody's day-by-day life since ancient Egypt. You have to choose what's worth knowing, and set up a hierarchy of importance among those data. Already, then, you see your ‘truth’ becoming a human construct. Second, you have to interpret. For instance, who really mattered more in the long-term course of events, the Greeks or the Persians? Third, man being what he is, moral judgments are inevitable. Was it right, was it desirable that Christianity take over Europe, or that it be later faced with such enemies as Mohammedanism and Communism?

"An adult, intellectually trained and emotionally mature, can debate these questions with pleasure and profit. A child cannot. Yet unless you raise the child with a sense of direction, of meaning, you'll never get the adult. You'll get an ignoramus, or else a spiritual starveling frantic for some True Belief—a potential revolutionary. Astra can't afford either kind."

“ The problem was foreseen,” Orloff puts in, “ but we purposely delayed considering it till we should have been en route for a while and gotten some feeling of how this unique community is shaping up."

"I see," I answer. “At least, I think I see.”

“ Details later,” Amspaugh says. “What we must arrive at is a basic educational philosophy. " He gives me a long look. “The original circle of us know each other quite well. I think, by and large, we can predict what stand everybody will take. That isn't good. We need a wider range of thoughts. lt’s a major reason why we're inviting new members in, you the latest.

"So would you like to open the discussion?"

From TALES OF THE FLYING MOUNTAINS (prologue) by Poul Anderson (1970)

Alter Metabolism

A variation of the "Increase Lifespan" technique was in Charles Sheffield's Between The Strokes Of Night. A technique was discovered that would allow human metabolism to enter the "S-state." In this state, humans age at a rate 1/2000th normal, and perceive things at the same rate. There was also a protocol that would return an S-state person back to normal metabolism.

So with ships traveling at a slow 10% light speed, the trip to Proxima Centauri seems to take only a few weeks to an S-state person. Of course to a human in normal state, the trip will take about forty years.

As far as the S-state person is concerned, the ships are travelling faster than light. As long as they always stay in S-state.

To an S-state person, a normal state person moves so fast that they are invisible. To a normal state person, an S-state person appears to be immobile, though they are actually moving very very slowely. Of course to an S-stater all those normal state persons grow old and die 2000 times faster.

Between the Strokes of Night

"I'll tell you one thing I still don't understand," Peron said. "When I was in S-space, I felt as though I was in a one-gee environment. Now we're in exactly the same part of the ship, but we're in freefall. I don't see how that can happen."

There was silence for a while, then Kallen made a little coughing noise. "T-squared effect," he said softly.


"He's quite right," Sy said calmly. "Good for you, Kallen. Don't you see what he's saying? Accelerations involve the square of the time—distance per second per second. Change the definition of a second, and of course you change the perceived speed. That's why they can travel light-years in what they regard as a few days. But you change perceived acceleration, too—and you change that even more. By the square of the relative time rates—"

"—which is another reason the Immortals don't go down to the surface of planets," said Lum. "They want to spend their time in S-space to increase their subjective lifespans, but then that forces them to live in a very weak acceleration field. They can't take gravity."

"Not even a weak field," added Rosanne. "They'd fall over before they even knew they were off balance. What did you say the time factor was?—two thousand to one? Then even a millionth of a gravity would be perceived by them as a four-gee field. They have to live in freefall. They have no choice about it. But they perceive a four-millionth of a gee as normal gravity."

Peron looked around him in disgust. "All right. So everybody saw it easily except me. Try another one. Tell me what's going on outside the ship. One reason I thought at first that S-space had to be some kind of hyperspace was the view from the ports. When you look out, you don't see stars at all. All you see is a sort of faint, glowing haze. It's yellow-white, and it's everywhere outside the ship."

This time there was not even a moment's pause.

"Frequency shift," said Sy at once. "Let's see. Two thousand to one. So the wavelengths your eyes could see would be two thousand times as long. Instead of yellow light at half a micrometer, you'd see yellow at a millimeter wavelength. Where would that put us?"

There was a hush.

"The Big Bang," whispered Kallen.

"The three degree cosmic background radiation," said Rosanne. "My Lord. Peron, you were seeing leftover radiation from the beginning of the Universe—actually seeing it directly with your eyes."

And it's uniform and close to isotropic," added Lum. "That's why it looked like a general foggy haze. At that wavelength you don't get a strong signal from stars or nebulae, just a continuous field."

"But it can't be that straightforward." Sy frowned. "The pupils of our eyes provide too small an aperture to deal with millimeter wavelengths. There has to be a lot more to S-space modification than the obvious changes."

From Between the Strokes of Night by Charles Sheffield (1985)

Increase Lifespan

Finally there is the "Methuselah" concept. Advances in medical technology might increase human lifespan to thousands of years. So prolonged interstellar trips are more a problem of boredom instead of life-span.

Laser Sail

In Dr. Robert L. Forward laid it all out in his classic paper "Roundtrip Interstellar Travel Using Laser-Pushed Lightsails," Journal of Spacecraft and Rockets 21 (1984), pp. 187–195.

The secret is using a Laser Sail, which you will recall is a photon sail beam-powered by a remote laser installation.

The advantage is the starship does not have to carry the mass of the engine and the propellant, you leave it at home. This makes the task of designing the starship merely incredibly difficult, instead of utterly impossible.

The home system can also add to the laser batteries gradually after the starship's journey starts, as needed (the inverse-square law will weaken the beam as the range increases). You cannot do this with a self-contained starship, all of its engines have to be built before the journey starts.

And if a home system's laser battery or two break down, no problem! The resources of the home system are available to fix it. If a self-contained starship engine breaks down on the other hand, they are in trouble. They do not have the resources of the home system to help, all alone in interstellar space. They have to fix it themselves with whatever spare parts they managed to bring along. Or die all alone in the night.

The disadvantage is the starship is at the mercy of whoever is in charge of the laser station back in the Solar System. If there is a revolution back home and the Luddites seize power, the starship and crew are up doo-doo pulsar with no gravity generator. Dr. Forward came up with two clever ways of using the home system's lasers to decelerate the starship into the target solar system. In Larry Niven and Jerry Pournelle's classic The Mote In God's Eye the Motie aliens laser sail starship rather pointedly do NOT use Dr. Forwards deceleration methods, because they absolutely do not trust the laser station controllers back at their homeworld. In Dr. Forward's The Flight of the Dragonfly aka "Rocheworld" political foot-dragging and short-sighted policies almost lead to disaster for the laser station and starship. In Buzz Aldrin and John Barnes's Encounter With Tiber politics does kill the laser station and starship.

Braking the Hard Way

“Light sail!” Rod shouted in sudden realization. “Good thinking.” The whole bridge crew turned to look at the Captain. “Renner! Did you say the intruder is moving faster than it ought to be?”

“Yes, sir,” Renner answered from his station across the bridge. “If it was launched from a habitable world circling the Mote.”

“Could it have used a battery of laser cannon?”

“Sure, why not?” Renner wheeled over. “In fact, you could launch with a small battery, then add more cannon as the vehicle got farther and farther away. You get a terrific advantage that way. If one of the cannon breaks down you’ve got it right there in your system to repair it.”

Like leaving your motor home,” Potter cried, “and you still able to use it.

“Well, there are efficiency problems. Depending on how tight the beam can be held,” Renner answered. “Pity you couldn’t use it for braking, too. Have you any reason to believe—”

“Captain, look,” he said, and threw a plot of the local stellar region on the screen. “The intruder came from here. Whoever launched it fired a laser cannon, or a set of laser cannon — probably a whole mess of them on asteroids, with mirrors to focus them — for about forty-five years, so the intruder would have a beam to travel on. The beam and the intruder both came straight in from the Mote.”

(ed note: the lightsail was accelerated by lasers from its homeworld. But it braked by diving into New Cal's sun.)

“But that’s the point: it’s not right, Captain,” Renner protested. “You see, it is possible to turn in interstellar space. What they should have done—

The new path left the Mote at a slight angle to the first. “Again they coast most of the way. At this point” — where the intruder would have been well past New Cal — “we charge the ship up to ten million volts. The background magnetic field of the Galaxy gives the ship a half turn, and it’s coming toward the New Caledonia system from behind. Meanwhile, whoever is operating the beam has turned it off for a hundred and fifty years. Now he turns it on again. The probe uses the beam for braking.

“You sure that magnetic effect would work?”

“It’s high school physics! And the interstellar magnetic fields, have been well mapped, Captain.”

“Well, then, why didn’t they use it?”

“I don’t know,” Renner cried in frustration. “Maybe they just didn’t think of it. Maybe they were afraid the lasers wouldn’t last. Maybe they didn’t trust whoever they left behind to run them. Captain, we just don’t know enough about them.”

(ed note: spoiler alert: the answer is they didn't trust who they left behind to run the laser cannons. So when it came to braking, they did it the hard way.)

(ed note: In Mallove and Matloff's The Starflight Handbook, they note that if the interstellar magnetic fields have not been well mapped, this scheme could potentially doom the starship to a lonely death.)

From The Mote In God's Eye by Larry Niven and Jerry Pournelle (1975)
At The Mercy Of Homeworld

(ed note: the laser transmitter lens was big enough to launch the starship. But the size of the lens has to be increased for the deceleration phase. Evil political hack Senator Winthrop manages to steal GNASA's budget for expanding the mirror so he can funnel it into tobacco farmer subsidies in his home state.)

Senator Beauregard Darlington Winthrop III was in his third term of office, and as Chairman of the Senate Appropriations Committee he wielded an influence only slightly less potent than the Senate Majority Leader. GNASA officials winced when they heard that budget-hearing time was coming around again.

"Now. Ah'm sure you honorable gentlemen realize that this nation, as rich and as glorious as it is, cannot afford every space boondoggle there is. Ah trust that you've come up with a budget that realizes that there are people here on the ground that desperately need money to keep their family businesses alive..."

"He probably means subsidies for the tobacco farmers," thought the Honorable Leroy Fresh, as he prepared to defend GNASA's budget before the committee.

"There is one item that the Chairman noticed in the preliminary reports that he would like to question the Honorable Dr. Fresh about, if he may." Without waiting for a reply, Winthrop continued. "I notice this line-item number one hundred eight, for four hundred million dollars to expand the transmitter lens for the Barnard laser propulsion system. I didn't notice that in the previous year's budget, and since the mission is not slated to reach Barnard for another twenty years or so, surely this item could be deferred a year or two to release a few funds to succor the poor people of this nation?"

Leroy was ready for this one. "May I remind the Chairman, the reason the item was not in last year's budget was that it was removed by the Senate Appropriations Committee, as it has each year for nearly the past decade. The transmitter lens doesn't have to be full size at the start of the mission, and can be built slowly as time passes and the Barnard expedition moves further away, but the lens must be made ready for the deceleration phase, which requires it to be at maximum diameter. The amount of money in the budget is that needed to bring us back on schedule."

"But the lasers are turned off, and the Barnard lightsail is merely coasting on its way to its destination. Surely we can defer work on the lens expansion since it's not being used. Especially since I notice in line-item one hundred ten the fifty million dollars for the construction of the Tau Ceti lens. The increase in diameter planned for each lens is fifty kilometers. Surely that indicates that they should have equal budgets. Perhaps we should just make those two lens-construction items both equal in size at fifty million?" Senator Winthrop looked around at his committee and smiled.

"Is that agreeable, gentlemen? ...Oh, yes. Excuse me, Madam Ledbetter. Is that agreeable, gentlemen and lady?" He raised a blue pencil and scratched away at his copy of the budget.

"But Senator Winthrop, Sir," Leroy protested. "The Ceti lens is going from a diameter of twenty kilometers to seventy kilometers, while the Barnard lens is going from three hundred and twenty to three hundred and seventy kilometers. Even though both have the same increase in diameter, the increase in area of the Barnard lens is eight times larger than that of the Ceti lens. The cost goes as the square of the diameter."

"Well, Ah must admit Ah'm a little 'square' when it comes to that scientific math, Dr. Fresh, but Ah'm pretty good at figures when they have a dollar sign in front of them." There was a polite laugh at the Chairman's joke from the committee and staff. Fresh was silent, knowing that he had lost another skirmish. "After all," said Senator Winthrop with a smile that seemed entirely sincere over the TV cameras. "That's what we have you scientific types at GNASA for, to take care of all that 'square root' and 'cube root' type math stuff. And Ah must say," he said, with only a trace of sarcasm, "You've been doing an excellent job on an austere budget—like the true Greater American patriots that you are. Now, let's go on to line-item one hundred thirty-three, the million-channel receiver to search for signals from aliens. Surely a single channel is all that you need. It's obvious. One receiving antenna, one receiving channel..."

(ed note: the stupid, it burns!)

(ed note: things start to unravel, and Evil Senator Winthrop frantically looks for a way to avoid his fate )

"As Chairman of the Senate Appropriations Committee, what are your plans for completing the transmitter lens for the Barnard star expedition so the crew can be brought safely to a halt?"

Winthrop didn't know the details, but he wasn't stupid. There was no way that the transmitter lens could be completed in time. Twenty years ago the construction of the lens had been stopped just short of one-third-diameter. The lens had to be nearly full-sized if the deceleration technique were to work. Since the diameter had to be tripled, the area had to increase nine times. Although the ship would not need to start decelerating at Barnard for nearly eight years, the light beam to carry out that deceleration had to be on its way across the six lightyears between here and there only twenty-four months from now. There wasn't enough time. That g*******d Gudunov was doomed.

(ed note: Evil Senator Winthrop gets his just desserts, and the new pro-space congress tries to repair the damage )

The first action of the new chairman was to call for testimony from the newly appointed head of the Space Agency, the Honorable Perry Hopkins.

"I'm pleased to have you with us today, Dr. Hopkins," said Senator Rockwell. "I know we're all concerned about our brave crew that are approaching Barnard, ready to stop. Now, in the past, this committee, under the leadership of our distinguished Minority Leader..." here Senator Rockwell turned to nod to Senator Winthrop down near the end of the table, "...found it expedient for the sake of the small farmers of this nation to defer certain items of expenditure for the space program. We realize that this may have caused you some problems in the past and we want you to know that the time has come for the space program to receive the resources that it needs to carry out its mission. Tell us. What do you need to bring this great nation's crew of astronauts to a successful conclusion of their epic voyage?"

"I wish I could tell you, Senator Rockwell," he started. "But I'm afraid I can't. And I can't because there is no answer. The previous GNASA administrators have reported to this committee an infinite number of times that work needed to be done on the Barnard transmitter lens. But that... (careful now, Perry, calm down)... the previous chairman always felt it could be postponed until some future date. Well, gentlemen, that date was two years ago."

"Do you mean to tell me that there is no way to allow our brave crew to come to a safe landing at their destination?" said Senator Rockwell.

"I don't mean to be melodramatic, Mr. Chairman. And I have exercised my staff for alternatives, but unless someone comes up with a miracle, that crew is as good as dead."

"But surely with a crash effort..."

"There are only so many robots in space, and due to the low demand for space robots, there is only one space robot factory," said Perry. "Even if we could speed up the production line by five times, and even if we had some magical way to transport those robots instantly over the ten astronomical units to the transmitter lens and put them all to work, there isn't enough web and plastic in the solar system to make up for twenty years of neglect. At best we could get the lens up to sixty percent of the necessary diameter. Even if the lasers were up to power, that would only suffice to strand the crew some two lightyears beyond Barnard, with no hope of getting back. I'm sorry to bring you such bad news, gentlemen, but it's the best I have!"

(ed note: spoiler alert: they managed to save the crew. Somebody invented a nonlinear material that would frequency triple, turning three infrared photons {at fifteen hundred nanometer frequency} into one green photon {at five hundred nanometer frequency}. If a laser beam has its wavelength cut by one-third then a lens of a given size can sent it three times as far. So while the existing transmitter lens is only 1/3rd the size it need be for an infrared laser beam, it is just the right size for a green laser beam.)

From The Flight of the Dragonfly aka "Rocheworld" by Dr. Robert E. Forward


Starwisp is an ultra-low mass interstellar probe, a tiny sail driven by a beam of microwaves. The concept was invented by Dr. Robert L. Forward, and expanded upon by Dr. Geoffrey A. Landis.

Dr. Forward assumed that the microwave beam would be efficiently reflected by starwisp, so he calculated it would be a superconducting metal mesh with a sail mass of 16 grams and a payload mass of 4 grams; total mass of probe is 20 grams. Dr. Landis found this turned out not to be the case, it would absorb quite a bit of microwaves and heat up (i.e., the design is thermally limited). In Dr. Landis' design the starwisp is woven out of carbon wires with a sail mass of 1,000 grams, a payload mass of 80 grams, and a diameter of 100 meters.

Acceleration is 24 m/s2, microwave lens 560 km in diameter transmitting 56 GW of power, accelerating the probe to 10% of the speed of light.

Yes, it probably could be weaponized. See Accelerando by Charles Stross.

Another form of beamed power propulsion uses beams of microwaves to drive the starship. Microwave energy has the great advantage that it can be made and transmitted at extremely high efficiencies, although it is difficult to make narrow beams that extend over long distances. Because of the short transmission range, the starship being pushed by the microwave beam must accelerate at a high rate to reach the high velocities needed for interstellar travel before the starship gets too far from the transmitting system (which means it can be weaponized). The accelerations required are larger than a human being can stand, so microwave pushed starships seem to be limited to use by robotic probes. There is one design that looks quite promising. I call it Starwisp, because of its extremely small mass.

Starwisp is a light-weight, high-speed interstellar flyby probe pushed by beamed microwaves. The basic structure of the Starwisp robotic starship is a wire mesh sail with microcircuits at the intersection of the wires. The microwave energy to power the starship is generated by a solar powered station orbiting Earth. The microwaves are formed into a beam by a large fresnel-zone-plate lens made of sparse metal mesh rings and empty rings. Such a lens has very low total mass and is easy to construct.

The microwaves in the beam have a wavelength that is much larger than the openings in the wire mesh of the Starwisp starship, so the very lightweight perforated wire mesh looks like a solid sheet of metal to the microwave beam. When the microwave beam strikes the wire mesh, the beam is reflected back in the opposite direction. During the reflection process, the microwave energy gives a push to the wire mesh sail. The amount of push is not large, but if the sail is light and the power in the microwave beam is high, the resultant acceleration of the starship can reach hundreds of times Earth gravity. The high acceleration of the starship by the microwave beam allows Starwisp to reach a coast velocity near that of light while the starship still close to the transmitting lens in the solar system.

Prior to the arrival of Starwisp at the target star, the microwave transmitter back in the solar system is turned on again and floods the star system with microwave energy. Using the wires in the mesh as microwave antennas, the microcircuits on Starwisp collect enough energy to power their optical detectors and logic circuits to form images of the planets in the system. The phase of the incoming microwaves is sensed at each point of the mesh and the phase information is used by the microcircuits to form the mesh into a retrodirective phased array microwave antenna that beams a signal back to Earth.

A minimal Starwisp would be a one kilometer mesh sail weighing only sixteen grams and carrying four grams of microcircuits. (The whole spacecraft weighs less than an ounce—you could fold it up and send it through the mail for the cost of first class postage.) This twenty gram starship would be accelerated at 115 times Earth gravity by a ten gigawatt (10,000,000,000 watt) microwave beam, reaching twenty percent of the speed of light in a few days. Upon arrival at Alpha Centauri some twenty years later, Starwisp would collect enough microwave power from the microwave flood beam from the solar system to return a series of high resolution color television pictures during its fly-through of the Alpha Centauri system.

Because of its small mass, the ten gigawatt beamed power level needed to drive a minimal Starwisp is about that planned for the microwave power output of a solar power satellite. Thus, if power satellites are constructed in the next few decades, they could be used to launch a squadron of Starwisp probes to the nearer stars during their "checkout" phase.

From Indistinguishable from Magic by Robert Forward (1995)

Laser-Pushed Lightsail

So your gigantic laser battery at home pushes the laser sail starship to its destination, accelerating it to about half the speed of light. Presumably you want to stop at your destination instead of streaking through it at 0.5c. But how?

If you were going about an order of magnitude slower, you might be able to use the sunlight from the destination star to put on the brakes. However that ain't gonna be enough at 0.5c. You'll just pancake into the star at a substantial fraction of the speed of light and be vaporized.

Dr. Philip Norem had a clever idea. Interstellar space has large magnetic fields. So one can use large electrical charges on the starship to make huge light-year wide sweeping turns by the Lorentz force.

Say you were going to Alpha Centauri. You aim the starship not at the destination, but instead off to one side. How far off depends upon the starship's turning radius. The laser battery back at the solar system pushed the starship up to relativistic velocities over the next 27 years or so. Then the lasers turn off.

The starship deploys one hundred metal cables, each about 50,000 kilometers long. It then charges them up to 800,000 volts and 3.7×104 coulombs. This is timed to interact with the interstellar magnetic field (as mapped) so that the starship makes a huge gradual turn, until it is approaching Alpha Centauri from the back door. That is, so that a line drawn from the starship to Alpha Centauri will pass directly through the solar system and the laser battery.

Meanwhile, the solar system laser battery starts up its barrage long enough in advance so that the leading edge of the laser wavefront will reach the starship just as it is aligned properly. It then continues the barrage for the years required to bring the starship to a halt exactly at Alpha Centauri.

In Mallove and Matloff's The Starflight Handbook, they note that if the interstellar magnetic fields have not been well mapped, this scheme could potentially doom the starship to a lonely death. If the starship misses the beam, it just goes sailing off into the Big Dark. The Starflight Handbook has the equations for a starship using the Lorentz force, if you are interested.

Laser sail propulsion is the one method for achieving star travel with human crews that is closest to reality. It will be some time before our engineering capabilities in space will be up to building the laser system needed, but there is no new physics involved, just a large scale engineering extrapolation of known technologies. In laser sail propulsion, light from a powerful laser is bounced off a large reflective sail surrounding the payload. The light pressure from the laser light pushes the sail and payload, providing the needed thrust. The laser sail starship is about as far from a rocket as is possible. The starship consists of nothing but the payload and the lightweight sail structure. The rocket engine of our starship is the laser, powered by an energy source such as the Sun. The reaction mass is the laser light itself.

For interplanetary operation and interstellar flight, the lasers would be in near-Earth space and powered by sunlight collected by large reflectors, sending their beams out to push the sails of the interplanetary fleet with the light pressure from their powerful beams. For pushing an interstellar starship, the lasers might work better if they were in orbit around Mercury. There is more sunlight there and the gravity attraction of Mercury would keep them from being "blown" away by the back reaction from their light beams. The lasers would use the abundant sunlight at Mercury's orbit to produce coherent laser light, which would then be combined into a single coherent beam and sent out to a transmitter lens floating between Saturn and Uranus.

The transmitter lens would be a fresnel-zone-plate lens with dimensions tuned to the laser frequency and consisting of wide rings of one-micrometer-thick plastic film alternating with empty rings. The transmitter lens would not be in orbit, but would either be freely falling (very slowly at that distance from the Sun), or "levitated" in place by rockets or by the momentum push from a portion of the laser light passing through it. The lens would be 1000 kilometers in diameter (as big as Texas) and mass about 560,000 tons. A lens this size can send a beam of laser light over forty lightyears before the beam starts to spread.

The first interstellar mission that could be performed with this laser and lens system would be a one-way flyby robotic probe mission to the nearest star system. The robotic probe would have a total mass of one metric ton, about one-third each of payload, support structure, and thin aluminum film reflecting panels. The sail portion of the probe would have a diameter of four kilometers.

The probe would be pushed at an acceleration of three percent of Earth gravity by an array of solar-pumped lasers with a total power of 65,000 megawatts or 65 gigawatts. While this is a great deal of laser power, it is well within our future capabilities. Power levels of this magnitude are generated by the Space Shuttle rocket engines during liftoff, and one of the ways to make a high power laser is to put mirrors across the exhaust of a high power rocket. If the acceleration is maintained for three years, the interstellar probe will reach the velocity of eleven percent of the speed of light at a distance of only one-sixth of a lightyear. At this distance it is still within range of the transmitter lens and all of the laser power is still focused on the sail. The laser is then turned off (or used to launch another robotic probe) and the robotic starship coasts to its target, flying through the Alpha Centauri system forty years after launch.

When I first invented the concept of laser-pushed lightsails back in 1962, I thought it was obvious that since all the laser can do is push the lightsail, it would not be possible to use a solar system laser to stop the lightsail at the target system. The idea seemed to be limited to fly-by precursor robotic probe missions. It wasn't until twenty years later, while trying to find a new way of traveling to the stars for a novel I was writing, I realized that if the lightsail were separated into two parts, then one part could be used as a mirror to reflect the laser light back toward the solar system. That retrodirected light could then be used to decelerate the other portion of the lightsail. When I worked out the equations and put numbers into it, I found that not only was it a good science fiction idea, but it would really work. The concept has since been published as a scientific paper in the Journal of Spacecraft and Rockets, and one of the references to prior work in the scientific paper is my novel, The Flight of the Dragonfly, later reissued by Baen Books in a much expanded version as Rocheworld.

If the reports from the unmanned probes are favorable, then the next phase would be to send a human crew on an interstellar exploration journey. More than just the nearest star system will ultimately need to be explored, so I designed the laser lightsail starship to allow a roundtrip exploration capability out to twelve lightyears, so Tau Ceti or Epsilon Eridani can be visited within a human lifetime. I assumed the diameter of the lightsail at launch to be 1000 kilometers in diameter, the same size as the transmitting lens. The total weight would be 80,000 tons, including 3,000 tons for the crew, their habitat, their supplies, and their exploration vehicles. The lightsail would be built with three stages. There would be a disc-shaped inner "return stage" portion, 100 kilometers in diameter, that would carry the payload and crew, and return them to Earth. This would be surrounded by a ring-shaped "accelerator stage" portion, 320 kilometers in diameter with a 100 kilometer diameter hole. Together, these two sails constitute the "rendezvous stage" that would stop at the target star. This in turn would be surrounded by the "decelerator stage", 1000 kilometers in diameter with a 320 kilometer diameter hole. [See Figure 7.]

