This page is for realistic scientifically plausible slower-than-light communication. For unrealistic science-fictional faster-than-light communication see this page.

This deck contains communication gear, perhaps even with something like a Morse code key for use when radio interference becomes a problem (If this was a Metalunan ship, this is where you'd find the interociter).

Sometimes this deck is called a "radio shack", manned by a communication officer whose nickname is "Sparks." Communication officers are the ship's ears and mouth. They direct incoming messages to the proper departments and send outgoing messages in the proper format to the proper channels. Communication noise must be monitored and auxiliary channels used if required. All messages must be logged.

Distress signals are sent to the watch officer, but never responded to without authorization. Responding binds the ship to render assistance, a decision reserved for the captain. John Reiher points out that given the reality of the spartan limitation on a ship's delta-V, there is probably little they could do to render assistance besides helpful advice over the radio. If they tried to match postion and vector they'd use up all their delta-V, so now there are two ships in distress. The best they can do is notify the Orbit Guard.

The communication officer must also maintain the ship's transponder, which broadcasts the ship's ID.

The communication officer may also be responsible for encrypted communications, using the proper keys to encrypt and decrypt. If this is a military spacecraft, the comm officer is responsible for destroying the code book if the ship is captured by a hostile power. Hit the red "incinerate" button to keep the one-time pad and Captain Midnight secret decoder pin from falling into enemy hands. The safe containing the code book may be on the communications deck, but on some ships it is in the captain's cabin.

(ed note: After the end of the Asteroid Revolutionary War, two former foes reminisce over a beer.)

"The message was important. They did capture the Vega in the battle off Sam's, you know. And maybe by now you also know they locked unto her code books. Pallas had to be told what your ciphers were. but we couldn’t risk a maser beam being intercepted. "

"Certainly not." Ulstad grimaced. "It took several disasters before we realized what must have happened."

From SAY IT WITH FLOWERS by Poul Anderson (1965)

Communication will generally be by radio or laser.

Back in the 1900's, Amateur Radio (aka "ham radio") was very popular. Back in those days people didn't travel very much. But if you got your amateur radio license and a radio rig, you could talk with people between 30 and 100 kilometers away. Or even over 400 kilometers away if you link to a radio repeater station. It was sort of like a more powerful and regulated version of the CB radio used by truckers. Science fiction writers like Poul Anderson postulate that rock rat asteroid miners would also be avid ham radio operators. A miner at their claim would probably be located at an inconveniently large distance and large delta V from the closest neighbor. Ham radio is a cheap way to keep in touch, and pass news & gossip. You probably can get more range if you use Morse code instead of speech.


(ed note: this is occuring during the horrors of Case Ragnarok)

Of course, calling any of the expedition's ships a "transport" was a bit excessive. For that matter, no one was certain Perez had actually ever been an officer in anyone's navy, much less a commodore. She'd never spoken about her own past, never explained where she'd been or what she'd done before she arrived in what was left of the Madras System with Noah and Ham and ordered all two hundred uninfected survivors of the dying planet of Sheldon aboard. Her face had been flint steel-hard as she refused deck space to anyone her own med staff couldn't guarantee was free of the bio weapon which had devoured Sheldon. She'd taken healthy children away from infected parents, left dying children behind and dragged uninfected parents forcibly aboard, and all the hatred of those she saved despite themselves couldn't turn her from her mission.

It was an impossible task from the outset. Everyone knew that. The two ships with which she'd