All three portions of the lightsail would be accelerated together at thirty percent of Earth gravity by 43,000 terawatts of laser power. At this acceleration, the lightsail would reach a velocity of half the speed of light in 1.6 years. The expedition would reach Epsilon Eridani in twenty years Earth time and seventeen years crew time, and it would be time to stop.

At a half-lightyear from the target star, the 320 kilometer rendezvous stage would be detached from the center of the lightsail and turned to face the large ring-shaped decelerator stage that remains. The laser light coming from the solar system would reflect from the decelerator stage acting as a retro-directive mirror. The reflected light would decelerate the smaller rendezvous sail and bring it to a halt at Epsilon Eridani.

After the crew explored the system for a few years (using their rendezvous stage lightsail as a solar sail), it would be time to bring them back. To do this, the 100 kilometer diameter return stage would be separated out from the center of the 320 kilometer ring-shaped accelerator stage. The laser light from the solar system would hit the accelerator stage and be reflected back on the return stage. The laser light would then accelerate the return stage and its payload back toward the solar system. As the return stage approached the solar system twenty Earth-years later, it would be brought to a halt by a final burst of laser power. The members of the crew would have been away 51 years (including five years of exploring), have aged 46 years, and would be ready to retire and write their memoirs.

From Indistinguishable from Magic by Robert Forward (1995)

Mechanical Reliability

A related issue is mechanical reliability. Currently the best space probe NASA can build cannot be guaranteed to properly function past about forty years. The starship will need an extensive self-repair capability or have some way of having humans periodically available to fix things.

Jumping The Gun

A common science fiction gag is the "jumping the gun" plot. A slower than light ship departs on a 500 year journey to Alpha Centauri. About 100 years after launch, some joker on Terra invents a faster-than-light starship. Fleets of FTL ships fly to Alpha Centauri and colonize the place. The slower than light ship arrives to find not the virgin planets they were expecting, but instead 400 year old colonies. Har, har, silly slowboat.

The earliest example of this trope that I could find was A. E. van Vogt's "Far Centaurus" (1944)

In 2006, scientist Andrew Kennedy actually studied the problem. He published his analysis in a paper called Interstellar Travel: The Wait Calculation and the Incentive Trap of Progress. In it, he introduced his solution: the Wait Calculation.

The Wait Calculation allows future space explorers to avoid the "jumping the gun" problem (and also avoid being paralyzed with indecision by terror of jumping the gun). The equation shows that, assuming technology develops in such a way that there is exponential growth in the velocity of travel, there is an optimal departure time for arriving earliest. Kennedy states that the equation applies even if somebody invents faster-than-light travel.

You see, there comes a time when although technological advances continues to produce higher speeds, the waiting time for that advance is too long to make up the velocity difference. If you wait too long for a higher speed, a slower ship launched sooner will have enough of a head start to beat you.

For details about the equation, see the Wikipedia article. It is also discussed in a blog post (and in the comment section) at Centauri Dreams.


When we get into space, we can note Voyager 1’s 17 kilometers per second as it leaves the Solar System. The Helios solar probes launched in 1974 and 1976 set the current record at 70.22 km/s. And looking forward, the Solar Probe Plus mission is to perform a close flyby of the Sun, reaching a top heliocentric speed of 195 kilometers per second, which works out to 6.5 × 10 −4 c. If Breakthrough Starshot realizes its goal, an interstellar lightsail may one day head for Proxima Centauri at fully 20 percent of the speed of light.

Part of what occupies René Heller in his new paper is the exponential growth law we can construct between the 1804 Penydarren locomotive and the 17 kilometers per second of Voyager 1 in 2015. From wind- to steam-driven ships and into the realm of automobiles, then aircraft and, finally, rockets, we can extrapolate speeds that may take us into interstellar probe territory some time in this century or the next. Given that an interstellar mission may take longer than the average human lifetime, we thus need to ask a key question. When do we launch?

For the problem, a classic in science fiction, is to work out the most efficient timing. If we launch a starship at a particular level in our technology, will it not be caught by a faster ship launched at a much later date? Given sufficient technological improvements, a later launch (incorporating the necessary ‘wait time’) could result in an earlier arrival.

Those who have read A. E. van Vogt’s story “Far Centaurus” will recall precisely that scenario, when an Alpha Centauri mission reaches destination only to find it populated by humans who arrived by faster means. It’s a theme that shows up in Heinlein’s Time for the Stars and many other places.

Heller calls this problem ‘the incentive trap.’ And he refers back to Andrew Kennedy’s 2006 paper, which looked at the problem with the assumption of an exponential growth of the interstellar travel speed. Kennedy was assuming a 1.4 % average growth rate, under which a minimum time to reach Barnard’s Star could be calculated: some 712 years from 2006.

What that means is this: There is a total time that includes the waiting time (waiting for improved technology) and the actual travel time, and we can calculate a minimum value for this total time by using our assumption about the exponential growth of the interstellar travel speed. Calculating the minimum value shows us when we can launch without fear of being overtaken by a faster future probe, in hopes of avoiding that “Far Centaurus” outcome.

But was Kennedy right? Heller’s own take on the incentive trap takes into account the possibility that Breakthrough Starshot may achieve a velocity of 20 percent of lightspeed within several decades, an outcome that would, in Heller’s words, “…fundamentally change both the assumptions and the implications of the incentive trap because the speed doubling and the compounded annual speed growth laws would collapse as v approaches c.” And whatever happens with Breakthrough Starshot, the speed growth of human-made vehicles turns out to be much faster than previously believed.

Intriguing results flow out of Heller’s re-examination of what Kennedy had called the ‘wait equation,’ and tomorrow I want to go deeper into the paper to explain how the scientist uses exponential growth law models to show us a velocity which, once we have attained it, will no longer be subject to the incentive trap of faster, later technologies. The results are surprising, particularly if Breakthrough Starshot achieves its goal in the planned 30 years. The implications for our reaching well beyond Alpha Centauri, as we’ll see, are striking.

The Heller paper is “Relativistic Generalization of the Incentive Trap of Interstellar Travel with Application to Breakthrough Starshot” (preprint). The Kennedy paper is “Interstellar Travel: The Wait Calculation and the Incentive Trap of Progress,” Journal of the British Interplanetary Society Vol. 59, No. 7 (July, 2006), pp. 239-247.


When to launch a starship, given that improvements in technology could lead to a much faster ship passing yours enroute? As we saw yesterday, the problem has been attacked anew by René Heller (Max Planck Institute for Solar System Research), who re-examined a 2006 paper from Andrew Kennedy on the matter. Heller defines what he calls ‘the incentive trap’ this way:

The time to reach interstellar targets is potentially larger than a human lifetime, and so the question arises of whether it is currently reasonable to develop the required technology and to launch the probe. Alternatively, one could effectively save time and wait for technological improvements that enable gains in the interstellar travel speed, which could ultimately result in a later launch with an earlier arrival.

All this reminds me of a conversation I had with Greg Matloff, author of the indispensable The Starflight Handbook (Wiley, 1989) about this matter. We were at Marshall Space Flight Center in 2003 and I was compiling notes for my Centauri Dreams book. I had mentioned A. E. van Vogt’s story “Far Centaurus,” originally published in 1944, in which a crew arrives at Alpha Centauri only to find its system inhabited by humans who launched from Earth centuries later. I alluded to this story yesterday.

Calling it a ‘terrific story,’ Matloff discussed it in terms of Robert Forward’s thinking:

“Bob had a couple of concepts of technological advancement. He had a famous plot of the velocity of human beings versus time. And he said if this is true, and you launch a thousand-year ship today, in a century somebody could fly the same mission in a hundred years. Theyre going to be passed and will probably have to go through customs when they get to Alpha Centauri A-2.”

Customs! Clearly, we’d rather not be on the slow starship that is superseded by new technologies. What Heller and Kennedy before him want to do is to figure out a rational way to decide when to launch. If we make assumptions about the exponential growth in speed over time, we can address the question by adding the time we spend waiting for better technology to the time of the actual journey. We can then calculate a minimum value for this figure based on the growth rates we find in our historical data.

This is how Kennedy came up with a minimum figure of 712 years (from 2006) to reach Barnard’s Star, which is about 6 light years away. The figure would include a long period of waiting for technological improvement as well as the time of the journey itself. Kennedy used a 1.4 percent annual growth in speed in arriving at this figure but, examining 211 years of data on historical speed records, Heller finds a higher annual growth, some 4.72 percent.

From the Penydarren steam locomotive of 1804 to Voyager 1, we see a speed growth of about four orders of magnitude. Growth like this maintained for another 112 years leads to 1 percent of lightspeed.

But how consistent should we expect the growth in speed over time to be? Heller points out that the introduction of new technologies invariably leads to jumps in speed. We are now in the early stages of conceptualizing the Breakthrough Starshot project, which could create exactly this kind of disruption in the trend. Starshot aims at reaching 20 percent of lightspeed.

Working with the exponential speed doubling law we began with, we would expect that a speed of 20 percent of c would not be achieved until the year 2191. But if Starshot achieves its goal in the anticipated time frame of several decades, its success would see us reaching interstellar speeds much faster than the trends indicate. Starshot, or a project like it, would if successful exert a transformative effect as a driver for interstellar exploration.

We know that speed doubling laws cannot go on forever as we push toward relativistic speeds (we can’t double values higher than 0.5 c). But as we move toward substantial percentages of the speed of light, we see powerful gains in speed as we increase the kinetic energy beamed to a small lightsail like Starshot’s. Thus Heller also presents a model based on the growth of kinetic energy, noting that today the Three Gorges Dam in China can reach power outputs of 22.5 GW. 100 seconds exposure to a beam this powerful would take a small sail probe to speeds of 7.1 percent of c. Further kinetic energy increases could allow relativistic speeds for at least gram-to-kilogram sized probes within a matter of decades.

Usefully, Heller’s calculations also show when we can stop worrying about wait times altogether. The minimum value for the wait plus travel time disappears for targets that we can reach earlier than a critical travel time which he calls the ‘incentive travel time.’ Considered in both relativistic and non-relativistic models, this figure (assuming a doubling of speed every 15 years) works out to be 21.6 years. In Heller’s words, “…targets that we can reach within about 22 yr of travel are not worth waiting for further speed improvements if speed doubles every 15 yr.”

Thus already short travel times mean there is little point in waiting for future speed improvements. And in terms of current thinking about Alpha Centauri missions, Heller notes that there is a critical interstellar speed above which gains in kinetic energy beamed to the probe would not result in smaller wait plus travel times. His equations result in a value of 19.6 percent of c, an interesting number given that Breakthrough Starshot’s baseline is a probe moving at 20 percent of c, for a 20-year travel time. Thus:

In terms of the optimal interstellar velocity for launch, the most nearby interstellar target α Cen will be worthy of sending a space probe as soon as about 20 % c can be achieved because future technological developments will not reduce the travel time by as much as the waiting time increases. This value is in agreement with the 20 % c proposed by Starshot for a journey to α Cen.

We can push this result into an analysis of stars beyond Alpha Centauri. Heller looks at speeds beyond which further speed improvements would not result in reduced wait times for ten of the nearest bright stars. The assumption here would be that Starshot or alternative technologies would be continuously upgraded according to historical trends. Plugging in that assumption, we wind up with speeds as high as 57 percent of lightspeed for 70 Ophiuchi at 16.6 light years.

Thus the conclusion: If something like Breakthrough Starshot’s beaming capabilities become available within 45 years — and assuming that the kinetic energy transferred to the probes it pushes could be increased at the historical rates traced here — then we can reach all ten of the nearest star systems with an interstellar probe within 100 years from today.

Just for fun let me conclude with a snippet from “Far Centaurus.” Here a ship is approaching the ‘slowboat’ that has just discovered that Alpha Centauri has been reached by humans long before. The crew has just puzzled out what happened:

I was sitting in the control chair an hour later when I saw the glint in the darkness. There was a flash of bright silver, that exploded into size. The next instant, an enormous spaceship had matched our velocity less than a mile away.

Blake and I looked at each other. “Did they say,” I said shakily, “that that ship left its hangar ten minutes ago?”

Blake nodded. ‘They can make the trip from Earth to Centauri in three hours,” he said.

I hadn’t heard that before. Something happened inside my brain. “What!” I shouted. “Why, it’s taken us five hund… ” I stopped. I sat there.

“Three hours!” I whispered. “How could we have forgotten human progress?”

The René Heller paper discussed in the last two posts is “Relativistic Generalization of the Incentive Trap of Interstellar Travel with Application to Breakthrough Starshot” (preprint).

(ed note: Bill, Blake, and Jim depart in the world's first slower-than-light starship on a trip to Alpha Centauri. They use hibernation to sleep for the 500 year trip.)

Blake rose to his feet. "Bill, after I'd read your reports about, and seen the photographs of, that burning ship, I got an idea. The Alpha suns were pretty close two weeks ago, only about six months away at our average speed of five hundred miles a second. I thought to myself: 'I'll see if I can tune in some of their radio stations.'"

"Well," he smiled wryly, "I got hundreds in a few minutes. They came in all over the seven wave dials, with bell-like clarity." He paused; he stared down at me, and his smile was a sickly thing. "Bill," he groaned, "we're the prize fools in creation. When I told Renfrew the truth, he folded up like ice melting into water."

Once more, he paused; the silence was too much for my straining nerves.

"For Heaven's sake, man" I began. And stopped. And lay there, very still. Just like that the lightning of understanding flashed on me. My blood seemed to thunder through my veins. At last, weakly, I said: "You mean ."

Blake nodded. "Yeah," he said. "That's the way it is. And they've already spotted us with their spy rays and energy screens. A ship's coming out to meet us.

"I only hope," he finished gloomily, "they can do something for Jim."

I was sitting in the control chair an hour later when I saw the glint in the darkness. There was a flash of bright silver, that exploded into size. The next instant, an enormous spaceship had matched our velocity less than a mile away.

Blake and I looked at each other. "Did they say," I said shakily, "that that ship left its hangar ten minutes ago?"

Blake nodded. 'They can make the trip from Earth to Centauri in three hours," he said.

I hadn't heard that before. Something happened inside my brain. "What!" I shouted. "Why, it's taken us five hund... " I stopped. I sat there. "Three hours!" I whispered. "How could we have forgotten human progress?"

From Far Centaurus by A. E. van Vogt (1944)
Vance Astro

"Exactly, Mr. Grimm. But I spent most of those years asleep, in suspended animation...aboard the first American rocket bound for the stars. I left the earth in 1988, travelling at a velocity of a million miles per hour. Even at that speed, the journey to Earth's nearest stellar neighbor required a millenium."

"But a mere 200 years after I left, a man named Harkov came along with a new theory of physics that made faster-than-light space travel a reality. When I came out of my ship on Centauri-IV, a colony of Earthmen was there to greet me!"

"I felt as you must have, Cap, when you were released from that iceburg back in 1964, after slumbering for two decades. Only I wasn't as lucky. As with you, my world, all the people I'd known and loved, were dead and gone. But you hadn't aged during your sleep — I had!"

"I am a prisoner of the copper foil suit that sustains my life. Once punctured, any exposure of my sky to fresh air ... and I crumble into dust. Martinex, our group's scientist, designed this additional outer shel I wear to prevent just that."

Lightspeed Leapfrog
Ten years we had been on our way when they found a hyper-drive
And man spread to a thousand stars while we were half alive
"Space is Dark", Bill Roper

The brave explorers or colonists set out in their spaceship to spread humankind to the stars. You can't travel faster than light, so they're going to spend most of the trip on a Sleeper Starship as Human Popsicles, or it's a Generation Ship and it'll be their descendants who step out at the other end of the trip. Either way, they're saying goodbye forever to everyone and everything they know. Decades and centuries pass, and eventually they arrive at their destination—

—and there's people there waiting for them. Turns out, Faster-Than-Light Travel is possible, and it got sorted out while they were in transit. Now the same trip that took them centuries can be done and be back in time for Christmas. And that planet you were all set to colonise? Done already, and actually we're not sure there's any room for you...

Expect the brave pioneers to be upset about this.

An in-universe Sub-Trope of Science Marches On. Can also be related to Humans Advance Swiftly.

(ed note: see TV Trope page for list of examples)

Go Fast

The second of Gordon Woodcock's methods of interstellar travel is "go fast".

Distance between stars is huge, traveling said distance slower-than-light will take a huge amount of time, human beings have a very limited lifespan. And it is much easier to travel at 10% the speed of light than it is to travel at 99.99999% the speed of light

"Go Fast" means to focus on traveling near the speed of light so that relativity will partially fix things. Time dialiation will allow the crew to experience only a few months passing while traveling to a star 50 light years away. Travleing back home to Terra will add a few more months to the crew's experience. Unfortunately they will discover that 50+50 = 100 years have passed n Terra during their round trip. But you can't have everything.

Naturally to the SF author, the more attractive option is to increase the speed of the starship. But this too has several serious problems.

First off, the equation for deltaV coupled with the huge velocities required imply some truly ugly mass ratios. We are talking about a crew cabin the size of a coffin strapped to the nose of a rocket ten times the size of the Empire State building. Or worse.

Secondly, that party-pooper Albert Einstein's theory of relativity more or less ruled out faster than light travel. And it inflicted extra difficulties for near-light travel.

And thirdly is the fact that space is not 100% empty. Remember Rick Robinson's First Law of Space Combat. At near light speeds hitting a dust speck will be like a contact explosion from a thermonuclear bomb. Indeed, individual protons will be transformed into deadly cosmic rays.



Einstein's theory of Special Relativity is an incredibly complicated topic, and I don't pretend to understand it all. Certainly I don't understand it enough to try and teach it. I'd advise you to go study the Wikipedia Special relativity for beginners or Jason Hinson's tutorial. If you want an intuitive feel for this: run, don't walk and get a copy of Poul Anderson's classic novel TAU ZERO.

But there are only a few specific implications of relativity that we have to worry about. Unless you are writing Gregory Benford style novels, in which case you know about the extra implications already.

First is of course the well-known fact that Special Relativity forbids any object possessing a rest mass from traveling at the speed of light in a vacuum (Which boils down to no FTL travel for you. Not yours. Science fiction authors have been cursing Einstein for decades over that one).

The second concern is "time dilation", crew members on a starship moving relativistically will age and experience time at a slower rate compared to people who stayed at home on Terra. The crew won't notice anything odd, until they return home to the Rip Van Winkle Experience. As a rule of thumb, you can figure the start of "moving relativistically" is arbitrarily when the the dilation effect gets bigger than 1/100th. This is when γ equals 1.01, which happens at about 14% c.

Thirdly it makes calculating transit times and mass ratios much more difficult.


In relativistic equations, a common factor called gamma (γ) appears often. Its value depends on the velocity of the starship.

γ = 1 / Sqrt[ 1 - (v2 / c2) ]


  • γ = gamma, the time dilation factor (dimensionless number)
  • Sqrt[x] = square root of x
  • v = current ship's velocity as measured in Terra's frame of reference (m/s)
  • c = speed of light in a vacuum = 3e8 m/s

Or more conveniently, you can make c = 1.0 and v the percentage of c, e.g., a starship moving at three-quarters light-speed would have v = 0.75. The ship's γ would be about 1.51.

How do you use gamma?

  • Time : A viewer on Terra will observe the crew of a starship moving relativistically relative to Terra living in slow motion. One unit of crew time will pass during one unit times gamma of Terra time

  • Mass : A viewer on Terra will observe a starship moving relativistically relative to Terra having an increased mass. The mass will be multiplied by gamma.

  • Length : A viewer on Terra will observe a starship moving relativistically relative to Terra having its length in the direction of motion shortened. The width and height will be the same, but the length will appear flattened. The length will be divided by gamma.

If a starship is moving at 0.99c relative to Terra, it's γ = 7.09. When the crew mark off one day passing inside the ship (the so-called "proper time"), 1 day * 7.09 = 7.09 days will pass on Terra. From the view point of people on Terra, the starship crew will be living and moving in slow motion, experiencing time at about 1/7th the rate on Terra (Due to the weird non-intuitive implications of relativity, from the viewpoint of the crew it will be the inhabitants of Terra who are moving in slow motion, but if you are not going to take the time to learn more about relativity you'd best ignore this).

With respects to a viewer on Terra, the starship's mass will increase by a factor of γ (which makes relativistic kinetic weapons quite deadly). The ship's length in the direction of travel will decreased by a factor of 1/γ, but nobody cares since this has little practical effect.


As a side note, at around 0.7 c the starship will be at Functional Lightspeed. This is because 0.7 c has a gamma of 1.4, and 1/1.4 is about 0.7 (unlike all the other values on the table). Well, actually after playing around on a spreadsheet it looks like it is closer to 0.707 c with a gamma of 1.414. But anyway:

What does this mean?

Say the good starship Breakaway is traveling to Alpha Centauri (distance 4.4 light-years) and is cruising at the Functional Lightspeed velocity of 0.7 c. The gamma is 1.4 and 1/γ = 0.7. From the viewpoint of the crew, the 4.4 light year distance appears to be only 3.1 light years (4.4×0.7=3.1 where 0.7 is 1/γ). The speed is still 0.7 c.

So for the crew, the trip will appear to take 4.4 years (3.1/0.7 where 0.7 is percent of lightspeed), where 4.4 is the quote "real" unquote distance to Alpha C in light-years. The crew will conclude they are traveling at one light-year per year, the speed of light, even though they are not. "Functional Lightspeed."


As another side note, the equation for gamma demonstrates how things go haywire when you calculate speed faster than light. Look at the formula for gamma above. If c = 1.0 and v = 2.0 (that is, a velocity of twice light speed), what is gamma?

Well, there is a problem there:

γ = 1 / Sqrt[1 - (v2 / c2)]
γ = 1 / Sqrt[1 - (2.02 / 1.02)]
γ = 1 / Sqrt[1 - (4 / 1)]
γ = 1 / Sqrt[1 - 4]
γ = 1 / Sqrt[-3]

The problem is when you try to take the square root of -3. If you try it on your calculator it will flash you INVALID INPUT! This is because there ain't no number you can multiply by itself to get a negative number (because a positive times a postive is a positive number, and a negative times a negative is also a positive). The only way you can get a negative number is by multiplying a negative by a postive, but by definition squaring a number means multiplying the same number together.

So if you try to calculate the gamma of a velocity faster than light, the equation blows up in your face.

Mathematicians have constructed towers of bizarre theories by saying "let's wave our hands and say there is weird number called i, such that i2 = -1." These are called, appropriately enough, imaginary numbers. The practical point is these numbers have been around since the 17th century, but they haven't helped much making a faster than light starship.


      “The latest development is the mass-conversion ship, such as the Mayflower, and it may be the final development—a mass-conversion ship is theoretically capable of approaching the speed of light. Take this trip: we accelerated at one gravity for about four hours and twenty minutes which brought us up to more than ninety miles a second. If we had held that drive for a trifle less than a year, we would approach the speed of light.
     “A mass-conversion ship has plenty of power to do just that. At one hundred per cent efficiency, it would use up about one per cent of her mass as energy and another one per cent as reaction mass. That’s what the Star Rover is going to do when it is finished.”

     One of the younger kids was waving his hand. “Mister Chief Engineer?”
     “Yes, son?”
     “Suppose it goes on a few weeks longer and passes the speed of light?”
     Mr. Ortega shook his head. “It can’t.”
     “Why not, sir?”
     “Eh, how far have you gone in mathematics, sonny?”
     “Just through grammer school calculus,” the kid answered. (ed note: egads!)
     ‘Tm afraid there is no use in trying to explain it, then. Just take it from me that the big brains are sure it can’t be done.”

     I had worried about that very point more than once. Why can’t you go faster than light? I know all that old double-talk about how the Einstein equations show that a speed faster than light is a meaningless quantity, like the weight of a song or the color of a sound, because it involves the square root of minus one—but all of that is just theory and if the course we had in history of science means anything at all, it means that scientists change their theories about as often as a snake changes his skin. I stuck up my hand.
     “Okay,” he says. “You with the cowlick. Speak up.”
     “Mr. Ortega, admitting that you can’t pass the speed of light, what would happen if the Star Rover got up close to the speed of light—and then the Captain suddenly stepped the drive up to about six g and held it there?”
     “Why, it would—No, let’s put it this way—” He broke off and grinned; it made him look real young. “See here, kid, don’t ask me questions like that. I’m an engineer with hairy ears, not a mathematical physicist.” He looked thoughtful and added, “Truthfully, I don’t know what would happen, but I would sure give a pretty to find out. Maybe we would find out what the square root of minus one looks like—from the inside.”

(ed note: in the real world what would happen is you'd continue to add more decimal 9s to your V/c, and your gamma would keep rising.)

From FARMER IN THE SKY by Robert Heinlein (1950)

In the following equations, note that a*T/c = (Ve / c) * ln(R)

Time elapsed (in Terra's frame of reference)

t = (c/a) * Sinh[a*T/c] (given acceleration and proper time)

t = (c/a) * Sinh[(Ve / c) * ln(R)] (to expend all propellant, given exhaust velocity and mass ratio)

t = sqrt[(d/c)2 + (2*d/a)] (given acceleration and distance)

Distanced traveled (in Terra's frame of reference)

d = (c2/a) * (Cosh[a*T/c] - 1) (given acceleration and proper time)

d = (c2/a) * (Cosh[(Ve / c) * ln(R)] - 1) (when all propellant is expended, given exhaust velocity and mass ratio)

d = (c2/a) (Sqrt[1 + (a*t/c)2] - 1) (given acceleration and Terra time)

Final Velocity (in Terra's frame of reference)

v = c * Tanh[a*T/c] (given acceleration and proper time)

Δv = c * Tanh[(Ve / c) * ln(R)] (given exhaust velocity and mass ratio)

v = (a*t) / Sqrt[1 + (a*t/c)2] (given acceleration and Terra time)

Time elapsed (in starship's frame of reference, "Proper time")

T = (c/a) * ArcSinh[a*t/c] (given acceleration and Terra time)

T = (c/a) * ArcCosh[a*d/(c2) + 1] (given acceleration and distance)

Gamma factor

γ = Cosh[a*T/c] (given acceleration and proper time)

γ = Cosh[(Ve / c) * ln(R)] (given exhaust velocity and mass ratio)

γ = Sqrt[1 + (a*t/c)2] (given acceleration and Terra time)

γ = a*d/(c2) + 1 (given acceleration and distance)


  • a = acceleration (m/s2) remember that 1 g = 9.81 m/s2
  • T = Proper Time, the slowed down time experienced by the crew of the rocket (s)
  • t = time experienced non-accelerating frame of reference in which they started (e.g., Terra) (s)
  • d = distance covered as measured in Terra's frame of reference (m)
  • v = final speed as measured in Terra's frame of reference (m/s)
  • c = speed of light in a vacuum = 3e8 m/s
  • Δv = rocket's total deltaV (m/s)
  • Ve = propulsion system's exhaust velocity (m/s)
  • R = rocket's mass ratio (dimensionless number)
  • γ = gamma, the time dilation factor (dimensionless number)
  • Sqrt[x] = square root of x
  • ln[x] = natural logarithm of x
  • Sinh[x] = hyperbolic Sine of x
  • Cosh[x] = hyperbolic Cosine of x
  • Tanh[x] = hyperbolic Tangent of x

The hyperbolic trigonometric functions should be present on a scientific calculator and available as functions in a spreadsheet.

In many cases it will be more convenient to have T and t in years, distance in light-years, c = 1 lyr/yr, and 1 g = 1.03 lyr/yr2.

Here are some typical results with a starship accelerating at one gravity.

T Proper time elapsedt Terra time elapsedd Distancev Final velocityγ Gamma
1 year1.19 years0.56 lyrs0.77c1.58

Of course, as a general rule starships want to slow down and stop at their destinations, not zip past them at 0.9999 of the speed of light. You need a standard torchship brachistochrone flight plan: accelerate to halfway, skew flip, then decelerate to the destination (which makes sense, since such starships will have to be torchships). To use the above equations, instead of using the full distance for d, divide the distance in half and use that instead. Run that through the equations, then take the resulting T or t and double it.


The good scout starship Peek-A-Boo is doing a 1 g brachistochrone for Vega, which is 27 light-years away. Half of that is 13.5 light-years. How long will the journey be from the crew's standpoint (the proper time)?

T = (c/a) * ArcCosh[a * d / (c2) + 1]
T = (1/1.03) * ArcCosh[1.03 * 13.5 / (12) + 1]
T = 0.971 * ArcCosh[13.9 / 1 + 1]
T = 0.971 * ArcCosh[13.9 + 1]
T = 0.971 * ArcCosh[14.9]
T = 0.971 * 3.39
T = 3.29 years
That's the crew time to the skew flip. The total time is twice this
T = 3.29 * 2
T = 6.58 years

But if you have more mathematical skills than I have, you could easily derive this short cut:

Tt = 1.94 * ArcCosh[dly/1.94 + 1]


  • Tt = Proper Time experienced during a brachistochrone flight (years)
  • dly = total distance to destination(light-years)

Remember this equation assumes a constant 1 g acceleration.

Extreme Relativistic Rocketry

In Stephen Baxter’s “Xeelee” tales the early days of human starflight (c.3600 AD), before the Squeem Invasion, FTL travel and the Qax Occupation, starships used “GUT-drives”. This presumably uses “Grand Unification Theory” physics to ‘create’ energy from the void, which allows a starship drive to by-pass the need to carry it’s own kinetic energy in its fuel. Charles Sheffield did something similar in his “MacAndrews” yarns (“All the Colors of the Vacuum”) and Arthur C. Clarke dubbed it the “quantum ramjet” in his 1985 novel-length reboot of his novella “The Songs of Distant Earth”.

Granting this possibility, what does this enable a starship to do? First, we need to look at the limitations of a standard rocket.

In Newton’s Universe, energy is ‘massless’ and doesn’t add to the mass carried by a rocket. Thanks to Einstein that changes – the energy of the propellant has a mass too, as spelled out by that famous equation:

For chemical propellants the energy comes from chemical potentials and is an almost immeasurably tiny fraction of their mass-energy. Even for nuclear fuels, like uranium or hydrogen, the fraction that can be converted into energy is less than 1%. Such rockets have particle speeds that max out at less than 12% of lightspeed – 36,000 km/s in everyday units. Once we start throwing antimatter into the propellant, then the fraction converted into energy goes up, all the way to 100%.

But… that means the fraction of reaction mass, propellant, that is just inert mass must go down, reaching zero at 100% conversion of mass into energy. The ‘particle velocity’ is lightspeed and a ‘perfect’ matter-antimatter starship is pushing itself with pure ‘light’ (uber energetic gamma-rays.)

For real rockets the particle velocity is always greater than the ‘effective exhaust velocity’ – the equivalent average velocity of the exhaust that is pushing the rocket forward. If a rocket energy converts mass into 100% energy perfectly, but 99% of that energy radiates away in all directions evenly, then the effective exhaust velocity is much less than lightspeed. Most matter-antimatter rockets are almost that ineffectual, with only the charged-pion fraction of the annihilation-reaction’s products producing useful thrust, and then with an efficiency of ~80% or so. Their effective exhaust velocity drops to ~0.33 c or so.

Friedwardt Winterberg has suggested that a gamma-ray laser than be created from a matter-antimatter reaction, with an almost perfect effective exhaust velocity of lightspeed. If so we then bump up against the ultimate limit – when the energy mass is the mass doing all the pushing. Being a rocket, the burn-out speed is limited by the Tsiolkovsky Equation:

(ed note: keeping in mind that such a gamma-ray laser plugged into the infinite power of the universe if used as a weapon would make the primary weapon of the Death Star look like a flashlight)

However we have to understand, in Einstein’s Relativity, that we’re looking at the rocket’s accelerating reference frame. From the perspective of the wider Universe the rocket’s clocks are moving slower and slower as it approaches lightspeed, c. Thus, in the rocket frame, a constant acceleration is, in the Universe frame, declining as the rocket approaches c.

To convert from one frame to the other also requires a different measurement for speed. On board a rocket an integrating accelerometer adds up measured increments of acceleration per unit time and it’s perfectly fine in the rocket’s frame for such a device to meter a speed faster-than-light. However, in the Universe frame, the speed is always less than c. If we designate the ship’s self-measured speed as and the Universe measured version of the same, , then we get the following:

[Note: the exhaust velocity, , is measured the same in both frames]


To give the above equations some meaning, let’s throw some numbers in. For a mass-ratio, of 10, exhaust velocity of c, the final velocities are = 2.3 c and = 0.98 c. What that means for a rocket with a constant acceleration, in its reference frame, is that it starts with a thrust 10 times higher than what it finishes with. To slow down again, the mass-ratio must be squared – thus it becomes 102=100. Clearly the numbers rapidly go up as lightspeed is approached ever closer.

A related question is how this translates into time and distances. In Newtonian mechanics constant acceleration (g) over a given displacement (motion from A to B, denoted as S) is related to the total travel time as follows, assuming no periods of coasting at a constant speed, while starting and finishing at zero velocity:

this can be solved for time quite simply as:

In the relativistic version of this equation we have to include the ‘time dimension’ of the displacement as well:

This is from the reference frame of the wider Universe. From the rocket-frame, we’ll use the convention that the total time is , and we get the following:

where arcosh(…) is the so-called inverse hyperbolic cosine.

Converting between the two differing time-frames is the Lorentz-factor or gamma, which relates the two time-flows – primed because they’re not the total trip-times used in the equation above, but the ‘instantaneous’ flow of time in the two frames – like so:

For a constant acceleration rocket, its is related to displacement by:

For very large factors, the rocket-frame total-time simplifies to:

The relationship between the Lorentz factor and distance has the interesting approximation that increases by ~1 for every light-year travelled at 1 gee. To see the answer why lies in the factors involved – gee = 9.80665 m/s2, light-year = (c) x 31,557,600 seconds (= 1 year), and c = 299,792,458 m/s. If we divide c by a year we get the ‘acceleration’ ~9.5 m/s2, which is very close to 1 gee.

This also highlights the dilemma faced by travellers wanting to decrease their apparent travel time by using relativistic time-contraction – they have to accelerate at bone-crushing gee-levels to do so. For example, if we travel to Alpha Centauri at 1 gee the apparent travel-time in the rocket-frame is 3.5 years. Increasing that acceleration to a punishing 10 gee means a travel-time of 0.75 years, or 39 weeks. Pushing to 20 gee means a 23 week trip, while 50 gee gets it down to 11 weeks. Being crushed by 50 times your own body-weight works for ants, but causes bones to break and internal organs to tear loose in humans and is generally a health-hazard. Yet theoretically much higher accelerations can be endured by equalising the body’s internal environment with an incompressible external environment. Gas is too compressible – instead the body needs to be filled with liquid at high pressure, inside and out, “stiffening” it against its own weight.

Once that biomedical wonder is achieved – and it has been for axolotls bred in centrifuges – we run up against the propulsion issue. A perfect matter-antimatter rocket might achieve a 1 gee flight to Alpha Centauri starts with a mass-ratio of 41.

How does a GUT-drive change that picture? As the energy of the propellant is no longer coming from the propellant mass itself, the propellant can provide much more “specific impulse”, , which can be greater than c. Specific Impulse is a rocketry concept – it’s the impulse (momentum x time) a unit mass of the propellant can produce. The units can be in seconds or in metres per second, depending on choice of conversion factors. For rockets carrying their own energy it’s equivalent to the effective exhaust velocity, but when the energy is piped in or ‘made fresh’ via GUT-physics, then the Specific Impulse can be significantly different. For example, if we expel the propellant carried at 0.995 c, relative to the rocket, then the Specific Impulse is ~10 c.

…where and are the propellant gamma-factor and its effective exhaust velocity respectively.

This modifies the Rocket Equation to:

Remember this is in the rocket’s frame of reference, where the speed can be measured, by internal integrating accelerometers, as greater than c. Stationary observers will see neither the rocket or its exhaust exceeding the speed of light.

To see what this means for a high-gee flight to Alpha Centauri, we need a way of converting between the displacement and the ship’s self-measured speed. We already have that in the equation:

which becomes:

As and , then we have

For the 4.37 light year trip to Alpha Centauri at 50 gee and an Isp of 10 c, then the mass-ratio is ~3. To travel the 2.5 million light years to Andromeda’s M31 Galaxy, the mass-ratio is just 42 for an Isp of 10c.

Of course the trick is creating energy via GUT physics…

From Extreme Relativistic Rocketry by Adam Crowl (2015)

Mass Ratio

As you may expect, the mass ratio for such rockets are generally absolutely outrageous. The "Relativistic Rocket" website made some estimates on the best possible mass ratios, assuming a 100% efficient photon rocket using constant acceleration.

Mass Ratio

R = (Mpt / Me) + 1, (1)

Mpt/Me = e(aT/c) - 1, (2)

Substituting (2) into (1):

R = e(a * T / c)


  • R = mass ratio (dimensionless number)
  • Mpt = Spacecraft's total propellant mass(kg)
  • Me = Spacecraft's empty (dry) mass (kg)
  • e = base of natural logarithms = 2.71828...(most calculators have an ex key, and spreadsheets have the exp() function)

What mass ratio will the Peek-A-Boo need for a fly-by, and for a brachistochrone? For a fly-by T = 3.94 years, for a brachistochrone T = 6.58 years.


R = e(a * T / c)
R = e(1.03 * 3.94 / 1.0)
R = e4.06
R = 57.97


R = e(1.03 * 6.58 / 1.0)
R = e6.78
R = 880.07

So for a brachistochrone the Peek-A-Boo will have to have 880.07 kilograms of propellant for every kilogram of ship that isn't propellant. Egad.

Why are these mass ratios absolutely outrageous? Because it is probably impossible to make a single-stage spacecraft with a mass ratio over about 20. And because the mass ratios that come out of the equation are the theoretical maximums of a 100% efficient photon drive. Since a real rocket is not going to be 100% efficient, and may not be a photon drive, the mass ratio will probably be much worse than what the equation suggests. It is also important to keep in mind that one g of constant acceleration is pretty huge. If the Peek-A-Boo only does 1/10th g, it will take 30 years of proper time to get to Vega, but it will only need a mass ratio of 21.

Other Relativistic Effects

The crew of a ship moving at relativistic velocities will notice some weird effects. The view of the sky will be distorted both fore and aft by relativistic aberration. Doppler shift will make the stars ahead look more blue, and the stars behind will appear more red. Back in the 1970's it was thought that the two effects would combine to make a sort of a rainbow of stars around the ship's destination. Alas, in 1980 a study published in the Journal of the British Interplanetary Society did the math and proved that it just wasn't going to happen.

Bussard Ramjet

So, there is the obscenely-huge-mass-ratio problem, and the deadly-space-junk problem. SF authors were depressed. Then in 1960, a brilliant physicist named Robert W. Bussard proposed to use these two problems to solve each other.

If your starship is moving fast enough, the widely scattered hydrogen atoms will hit your hull like cosmic rays, and damage both the ship and the crew. One can theoretically use magnetic or electrostatic fields to sweep the hydrogen atoms out of the way so the ship doesn't hit them.

But wait a minute. Hydrogen is propellant, and could also be fusion fuel. Instead of sweeping it away, how about gathering it?

And if you are gathering your propellant instead of carrying it along with you, your mass ratio becomes infinity. This means you could theoretically accelerate forever.

This is the legendary "Bussard Interstellar Ramjet." No mass ratio problems, and no space junk problems. Pretty slick, eh? Accelerating at 1 g a Bussard ramjet could reach the center of the galaxy in a mere twenty years of proper time, and could theoretically circumnavigate the entire visible universe in less than a hundred years.

(Keep in mind that twenty years to the galactic core is in terms of "proper time", that is, the time as experienced by the crew. The people who stay at home on Earth will still see the Bussard ramjet taking the better part of 25,000 years to make the trip.)


Acceleration of a Ramjet

Consider a ramjet moving through the interstellar medium at speed u. Translating to the ramjet's frame of reference, this is equivalent to the medium flowing past a stationary ramjet at speed u. Assume that whatever mass is collected in the intake funnel is ejected from the rear of the ramjet at speed v (relative to the ramjet), which is naturally greater than u. The change in momentum of a given mass m of interstellar medium on passing through the ramjet is:
Δ momentum = m (v - u)
By the conservation of momentum, this is equal to the change in momentum of the ramjet:
m (v - u) = M Δ V
M = the mass of the ramjet ship
Δ V = the change in velocity of the ramjet ship.
Note: This equation is an approximation which neglects the small amount of collected mass which is converted into energy by the nuclear fusion reaction. For hydrogen fusion, less than 1% of the mass is lost in this way, so any error is quite small. The acceleration of the ramjet a is then given by:
a = dV / dt = m (v - u) / M dt
dt = an "infinitesimal change in time" (I am not bothering with strict formalities of calculus here).
Now, the change in kinetic energy of the interstellar medium material Δ (m v2) / 2 is equal to the generated engine power P multiplied by the change in time:
P dt = Δ (m v2) / 2
      = m (v2 - u2) / 2
      = m (v - u) (v + u) / 2
But (v + u) / 2 is the average speed V of the ramjet relative to the interstellar medium over the time increment in question. Substituting this, and the acceleration formula above:
P dt = a M dt V
P = a M V
Now consider the volume of interstellar medium swept up by the ramjet funnel. If the effective funnel (including any electromagnetic attraction fields) is circular, with a radius r, then in a time dt it sweeps through a volume of:
π r2 V dt
If the density of hydrogen nuclei in the interstellar medium is ρ (in mass per unit volume units), then the mass of hydrogen nuclei swept up in time dt is:
π r2 V ρ dt
This mass is available for conversion into energy, with a nuclear fusion efficiency η (η is 0.753% for hydrogen fusion), so:
E = m c2
P dt = π r2 V ρ η c2 dt
c = the speed of light.
Substituting the formula for power above and rearranging:
a = π r2 ρ η c2 / M

This means the acceleration of a ramjet is dependent only on the size of the collecting funnel, density of the interstellar medium, efficiency of the nuclear fusion reaction, and mass of the ship, and is a constant over time. In other words, the ship's velocity will increase linearly with time.

The limit to this velocity increase is the speed of light, and close to the speed of light the equation derived above will break down due to the effects of special relativity.

Threshold Speed

Normally a Bussard ramjet needs to be moving at a certain threshold speed before the ramjet engine can begin operation. If the ship is moving too slowly, hydrogen may be swept up at too slow a rate to sustain the nuclear fusion reaction.

If we assume a threshold mass-collection rate dm/dt (the units are mass per unit time), then the rate of mass collection by the funnel π r2 ρ V needs to be greater than the threshold. This gives a threshold velocity:

Vt   >   (dm/dt) / ( π r2 ρ )
Below this velocity, the ramjet engine will not work.

In order to get up to the threshold velocity, a ramjet may be equipped with a reaction engine with its own power and reaction mass supply. This engine can be switched off once the ramjet begins to work.

Slowing Down

A ramjet which needs to slow down can utilise its mass collection system as a brake by simply collecting the incoming matter rather than fusing and ejecting it.

Consider a ramjet moving at speed V with respect to the interstellar medium. If matter collected by the funnel is stored in the ship, then in a time increment dt an amount of mass dm is given a change in momentum equal to the change in momentum of the ship, but in the opposite direction:

dm V = - M dV
But the mass collected in this time interval is as given under Acceleration of a Ramjet above, so:
π r2 ρ V2 dt = - M dV
dt / dV = - M / ( π r2 ρ V2 )
Integrating with respect to V from time to when speed is Vo to time t when speed is V:
t = [ M / ( π r2 ρ ) ] (1 / V - 1 / Vo)
Rearranging to make speed the subject as a function of time:
V = M Vo / (M + π r2 ρ Vo t )
Note that the drag generated on the ship by the incoming interstellar medium does not affect the acceleration calculated above, since only the total change of momentum is relevant (and is how the acceleration was calculated).
From BUSSARD RAMJETS by David Morgan-Mar (2004)

Ramjet Problems

Of course not everything is rainbows and unicorns, there are a few problems.

The density of the vacuum of space is about 10e-21 kg/m3. This means you have to scoop a gargantuan 10e18 cubic meters in order to harvest a single gram of hydrogen. Bussard, working with an estimate of one hydrogen atom per cubic centimeter, and desiring a 1,000-ton spacecraft with an acceleration of 1 g, figured that the scoop mouth will need a frontal collecting area of nearly 10,000 km2. Assuming the scoop mouth is circular, I figure the mouth will have to be about 56 kilometers radius or 112 kilometers diameter. Other estimates have the scoop orders of magnitude larger. It is probably out of the question to build a physical scoop of such size, so it will have to be an immaterial scoop composed of magnetic or electrostatic fields.

Hydrogen ignores magnetic and electrostatic fields unless it is ionized. This means you will need a powerful ultraviolet beam or strong laser to ionize the hydrogen heading for the scoop.

A Bussard ramjet has to be boosted to a certain minimum speed before the scoop can operate. Estimates range from 1% to 6% of c, which is pretty awful. There is an equation here but it depends upon other assumptions about the minimum mass-collection rate.

The Sun has the misfortune to be located near the center of a huge region about 330 to 490 light-years in diameter called "The Local Bubble". The interstellar medium within the Local Bubble has a density of about 0.07 atoms/cm3, which is about ten times lower than in the rest of the galaxy. This makes a thin fuel source for a Bussard ramjet. The Local Bubble is thought to have been caused when the star Geminga went supernova about 300,000 years ago.

And to top it off, trying to use hydrogen in a fusion reactor would require mastery of proton-proton fusion, which is so much more difficult than deuterium fusion that some scientist doubt that we will ever learn how to do it.

But none of these were show-stoppers. There was a Renaissance of science fiction novels written using Bussard ramjets. Arguably the best is the classic Tau Zero by Poul Anderson, which you absolutely must read if you haven't already. Other include Larry Niven's Protector and short stories set in his "Known Space" series, Footfall by Larry Niven and Jerry Pournelle, A Deepness in the Sky by Vernor Vinge, and The Outcasts of Heaven's Belt by Joan Vinge.

Ramjet Show Stopper

Things started to unravel in 1978. T. A. Heppenheimer wrote an article in Journal of the British Interplanetary Society entitled "On the Infeasibility of Interstellar Ramjets." Heppenheimer applies radiative gas dynamics to ramjet design and proves that radiative losses (via bremsstrahlung and other similar synchrotron radiation-type mechanisms) from attempting to compress the ram flow for a fusion burn would exceed the fusion energy generated by nine orders of magnitude, that is, one billion times. The energy losses will probably show up as drag. This was confirmed by Dana Andrews and Robert Zubrin in 1989.

The effect of drag? What it boiled down to was that the ramjet had a maximum speed, where the relative velocity of the incoming hydrogen equaled the drive's exhaust velocity. It has a "terminal velocity", in other words.

A proton-proton fusion drive has an exhaust velocity of 12% c, so a proton-proton fusion Bussard Ramjet would have a maximum speed of 12% c. You may remember that a spacecraft with a mass ratio that equals e (that is, 2.71828...) will have a total deltaV is exactly equal to the exhaust velocity. So if a conventional fusion rocket with a mass ratio of 3 or more has a better deltaV than a Bussard Ramjet, what's the point of using a ramjet?


The magsail was invented by Dana Andrews and I working in collaboration. What happened was this; Dana had an idea for a magnetic ramscoop that would gather interplanetary hydrogen and then feed it to a nuclear electric ion drive, thus avoiding the necessity of the p-p fusion reaction in the classic Bussard scoop. The problem was, according to Dana's rough back of the envelope calculations, he was getting more drag than thrust. Dana asked me to help him on it, hoping that a more expect calculation would give a more favorable result. I wrote a code and modeled the system as a Monte-Carlo problem, and discovered that Dana was wrong: he was not getting more drag than thrust, he was getting MUCH MUCH more drag than thrust. At that point I made the suggestion to Dana that we abandon the ion thruster and just use the collection device as a sail. He agreed. Based on the Monte Carlo results, we calculated total system drag and wrote a IAF paper in Oct. 1988 showing the value of the magsail as an interstellar drag device. Then, in early 1989 I derived a closed form analytic solution to the magsail drag problem, and also a set of equations governing magsail motion in the gravitational field of the Sun, and published this together with some mission analysis by Dana as a AIAA paper in July 1989 (republished in referred form in Journal of Spacecraft and Rockets, March-April 1991).

Up to this point (Dr. Robert) Forward had not been involved. However, after the presentation of the 1989 paper Forward suggested to me that I take a look at how the magsail would operate inside the Earth's magnetosphere — i.e. how it would interact with the Earth's magnetic poles — could this be used for orbit raising. I derived all the equations for this and published it as an AIAA paper AIAA-91-3352 in 1991, and republished it in JBIS later (in 1992, I think) Someone then sent me a letter pointing out that in 1963, Joe Engleberger had patented a concept for using a magnetic device to pump against the Earth's magnetic poles to raise orbits. I got hold of Engleberger's patent and sure enough, he had addressed that aspect of magsail capability. However Engleberger's equations in his patent are incorrect (get hold of his patent #3,504,868 — you can see that he's wrong by inspection) and of course, no one in 1963 had any viable technology to offer to allow such a propulsion system to be built — that was not made possible until 1987 when Chu introduced high T superconductivity. For these reasons, an USAF review of advanced propulsion systems done in 1972 rejected Engleberger's work. Interestingly, the attempt made in that USAF review (Meade et-al AFRPL-TR-72-31) to correct Engleberger's equations also resulted in a incorrect solution, although the error in the USAF derivation is harder to spot.

Around 1992, Dana did some further work on the Magsail together with Steve Love, and they showed that a magsail could be used to brake a spacecraft returning from the moon in the Earth's magnetosphere, i.e. a low stress alternative to aerobraking. Also in 1992, G.Vulpetti, of Italy, published some analysis of trajectory capabilities of spacecraft that combined magsails with light sails.Vulpetti's work was explicitly based upon the prior work by Dana and I, and referenced as such.

To my knowledge, which is based upon a pretty thorough literature search at this point, these are the only quantitative work done on magsails to date. People did know by the 1970's of course, that ramscoops would create some drag that would interfere with a Bussard scoop's performance, but no one had quantified this and thus the possibility of using a magnetic field as a propulsive sail was not seriously discussed .Occasionally I run into people who tell me that they "thought of" the magsail years ago, but they never published their "idea." I believe that without quantification and publication such intuitions, assuming they actually occurred, do not constitute invention. Invention requires real work, and real publication, and a real fight to prove the validly of an idea- not just idle musing within the confines of ones own daydreams.

For these reasons, I believe that the claim of Dana Andrews and I to be the co-inventors of the magsail are fully justified. Until someone can present a prior publication for a magsail, including a competent calculation of its performance, all claims to the contrary have to be regarded as nebulous.

Robert Zubrin (1994)

Ramjet Show Starter

Things look bleak for the Bussard Ramjet, but it isn't quite dead yet. First off, Dr. Andrews and Dr. Zubrin's analysis depends upon certain assumptions. But even if the drag problem is as severe as calculated, there may be ways to avoid it.

Bussard Scramjet

The drag is caused by bremstrahlung and synchrotron radiation produced by the motion of the charged particles as they spiral through your collector fields and into your fusion chamber. It is theoretically possible to recover energy instead of it being wasted as drag. Then the energy could be added to the fusion energy and used to accelerate the exhaust stream, thus defeating the drag.

It would be a Bussard Scramjet, in other words.

But only theoretically. It is incredibly difficult, as in "we might not manage to do it with five hundred years of research" level of difficult.

  • Subject: Bussard Ramjet woes
  • From: Nyrath the nearly wise
  • Date: Mon, 26 Nov 2001 02:41:46 GMT
  • Newsgroups:

According to my understanding of the legendary Bussard Ramjet, it has a terminal velocity. This is when the velocity of the incoming hydrogen relative to the scoop is equal to the exhaust velocity.

Assume that the ramjet has enough technomagic to manage real live proton-proton fusion.

The question is: does anybody have a ballpark estimate of what this terminal velocity is likely to be?

Extra credit question: I understand that the terminal velocity constraint can be by-passed if the ramjet can use even more technomagic to somehow gather and fuse the hydrogen without affecting the hydrogen's vector.

  • Is this:
  • [1] not even theoretically possible
  • [2] not impossible, given about ten thousand years of research
  • [3] possible with about 500 years of research

  • Subject: Re: Bussard Ramjet woes
  • From: "Ray Drouillard"
  • Date: Sun, 25 Nov 2001 23:20:26 -0500
  • Newsgroups:

I came up with about 12% of C. I forgot what I assumed as an efficiency.

The terminal velocity assumption is true IF the incoming hydrogen has to be stopped relative to the ship (IOW, sped up). If it is merely gathered, compressed, then shot out the back, I see no reason for a terminal velocity. of course, the exhaust speed will be very high relative to the ship. It will be 0.12C (or whatever) relative to the original "stationary" interstellar hydrogen. (Note the quotes around "stationary" and don't give me any grief about relativity).

Note 2: The engineering details will be pretty nasty :-)

  • Subject: Re: Bussard Ramjet woes
  • From: "Geoffrey A. Landis"
  • Date: Mon, 26 Nov 2001 11:05:19 -0500
  • Newsgroups:

This is *vastly* dependent on the assumptions you make.

Can you harvest the energy released by stopping the protons?

The primary energy loss mechanism seems to be bremstrahlung and synchrotron radiation produced by the motion of the charged particles as they spiral through your collector fields and into your fusion chamber.

In the worst case, all of the original energy of the particles (in your frame of reference) is lost; in the best case— well, how big do you want to assume your collector is?

  • Geoffrey A. Landis

  • Subject: Re: Bussard Ramjet woes
  • From: schillin@xxxxxxxxxxxxx (John Schilling)
  • Date: 26 Nov 2001 11:02:35 -0800
  • Newsgroups:
  • Organization: University of Southern California, Los Angeles, CA

Nyrath the nearly wise writes:

The question is: does anybody have a ballpark estimate of what this terminal velocity is likely to be?

I get 0.120c using a simple non-relativistic calculation, should be good to within a few percent. With such a limit, it is not worth the trouble of using a ramjet at all. A simple fusion rocket, with the fuel carried in tanks, can do the same job much easier.

Extra credit question: I understand that the terminal velocity constraint can be by-passed if the ramjet can use even more technomagic to somehow gather and fuse the hydrogen without affecting the hydrogen's vector.

Or if you can recover the energy associated with decelerating the incoming fuel, and pump it back into the exhaust stream.

For example, if one can collect the fuel without decelerating it, feeding the relativistic plasma jet through a suitable MHD generator would produce *enormous* ammounts of power. Add this to the power produced by fusing the hydrogen and use the combined total to accelerate the exhaust.

  • Is this:
  • [1] not even theoretically possible
  • [2] not impossible, given about ten thousand years of research
  • [3] possible with about 500 years of research

It is theoretically possible. Anyone who imagines they can predict the results of five hundred, much less ten thousand, years of research, is using a much higher grade of LSD than I have ever heard of. It would require an indeterminate ammount of research and an unknown number of theoretical breakthroughs, which means that it could take anywhere from ten years to forever.

*John Schilling                    * "Anything worth doing,         *
*Member:AIAA,NRA,ACLU,SAS,LP       *  is worth doing for money"     *
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thread Bussard Ramjet woes on (2001)
Toroidal-Field Ramscoop

The information here is mostly from Deep Space Probes: To the Outer Solar System and Beyond by Dr. Gregory Matloff.

In the late 1990s Brice Cassenti tried to salvage the Bussard Ramjet concept.

The good news is that he managed to drastically reduce the drag.

The bad news is it uses an array of flimsy superconducting wires in front of the spacecraft. Which means only accelerations on the order of 0.04g are possible. That is about 50 snails worth of acceleration, which is pathetic.

Earlier attempts to stop drag used electrostatic fields for the scoop. But the Debye-Hückel screening effect raised its ugly head. The interstellar ions are charged (otherwise they wouldn't be ions), so are attracted to the electrostatic scoop. The trouble is the charge on the ions is also an electrostatic field. The huge cloud of attracted ions that gather in front of the scoop make a huge electrostatic field of their own (of opposite polarity), perfectly positioned to totally mask the scoop field. This screening ensures that ions further away do not even see the scoop field, forget about them actually being scooped up.

Others tried to eliminate the drag with standard Bussard electromagnetic fields by playing around with the geometry. Alas, most of the designs were better at reflecting away the ions instead of gathering them. Talk about counter-productive.

Cassenti's design was electromagnetic not electrostatic. Thus avoiding the heartbreak of Debye-Hückel screening.

And Cassenti's design did not affect the ions until they are actually inside the scoop, so there would be little or no ion reflection.

The scoop is a torus (donut shape) with a superconducting wire wound around the circumference. Depending upon the current direction and ion charge, an ion entering the torus will either be deflected to the center or the circumference. The idea is to deflect to center, so eventually they will enter the engine intake. Deflect to the cirumference would also be counter-productive.

So an ion in the interstellar media is just sitting around, minding its own business. Here comes the ramjet starship traveling at a sizable fraction of c. As the ion passes through the torus, it gets an electromagnetic shove to the center. As the ion passes further on, it gets closer and closer to the thrust axis. Using this information one can calculate the point where the ion hits the axis. This is where you put the engine fuel intake.

Cassenti analyzed a sample design of a ramjet with a scoop radius of 400 kilometers (800 km diameter), a supercurrent of 3×105 amps, twelve wire turns, traveling through the interstellar medium at 0.1 c. Using some hideous equations that I won't scare you with, Cassenti calculated that an ion entering the torus 200 kilometers from torus center would travel about 170 kilometers parallel to the thrust axis before it moved laterally enough to hit it.

Translation: the scoop torus will have to be held 170 kilometers in front of the engine fuel intake or the ions will miss the intake.

Since every gram counts and the freaking scoop is too huge to fit between New York and Cleveland the wire structure will have to be dangerously flimsy. Cassenti's design uses rotation produced centripetal force and minimal supporting structure, but still would collapse if the acceleration got above a measly 0.04 g.

His design had a featherweight mass of a few hundred thousand kilograms, making it very un-dense. This is about the mass of the International Space Station. But the scoop is more than seven thousand times as wide. Same mass but bigger means the torus is less dense than the ISS. And the station wasn't that dense to start with. Unfortunately very low density usually means flimsy, weak, and vulnerable to strong acceleration.

Cassenti looked into supporting the scoop with ion drive thrusters and/or laser beam radiation pressure (where part of the support structure is composed of beams of radiation with zero or almost zero mass), but one rapidly gets to the point of diminishing returns with that sort of thing.

A helpful reader named Yoel Mizrahi (יואל מזרחי) contacted me explaining that I had the design incorrect. Not surprising considering the sparse details I had. Mr. Mizrahi said Dr. Cassenti's design did not use fusion for propulsion. Instead it utilized beamed power. A large power plant back home at Sol energized a free-electron x-ray laser whose beam was sent to the light-years distant toroidal-field ramscoop. But instead of the laser beam pushing a laser sail, it is turned into electricity and used to accelerate the hydrogen scooped up.

So it is like a beam-powered RAIR with a no-drag scoop. The advantage of the toroidal scoop is that a conventional RAIR requires mass for fusion fuel and mass for the fusion reactor. In addition the conventional RAIR scoop suffers from drag. Beamed power is a good way to drastically reduced the mass of the propulsion system. The main drawback is that the starship is at the mercy of whoever back home controls the x-ray laser.

Advantages of toroidal scoop: does not waste mass on carrying propellant or energy. And the scoop is drag free.

Disadvantage: the acceleration of a toroidal scoop will be limited to about 0.4 m/s2 (0.04 g). The scoop does not gather a lot of hydrogen propellant due to the thinness of the interstellar medium, and due to the relatively small scoop radius. The exhaust consists of only light ions. More importantly, if the acceleration climbs above 0.4 m/s2 the flimsy scoop will buckle and collapse.

Technical challenges: designing a high-efficiency low mass x-ray power system. Figuring out how to use electricity to efficiently accelerate the scooped propellant.

Dr. Cassenti is going to send a copy of the scientific paper to Mr. Mizrahi, so stay tuned for more details. In the meanwhile, Mr. Mizrahi gave me these images:

I made some quick images with Blender 3D to figure out how the rigging worked:

Bussard Ramjet Combat

Orion Wargame

This Bussard ramjet is from a science fiction boardgame/wargame called ORION Combat Near the Speed of Light (1987) by Alan Sherwood and David Cohn (Monash Games).

...The large map ... is a 2-dimensional representation of the Great Nebula of Orion... Regions A to D are ionized gas (H-II regions), A being the Strömgren zone, and E and F are dusty molecular clouds...

...The ramships in this game are envisaged as vehicles of about 10,000 tonnes mass, with a magnetic field acting as the ramscoop extending out to about 1000 km radius. The field would be produced by magnetic coils of about 1 km radius. Protons (ionized hydrogen) collected by the field are fed into a nuclear fusion reactor, and the reactions products exhausted out the rear to produce thrust. Turning and braking are done by directing either this exhaust or the incoming stream of protons by magnetic fields (so the ramship can brake and turn without using the reactor). Induced drag results from this redirection of the gas stream. In low density gas, it must be redirected further, causing more drag. When traveling through un-ionized gas, the ramship shines an ultraviolet light ahead to ionize the gas in its path.

Performance is limited by the reactor power (which limits acceleration), structural g limits (limits turning and braking), and the gas density (which reduced all performance in low density regions)...

COMBAT Combat in interstellar space can occur between ramships that come within weapons range, which of course will be very small compared to interstellar distances, or even a single Mapsheet hex (1/6 light-year diameter). Range is envisaged to be limited by Beam weapons to about 100,000 km. Note this means that at closing speeds near to light, the battle may last less than a second, so there is no time for any manoeuvre in battle (although it would have been preceded by years of manoeuvring).

Once an encounter has been arranged, the most important parameter (apart from number of ramships involved) is the relative velocity, which is the closing speed of one ramship relative to the other. Except for its effect on manoeuvrability, the speed of each ramship through the nebula is not relevant; the two ramships are equivalent and neither has any advantage. This reflects the fundamental principle (in fact The Principle of Relativity) that all inertial (i.e., traveling at or approximately at constant velocity) observers are equivalent.

Before the encounter, a ramship would detach its Fighter, and then stand off from the battle while the Fighter pursued the enemy ramship. The Fighter is essentially a small ramscoop carrying only weapons and guidance systems that can manoeuvre much better than a ramship, without the extra weight of the reactor and life support systems. This necessity for a Fighter is a unique feature of interstellar combat. It results from the fact that when observing an enemy ramship from a great distance you are seeing it in the past, due to the finite speed of light. Thus, you do not see any of its evasive manoeuvres until some time later, and the counter-manoeuvres of your ramship will come too late to catch it. To catch an evading enemy, your ramship's manoeuvrability must be greater by the Pursuit Factor, which becomes quite large at even modest relative velocities. It is reasonable to assume that ramships would not differ much in their manoeuvrabilities, so if it was only ramship against ramship, an opponent who didn't want to fight would always escape. Thus, to be an effective fighting vehicle a ramship must carry a Fighter.

(ed. note: this means in at the start of a combat situation, all involved ramships must decide if they send their Fighters to attack enemy ramships or keep their Fighters with them to defend against enemy Fighters.)

The weapons envisaged to be carried by the Fighter are:

  1. Missiles: merely lumps of any matter thrown out in the path of the enemy. The kinetic energy released from an impact at such high speeds makes even nuclear warheads unnecessary. They would be thrown out in a large cloud of sand-sized particles to ensure a hit - this is how each missile can attack all opposing ramships. Missiles naturally do more damage at higher relative velocity due to their greater kinetic energy. The ramship would have frontal armor for protection, and only when missiles have enough energy to penetrate this do they become effective weapons.
  2. Beam weapons: Probably X- or Gamma-ray lasers - the shortest possible wavelength would be used to get the long range.
From ORION Combat Near the Speed of Light

Winchell Chung: If you have two bussard ramjet ships with nearly identical propulsion performance, moving at relativistic velocities, and seeing only where the enemy was but not where it is now (due to lightspeed lag), well, if one of the ships wants to evade, there is no way the other can catch it.

David Iwancio: It seems like your evading would be more difficult to pull off in Einsteinian space than Newtonian. If the ability to evade relies on how far away from your present course you can "jink," your energy/thrust reqirements go up exponentially with the size of your "jink" (what with the increase in your mass and all).

Where your target might be after time T can be expressed as a sphere of a certain radius, and the radius increases with T. In Newtonian space, the radius increases linearly with T, so you can kind of visualize a cone centered about the target's current path of travel. However, in Einsteinian space the radius of the sphere increases only logarithmicly, giving you a smaller (usually much smaller) sphere radius than Newtonian space.

Wouldn't this kinda counteract the problems of light/sensor lag a bit?

Ken Burnside: I call this the trumpet bell effect, and it becomes much more noticeable when slinging ballistic weapons in 3-D play.

Provided your ballistic weapon's rate of closure is greater than the lateral velocity of the target, you get a trumpet bell, or manifold shape. As the projectile's velocity increases, the skinny part of the trumpet bell elongates — but it also thins out. The volume described by the surface of the trumpet and the centerline of the trumpet remains constant along the time axis, provided the ability to laterally accelerate remains constant.

In short, if you've GOT a good shot lined up, it's harder to dodge it by "jinking". If you've got a fuzzy shot that gets refined as you approach (which is roughly how Attack Vector: Tactical does it, because it's easier than having people pretend to be targeting computers in 3-D vector space), higher speeds on the shells can reach a threshold effect, where a small error that could be corrected for at a low closing velocity can't be corrected for at a high closing velocity.

A bit of practice renders this moot, but without that practice in the mechanics of doing vector ballistics (let alone 3-D vector ballistics), they can get very frustrating to use.

(somebody asks if sensor lag will prevent the trumpet bell effect)

My suspicion is that it's still going to be a trumpet bell effect. While there's sensor lag, if they're moving at 0.92 c (about where relativity becomes noticeable), the "trumpet bell" of the target's possible positions is also very long and skinny.

One thing you learn in Attack Vector: Tactical is that velocities past about 30 hexes/turn (300 km/64 seconds) actually make you EASIER to hit with ballistic weapons, because your ability to change your vector is so dramatically reduced. What you want for dodging missiles is a low enough velocity that you can swing around and thrust in an unanticipated direction and throw off the ballistic weapon's accuracy.

From thread on sfconsim-l (2002)

The Flying Dutchman was a matrix of rock, mostly hollow. Three great hollows held the components of a Pak-style Bussard ramjet ship. Brennan called it Protector. Another had been enlarged to house Roy Truesdale's cargo ship. Other hollows were rooms.

The inside of the teardrop-shaped cargo pod was nothing like that of the alien ship that had come plowing into the solar system two centuries ago. Its cargo was death. It could sprout heavy attitude jets and fight itself. Its long axis was an X-ray laser. A thick tube parallel to the laser would generate a directed magnetic field. "It should foul up the fields in a monopole-based Bussard ramjet. Of course that might not hurt him enough unless your timing was right." When Roy had learned how to use it— and that took time; he knew little about field theory— Brennan started drilling him on when.

A directed magnetic field would churn the interstellar plasma as it was guided into a Bussard ramjet. As a weapon it might be made to guide the plasma flow across the ship itself. The gunner would have to vary his shots, or an enemy pilot could compensate for the weapon's effect. If the local hydrogen density were uneven, that would hurt him. If the plasma were dense enough locally, the enemy could not even turn off his drive without being cremated. Part of the purpose of the ram fields was to shield the ship from the gamma ray particles it was burning for fuel.

"Hit him near a star, if you get the choice," said Brennan. "And don't let him do that to you."

The laser was surer death, if it hit a ship. But an enemy ship would be at least light-seconds away at the start of a battle. It would make a small, elusive target, its image delayed seconds or minutes. The thousand mile wings of a ram field would be easier to hit.

The guided bombs were many and varied. Some were simple fusion bombs. Others would throw bursts of hot plasma through a ram field, or carbon vapor to produce sudden surges in the burn rate, or half a ton of pressurized radon gas in a stasis field. Simple death or complicated. Some were mere decoys, silvered balloons.

Lately he had come to enjoy these simulated battles, but he wasn't enjoying this one. Brennan was throwing everything at him. The Pak scouts had used a three gee drive until they crossed his wake, and then Wham! Six gees and closing. Some of his missiles were going wild; the scouts were doing something to the guidance. The pair dodged his laser with such ease that he'd turned the damn thing off. They'd used lasers on him, firing not only at his ship but at the field constriction behind him where hydrogen atoms met and fused, so that Protector surged unevenly and he had to worry for the generator mountings. They threw bombs at unreasonable velocities, probably through a linear accelerator. He had to dodge in slow random curves. Protector was not what you'd call maneuverable.

He tried some of his weaponry on the lone ship behind him.

Then half his weapons board was red, and he had to guess what had exploded in the trailing pod. Probably that idiot projector: he'd been trying to punch a hole in the lone ship's ram field. He bet his ship he was right, and gambled further that the explosion had wrecked his laser, which might otherwise have been of some use. He fired a flurry of bombs from the side of the cargo pod opposite the explosion. The lead ship of the remaining pair flared and died.

That left two, each the trailing ship of a pair, making less than his own acceleration. He dithered a bit, then ran for it. He continued to dodge missiles and laser beams.

He dropped two half-tons of radon with the drives disconnected.

Radon has a short half-life: it has to be kept in stasis. The generator was outside the bomb shell, and was partly soft iron. The enemy's ram field tore it apart. A minute later the radon was in the constriction, and incredible things were happening: radon fusing to transuranian elements, then fissioning immediately. The constriction exploded. The ram field sparkled like a department store Xmas tree gone manic. The Pak ship flared into a small white point, fading.

Brennan made pictures on the screen: ... He spread a wide cone before the lead ship, converging it almost to a point behind the ship. A needle shape with the ship in its point — the ship's protective shield — brought the incoming hydrogen into a ring shaped constriction.

"You depend too much on those long, slow turns," he said. "The way to dodge Pak weaponry is to vary your thrust. Keep opening and closing the constriction in the ram field. When they throw something like a laser pulse into the constriction, open it. Nothing's going to fuse if you don't squeeze the plasma tight enough."

Roy wasn't flustered. He was getting used to Brennan's habit of resuming a subject that may have been broken off days ago. He said, "That last ship could have done that when I threw radon at him."

"Sure, if he did it fast enough. At good ramscoop velocities the s**t should be in the constriction before he knows it's reached the ram field, especially as you didn't put any rocket thrust on it. That was good thinking, Roy. Memo for you: don't ever follow a ship that's running. There are too many things he can throw into your ram field. Hopefully we'll be doing the running in any battle."

"Then these scouts are tougher than what I fought."

"And there are three of them."


"They're coming in a cone, through— you remember that map of the space around Sol? There's a region that's almost all red dwarfs, and they're coming through that. I think the idea is to map an escape route for the fleet, in case something goes wrong at Sol. Otherwise they'll see to it that Sol is clean, then go on to other yellow dwarf stars. At the moment they're all about a light year from Sol and about eight light-months apart."

In the 'scope screen the Pak scouts showed as tiny green lights, a good distance from each other, and measurably closer to Sol. Brennan seemed to know just where to find them, but then he'd been observing them for two months. "Still making three gravities," he said. "They'll be at rest when they reach Sol. I've been right about them so far. Let's see how far I can carry it."

"Isn't it about time you told me what you've got in mind?"

"Right. We're leaving the Flying Dutchman, now. The hell with convincing them I'm coming from Van Maanen's Star. They're seeing us from the wrong angle anyway. I'll take off for Wunderland at one point aught eight gee, hold for a month or so, then boost to two gee and start my turn away from them. If they spot me in that time, they'll turn after me, if I can make them think I'm dangerous enough."

"Why," he started to ask, before he remembered that one point aught eight was the surface gravity of Home.

"I don't want them to think I'm a Pak. Not now. They're more likely to chase an alien capable of building or stealing a Pak ship. And I don't want to use Earth gravity. It'd be a giveaway."

"Okay, but now they'll think you came from Home. Do you want that?"

"I think I do."

Home wasn't getting much choice about entering the war. Roy sighed. Who was? He said, "What if two of them go on to Sol and the other comes after us?"

"That's the beauty of it. They're still eight light-months apart. Each of them has to make his turn eight months before he sees the others make theirs. Turning back could cost them another year and a half. By then they may just decide I'm too dangerous to get away." Brennan looked up from the screen. "You don't share my enthusiasm."

"Brennan, it'll be two bloody years before you even know if they've turned after you. One year for them to spot you, one year before you see them make the turn."

"Not quite two years. Close enough." Brennan's eyes were dark beneath their shelf of bone. "Just how much boredom can you stand?"

From Protector by Larry Niven (1973)

This is from a discussion entitled Bussard Ramjet Evasion started at March 1st 2002.


A couple of acquaintances of mine have a disagreement. Perhaps the r.a.s.s. massmind can provide some input. Start off with the (implausible) postulate that Bussard Ramjets are practical.

Given two Bussard ramjets with identical propulsion performance, about one light year of separation, moving at relativistic velocities towards each other. Both ramjets armed to their cute little teeth.

Acquaintance #1 maintains that if one ramjet wished to avoid combat, it is impossible for the other ramjet to force combat. (combat being loosely defined as maneuvering such that the opposing ship is within one's weapons' footprint)

The argument is along the lines of the lightspeed delay in observing the position and vector of the enemy ramship coupled with relativistic velocity and parity in maneuverability will make it always possible for the enemy to dodge out of the way.

Acquaintance #2 argues that as a ship's speed increases, the maximum possible angular change in the ships vector decreases (given the same deltaV). So at relativistic velocities, any ship will have very limited maneuverability. Therefore they cannot avoid being caught.

My gut level feeling is that neither of my acquaintances are right or wrong, but that the answer depends upon the situation, e.g., ship's velocity compaired to ship's deltaV, size of weapon's footprint, etc.

Any thoughts?


My thought: Sounds like the scenario in Niven's Ethics of Madness short story, though there it was one chasing the other. In your scenario much depends on what is meant by 'weapon footprint'.

Erik Max Francis

I think the answer really comes down to the actual maneuverability, velocities, and weapons ranges of the ships in question.

Mike Williams

I reckon that for relativistic velocities to be practical in your Bussard ramjet, then they should be capable of sustained accelerations of at least 0.1 g. If they can't do that, then it's going to take them over a decade to achieve relativistic speed, which I don't consider very practical.

The first ramjet starts to thrust sideways at a constant 0.1 g in a random direction. The second ship can't possibly observe which way they've gone for more than 6 months, by which time the first ship would have moved sideways by 125,000,000,000 km, and have accumulated a sideways velocity component of 15,800,000 m/s. That's only 0.013 of a light year off the original track, so the angular deflection is only about a degree and a half.

The second ship can't guarantee to come closer than about 5 light days from the first ship, so it's going to need an awfully big weapons footprint in order to engage it.

Hop David

The light year separation is observed from whose frame? What relativistic velocity are they moving towards each other?


I dunno, this exceeds my meager knowledge of relativity.

The key factor seems to be "relative velocity", that is, for each ramjet, the velocity of the enemy ramjet in the frame of reference of the friendly ramjet.

Hop David

By "what relativistic velocity" I meant whaf fraction of c. I believe observers on both ships would see an approach of the same velocity as the other, but a third observer might see something different.

If they are going a very good clip, the spatial distance could also be quite different depending on whose measuring. One observer's lightyear may be another observer's mile.


As far as I remember from huge Relativistic Kill Vehicles (RKV)/planet killers thread it's more or less consensus that maneuverable relativistic target could not be practically intersepted with single interceptor.


Oh, I agree that if the target is a planet, there is no way it is going to stop a relativistic weapon aimed at it.

However, is that true if the target is capable of the same propulsion performance as the weapon?

And is it true if the target's performance is an order of magnitude better than the weapon?


{ target propulsion the same } Target evades if it far enough from interceptor and have comparable fuel resource.

{ target propulsion order of magnitude superior} In this case target evades without any doubt.

Isaac Kuo

Actually, I calculated that a dumb brute force approach works really well if you know more or less the direction and time of the attack (i.e. seeing the incredibly bright launch signature of the multi-hour acceleration phase in the attacker's system).

The dumb brute force approach is to throw a planet-sized wall in the vague direction of the attack. This wall is actually a puff of gas generated by everything from particle beams to rocket exhausts—whatever creates gas (which will spread evenly without gaps) and can be directed more or less in the correct direction during the hours warning time.

This really really really thin spread wall looks like a dense disc moving at near-c velocities to the incoming munitions. It vaporizes the munitions instantly upon impact.

To a rough approximation, the amount of gaseous material the defenders need to throw up is about the same mass as the total mass of the incoming munitions. The fact that this mass is spread out over an area the size of a planet is roughly balanced out by the fact that the incoming munitions have the kinetic energy necessary to devastate and entire planet's surface.

Assuming the defenders have anything vaguely like the capabilities of the attackers, they could more plausibly throw up a planetary wall many orders of magnitude more massive than the incoming munitions.

{ However, is that true if the target is capable of the same propulsion performance as the weapon? } Umm...Serg is saying the opposite of what I think you think he's saying. He's saying that our conclusion was that a near-c interceptor probably could not intercept a maneuverable target. In other words, a near-c attacker could not intercept a near-c target (or any other target which was maneuverable).

I think you're going an extra unnecessary step, thinking that this means it's impossible for the defender to shoot down near-c missiles from the attacker. This is true...but it's a moot point since those missiles from the attacker can't hit the defender anyway.

Basically, it's the defender's game either way.

I haven't thought of a way to make near-c weaponry workable. They just give too much "free energy" to the defender to vaporize your munitions with their own incredible kinetic energy. Roughly, you stick to a munition velocity low enough so you can overwhelm defenses with sheer weight of fire.

Brian McGuinness

So instead of a missile you now have a gas with nearly the same momentum and kinetic energy approaching the planet. Why is this an improvement?

Isaac Kuo

Because it isn't nearly the same momentum and energy nor is it approaching the planet, except for a very tiny fraction of it.

When the small mass of the incoming near-c munition hits the much larger mass of the nebulous defense cloud, it explodes more or less evenly in every direction. Actually, when it first hits the closest layers of the defense cloud, it merely expands into a narrow cone. However, this defense cloud is many planet diameters deep—the cone balloons out into a trumpet shape and then to a spherical expanding explosion quickly.

Very little of this explosion will impact the planet, depending upon how far away the defense cloud is from the planet. For example, if this defense cloud is being thrown from near the planet itself with crude chemical rocket exhausts, the cloud would plausibly be around 20+ planet diameters away. About 1% of the explosion would impact the planet. With more sophisticated plasma thrusters, the cloud could be 20 times further away—for 0.003% of the explosion impacting the planet.

Timothy Little

{ When the small mass of the incoming near-c munition hits the much larger mass of the nebulous defense cloud, it explodes more or less evenly in every direction. }

This does not at all square with your previous assertion that "the amount of gaseous material the defenders need to throw up is about the same mass as the total mass of the incoming munitions".

Furthermore, you are forgetting that relativistic collisions should be handled in the center-of-mass frame, which is still very highly relativistic.

Using your generous figures of (say) 2 Gm interception distance, an assumed incoming speed of 0.999c or more (based on the fact that the cloud "looks like a very dense disk"), an attack of 10 RKVs with an assumed mass of say 104 kg each with 0.1 m2 cross-section. I'll assume the defenders have the same energy budget and 100 hours warning, and hence can disperse about 1015 kg of gas and dust into the path with the same energy budget (assuming it doesn't have to be lifted off planet, but is available from some convenient moon).

The cloud is say 20 Mm wide (enough to shield the planet), and 100 Mm deep ("many planet diameters"). The cross-sectional density is thus 3 kg/m2. To model the interaction, it is best to consider the RKV to be a collection of independent nuclei; certainly its chemical binding energy is negligible. With this area density and these energies, the probability of significant interaction between RKV and cloud nuclei is somewhere around 0.03% to 1%, depending upon materials used, say 0.3%. Hence 99.7% of the RKV nuclei are affected only by mere chemical energies, say up to 1 keV per nucleon (to give a gross overestimate).

This imparts an average deflection of up to 400 km/s, so by the time it reaches the planet it misses its target by about 100 km. Hence with even 4 days to prepare, and the same energy availability as the attacker, the defender's 1015 kg cloud is grossly insufficient to prevent the RKV from hitting the planet.

With less time, quadratically more energy would be required to get the cloud into position. Furthermore, it is likely that the defender's available energy is somewhat proportional to how much time they have.

Hence, I conclude that for a 0.999c RKV, the defender needs at least 100 times the attacker's energy budget and/or at least a few weeks warning before they have a reasonable chance of protecting their planet.

Isaac Kuo

{ This does not *at all* square with your previous assertion that "the amount of gaseous material the defenders need to throw up is about the same mass as the total mass of the incoming munitions". }

That's the minimum amount of mass required to obliterate the incoming munitions. In reality, the defenders can afford to put up many orders of magnitude more mass—as I stated in the first posting.

{ Furthermore, you are forgetting that relativistic collisions should be handled in the center-of-mass frame, which is still very highly relativistic. Using your generous figures of (say) 2 Gm interception distance, an assumed incoming speed of 0.999c or more (based on the fact that the cloud "looks like a very dense disk"), }

Very dense disk is a relative term. Something a hundred kilometers deep by 10,000km in diameter is a thin dense disc compared to the same mass in 50,000km deep by 10,000km in diameter.

What launch mechanism do you have in mind with which to acheive 0.999c in an attacking munition across interstellar distances?

More or less, there are only three possibilities:

  1. A honking huge particle accelerator. This one won't work because it's not plausible to focus a particle beam over interplanetary distances, much less interstellar distances.
  2. An antimatter rocket. This can work, but the pathetically low acceleration implies launch acceleration runs on the order of centuries or much longer. This gives the defenders a very very long time to do something about it. Also, the minimum resources required to create this antimatter rocket are daunting, and the inefficiency in antimatter generation is a factor.
  3. Laser sail. This can work, with reasonably high accelerations, but once you get up to near-c velocities things become very problematic. With the sail travelling away from the beam, the beam is just barely able to keep up with the sail. The final hours or weeks of acceleration is provided by the beam generated in the final seconds or minutes of beam generation. What's worse, this beam is severely red-shifted, reducing its effectiveness. The effect is bad enough at 0.95c. I could see it going up to 0.99c, but not really further than that.

Note that whatever acceleration mechanism you use, it MUST accelerate the munitions without vaporizing them. If the munitions accept even the tiniest fraction of waste energy from the acceleration mechanism, it will melt and evaporate and disperse into a multi-AU conical beam by the time it reaches the target system.

Realistically, the only plausible way to deal with this problem is to accelerate the munitions slowly enough that they can radiate away what waste heat they do absorb. For interstellar laser sails, the numbers used seem to limit themselves to 1000m/s2 or lower. Realistically, even 1000m/s2 is highly optimistic for the sail not to instantly rip apart from slightly uneven acceleration.

If you've got a laser powerful enough to go the interstellar distances to accelerate a 0.999c sail, then it probably makes more sense to just use the laser itself as an interstellar weapon. Unlike the sail weapon, the victims will have NO preperation time—no brightly visible lengthy acceleration run is required.

{ I'll assume the defenders have the same energy budget and 100 hours warning, and hence can disperse about 10^15 kg of gas and dust into the path with the same energy budget (assuming it doesn't have to be lifted off planet, but is available from some convenient moon). }

What munition mass do you assume? What velocity of the defending gas cloud do you assume?

When calculating the energy budget, did you consider the inefficiencies in the launch mechanism vs the final warhead energy? Did you consider the budget required for the infrastructure? For example, laser launch requires a truly astronomically sized space laser to be built.

In contrast, the defenders can use existing rockets and their rocket nozzles, probably already in abundance for mundane purposes. At the low exhaust velocities ideal for interplanetary uses, rocket nozzles are pretty energy efficient (much better than 50%). OTOH, energy budget is not the limiting factor. Mass "budget" is.

Timothy Little

{ Very dense disk is a relative term. Something a hundred kilometers deep by 10,000km in diameter is a thin dense disc compared to the same mass in 50,000km deep by 10,000km in diameter. }

It is the latter case that you were proposing for the defending cloud, and I was basing my estimate of the speed on your post. To make the 50Mm deep cloud look like a "disc", you need a gamma of about 20 or so, hence 0.999c.

{ What launch mechanism do you have in mind with which to acheive .999c in an attacking munition across interstellar distances? }

I don't think it is feasible at all. I was simply countering your assertion that if one happened along, then you could easily defend against it, expending much less energy to do so.

{ If you've got a laser powerful enough to go the interstellar distances to accelerate a 0.999c sail, then it probably makes more sense to just use the laser itself as an interstellar weapon. }

I fully agree. I wasn't proposing that RKVs are useful weapons, just that defending against them involves a lot more than just blowing rocket exhaust at them. You need to intercept them with a few tonnes per square metre of something, or else a significant fraction of the nuclei pass straight through without interacting and hit the planet anyway. Note — this is just as true for 0.5c as for 0.999c. At GeV energies and above, nuclei have to get very close before they interact significantly.

{ What munition mass do you assume? What velocity of the defending gas cloud do you assume? }

Both were stated earlier in the post: munition mass 10 Mg (×10 munitions, total 100 Mg), defending gas cloud moving with the minimum speed needed to get it to the interception range in the time available. Neither are especially relevant.

{ When calculating the energy budget, did you consider the inefficiencies in the launch mechanism vs the final warhead energy? }

So long as the efficiency is more than about 1%, it doesn't much matter. I can't think of any that are that poor. For example, even the lightsail approach should be at least 10% efficient, and there is no theoretical reason why it couldn't approach 100%. Light reflecting from even a greatly red-shifted object still delivers its full momentum (and then some). In fact, energy efficiency of lightsails increases with speed.

{ Did you consider the budget required for the infrastructure? For example, laser launch requires a truly astronomically sized space laser to be built. }

So long as the equipment can deliver at least a significant fraction of the energy required for its assembly to projectiles over its working lifetime, I don't care. e.g., if a single 1 MW laser launcher module with associated power production and distribution costs 1 TJ in energy (or equivalent) to assemble, then its assembly cost becomes relatively insignificant in about 2 weeks of operation as far as energy budget goes.


Thank you for a fascinating post — I've been trying to think of some intelligent questions to ask.

How do you calculate or estimate the cross-sections for the interactions? Is the columb force law good enough at these energies? If not, what do you do?

I suppose the real trick is to figure out how many ev you have to impart to the nucleus to have it miss the planet, then estimate the cross section for that interaction.

Is the direct nucleon-nucleon interaction really going to be the dominant way that deflection happens? (As opposed to some indirect mechanism, in which generated particles or radiation produce the deflection indirectly, rather than it being produced directly by a nucleon-nucleon interaction).

Timothy Little

{ How do you calculate or estimate the cross-sections for the interactions? Is the columb force law good enough at these energies? If not, what do you do? }

What I personally do is look at experiments and read papers by people closer to the source than I :)

A nucleon travelling at 0.999c has an energy of about 20 GeV. There are plenty of experiments probing this energy region, so you can usually find some relevant data, including collisions with heavy nuclei. Often, such papers determine empirical formulae for cross-section based on various properties, and propose theoretical models to explain them. Even if not demonstrated to be correct, it is usually a fair bet that some professional nuclear physicists have put a fair bit of brainpower into these models and they probably aren't grossly wrong. That suffices for Usenet :)

{ I suppose the real trick is to figure out how many ev you have to impart to the nucleus to have it miss the planet, then estimate the cross section for that interaction. }

That would work in a more general case, yes. I was more interested in the specific case of trying to hit a target region on the planet.

{ Is the direct nucleon-nucleon interaction really going to be the dominant way that deflection happens? (As opposed to some indirect mechanism, in which generated particles or radiation produce the deflection indirectly, rather than it being produced directly by a nucleon-nucleon interaction). }

I think so. Obviously there isn't any data on relativistic interactions between macroscopic objects, so I can't be sure :)

It seems to me that indirect interactions might initially play a part, but by the time the projectile matter has spread to even a few tens of metres across (i.e. to a millionth of the density), any such secondary processes become completely negligible.

My only remaining concern is that maybe the electrons, despite making up less than 0.05% of the overall energy, could interact orders of magnitude more strongly and transfer their momentum to the nucleons via electromagnetic coupling. In relativistic ion experiements they contribute pretty much negligible energy, but Coulomb energies go with the square of the number of separated charges. This might be a case where particle accelerator results can't simply be scaled up. A 10-tonne projectile has a hell of a lot of electrons that might try to separate...

It's an interesting problem, and one that may well affect my answer to Isaac's post. I'm more interested in finding out the correct answer than appearing to be correct, so I may have to post a retraction :)

{ My only remaining concern is that maybe the electrons, despite making up less than 0.05% of the overall energy, could interact orders of magnitude more strongly and transfer their momentum to the nucleons via electromagnetic coupling. }

This appears to be the case.

In my quantification of Isaac's scenario, on average the electrons pick up deflection energies of about 100 keV each just by electromagnetic interactions. Now obviously the electrons can't just nick off and leave the nucleons behind due to electromagnetic forces. So the RKV rapidly (on the order of microseconds) thermalizes into a plasma which is only partly constrained by its own magnetic fields. Using a reference for high energy plasmas that I don't fully understand [:(], it looks like the mean dispersion will be on the order of 400 km/sec for a single-layer impact.

In the the diffuse cloud case, it initially expands more slowly, with the rate increasing as it encounters more total mass. However, as it becomes more diffuse it does interact more weakly (the plasma is still moving at negligibly diminished relativistic speeds). The dispersion rate appears to approach a maximum around 2-3 Mm/s, independent of depth of the defending cloud but merely dependent upon its area density. In this scenario, the RKV plasma cloud impacts upon the planet across a region about 1500 km in diameter (instead of 100 km). The next 9 RKVs will do likewise.

So I conclude that the RKV does still deposit its total energy upon a region of the planet's surface and vaporize surface features down to bedrock, but with this dispersion it may be insufficient to guarantee destruction of a particular hardened target within the region. This may mean that the defender has acheived some benefit from throwing the shield cloud into place.

The energy requirements on both sides are rather staggering however: I've allocated both sides 2×1023 joules each. That's something like a few thousand years of energy at our civilization's current rate of production. Any civilization capable of mustering such energies within weeks or months no doubt has much better ways of using it than either RKVs or rocket exhaust.


The lure of infinite fuel is too big a prize to let go without a fight. The Bussard ramjet concept has gotten a lot of scrutiny, trying to derive a spin-off concept without the crippling flaws but with most of the benefits.

In 1974, Alan Bond proposed the Ram-Augmented Interstellar Rocket (RAIR). RAIR attempts to deal with the drag problem and the difficulty of sustaining a proton-proton fusion reaction.

Basically, a RAIR carries its own fuel, but does not carry its own reaction mass.

Remember that fuel and reaction mass are generally not the same thing (unless you are dealing with a chemical rocket). For instance, in a nuclear thermal rocket, the fuel is the uranium or plutonium rods, and the reaction mass is the hydrogen propellant.

So the RAIR carries fusion fuel, feeding it to a fusion reactor in order to generate energy used to accelerate hydrogen gathered by the scoopfield. Since the RAIR carries its own fuel, it is not required to do proton-proton fusion, it is free to use whatever fusion fuel it wants.

The drag problem does not go away, but it is reduced. In a pure Bussard ramjet, the hydrogen scooped up has to be braked to a stop, creating drag (unless you can manage to make the hydrogen fuse while it is still travelling at whatever percentage of lightspeed the starship is travelling, which is pretty darn close to being impossible). In a RAIR, you do not have to slow the propellant down. You are left with the lesser problem of dealing with the braking effect of bremstrahlung and synchrotron radiation.

A related concept is the "Catalyzed RAIR." You still use a fusion rocket with internal fuel to get up to speed. But instead of heating the gathered hydrogen with the internal fusion reactor, you get it to do a low-grade reaction by itself.

You stick a target made of lithium or boron into the scooped hydrogen stream, as if it were the beam from a particle reactor. This will initiate a low-level lithium-hydrogen fusion reaction which will heat up and accelerate the rest of the stream. Lithium or boron fusion has the advantage of being almost totally without pesky neutron radiation.

Or if you want the ultimate Catalyzed RAIR, you just inject a steady flow of antimatter into the hydrogen stream. That will heat it up without requiring it to be braked to a stop first.

The draw-back to the RAIR is the fact that while the supply of propellant is infinite, the supply of fuel is not.

Quark Nuggets

A Quark Nugget is a chunk of "strange matter", which is composed of "strangelets", which are composed of roughly equal numbers of up, down, and strange quarks. In technical science speak it is described as Compact Composite Objects (CCOs) nuggets of dense Color-Flavor-Locked Superconducting quark matter created before or during the Quantum ChromoDynamics phase transition in the early universe. Now you know as much as I do.

Suffice to say that it is weird stuff.

Some scientist have become fascinated by the concept because:

  • It can explain Dark Matter (or why is there over five times as much gravity in the universe than can be accounted for with observed matter?)
  • It can explain the observed cosmological baryon asymmetry (that is, why isn't the universe half matter and half antimatter and thus suffering cosmic explosions every ten seconds?)
  • It can explain both of the above within exisiting physics, you do not need to postulate some bizarre new particle.

Thomas Marshall Eubanks examined the concept and wrote a scientific paper about them. You can tell it is relevant to our interests by the title: Powering Starships with Compact Condensed Quark Matter.

He calculates that this stuff is everywhere, left over from the Big Bang. There must be tons and tons of it, because it causes Dark Matter gravity. The point being it should be readily available in our own solar system. Now due to the incredible density of quark nuggets, it is all going to be at the core of various solar system objects. We won't be able to mine any at the core of Sol, the planets, or the moons, but asteroids are a different mattter. Eubanks notes there do exist so-called Very Fast Rotating asteroids, the little whirling dervishes have rotation periods measured in tens of seconds. This is consistent with strange matter asteroids with core masses between 1010 and 1011 kilograms (50 million metric tons). The cores can be extracted and used (but alas cannot be subdivided, the mutual attraction is too strong). The cores will typically be about one millimeter in radius.

Why do we care?

Because such quark nuggets can be used as super-efficient antimatter factories, that's why.

Using Andreev reflection you could create about 109 kilograms (1 million metric tons) of antimatter before the nugget wore out. You bombard the nugget with a 100 MeV particle stream and some of the particles will transform into their antiparticle (it is actually more complicated than that, but who cares?). Each 1010 kg of quark nugget can produce 109 kg of antimatter.

One the one hand it is far easier to generate antimatter as you need it, instead of trying to carry a million tons of touchy antimatter. Especially since an antimatter containment failure would make an explosion big enough to obliterate an entire solar system.

On the other hand it will be a major engineering feat to drag along a quark nugget with a mass that is a substantial fraction of the weight of Mount Everest. That's why I filed this here in the "Starship" page instead of the "Engines You Can Use Within The Solar System" page.


Marshall Eubanks has posited the presence of million tonne masses of stable quark matter inside solar system objects – potentially both matter and antimatter forms of it, with the antimatter version protected from annihilation by a 100 MeV Colour-Force potential well.

Powering Starships with Compact Condensed Quark Matter

While pure antimatter/matter propulsion promises high exhaust velocities (~c) the difficulties of achieving that ultimate performance are considerable. But what if we use something else for reaction mass and use antimatter to energise that? And, instead of using it in a rocket, we use a magnetic scoop to draw in reaction mass from the interstellar medium? This is the Ram-Augmented Interstellar ‘Rocket’ – though technically a rocket carries all its reaction mass – and it promises high performance without all the disadvantages of exponentially rising mass-ratios. Mixing 1% antimatter into the matter flow could, in theory, produce an exhaust velocity of ~0.2 c. Scooping and energising the equivalent mass of ~100 times the mass of the starship would allow a top-speed of 0.999999996 c to be achieved, before braking to a halt using half that mass. This would allow, at 1 gee acceleration, a journey of ~20,000 light-years. The nearby stars would be accessible at a much lower antimatter budget.

Quark Matter in the Solar System : Evidence for a Game-Changing Space Resource

Very Rapid Rotating asteroids might be held together by the additional gravity of a mm-sized million tonne quark nugget.

Primordial Capture of Dark Matter in the Formation of Planetary Systems

Evidence for Condensed Quark Matter in the Solar System

Observational Constraints on Ultra-Dense Dark Matter

Such quark nuggets would be made in the Big Bang potentially, if antimatter is squirrelled away in such a form, the explanation of the observed lack of free-antimatter in the Universe. The abundance of such ultra-dense tiny specks, to be compatible with microlensing observations, would be in the ‘interesting’ mass-range suggested by the Solar System evidence.

From ANTIMATTER AT HAND? by Adam Crowl (2014)

Pondering Marshall Eubanks’s concept of quark nuggets for making antimatter and hunting for such inside NEOs and comets, I thought of what an antimatter starship would require. The difficulty of storing anti-hydrogen led to me reason that carrying an antimatter source, like a quark nugget, made more sense than refining the stuff, then trying to store it safely. Make it as you use it seems the best approach.

That does imply that starships will mass millions of tons, to match the quark nugget. Depending on how the antimatter is mixed into the propellant stream, I suspect an antimatter rocket will be a comet adapted to the purpose, blasting out a jet of energised water as the main reaction drive. I’d hazard to guess the efficiency of such a rocket, since mixing annihilation energy into a reaction stream is incredibly difficult. However an exhaust velocity of 0.1 to 0.2 c seems reasonable.

When drives are power limited, based on the endurance of the engine rather than the energy of the fuel, there’s a simple relationship between the mission velocity, exhaust velocity and cruise velocity, with an overall mass ratio of ~4.42. The cruise velocity – the speed at which the vehicle coasts – would be somewhere between 0.075 c to 0.15 c, while the mission velocity would be 0.05 c – 0.1 c.

In the Oort Cloud there’s about 100 billion comets in far-flung orbits. One for every star in the Galaxy. If each formed around a quark nugget, then that would be 100 billion potential starships. Launching forth to every star in the Milky Way at 0.1 c, they would take ~750,000 years to reach the stars on the opposite side of the Milky Way to us. To reach every Globular Cluster in the Milky Way’s vast halo might take 1.5 – 3 million years.

Not every star has an Oort Cloud, ours being one of the few to keep its Cloud, as passages through Molecular Clouds and tight star clusters can lure the far-flung comets away with their gravity. Yet there are enough Oort Clouds that Others might have done the same before us. If Other Civilizations came to the same conclusion, as my musings above, and launched forth thus-like, what would a Galaxy in the throes of such a “Life Burst” look like from far away? Would we see the unique signs of antimatter annihilation spraying forth from that Galaxy? Could we see it with the right gamma-ray telescopes?


Starship Bumpers

If you had ever studied kinetic energy weapons, you'd be aware that their destructive energy is equal to ½v2m, that is, 0.5 times the square of the velocity v times the mass m. This means if you get hit in the head by a 1 kilogram brick traveling at 1 meter per second; if the brick was reduced to 0.1 kilograms, to get the same sized headache you'd only have to increase the velocity to 3 m/s. Not to 10 m/s like you'd expect, because just a little bit of velocity increase makes a big difference in the destructive energy. You might have noticed in the equation that while the mass is just in there plain, the velocity is squared.

Also note that as far as your headache severity goes, it does not matter if you are quote "standing still" unquote and the brick is traveling at 1 m/s or if you are moving at 1 m/s and the brick is "standing still." In both cases you will be damaged by the exact same amount of energy. Actually according to Einstein's relativity, both cases are just two ways of saying the same thing, but we won't get into that.

What does this have to do with starships? Well, first off if the starship is moving at relativistic velocities (above 0.1 c), squaring that velocity is going to make a huge number. And secondly, space ain't 100% empty. Yes, the interstellar medium is pretty darn close to being a perfect vacuum, but that is not the same as zero atoms. When you are multiplying this by a relativistic velocity squared, every atom counts.

In other words, a starship traveling relativistically will suffer as if it was under bombardment by a particle beam weapon. Over every square centimeter of frontal surface area. For decades.

Within about 200 light-years of Sol the density is around 7×10-2 atoms/cm3, because Sol in inside a bubble. Elsewhere it varies from 10-4 to 106 atoms/cm3 depending upon what sort of space you are in.

Remember, this was one of the problems a Bussard Ramjet was designed to solve.

While the bombardment will erode away the solid metal of the leading edge of the starship, the main threat is the radiation. The bombardment will be functional equivalent of you basking your unprotected body in the radioactive glow of twenty unshielded nuclear reactors. According to Dr. Oleg Semyonov, the estimated radiation dose is about:

D = 1.67×10-8 × Q × n × S × β × c × H(β) × d(β) / M


D = radiation dose (rem/s)
Q = radiation quality factor (for protons Q = 10)
n = concentration of interstellar gas (cm-3) varies from 104 cm-3 om galactic clouds to less than 1 cm-3 between clouds. Around Sol 0.2 cm-3
S = cross-section of a human body (cm2) ≈ 104 was used in the paper
H = stopping power of particles in human tissue (MeV cm2/g)
d = EITHER penetration depth of particles in human tissue OR thickness of human body in direction of motion (35 cm), whatever is less (g/cm2)
M = mass of individual (g)
c = speed of light in a vacuum (cm/s) = 29,979,245,800 cm/s
β = percentage of the speed of light, v/c

Paper says The data for H and d as functions of energies of nucleons are taken from the NIST (National Institute of Standards and Technology) online database.

A safe dose is about 3×10-7 rem/s or about 10 rems/year.

The paper estimates that up to 0.3 c the radiation can be controlled with a titanium radiation shield about 2 cm thick. Above 0.3 c the thickness increases "dramatically". Around 0.8 c the titanium shield will have to be several meters thick.

You can find details and other equations in Radiation Hazard of Relativistic Interstellar Flight by Oleg Semyonov.

Shields for Icarus: Part 2 – Navigational Deflectors for Real

(ed note: Icarus was to have a maximum velocity of 0.2 c)

A proposed means of decelerating from interstellar speeds is the magnetic-sail, which is a large loop of superconducting wire producing an artificial magnetosphere around the moving spacecraft. By deflecting interstellar ions, the magnetic field forms a semi-spherical zone forward of the vehicle where the magnetic pressure of the field and the pressure of colliding ions are evenly balanced. A magnetopause forms, in which ions are reversed in direction and their change in momentum produces an equal, but opposed, change in momentum in the magnetic-sail, and thus the spacecraft to which it is attached.

Interestingly the Sun’s magnetosphere already acts like a deflector shield, forcing the ions and small charged particles of dust to flow around the Sun as it moves against the average flow of the Galaxy. Exposed to energetic photons (ultraviolet and x-ray) and high-energy ions (cosmic rays) the interstellar dust is charged. The very smallest dust particles, up to a certain diameter, are completely excluded from the inner Solar System by the Sun’s magnetosphere, while particles a bit larger are significantly deflected. Only the high-end of the dust size range is able to penetrate.

In the case of a moving magnetic-sail, the atoms of the Interstellar Medium (about 90%-50% of the ISM) are actually ionized by its rapidly changing magnetic-field strength, in a process akin to that used to ionize gas in a Pulsed Inductive Thruster. If you imagine an atom drifting through space at typically 15 km/s, to then encounter a magnetic field approaching at 60,000 km/s is to experience a change in field sufficiently quick enough to ionize the atom. In effect the ship is creating a shock-wave in the ISM which is producing a lot of extra charge as atoms are ionized. All those suddenly energetic electrons could be sufficient to increase the charge on the ISM dust, thus increasing the deflector effect.

The question needing investigation is whether this is sufficient to provide protection against all the ISM dust, or whether some additional defences will be needed. Cosmic sand-grains, with the kinetic energy of 100 pound bombs, while rare, will perhaps still need some means of interception by “Icarus”. The original “Daedalus” study proposed an artificial dust cloud moving 200 kilometres ahead of the main vehicle and this might prove sufficiently effective.

Alternatively newer materials have become available which might provide multilayer protection – carbon allotropes, the most exciting of which is graphene. Graphene is basically a single sheet of graphite – a hexagonal grid form of carbon in the form of immensely strong sheets of covalently bonded carbon atoms, but held together between the sheets via via weak hydrogen bonds to make graphite. Peeling away single layers of graphene has now become possible and it has all sorts of surprising properties.

What I’m interested in for shielding is making a large, low-mass “bumper” which cosmic sand-grains run into before hitting the craft. After passing through several layers of graphene the offending mass is totally ionized and forms a high-energy spray of particles, but particles that can now be deflected by the vehicle’s cosmic-ray defences (akin to the mag-sail, but smaller with a higher current) and safely diverted away from sensitive parts. To put the bumper in place, perhaps 100 kilometres ahead, it can be deployed via a small sub-vehicle – sheets made from carbon fibre are surprisingly springy and can self-unfold from a small volume. Once in place it might be kept in place by firing lasers at super-reflective patches on the bumper. Via reflecting ~2,000 times the laser achieves far more push than a single pulse of energy can achieve.

Circuitry is being made from graphene in laboratories around the world, thus the bumper isn’t a passive mass. Multiple layers could work together to track any grains that pass through without being totally ionized. This causes a signal to be sent back to the vehicle which then activates its final layer of defence, high-powered lasers. In microseconds the lasers either utterly ionize the target or give it a sideways nudge via ablation – blowing it violently to the side via a blast of plasma. Such an active tracking bumper would need to be further away than 100 km to give the laser defence time to react, though 1/600th of a second can be a lot of computer cycles for a fast artificial intelligence. The lasers might use advanced metamaterials to focus the beam onto a speck at ~100 km, without needing to physically turn the laser itself in such a split-second. Highly directional, high-powered microwave phased arrays exist which already do so purely electronically and an optical phased-array isn’t a stretch beyond current technology.


‘You’ll see that the ship is roughly cylindrical — length four kilometres, diameter one. Because our propulsion system taps the energies of space itself, there’s no theoretical limit to speed, up to the velocity of light. But in practice, we run into trouble at about a fifth of that speed (0.2 c), owing to interstellar dust and gas. Tenuous though that is, an object moving through it at sixty thousand kilometres a second or more hits a surprising amount of material — and at that velocity even a single hydrogen atom can do appreciable damage.

‘So Magellan, just like the first primitive spaceships, carries an ablation shield ahead of it. Almost any material would do, as long as we use enough of it. And at the near-zero temperature between the stars, it’s hard to find anything better than ice. Cheap, easily worked, and surprisingly strong! This blunt cone is what our little iceberg looked like when we left the solar system, two hundred years ago. And this is what it’s like now.’

The image flickered, then reappeared. The ship was unchanged, but the cone floating ahead of it had shrunk to a thin disc.

‘That’s the result of drilling a hole fifty light-years long, through this rather dusty sector of the galaxy. I’m pleased to say the rate of ablation is within five per cent of estimate, so we were never in any danger — though of course there was always the remote possibility that we might hit something really big. No shield could protect us against that — whether it was made of ice, or the best armour-plate steel.

‘We’re still good for another ten light-years, but that’s not enough. Our final destination is the planet Sagan 2 — seventy-five lights to go.

‘So now you understand, Mr. President, why we stopped at Thalassa. We would like to borrow — well, beg, since we can hardly promise to return it — a hundred or so thousand tons of water from you. We must build another iceberg, up there in orbit, to sweep the path ahead of us when we go on to the stars.’

‘But why is it that shape?’ the president asked.

Deputy Captain Malina sighed; he was quite sure that this had already been explained several times.

‘It’s the old problem of covering any surface with identical tiles,’ he said patiently. ‘You have only three choices — squares, triangles, or hexagons. In our case, the hex is slightly more efficient and easier to handle. The blocks — over two hundred of them, each weighing six hundred tons — will be keyed into each other to build up the shield. It will be a kind of ice-sandwich three layers thick. When we accelerate, all the blocks will fuse together to make a single huge disk. Or a blunt cone, to be precise.’

For that matter, they might never reach Sagan 2. Although the ship’s operational reliability was still estimated to be ninety-eight per cent, there were external hazards which no one could predict. Only a few of his most trusted officers knew about the section of the ice-shield that had been lost somewhere around light-year 48. If that interstellar meteoroid, or whatever it was, had been just a few metres closer …

From THE SONGS OF DISTANT EARTH by Arthur C. Clarke (1985)



The URSS Alabama is a fictional Bussard Ramjet starship from Alan Steele's novel Coyote (2002). It was the first starship, build by the authoritarian conservative regime which took over after the fall of the United States. At its dedication ceremony, it is hijacked by the captain, and escapes the regime by travelling to 47 Ursae Majoris. The 46 light-year journey takes 230 years cruising at 0.2c, with the crew and colonists in biostasis.

Avatar ISV Venture Star

RocketCat sez

Much as I hate to admit it, the Venture Star is arguably the most scientifically accurate spacecraft in the history of Hollywood. It is a beautiful piece of work, with all the major problems solved. And it has heat radiators!

The good starship ISV Venture Star from the movie Avatar is one of the most scientifically accurate movie spaceships it has ever been my pleasure to see. When I read the description of the ship, I got a nagging feeling that something was familiar. A ship with the engines on the nose, towing the rest of the ship like a water-skier? Wait a minute, that sounds like Charles Pellegrino and Jim Powell's Valkyrie starship.

Well, as it turns out, there was a good reason for that. James Cameron likes scientific accuracy in his movies. So he looked for a scientist who had experience with designing starships. Cameron didn't have to look far. As it turns out he already knew Dr. Pellegrino. This is because Dr. Pellegrino had worked with Cameron on a prior movie, since Dr. Pellegrino is one of the worlds greatest living experts on the Titanic.

After James Cameron had designed all the technical parameters of the Venture Star, master artist Ben Procter worked within those parameters to bring it to life.

Departing from Earth

In the upper diagram is a green arrow at the ship's nose, indicating the direction of flight. The ship is 1.5 kilometers long. In the Sol departure phase, a battery of orbital lasers illuminates a 16 kilometer diameter photon sail attached to the ship's nose (sail not shown). A mirror shield on the ship's rear prevents the laser beams from damaging the ship. The lasers accelerate the ship at 1.5 g for 0.46 year. At the end of this the ship is moving at 70% the speed of light (210,000 kilometers per second).

Keep in mind that battery of orbital lasers is going to have to be absolutely huge if it is going to push a lightsail at 1.5 g. This is not going to be a tiny satellite in LEO.

I cannot calculate the exact power rating since figures on the mass of the ISV Venture Star are conspicuous by their absence. The equation is Vs = (2 * Ev) / (Ms * c) where Vs is the starship acceleration, Eb is the energy of the beam, Ms is the mass of the starship, and c is the speed of light in a vacuum. Dr. Geoffrey Landis says is boils down to 6.7 newtons per gigawatt.

In Dr. Robert Forward's The Flight of the Dragonfly (aka Rocheworld), his starship's light sail is illuminated by a composite laser beam with a strength of 1500 terawatts. This pushes the starship with an acceleration of 0.01g (about 150 times as weak as the acceleration on the Venture Star). The beam is produced by one thousand laser stations in orbit around Mercury (where solar power is readily available in titanic amounts). Each station can produce a 1.5 terawatt beam, 1500 terawatts total. By way of comparison, in the year 2008, the entire Earth consumed electricity at a rate of about 15 terawatts. Since the Venture Star appears to be more massive than Forward's starship, and is accelerating 150 times as fast, presumably its battery of laser cannons is orders of magnitude larger.

As a side note, it is good to remember Jon's Law for SF authors. and The Kzinti Lesson. While technically this laser array is a component of a propulsion system, not a weapon; in practice it will have little difficulty vaporizing an invading alien battlefleet. Or hostile human battlefleet, for that matter (with the definition of "hostile" depending upon who actually controls the laser array). As Commander Susan Ivanova said in the Babylon 5 episode Deathwalker: "Our gun arrays are locked on to your ship, and will fire the instant you come into range. You will find their firepower most impressive ... for a few seconds."

Anyway, after the laser boost period is over, the sail is then collapsed along molecular fold lines by service bots, and stowed in the cargo area. The ship then coasts for the next 5.83 years to Alpha Centauri.

Braking at Alpha Centauri

There are no batteries of laser cannon at Alpha Centauri so the lightsail cannot be used to brake to a halt. Instead, the twin hybrid fusion/matter-antimatter engines are used. These engines are not used for the Sol departure phase because that would increase the propellant requirement by about four times with a corresponding decrease in cargo capacity. The engines burn for 0.46 year, producing 1.5 g of thrust, thus braking the ship from a velocity of 70% c to zero.

Matter and antimatter is annihilated, and the energy release is used both in the form of photons and to heat up hydrogen propellant for thrust. A series of thermal shields near the engines protect the ship's structure from the exhaust heat. The engines are angled outwards a few degrees so that the exhaust does not torch the rest of the ship (exhaust path indicated in diagram by red arrows). This does reduce the effective thrust by an amount proportional to the cosine of the angle but is acceptable.

Why is most of the ship behind the engine exhaust? Because this reduces the mass of the ship. And when you are delta-Ving a ship up to and down from 70% c, every single gram counts. Conventional spacecraft have the engines on the bottom and the rest of the ship build on top like a sky scraper. This design has the engines on the top and the rest of the ship is dragged behind on a long tether (the "tensile truss" on the diagram). The result is a massive reduction in structural mass.

The engines are topped by monumental heat radiators used to get rid of waste heat from the matter-antimatter reaction. According to the description, after the burn is finished, the radiators will glow dull red for a full two weeks.

Cargo Modules

Immediately stern ward of the engines is the cargo section. It is arranged in four ranks of four modules each. Each module contains 6 cargo pods. A mobile transporter with a long arm moves within the cargo section in order to load and unload the shuttles.

Interface Craft

Next comes Two Valkyrie trans-atmospheric vehicles, aka "surface to orbit shuttles." They are docked to pressurized tunnels connected to the habitation section. Each is capable of transporting either:

  1. the contents of two cargo pods and 100 passengers OR
  2. the contents of six cargo pods and no passengers
Habitation Modules

Next come the habitation module. This holds the passengers in suspended animation for the duration of the trip. This is constructed almost totally from non-metallic materials, to prevent secondary radiation from galactic cosmic radiation.

The habitation module's life support system can only support all the passengers being awake for a limited time. There is no problem for the short period when the passengers are woken up and shuttled to the planet's surface. However, if the suspended animation system malfunctioned half-way through the multi-year voyage, life support could not handle it. In theis case, the passengers would be "euthanized" instead of being awakened.

Crew Modules

Next is the two on-duty crew modules. These are spun on the ends of arms to provide artificial gravity. When the ship is under thrust, the spin is taken off, and the arms are folded down along their hinges so that the direction of gravity is in the proper direction.


Finally comes the shield. While the ship is being boosted by the laser batteries, the shield protect the ship (but not the sail) from the laser beams. After boost, while the ship is coasting at 70% c, the ship is rotated so that the shield is in the direction of travel. The shield is constructed as a Whipple shield, and protects the ship from being damage by grains of dust.

At 70% c relative, each dust grain would have 4,900,000,000 freaking Ricks of damage. This means a typical interstellar dust grain with a mass of 4 x 10-6 grams will hit with the force of 20 kilograms of TNT, or about the force of four anti-tank mines.

When the ship wants to depart Alpha Centauri and return to Sol, it re-fills its antimatter and propellant tanks from the local fueling stations, uses the matter-antimatter engines to boost up to 70% c again, coasts for five-odd years, and is decelerated to a halt by the laser batteries at Sol.

Encounter With Tiber

There is not one, not two, but three different slower-than-light starships in Encounter With Tiber by Buzz Aldrin and John Barnes.

9,000 years ago, the aliens ("Tiberans") living around Alpha Centauri A become aware of a rogue planet that is going to drastically lower the property values of their home planet. They need to migrate their civilization to another planet, and their is not any suitable candidates in the Alpha Centauri star system. So they take a look at our Solar System.

In the 73rd century BCE they mount an interstellar scouting mission to Terra, using the starship Wahkopem Zomos. The mission mysteriously fails. In the 72nd century BCE a follow-up mission is sent, using the starship Egalitarian Republic. It fails as well.

Around 2030 we humans discover artifacts from the two alien mission on Luna's south pole and on Mars. In 2069 a mission is sent to Alpha Centauri to make first contact, using the starship Tenacity.

Wahkopem Zomos

A plasma-core antimatter booster section sends the starship Wahkopen Zomos into a close perihelion approach to the primary star (Alpha Centauri A). A 1000 kilometer diameter solar sail is unfurled. This accelerates the ship to a close approach to Alpha Centauri B for a second perihelion manoeuvre. It is then further accelerated by lasers until it reaches a velocity of 0.4c. It then cruises to Sol for about 18 m (alien) years.

Approaching Sol, it deploys a "brakeloop" of superconducting wire 100 kilometers in diameter. This converts the ship's kinetic energy into heat in the interstellar medium. Two years of braking is enough to slow the starship into the solar system.

The plan was for the homeworld to launch a 5000 kilometer laser sail and guide it into the solar system. Then it could reflect laser beams on to the Wahkopen Zomos' sail and return it to Alpha Centauri A. Unfortunately politics at home led to abandoning this plan, thus stranding the Wahkopen Zomos. This is an occupational hazard for laser lightsail starships. The advantage is you leave at home your engine and its inconvenient mass. The disadvantage is you are at the mercy of the people at home (and their political parties) who control the engine.

Wahkopem Zomos was a stubby cylinder, wider than it I was long and about as tall as a four-story building, inside a wide ring that surrounded its base and extended about one-fourth as high as the cylinder. We lived in the ring—once we were on our way, the ship would be spun along its axis, so that the outer edge of the ring formed the outer deck, with the gravity we were used to, and an interior deck formed the inner deck, with about four-fifths of Nisuan (Tiberian Homeworld) gravity, a little less than the gravity would be on Setepos (Terra) when we got there.

Yet despite the apparent size from the outside, quarters were fairly cramped inside. The entire central cylinder was devoted to the ship's farm, sail room, power plant, and lander storage; life support, waste recycling, general storage, and everything else took up more than half of the ring. So we actually only lived in the outermost part of the ring, on a double deck that barely had head clearance for Poiparesis. And many parts of the living space were things like the cockpit and the biological laboratory that couldn't be used for much of anything else and weren't used most of the time. Even with all that space in the ship, at the time, as a child, I could already span my compartment with my outstretched hands.

Inside the central cylinder were the power plant, the reaction engines, the recycling system, the ship's farm, and the squat, dark forms of the two landers, Gurix and Rutnaz. Though it would be almost twenty-four years until we used them, they were always there, reminding us of what we were intending to do. The forward third of the cylinder was taken up with the sail, brakeloop, shrouds, and winches to operate them.

"Boost imminent," Osepok's voice said from the intercom.

We turned to look out the big view port. Behind the ship, connected by a long, thin pole, was a big structure of struts and tanks, a third as wide as the ship and five times as long: the booster. It filled most of the window, shining silver in the harsh light of space. For a long breath or two nothing happened. Then a glow appeared behind the booster and spread to fill the rest of the window.

There was no sound, of course, with no air to carry it; just the purplish-white glow. We sank into the webbing, and the hammocks swung around so that our faces were pointed down toward the view port as the ship began to accelerate. Moment by moment, we felt ourselves gaining weight, sinking deeper into the hammocks.

Ordinary spacecraft had to take off from Nisu's surface, starting with no velocity and fighting directly against gravity; they had to accelerate at about one and a half times the acceleration of gravity, increasing to three gravities, for periods of a thirty-second of a day (45 minutes) or more, to leap up to orbit. But Wahkopen Zomos was already in orbit around Nisu, and Nisu was orbiting Sosahy; we could begin with a gentler thrust and let it run for a third of a day (8 hours).

At first the thrust was pushing Wahkopen Zomos, plus all those tanks and struts in the booster, plus the immense weight of fuel, thirteen times the weight of the ship itself (mass ratio of 14, which is a little excessive). The ship and booster sped up very slowly. But with each passing instant, more of the fuel was gone, and yet the engines pushed just as hard. Acceleration increased, and the webs pressed harder against our faces.

The glare we saw was hydrogen plasma, heated far beyond the point where its electrons and protons stayed together, so that it was a mere thin wisp of atomic particles. By weight the booster was almost all liquid hydrogen, and the rest was the assembly of girders, tanks, and pipes that held it together—but a tiny fraction of the total mass, held in one small compartment that any of us could have picked up and carried with one hand, was the key to the whole thing: antimatter. Mix liquid hydrogen just above absolute zero with one millionth of its weight in antimatter, and it became hydrogen plasma hotter than the core of the sun (plasma-core antimatter rocket).

Had we been outside, looking directly at the glow instead of seeing it through a shielded viewport, we would have been blinded; on Nisu below us, people had to be warned not to look directly at our boost out of orbit, and we briefly lit up the sky ss brightly that night animals went back to their dens and plants opened their leaves to what they thought was sunrise.

(they make a close perihelion approach to Alpha Centauri A, deploying a photon sail)

Long ages crept by and I watched my screen. I itched in a couple of places and quickly scratched those, always watching the clock on the screen to make sure that it wasn't too close to sail deployment. Poiparesis had told us that if we got a hand trapped under ourselves, very likely we would break every bone in that hand and in our wrists, and give ourselves deep bruises in whatever flesh lay across the hand.

Time crawled by slowly. The screen showed the sun bloated and swollen, almost as large as Sosahy seen from Nisu's surface; the filters over the cameras meant we were seeing less tan one ten-millionth of the actual brightness outside, and yet the screen was becoming uncomfortably bright to look at.

If we had tried to use a rocket, to have made the trip to Setepos and returned within our lifetime would have taken a vastly larger ship that would have had to be almost all antimatter. As it was we had burned virtually all the antimatter of Nisu, nine years of production, in our booster at takeoff, and the speed it had gotten us up to would have taken tens of thousands of years to get us to Setepos. We needed more power than all of Nisu produced in a year, and we needed it early in the trip so that we could travel as much of it as possible at high speed.

From Encounter With Tiber by Buzz Aldrin and John Barnes (1996)
Egalitarian Republic

Photon drive powered by vacuum energy.


Enzmann Starship

The Enzmann starship is a concept for a manned interstellar spacecraft proposed in 1964 (date is disputed) by Dr. Robert Enzmann. Over the years the basic design has evolved, and there were several types in the initial design. It was quite popular in the science fiction community. An analysis of the Enzmann starship can be found here.

In 1972 space artists Don Davis and Rick Sternbach worked with Dr. Enzmann to develop the idea. This refined the "lollypop" look of the ship. For some odd reason most paintings of the Enzmann starship show two of them in formation. The original design had a naked sphere of frozen deuterium as fuel. Calculations with Sternbach and Davis revealed that the deuterium could not be kept frozen and was too structurally weak to be accelerated. So the redesign encased the deuterium in a huge tank.

The Enzmann exploded into the science fiction community with the October 1973 issue of Analog magazine. G. Harry Stine wrote an extensive article about the concept, accompanied by a stunning piece of cover art by Rick Sternbach. Stine said the ships were 12 million tonnes, could reach 0.30 c (highly unlikely), had 8 engines, and used spinning habitats for artificial gravity.

  • Command center 30 meters in diameter
  • Central core load bearing struture 15 meters diameter
  • Frozen deuterium 300 meters diameter
  • Living modules 90 meters diameter × 90 meters long
  • Engineering compartments 70 meters diameter

In Science Digest, Rick Sternbach's 1972 piece depicts a pair of Enzmanns departing from an asteroid factory. The number of engines was increased from 8 in the original design to 24. Modular sections were created that can separate from the starship. Height of starship was 690 meters. 3 million tonnes of deuterium, with metal shell (doubling as a radiation shield). Magnetic confined fusion propulsion. 20 decks per habitat, 100 rooms per deck. Cruising speed 0.09c.

Thomas Schroeder wrote an article entitled "Slow Boat to Centauri" in Astronomy Magazine. Claimed a cruising speed of 0.1c, and an advanced design might reach 0.3c. 12 million tonnes deuterium. The outer layers of the habitats were composed of bulk material as radiation shielding for the inner layers. Bulk means nuclear reactor, store rooms, heat exchangers, airlocks, landing craft, observation areas, communication equipment. Eight Project Orion nuclear pulse units.

  • Frozen deuterium 305 meters diameter
  • Height 609.6 meters
  • Living modules 91.5 meters diameter × 91.5 meters long
  • From bottom of fuel sphere to top of Orion engines 305 meters

In the 1980's Dr. Enzmann started designing variants.

In 2011, K. F. Long, A. Crowl, and R. Obousy did a study on the Enzmann starship, and tried to rationalize it with recent developments in astronautics. First they took the historical concepts:

Historical Concepts
Length610 msamesame
Sphere Diameter305 msamesame
Total Habitat Length273 msamesame
Individual Habitat length91 msamesame
Habitat Diameter91 msamesame
Core Diameter15 msamesame
Num Habitats113
Num Engines8824
Exhaust VelocityUnspecifiedUnspecifiedUnspecified
Specific ImpulseUnspecifiedUnspecifiedUnspecified
Structural MassUnspecifiedUnspecifiedUnspecified
Propellant Mass3×106 metric tons12×106 metric tons3×106 metric tons
Cruise Speed27,000 km/s
90,000 km/s
27,000 km/s
Starting Colony200samesame
Final Colony2000samesame

Long et al created three variants. The primary difference is the size of the population carried. The rest of the design was re-sized to handled the population. As near as I can calculate, the cruise and mission times for all three are for a mission to Alpha Centauri.

Specific Power11.5 MW/kg
Thrust Power344 terawatts
Length620 m979 m1752 m
Dry Mass30,000 MT300,000 MT3,000,000 MT
Propellant Mass3 × 106 MT3 × 106 MT3 × 106 MT
Mass Ratio
Mass Ratio10.053.321.41
Exhaust Velocity11,700 km/s11,260 km/s12,119 km/s
ΔV54,000 km/s
27,000 km/s
8,400 km/s
Cruise Velocity27,000 km/s
13,500 km/s
4,200 km/s
Total Acceleration
18.95 yrs98.67 yrs84.9 yrs
Total Cruise
41.05 yrs51.33 yrs265.1 yrs
Total Mission
60 yrs150 yrs350 yrs
Mass Flow Rate5.02 kg/s0.96 kg/s1.12 kg/s
0.019 m/s2
0.003 m/s2
0.004 m/s2
Thrust58,730 kN10,810 kN13,573 kN

Firefly Starship

Firefly Starship
2013 design
ΔV2.698×107 m/s
Wet Mass17,800 metric tons
Dry Mass2,365 metric tons
Mass Ratio7.526
Payload150 metric tons
PropulsionZ-Pinch DD Fusion
Exhaust Velocity1.289×107 m/s
Thrust1.9×106 N
Acceleration0.11 m/s
(0.01 g)
Accel time4 years
Coast time93 years
Decel time1 years
Firefly Starship
2014 design
ΔV2.998×107 m/s
Wet Mass45,000 metric tons
Dry Mass3,000 metric tons
Mass Ratio15.0
Payload150 metric tons
PropulsionZ-Pinch DD Fusion
Specific Impulseone million seconds
Thrust855,000 N
Acceleration0.019 m/s
(0.002 g)
Accel time25 years
Coast time70 years
Decel time5 years
Length~1,0000 m

Icarus Interstellar has a project to design a fusion-rocket based interstellar spacecraft. They call it "Firefly". The technical lead director is Robert Freeland.

Most of the other Icarus fusion designs use inertial confinement fusion. That's because IC fusion is easier to get halfway worthwhile power levels. Magnetic confinement fusion would be nicer but once you get enough nuclear fusion going to to be worthwhile, the magnetic bubble pops like a cheap balloon.

The drawback to IC fusion is that the confinement time is pathetic. The longer you confine the fusion reaction, the more of the fusion fuel actually burns and generates energy. But in IC fusion the first bit of fusion acts to blast the pellet apart, scattering the un-burnt fuel to the four winds.

Back in the olden days of fusion research, the darling was Z-Pinch fusion. You send a bolt of electricity (about 5 mega-amps) down the center of a long tube full of ionized plasma, creating magnetic field which compresses the plasma enough to ignite nuclear fusion. One of the big advantages with Z-Pinch was that the confinement time (and net energy output from the burn) can be increased by simply making the reaction chamber longer.

Unfortunatley, the disadvantage is that Z-Pinch fusion suffers from several hydrodynamic instabilities which disrupt the plasma. So researchers stopped working on it in.

But in 1998 Dr. Uri Shumlak discovered you could eliminate the instabilities if you made the plasma move at high velocities. Based on this work, Z-Pinch was selected for the Icarus design.

The Firefly's long thin tail is the Z-Pinch tube, frantically fusing and radiating x-rays like a supernova. So the starship was given its name for similar reasons as the one on the TV show: it is a flying thing whose tail lights up.

The spacecraft profile is long and skinny, for two reasons:

  • Its cruise velocity is a substantial fraction of the speed of light (4.5% c for the 2013 version). This make interstellar dust grains impact with about 9.1×10-4 joules worth of damage, the equivalent of 46,000 cosmic ray photons. You want to reduce the ship's cross section as much as possible to minimize the number of grain impact events.
  • The longer the ship is, the farther the payload can be placed from the deadly radioactive Z-Pinch drive, taking advantage of distance shielding.

Many other starship designs use 3He-D fusion, because all the reaction products are charged particles that can be easily shieldied. The drawback is that 3He is rare, you'd have to harvest the atmosphere of Jupiter for twenty years in order to get enough.

Instead, Firefly uses D-D fusion, since deuterium can be easily found in common seawater. Of course then you have to deal with all the nasty neutrons and x-rays produced by that reaction. Firefly's approach is to forgo the use of massive radiation shields, and instead try to let as much of the radiation escape into space. The Z-Pinch core is almost totally open to space with only a triad of support rails connecting the aft electrode and magnetic nozzle to the rest of the vessel.

Even with that, the waste heat is going to be titanic. That's where the heat radiators come in. Notice how they are the bulk of the ship. Makes the thing look like a garantuan lawn-dart. The radiators use beryllium phase-change technology, and are positioned as close as possible to the heat loads on the tail.

A long conical shield forwards of the reactor core deflects x-rays away from the payload using shallow-angle effects. The electrodes, rails, and other structure near the core are constructed of zirconium carbide (which is capable of surviving the intensely radioactive environment.

The 2014 design had a total length of just under one kilometer, half of which is the fuel tanks. The forward part of the ship uses the old fuel tank in lieu of spine trick in an effort to save on ship mass.

A fission reactor provides secondary power.

Frisbee Antimatter Starship

Antimatter Starship
(one stage)
Beam Core
ΔV7.5×107 m/s
Exhaust Velocity9.99×107 m/s
Thrust1.174×107 N
Thrust Power587.4 TW
Average Accel0.098 m/s
(0.01 g)
Gamma radiation996.3 TW
Mass Ratio5.45
Dry Mass
Dust Shield6,530 MT
Power Systems1,064.6 MT
Payload100 MT
Misc.100 MT
Propellant tanks,
feed system
26,698.8 MT
Propellant tank
104.7 MT
Payload Rad Shield
w/ radiator
361.6 MT
Radiator Rad Shield6.4 MT
Magnet Rad Shield103.3 MT
R. R. +
Magnet Shield
15,533.7 MT
Magnet, structure,
282.3 MT
3,707.4 MT
30% Contingency
16,347.8 MT
Total Dry Mass70,940.6 MT
Propellant Mass
Propellant Total
matter LH2
159,450 MT
boiloff loss
matter LH2
1,579 MT
Propellant Usable
matter LH2
157,872 MT
Propellant Total
antimatter LH2
165,765 MT
boiloff loss
antimatter LH2
7,894 MT
Propellant Usable
antimatter LH2
157,872 MT
Engine Magnet Radiation Shield
mass103 MT
volume5.337 m3
thickness0.173 m
cross-section area0.088 m2
minimum distance
to ignition point
10.639 m
center distance
to ignition point
11.038 m
fraction of gamma
flux intercepted
gamma power
1.455×104 GW
Radiator Radiation Shield
mass6.434 MT
volume0.332 m3
diameter19.9 m
height0.125 m
cross-section area2.488 m2
minimum distance
to ignition point
along hypotenuse
11.455 m
minimum distance
to ignition point
along x-axis
5.123 m
fraction of gamma
flux intercepted
gamma power
1.503×103 GW
System & Payload
Radiation Shield
mass361.09 MT
volume18.661 m3
diameter19.9 m
height0.064 m
cross-section area311.026 m2
minimum distance
to ignition point
along hypotenuse
5.152×105 m
fraction of gamma
flux intercepted
gamma power
9.29×10-5 GW
Shield Radiator
gamma power
to radiate
16,052 GW
2-sided area1.025×107 m2
width19.9 m
height515,189 m
(515 kilometers)
mass15,533.7 MT

This is from AIAA 2003-4676 How To Build an Antimatter Rocket For Interstellar Missions by Robert H. Frisbee. The basic spacecraft has a delta V of one-quarter the speed of light and an acceleration of 0.01 g. The freaking thing is about 700 kilometers long (about the distance between Washington DC and Montpelier Vermont), due to the off-the-chart levels of gamma radiation and the 500 kilometers of heat radiators required to keep the ship from vaporizing.

Most of the 500 km of heat radiators is to reject the gamma-ray heat absorbed by the radiation shields.

The superconducting magnet in the engine proper is kept cool to 100 Kelvin, the liquid hydrogen is cooled to 20 K, and the solid anti-hydrogen pellets are cooled to 1 K.

On the nose is the dust impact shield, which protects against interstellar dust impacts. Because at 0.25 c even a speck of dust is going to hurt.

Everything you hit will have about 625 mega-Ricks worth of damage. This means if you hit a grain of sand that had a mass of one milligram (10-3 kg), it would explode with about the force of 625 metric tons of TNT. Now your average interstellar dust grain has only a mass of 10-17 kg which makes the boom much smaller. Unfortunately the interstellar medium has a dust density of 10−6 × dust grain/m3, and there are a lot of meters in a light year.

My slide rule says a cylinder with a diameter of 19.9 meters and a length of one light-year will contain about 2.94×1018 m3. This is the volume the nose of the starship will plow through per one light-year of travel. At a dust density of 10−6 grain/m3 means the nose will hit 2.94×1012 dust grains. 10-17 kg per grain means total mass impacting the shield per light year is 2.94×10-5 kg. At 625 mega-Ricks this means it will only subject the dust shield to the equvalent of an explosion of 18.4 metric tons of TNT. Per light year.

The design specs called for a cruising velocity of 0.5 c, which means you'd need four stages, that is, a stack of four of these monsters. One stage to boost up to the coasting speed of 0.5 c, second stage to brake from 0.5 c to halt at the destination, third stage to boost to 0.5 c for the trip home, and 0.5 c to brake to a halt at Terra. The four stage vehicle will have a length between 1,900 and 7400 kilometers, depending upon the technology assumptions. Egads.

As it turns out the starship needs a minimum acceleration or it will take a century to get up to speed. Dr. Frisbee drew up the above chart and figured if you wanted to maximize the mission time spent at peak velocity the starship would have to be capable of accelerating and decelerating at about 0.01 gee minimum. The trouble is that beam core antimatter drives are classic high specific impulse/low thrust rockets. This means you have to really crank up the propellant mass flow if you want to get 0.01 g. Which means the engine mass will skyrocket.

Another problem with using proton-antiproton antimatter rockets is that only 22% of the propellant mass actually propels the starship. The rest is wasted. This means that the standard delta V equation has to be modified to take this into account. It needs to be modified further for relativity if the delta V is substantial fractions of the speed of light. The equation was use to draw the graph above. The equation itself is below.

So a normal rocket that does not annihilate its reaction mass so that 100% of it propels the starship uses the standard delta V equation. This says if the specific impulse is 0.33 c and the delta V is 0.25 c, the mass ratio would be a modest 2.15. But for this antimatter rocket with only 22% of the propellant working (a=0.22), the mass ratio climbs to 5.45. By doing some estimates on the minimum tankage masses, Dr. Frisbee concludes that 0.25 c is the maximum delta V per stage of the starship. You can read his reasoning in the report.

It is bad that only 22% of the propellant is doing its job. What is worse, 38% of the propellant mass is turned into deadly gamma rays that will fry anything unprotected from their deadly shine. This means heavy radiation shields, which need 500 kilometers of heat radiators to keep the gamma-ray heat from vaporizing them. This also forces the vehicle to be long and narrow to minimize the solid angle of intercepted gamma radiation from the engine.

Ghost Ship

Ghost Ship
Payload150,000 kg
Dry Mass3,800,000 kg
Propellant Mass150,000,000 kg
Wet Mass153,800,000 kg
Mass Ratio40.5
EngineIC Fusion
Pellet Mass2 grams @
Ignition Rate150 Hz
ΔV19,607,277 m/s
Exhaust Velocity5,297,400 m/s
Specific Impulse540,000 s
Thrust1,600,000 N
0.01 m/s

This was the winner in Project Icarus' 2013 contest to design an interstellar starship using current technology. The entry was created by the Munich Ghost Team headed up by Andreas Hein. The basic rules were to design a spacecraft which was mainly fusion powered and on a mission to Alpha Centauri carrying a 100 to 150 tonnes payload and reaching the destination in no more than 100 years.

The design uses Deuterium-Deuterium fuel, even though it has only half the exhaust velocity of Deuterium-Tritium and Deuterium-Helium3, and about 38% of the energy expresses itself as nasty neutron radiation. They rejected D-T because blasted Tritium has a freaking half-life of only 12 years so most of it would decay away during the 15.6 year acceleration phase and the 54 year coast phase. You'd have to carry a huge excess penalty mass of extra Tritium to allow for decay. They also rejected D-He3 because there probably isn't enough He3 on all of Luna, and harvesting it from a gas giant's atmosphere would require a huge space infrastructure.

Forced to use D-D, the designers looked for ways to turn that pesky neutron flux from a liability into an asset.

Standard inertial-confinement fusion engines use a circular firing squad of lasers to implode the fuel pellet. The compression ignites the fusion fuel. The designers note this is a bit inefficient. By analogy, it is possible to detonate a stick of TNT by squeezing it but you have to squeeze real hard. It takes a lot less energy to use a match to light the fuse on the TNT.

So the designers used a so-called "fast ignition scheme". The circular firing squad just has enough laser power to confine the fusion fuel, but not the extra energy needed to compress it to ignition. A secondary high-powered laser acts as the fuse, piercing the pellet and igniting it. You get the same energy from the pellet, but you need a whole lot less input laser energy.

Alas, a "whole lot less" is still freaking huge. Lasers are power hogs.

The standard method is to harvest some of the fusion energy and convert it into electricity. This is stored in huge banks of heavy capacitors, to be used for the next laser pulse. The initial capacitor charge comes from a nuclear reactor or something which trickle charges the capacitor banks. The problem is the mass of all those capacitors is a punishing amount of penalty-weight.

That's when the designers turned the D-D waste neutron flux into an asset. Have you ever heard of Nuclear-pumped lasers?

All lasers consist of a lasing medium which emits laser light when it is pumped. Conventional lasers pump the medium with electricity or light. Nuclear lasers on the other hand pump with the awesome might of nuclear fission. Uranium-235 is exposed to neutrons, undergoes fission, the energy pumps the lasing medium, and a rather powerful laser beam emerges.

The main draw-back of nuclear-pumped lasers is the lack of convenient sources of high neutron flux. A nuclear reactor can provide a bit of neutrons, and in theory a fission warhead detonation can provide lottsa neutrons (see Bomb-Pumped Lasers). The light-bulb went off over most of your heads while you were reading the previous paragraph. Yes, the neutron flux from the detonating D-D fuel pellets would work splendidly.

The designers used a solid-core nuclear laser instead of liquid-core, since solid-core is more suitable for generating extremely short high-powered pulses. A ring-shaped chamber circles the thrust chamber, centered on the fusion pellet detonation point. The chamber is filled with a uranium dioxide aerosol and a fluorescent gas acting as the lasing medium. Some of the neutrons from the fusion detonation enter the chamber, causing fission reaction with the uranium 235 atoms, the fissile products then excite the fluorescent gas thus pumping it. The light flash from the fluorescent gas is transmitted through a light pipe into the laser amplifier. This creates the laser beam.

This system is about 8% efficient, which is pathetic for a general device but actually fantastic for a laser. And it is using all those otherwise worthless neutrons. The drawback is the uranium dioxide aerosol and fluorescent gas are expended with each laser bolt, that is, they are consumables. Which adds to the mass load.

However, even with the consumables the total mass of the ignition system is less than 1,000,000 kilograms, which is far less than all those banks of capacitors.

The report states an exhaust velocity of 0.018 c, which is considerably smaller than the theoretical maximum of D-D fusion (0.043 c). Which is probably very realistic.

As with all high-energy propulsion there is huge amounts of waste heat to get rid of, and this system does not lend itself to open-cycle cooling. Meaning you actually need plenty of heat radiators. The design use liquid-droplet radiators with a total area of 7.6 square kilometers. It has a very high heat rejection rate of 500 kW/kg by using liquid aluminum as the heat-conducting liquid.

The spine of the ship is a cylindrical truss structure composed of carbon nanotubes. This material has an exceptionally high tensile strength at a very low density, prime spacecraft building material. And you are going to need it. An engine thrust of 1.6 megaNewtons is 160 metric tons of compressive force which the spine has to endure for a bit more than 15 years. Thin spines tend to buckle so the design has a spine with a fat diameter of 100 meters. I did some analysis of the above image, if the spine is 100m in diameter, the pictured ship is about 1.4 kilometers long.

Mission Profile

Starting wet mass is 153,800,000 kg, of which 150,000,000 kg is propellant. Outrageous mass ratio of freaking 40.5!

It accelerates for 15.6 years, reaching a velocity of 0.06c. Only 1,356,000 kg of propellant is left, the total ship mass is now 5,156,000 kg. It then coasts for 54 years.

Upon approaching Alpha Centauri, it deploys a magnetic sail. This drags on the interstellar medium, decelerating the spacecraft. Once the velocity is down to 0.005c the fusion engine is used to finish the job of bringing the ship to a halt. The ship is now totally out of fuel. It then deploys lots of scientific probes and drones to gather as much scientific information as it possibly can, and transmits it back to Terra.


The Prometheus is a laser sail starship from The Flight of the Dragonfly aka Rocheworld, written by Dr. Robert L. Forward who wrote the classic real-world scientific reseach paper on the subject.

Interstellar Laser Propulsion System 

The payload sent to the Barnard system consisted of the crew of twenty persons and their consumables, totalling about 300 metric tons; four landing rockets for the various planets and moons at 500 tons each; four nuclear powered VTOL exploration airplanes at 80 tons each; and the interstellar habitat for the crew that made up the remainder of the 3500 tons that needed to be transported to the star system.

This payload was carried by a large light sail 300 kilometers in diameter. The sail was of very light construction, a thin film of finely perforated metal, stretched over a lightweight frame. Although the sail averaged only one-tenth of a gram per square meter of area, the total mass of the sail was over 7000 tons. The payload sail was not only used to decelerate the payload at the Barnard system, but also for propulsion within the Barnard system.

The 300 kilometer payload sail was surrounded by a larger ring sail, 1000 kilometers in diameter, with a hole in the center where the payload sail was attached during launch from the solar system. The ring sail had a total mass of 71,500 tons, giving a total launch weight of the sails and the payload of over 82,000 tons.

The laser power needed to accelerate the 82,000 ton interstellar vehicle at one percent of earth gravity was just over 1300 terawatts. As is shown in Figure 5, this was obtained from an array of 1000 laser generators orbiting around Mercury. Each laser generator used a thirty kilometer diameter lightweight reflector that collected 6.5 terawatts of sunlight and reflected into its solar-pumped laser the 1.5 terawatts of sunlight that was at the right wavelength for the laser to use.

When fed the right pumping light, the lasers were very efficient and produced 1.3 terawatts of laser light at an infrared wavelength of 1.5 microns. The output aperture of the lasers was 100 meters in diameter, so the flux that the laser mirrors had to handle was only about 12 suns. The lasers and their collectors were in sun-synchronous orbit around Mercury to keep them from being moved about by the light pressure from the intercepted sunlight and the transmitted laser beam.

The 1000 beams from the laser generators were transmitted out to the L-2 point of Mercury where they were collected, phase shifted until they were all in phase, then combined into a single coherent beam about 3.5 kilometers across. This beam was deflected from a final mirror that was tilted at 4.5 degrees above the ecliptic to match Barnard's elevation, and rotated so as to always face the direction to Barnard.

The crew to construct and maintain the laser generators were housed in the Mercury Laser Propulsion Construction, Command, and Control Center. The station was not in orbit about Mercury, but hung below the "sunhook," a large ring sail that straddled the shadow cone of Mercury about halfway up the cone.

The final transmitter lens for the laser propulsion system was a thin film of plastic net, with alternating circular zones that either were empty or covered with a thin film of plastic that caused a half-wavelength phase delay in the 1.5-micron laser light. (During the deceleration phase, when the laser frequency was tripled to produce 0.5-micron green laser light, the phase delay was three half-wavelengths.) This huge Fresnel zone plate acted as a final lens for the beam coming from Mercury. Since the focal length of the Fresnel zone plate was very long, the changes in shape or position of the billowing plastic net lens had almost no effect on the transmitted beam. The zone plate was rotated slowly to keep it stretched and an array of controllable mirrors around the periphery used the small amount of laser light that missed the lens to counteract the gravity pull of the distant Sun and keep the huge sail fixed in space along the Sun-Barnard axis. The configuration of the lasers, lens, and sail during the launch and deceleration phases can be seen in Figure 6.

The accelerating lasers were left on for eighteen years while the spacecraft continued to gain speed. The lasers were turned off, back in the solar system, in 2044. The last of the light from the lasers traveled for two more years before it finally reached the interstellar spacecraft. Thrust at the spacecraft stopped in 2046, just short of twenty years after launch. The spacecraft was now at two lightyears distance from the Sun and four lightyears from Barnard, and was traveling at twenty percent of the speed of light. The mission now entered the coast phase.

For the next 20 years the spacecraft and its drugged crew coasted through interstellar space, covering a lightyear every five years. Back in the solar system, the laser array was used to launch another manned interstellar expedition. During this period, the Barnard lens was increased in diameter to 300 kilometers. Then, in 2060, the laser array was turned on again at a power level of 1500 terawatts and a tripled frequency. The combined beams from the lasers filled the 300 kilometer diameter Fresnel lens and beamed out toward the distant star. After two years, the lasers were turned off, and used elsewhere. The two-light-year long pulse of high energy laser light traveled across the six lightyears to the Barnard system, where it caught up with the spacecraft as it was 0.2 lightyears away from its destination.

Before the pulse of laser light had reached the interstellar vehicle, the vehicle had separated into two pieces. The inner 300 kilometer payload sail detached itself and turned around to face the ring-shaped sail. The ring sail had computer-controlled actuators to give it the proper optical curvature. When the laser beam from the distant solar system arrived at the spacecraft, the beam struck the large 1000 kilometer ring sail, bounced off the mirrored surface, and was focused onto the smaller 300 kilometer payload sail as shown in the lower portion of Figure 6. The laser light accelerated the massive 71,500 ton ring sail at 1.2 percent of Earth gravity and during the two year period the ring sail increased its velocity slightly. The same laser power reflecting back on the much lighter payload sail, however, decelerated the smaller sail and the exploration crew at nearly ten percent of Earth gravity. In the two years that the laser beam was on, the payload sail slowed from its interstellar velocity of twenty percent of the speed of light to come to rest in the Barnard system. Meanwhile, the ring sail sped on into deep space, its job done.



The interstellar spacecraft that took the exploration crew to the Barnard system was called Prometheus, the bringer of light. Its configuration is shown in Figure 7. Although quite large, from a distance it would be difficult to see Prometheus in the vast expanse of shining sail that carried it to the stars.

A major fraction of the spacecraft volume was taken up by four units. They consisted of a planetary lander called the Surface Landing and Ascent Module (SLAM), holding within itself a winged Surface Excursion Module (SEM). Each SLAM rocket is forty-six meters long and six meters in diameter, and masses 600 tons including the SEM.

Running all the way through the center of Prometheus is a four-meter-diameter, sixty-meter-long shaft with an elevator platform. Capping the top of Prometheus on the side toward the direction of travel is a huge double-decked compartmented area that holds the various consumables that will be used in the 50-year mission as well as the workshop for the spaceship's computer motile. At the very center of starside is a small port with a thick glass dome that is used by the star-science instruments to investigate the star system they are moving toward. There is enough room for one or two people under the dome, but the radiation level is high enough that the port is mostly used by machines, not people.

At the base of Prometheus were five decks. These were the home for the crew. Each deck is a flat cylinder twenty meters in diameter and three meters thick. The bottom control deck contains the consoles that run the lightcraft, with the earthside science dome at the center. The living area deck is next. This contains the communal dining room, lounge, and recreational facilities. The next two decks are the crew quarters decks that are fitted out with individual living quarters for each of the twenty crew members. Above that is the hydroponics deck with four air locks that allow access to the four SLAM spacecraft. The water in the hydroponics tanks added to the radiation shielding for the crew quarters below.

From The Flight of the Dragonfly aka "Rocheworld" by Dr. Robert E. Forward

Shepherd Generation Starship

This is from the paper Interstellar Flight JBIS Vol. 11 (1952) by the legendary Les Shepherd The Journal of the British Interplanetary Society (JBIS) proudly proclaims this to be the first technical paper on interstellar flight.

Lamentably I am still trying to obtain a copy. In the meantime I will make do with Mr. Shepherd's popularization of his paper which appeared in Science-Fiction Plus April 1953.

Mr. Shepherd points out that when it comes to interstellar colonization, the problem is not transporting a man across stellar distances, it is more a problem of transporting an adequate community. If the transport is not moving at relativistic velocities you are probably talking about a generation ship (I suspect the sleeper ship concept had not been conceived of as early as 1952). Shepherd opines that interstellar explorers or colonists, faced with the knowledge that they will not only never see Terra again but also never see their destination, should adopt a similar philosophy to that of a soldier setting out on a suicide raid. There will be no personal gain, but instead the dying knowledge that some will survive to benefit from their action. This is calling for the sacrifice of entire generations in the depths of space, which admittedly will require a revolution in society. But Shepherd says this may be necessary if we are ever to become a galactic people.

Shepherd does not mention the problem of generation born en route being angry at their forebearers presumptuously committing them to this role. He does point out that the society will have to be a bit regimented. There will be specific population goals (overpopulation or underpopulation is a problem) so procreation is strictly regulated. Civilization has to be preserved, knowledge and culture will have to be carefully handed from generation to generation. New developments in science and art will be needed since Shepherd is of the opinion that "stagnation is the first step to degradation."

Shepherd figures that generation ships should not be used until the state-of-the-art allows transit times less than one thousand years. However he apparently didn't think of the "jumping the gun" problem.

For the journey to Alpha Centauri Shepherd figures that a fission-powered generation ship with an amount of fission fuel equal to 2.4 times the ship's dry mass, plus enough hydrogen propellant to rase the total mass ratio to 5.0 would have an exhaust velocity of 6,000 km/s and a deltaV of 10,000 km/s (about 0.03c). It accelerates to 5,000 km/s, cruises, then brakes down to zero at Alpha C. The transit time should be about 250 years.

If the transit time was lengthened to 350 years, the acceleration and deceleration phases could be increased to 50 years each, with 250 years of coasting in the middle. This would reduce the required acceleration to 0.00327 m/s2 (about 1/3000 g). Which would reduce the required engine power output per short ton of wet mass (specific exhaust power) to "only" 10 megawatts. E.g., if the wet mass was 10,000 short tons the engine power would be 100,000 megawatts (100 gigawatts). Shepherd admits that designing engines which can crank out 100 gigawatts for fifty years will be a bit of a challenge.

Shepherd says the transit time can be cut to 140 years if "lithium-hydrogen" fusion is used, but I think Mr. Shepherd was unaware that there are much better fusion reactions that can be used. I'd be more sure if I could read his actual paper.

Shepherd tries to put a spin on matters, pointing out that while a thousand years sounds like a long time to us, it is actually a small interval in terms of geological time. Which will fool nobody with an I.Q. higher than room temperature. He also mentioned that it would be a real good idea if astronomers made quite sure that the target star indeed had a habitable planet. Otherwise it would be a most tragically ironic ending to a very long mission.

Shepherd also acknowledges that the biological problem of maintaining a life support system for thirty generations is a major engineering challenge, but that isn't his department. Conservation of resources is important since losses can really add up over a thousand years. E.g., a million ton vessel losing 100 milligrams of air per second doesn't sound too serious, but over a 1,000 years that adds up to about three thousand tons of atmosphere loss.

The ship should also transport an entire ecosystem to be transplanted to the new world. This would turn the ship into a veritable Noah's Ark, and might force the ship to be a hollowed-out asteroid in order to carry everything. An asteroid has such a large radius that it could be spun up for artificial gravity at a low enough rate to prevent spin nausea.

Tau Zero

Seen from one of the shuttles that brought her crew to her, Leonora Christine resembled a dagger pointed at the stars.

Her hull was a conoid, tapering toward the bow. Its burnished smoothness seemed ornamented rather than broken by the exterior fittings. These were locks and hatches; sensors for instruments; housings for the two boats that would make the planetfalls for which she herself was not designed; and the web of the Bussard drive, now folded flat. The base of the conoid was quite broad, since it contained the reaction mass among other things; but the length was too great for this to be particularly noticeable.

At the top of the dagger blade, a structure fanned out which you might have imagined to be the guard of a basket hilt. Its rim supported eight skeletal cylinders pointing aft. These were the thrust tubes, that accelerated the reaction mass backward when the ship moved at merely interplanetary speeds. The "basket" enclosed their controls and power plant."

Beyond this, darker in hue, extended the haft of the dagger, ending finally in an intricate pommel. The latter was the Bussard engine; the rest was shielding against its radiation when it should be activated.

Thus Leonora Christine, seventh, and youngest of her class. Her outward simplicity was required by the nature of her mission and was as deceptive as a human skin; inside, she was very nearly as complex and subtle. The time since the basic idea of her was first conceived, in the middle twentieth century, had included perhaps a million man-years of thought and work directed toward achieving the reality; and some of those men had possessed intellects equal to any that had ever existed. Though practical experience and essential tools had already been gotten when construction was begun upon her, and though technological civilization had reached its fantastic flowering (and finally, for a while, was not burdened by war or the threat of war) —nevertheless, her cost was by no means negligible, had indeed provoked widespread complaint. All this, to send fifty people to one practically next-door star?

Right. That's the size of the universe...

..." — zero!" The ion drive came to life. No man could have gone behind its thick shielding to watch it and survived. Nor could he listen to it, or feel any vibration of its power. It was too efficient for that. In the so-called engine room, which was actually an electronic nerve center, men did hear the faint throb of pumps feeding reaction mass from the tanks. They hardly noticed, being intent on the meters, displays, readouts, and code signals which monitored the system. Boris Fedoroff's hand was never distant from the primary cutoff switch. Between him and Captain Telander in the command bridge flowed a mutter of observations. It was not necessary to Leonora Christine. Far less sophisticated craft than she could operate themselves. And she was in fact doing so. Her intermeshing built-in robots worked with more speed and precision — more flexibility, even, within the limits of their programming — than mortal flesh could hope for. But to stand by was a necessity for the men themselves...

...Reaction mass entered the fire chamber. Thermonuclear generators energized the furious electric arcs that stripped those atoms down to ions; the magnetic fields that separated positive and negative particles; the forces that focused them into beams; the pulses that lashed them to ever higher velocities as they sped down the rings of the thrust tubes, until they emerged scarcely less fast than light itself. Their blast was invisible. No energy was wasted on flames. Instead, everything that the laws of physics permitted was spent on driving Leonora Christine outward...

(ed note: the ion drive is used to boost the ship up to the minimum velocity required for the Bussard ramjet to operate)

...Practical problems arose. Where was the mass-energy to do this coming from? Even in a Newtonian universe, the thought of a rocket, carrying that much fuel along from the start, would be ludicrous. Still more so was it in the true, Einsteinian cosmos, where the mass of ship and payload increased with speed, climbing toward infinity as that speed approached light's.

But fuel and reaction mass were there in space! It was pervaded with hydrogen. Granted, the concentration was not great by terrestrial standards: about one atom per cubic centimeter in the galactic vicinity of Sol. Nevertheless, this made thirty billion atoms per second, striking every square centimeter of the ship's cross section, when she approximated light velocity. (The figure was comparable at earlier stages of her voyage, since the interstellar medium was denser close to a star.) The energies were appalling. Megaroentgens of hard radiation would be released by impact; and less than a thousand r within an hour are fatal. No material shielding would help. Even supposing it impossibly thick to start with, it would soon be eroded away.

However, in the days of Leonora Christine non-material means were available: magnetohydrodynamic fields, whose pulses reached forth across millions of kilometers to seize atoms by their dipoles — no need for ionization — and control their streaming. These fields did not serve passively, as mere armor. They deflected dust, yes, and all gases except the dominant hydrogen. But this latter was forced aft — in long curves that avoided the hull by a safe margin — until it entered a vortex of compressing, kindling electromagnetism centered on the Bussard engine.

(ed note: seizing atoms by their dipoles is handwavium)

The ship was not small. Yet she was the barest glint of metal in that vast web of forces which surrounded her. She herself no longer generated them. She had initiated the process when she attained minimum ramjet speed; but it became too huge, too swift, until it could only be created and sustained by itself. The primary thermonuclear reactors (a separate system would be used to decelerate), the venturi tubes, the entire complex which thrust her was not contained inboard. Most of it was not material at all, but a resultant of cosmic-scale vectors. The ship's control devices, under computer direction, were not remotely analogous to autopilots. They were like catalysts which, judiciously used, could affect the course of those monstrous reactions, could build them up, in time slow them down and snuff them out — but not fast.

Starlike burned the hydrogen fusion, aft of the Bussard module that focused the electromagnetism which contained it. A titanic gas-laser effect aimed photons themselves in a beam whose reaction pushed the ship forward — and which would have vaporized any solid body it struck. The process was not 100 per cent efficient. But most of the stray energy went to ionize the hydrogen which escaped nuclear combustion. These protons and electrons, together with the fusion products, were also hurled backward by the force fields, a gale of plasma adding its own increment of momentum.

The process was not steady. Rather, it shared the instability of living metabolism and danced always on the same edge of disaster. Unpredictable variations occurred in the matter content of space. The extent, intensity, and configuration of the force fields must be adjusted accordingly — a problem in? million factors which only a computer could solve fast enough. Incoming data and outgoing signals traveled at light speed: finite speed, requiring a whole three and a third seconds to cross a million kilometers. Response could be fatally slow. This danger would increase as Leonora Christine got so close to ultimate velocity that time rates began measurably changing.

From Tau Zero by Poul Anderson (1970)

Ultra-Dense Deuterium

Deuterium Starship
EngineIC Fusion
Ignition Laser
3 kJ
Ignition Laser
1 PW
Specific Impulse550,000 s
Exhaust Velocity5,400,000 m/s
Propellant Mass75,000,000 kg
Cruising Velocity15,000,000 m/s

This was the fourth entry in Project Icarus' 2013 contest to design an interstellar starship using current technology (it didn't win, the Ghost ship did). The basic rules were to design a spacecraft which was mainly fusion powered and on a mission to Alpha Centauri carrying a 100 to 150 tonnes payload and reaching the destination in no more than 100 years.

For reasons similar to those raised by the Ghost ship, this design also uses the relatively feeble Deuterium-Deuterium fusion reaction. Both designs use laser ignited inertial confinement fusion engines.

The main drawback to IC fusion engines is since beams of light do not push very hard, you need metric-assloads of laser energy to crush the fuel pellet to fusion ignition. Which requires lots of heavy lasers, savagely cutting into your payload mass budget. Since the laser pulse has to be microscopically short, the lasers have to be powered by huge banks of weighty capacitors, further slashing your payload budget.

The Ghost ship gets around this by replacing the capacitor banks with nuclear-pumped lasers, using the waste neutrons from the prior detonation.

The Ultra-Dense Deuterium starship gets around this with something even more tricky. It uses a weird fuel called, you guessed it, ultra-dense deuterium.

Ultra-Dense Deuterium

Ultra-dense deuterium (UDD) is an exotic form of metallic hydrogen called Rydberg matter. As you can probably figure out from the name the stuff is dense. Real dense. As in 1028 to 1029 grams per cubic centimeter dense. About a million times denser than frozen deuterium.

For our purposes the interesting point is it is about 150 times as dense as your average pellet of fusion fuel when laser-compressed to peak compression. Yes, this means do you not need metric-assloads of laser energy to crush the fuel pellet, a pellet just sitting on the table is already at 150 times the needed compression. It is pre-compressed. All you need is a miniscule 3 kilojoules worth of laser energy to ignite the stuff. That is pocket-change compared to what 200-odd compression lasers require. In fact it is so little that a single laser can handle the job. This results in a vast savings on laser mass and capacitor mass.

The laser pulse has to be quick, so the power rating is a scary 1 petawatt. But by the same token since the pulse is quick, it only require the aforesaid 3 kilojoules of energy.

Since you do not have to compress the fuel you can avoid all sorts of inconvienient hydrodynamic instabilities and plasma-laser interation problems. You also have virtually unlimited "fusion gain". Meaning that with a conventional IC fusion engine there is a maximum fuel pellet size due to the hydrodynamic instabilities and the geometric increase in compression laser power. With UDD you can make the fuel pellet as large as you want (well, as large as the engine can handle without blowing up at any rate).

An important safety tip: since UDD has such absurdly low ignition energy, there is a statistical change a large number of UDD atoms would undergo fusion spontaneously. This dangerous instability means the spacecraft will carry ordinary deuterium fuel and only convert it into UDD immediatly before use.

The cherry on top of the sundae is UDD fusion does not produce deadly neutron radiation. Instead it produces charged muons, which are not only easier to deal with, but also can be directly converted into electricity. Left alone, the muons quickly decay into ordinary electrons and similar particles.

And since deuterium is plentiful in ordinary seawater, you do not have to go strip mining Lunar Regiolith or set up atmospheric scoop operations around Jupiter were you to use a fusion reaction requiring Helium-3.

Sounds too good to be true, I hear you say. Well, there are a couple of drawbacks.

The minor drawback is that D-D fusion has a specific impulse (and exhaust velocity) which is about half of what you can get out of D-T fusion or D-He3 fusion. This drastically increases the mass ratio required for a given mission delta-V. Having said that it is still much better than what you'll get out of chemical or fission engines.

But the major drawback is UDD might not even have that magic ultra-density.

You see, the vast majority of the UDD-related papers has been published by a single scientific group at University of Gothenburg, Sweden, led by Dr. L. Holmlid. Currently there are no third-party confirmations about UDD observations and generally very few discussions about it in the scientific community. Until the density figure is confirmed, it might be all a pipe-dream.

The Mision

The spacecraft has two stages, kinda-sorta.

It accelerates for ten years using Stage One, reaching a velocity of 0.04c. Stage One is then jettisioned.

It accelerates for an additional two years using Stage Two, reaching a velocity of 0.04c. Stage Two stops burning, it still has fuel left. It jettisons about 68% of its heat radiator mass which is no longer needed.

The spacecraft proceeds to coast for the next seventy-five years.

At the end of the coast phase, the spacecraft is 0.378 light-years (0.368 + 0.010) away from the destination (Alpha Centauri). About 24,000 astronomical units. It then deploys a Magsail drag (with a mass of 238,000 kg). Over the next twelve years it decelerates the spacecraft to a velocity of 0.012c.

The spacecraft is now 0.01 light-years from destination. It jettisions the Magsail, bringing the spacecraft mass down to 612,000 kg. Stage Two's engine starts burning (in the diagram this is marked as "3rd Stage"). It burns for the next two years, bringing the spacecraft to halt at the destination. The spacecraft mass is now 320,000 kg, of which 150,000 kg is scientific payload.

Valkyrie Antimatter Starship

Noted polymath Charles Pellegrino and Brookhaven physicist Jim Powell have an innovative antimatter powered starship design called a Valkyrie. They say that current designs are guilty of "putting the cart before the horse", which create ships that are much more massive than they need be. Their "spaceship-on-a-string" starship is capable of accelerating up to ninety-two percent the speed of light and decelerating back down to stationary. At this velocity, relativity mandates that time on board the ship will travel at one-third the rate of the stay at home people on Terra (actually it's closer to 1/2.55). They figure this will be adequate for visiting stars up to about twelve light-years from Terra, without using up excessive amounts of the crew's lifespan.

Dr. Pellegrino served as a scientific consultant on James Cameron's Avatar movie. The interstellar vehicles seen in the film are based on the designs of Pellegrino and Powell's Valkyrie rockets, fused with Robert L. Forward's designs. I figured this out when I noticed that the Avatar starship had the engine in the front, which is a unique feature of the Valkyrie.

...For propulsion purposes, microfusion bursts triggered by antihydrogen-hydrogen annihilation (possibly with a component of lithium added) will prove efficient up to ship-cruising speeds approaching twelve percent the speed of light, owing to jets of relatively slow, massive particles. Above twelve percent lightspeed, propulsion shifts from antimatter-triggered fusion jets to straightforward matter-antimatter annihilation, which produces a lower mass thrust than fusion, but provides particles with the high-exhaust velocities necessary to push the ship to a high fraction of lightspeed.

How much antimatter might be needed for a trip to Alpha Centauri — assuming that Asimov Arrays or something very much like them will eventually provide humanity with the excess energy required for its large-scale production? We have estimated that the fuel stores (both antimatter and matter combined) might be equal to roughly half the mass of the rest of the spacecraft, or about one hundred tons (to assure "burning" of all available antimatter, an as-yet-undetermined excess of matter will be required).

From Flying To Valhalla by Charles Pellegrino (1993)

If I am reading this correctly, this is a mass ratio of 1.5, which I find a little difficult to believe. The equations above seem to say that accelerating up to 92%c and back down to zero will require a mass ratio around 22.

Adam Crowl got in touch with Mr. Pellegrino on this matter. As it turns out, the mass ratio of 1.5 only applies to a Valkyrie capable of approaching ten percent lightspeed.

Mr. Pellegrino's response to Adam Crowl:

On Valkyrie, the lower mass of material you were quoting was for up to 10%c - much lower than the mass for giants like Daedalus, and other such nonsense. The mass of propellant is kept low because up to about 10% c you can go with the lower exhaust velocities of antiproton-triggered fusion. (As an aside, during a bowling game with Engineer Ed Bishop and my kids, last winter, I suddenly got a warning alarm screaming up from my subconscious - in 3-D with the berilium windows failing terribly. That's all I could think of as I bowled [I'm usually lousey at the game], and I have still not adequately solved the problem - - but for the only time in my life, and with my mind not at all in the game, I hit series of perfect strikes after series of perfect strikes.

In any case, the antiproton triggered fusion system, scaled down to Valkyrie Mark II, is wonderfully practical for getting around the solar system at a mere 750 km/sec. (this velocity would eventually be practical for Project Spaceguard: the kinetic force of merely ten Toyota masses impacting a comet or asteroid at this velocity (diameter 1/4 mile) would completely "dust" the object.

In answer to original question, for a true, Valkyrie Mark III (requiring direct proton-antiproton annihilation after 15%c), interstellar crewed mission, the propellant mass would of course exceed the ship mass. After 92%c, the excess becomes too extreme - which is a main reason that, although we could deal with particle collisions (dust) at 95%c (halving apparent travel time at this cruise velocity), that 92% becomes close to being a pretty solid speed limit. The time dialation effect gain is simply not worth the mass-energy cost.

Charles Pellegrino

Anyway, back to the main description:

Others have been more pessimistic, including an earlier study by space scientists Donald Goldsmith and Tobias Owen which yielded an estimate that a journey to Alpha Centauri would require four hundred million tons of matter-antimatter fuel. Such estimates arise from assumptions that the spacecraft will be huge, with powerful engines mounted in the rear. Everything forwards of the engines becomes, in essence, a massive, rocketlike tower, requiring enormous amounts of shielding from the rocket's gamma ray shine, supplemented by complex (and massive) cooling systems to shed intercepted engine heat (and a traditional rocket configuration must absorb most of the head-depositing gamma rays, even if they do, like X rays, have a tendency to pass through things). The addition of each layer of shielding and cooling equipment placed on top of the engine becomes increasingly prohibitive as ship mass increases, requiring higher burn rates, which in turn requires more cooling and shielding, which increases ship mass and burn rates, and so on.

With our elongated, two-crew-member ship on a string, gamma shine and heat are spilled directly into the unfillable sink of outer space. A pulling rather than a pushing engine eliminates most of the structural girders that would not only, by their mere existence, add unwarranted mass, but would multiply that mass many times over by their need for shields and coolers. Valkyrie, in effect, is a fuel-efficient, twenty-first-century version of today's "ultralight" aircraft...

...Since antimatter and matter annihilate each other on contact, releasing enormous bursts of energy from literally microscopic amounts of propellant, you cannot simply fill a shuttle tank with liquid antihydrogen and let it slosh around inside.

The only storage method that has a hope of working is solid antihydrogen, supercooled within one degree of absolute zero (within one Kelvin of -273 degrees C). At this temperature, antihydrogen condenses into "white flake," with an extremely low evaporation rate.

Particles of solid antihydrogen will be suspended and held away from the "pod" walls, probably by electrostatic forces and/or magnetism. According to our latest models, near 0.0005° K, antihydrogen should be sufficiently stable as to allow, in the form of matter-antimatter micropellets or wafers (we are presently working to determine which design, layered pellets or wafers, will provide optimal thrust). With one-fifty thousandth of a degree Kelvin, matter-antimatter storage becomes thinkable because wave functions do not overlap enough to produce an appreciable reaction, at least in principle.

(And in practice?)

We do not know. It has not been practiced yet, and can only be verified by experimentation. Personally, carrying matter-antimatter pellets already assembled, even at 0.0005° K, gives me nightmares. I keep seeing a cosmic ray particle stopping at the matter-antimatter interface, giving off its heat, and triggering a horrible chain reaction... Jim says we can prevent that, but I am still opting for storing our antihydrogen in complete isolation from matter until virtually the moment it is needed. I am reminded of that scene from the movie version of 2010, in which Roy Scheider describes the aerobraking maneuver his ship is about to make through Jupiter's atmosphere. "It's dynamite on paper," he says. "Of course, the people who came up with the numbers on paper aren't here."...

...Upon warming, electrons and positrons self-annihilate to produce small bursts of gamma rays which, in terms of thrust, can be totally ignored. The positrons are there simply for stability's sake. The proton-antiproton pair, however, produce three varieties of elementary particles called pi-mesons...

...The charged pions and muons are the particles we want and when not being used below twelve percent lightspeed to immediately trigger fusion explosions (a matter of simply modifying the type of pellet or flake used), we want to simply bounce the pions off the outermost fringes of the engine's magnetic field, and thus steal whatever thrust they have to contribute, before a significant fraction of them have traveled twenty-one meters and shed part of their energy as useless neutrinos. The engine we have designed ejects pions and muons (and, at lower velocities, pion- and muon-triggered fusion products) along a diverging magnetic field nozzle to produce thrust, in much the same fashion as hot, expanding gases in a conventional rocket impact against the solid wall or pusher plate at the back of the ship, propelling the entire assembly forwards. Since the pions and muons are acting only against a magnetic field, they can propel the Valkyrie without ablating or wearing down the engine walls (as does space shuttle propellant, with the result that the engines must be rebuilt after every flight, and eventually thrown away). However, gamma rays emitted by the decay of neutral pions will knock atoms out of position in structures near the antimatter reaction zones, making the material stronger, yet brittle. One solution is to add structures called shadow shields wherever practical. (Shadow shields are nifty little devices already being used in certain very advanced nuclear reactors. They are a major component of Valkyrie, so stay with me and I will get around to describing them in just a few moments.) Another, supplemental solution is to weave most structures residing within four kilometers of the reaction zone from hundreds of filaments, and to send electric currents through the filaments, heating them, one at a time, to several hundred degrees below their melting point. Gamma ray displacements in the wires are thus rearranged, and the atoms can reestablish their normal positions. (ed. note: this is called "In-Site Annealing")

There appears to be nothing we can do to prevent the occasional transmutation of atoms into other elements. Fly far enough with your engines burning at full throttle, and your ship will turn slowly into gold, plus lithium arsenic, chlorine, and a lot of other elements that were not aboard when you left. These new substances will be concentrated around the antimatter reaction zone, and it is important to note that advanced composite materials already coming into existence dictate that our Valkyrie, even at this early design stage, will be built mostly from organic and ceramic materials, rather than from metals. It is conceivable that expanding knowledge of composites can be taken into account by the time relativistic flight becomes a reality, so that the ship actually incorporates the transmuted elements into its filaments in a manner that ultimately results in structural improvements for a ship designed to essentially rebuild itself as it flies. Exploiting what at first glance seems to be a disadvantage (transmutation) is simply a matter of anticipating the "disadvantage" before you begin to build. It's the disadvantages unforeseen or unaddressed that will get you in the end.

The gamma ray flare from the engine dictates other major features of ship design. In particular, it has caused us to turn rocketry literally inside out.

Riding an antimatter rocket is like riding a giant death-ray bomb. An unshielded man standing a hundred kilometers away from the engine will receive a lethal dose of gamma radiation within microseconds. In designing spacecraft, even when considering propellant as efficient as antimatter, RULE NUMBER ONE is to keep the mass of the ship as low as possible. Even an added gram means extra fuel.

Here's how we can shave off many tons of shielding.

Put the engine up front and carry the crew compartment ten kilometers behind the engine, on the end of a tether. Let the engine pull the ship along, much like a motorboat pulling a water skier, and let the distance between the gamma ray source and the crew compartment, as the rays stream out in every direction, provide part of the gamma ray protection - with almost no weight penalty at all. (ed. note: this should remind you of "Helios") We can easily direct the pion/muon thrust around the tether and its supporting structures, and we can strap a tiny block of (let's say) tungsten to the tether, about one hundred meters behind the engine. Gamma rays are attenuated by a factor of ten for every two centimeters of tungsten they pass through. Therefore, a block of tungsten twenty centimeters deep will reduce the gamma dose to anything behind it by a factor of ten to the tenth power (1010). An important shielding advantage provided by a ten-kilometer-long tether is that, by locating the tungsten shield one hundred times closer to the engine than the crew, the diameter of the shield need be only one-hundredth the diameter of the gamma ray shadow you want to cast over and around the crew compartment. The weight of the shielding system then becomes trivial.

(ed note: This is the Waterskiing school of spacecraft design)

The tether system requires that the elements of the ship must be designed to climb "up" and "down" the lines, somewhat like elevators on tracks.

We can even locate the hydrogen between the tungsten shadow shield and the antihydrogen, to provide even more shielding for both the crew and the antihydrogen.

There is an irony involved in this configuration. Our "inside-out" rocket, the most highly evolved rocket yet conceived, is nothing new. We have simply come full circle and rediscovered Robert Goddard's original rocket configuration: with engines ahead of the fuel tanks and the fuel tanks ahead of the payload. Nor is the engine itself an entirely new creation. It guides and focuses jets of subatomic particles the same way the tool of choice among most microbiologists guides streams of electrons through magnetic lenses. Valkyrie, in essence, is little more than a glorified electron microscope.

In addition to shielding against gamma shine and avoiding the absorption of engine heat, another major design consideration is shielding against interstellar dust grains. Flying through space at significant fractions of lightspeed is like looking through the barrel of a super particle collider. Even an isolated proton has a sting, and grains of sand begin to look like torpedoes. Judging from what is presently known about the nature of interstellar space, such torpedoes will certainly be encountered, perhaps as frequently as once a day. Add to this the fact that as energy from the matter-antimatter reaction zone (particularly gamma radiation) shines through the tungsten shields and other ship components, the heat it deposits must be ejected.

Jim Powell and I have a system that can perform both services (particle shielding and heat shedding), at least during the acceleration and coast phases of flight. We can dump intercepted engine heat into a fluid (chiefly organic material with metallic inclusions) and throw streams of hot droplets out ahead of the ship. The droplets radiate their heat load into space before the ship accelerates into and recaptures them in magnetic funnels for eventual reuse. These same heat-shedding droplets can ionize most of the atoms they encounter by stripping off their electrons. The rocket itself then shuts the resulting shower of charged particles - protons and electrons - off to either side of its magnetic field, much the same as when a boat's prow pushes aside water.

The power generated by occasional dust grains should range from the equivalent of rifle shots to (rarely) small bombs. These detonate in the shield, harmlessly, far ahead of the ship. Fortunately, almost all of the interstellar particles likely to be encountered are fewer than 20 microns across (10,000 microns = 1 centimeter), and we should expect no more than one impact per day per square meter of Valkyrie's flight path profile...

...One of the great advantages of a droplet shield is that it is constantly renewing itself. Put a dent in it, and the cavity is immediately filled by outrushing spray.

If a dust grain passes into the shield, many of the shield's droplets are bound to be exploded. Some of the scattered droplet fluid will be absorbed and recovered by surrounding droplets, but some fluid is bound to be hurled out of the droplet stream, which means that we must add the weight of droplets to be replaced to the ship's initial mass.

In addition to spare droplet fluid, our preliminary design calls for a spare engine. Both engines will be located at opposite ends of the tether. The forward engine pulls the ship along during the acceleration phase of flight. It also fires during the cruise phase, but only at one-hundredth thousandth of a gravity, keeping the tether taut and permitting recapture of forward flying droplets. At the end of the cruise phase, the rear engine kicks in for deceleration (as we cannot simply swing a ten-kilometer-long ship broadside to relativistic bombardment in order to turn the engine around and fire in reverse).

In normal use, the rear engine is turned on only to decelerate the ship, or to maneuver the crew compartment into the center of the forward engine's gamma ray shadow. Nudging the crew compartment, from behind, to one side or the other will be necessary during major course changes, because the crew compartment, much like a water skier, cannot turn simultaneously with the motor that pulls it and might otherwise drift out of the protective shadow. A spare engine also provides some insurance against the chilling possibility of irreparable damage to the leading engine or, worse, a break in the tether. In the former case, identical engine parts could be ferried up and down the tether and exchanged as necessary. In the latter, depending upon where the break occurs, with careful rearrangement of the ship's components along the tether, the remaining coil can be safely used to finish the outbound leg of the mission.

At the end of the cruise phase, with nearly half of the ship's fuel exhausted, empty fuel tanks can be ground up into ultrafine dust, for dumping overboard (we see no reason to expend extra energy decelerating tons of equipment, no longer in use, which can easily be remanufactured and replaced at the destination solar system). At up to ninety-two percent the speed of light, the dust will fly ahead of the decelerating ship, exploding interstellar particles and clearing a temporary path (trajectories must be such that the relativistic dust will fly out of the galaxy without passing near stars and detonating in the atmospheres of planets). This fist of relativistic dust is the first line of defense against particles encountered during final approach. With the rear engine firing into the direction of flight, droplet shields will be come useful only for expelling heat from the rear engine, for along the tether, "up" has now become "down," and droplets can only be sprayed "up" behind the engine, where, traveling at uniform speed, they will fall back upon the decelerating ship. To shield against particles ahead of the ship, ultrathin "umbrellas" made of organic polymers similar to Mylar and stacked thousands of layers deep are lowered into the direction of flight. This is the second line of defense - against particles moving into the ever-lengthening space between the ship and the fist. The umbrellas will behave much like the droplet shield and, in like fashion, they will be designed with rapid self-repair in mind. Throughout the ship, repair and restructuring will be assisted (where such repair abilities as self-annealing filaments are not already built into ship components) by small, mouselike robots capable of climbing up and down tethers and rigging.

From Flying To Valhalla by Charles Pellegrino (1993)

Go Tricky

The third of Gordon Woodcock's methods of interstellar travel is "go tricky".

This means to cheat and find a way to travel to the stars faster than light.

This is such can of worms that it has an entire page to itself.

Atomic Rockets notices

This week's featured addition is MOVERS Orbital Transfer Vehicle

This week's featured addition is UM Lunar Transport

This week's featured addition is Afterburner fission-fragment rocket engine

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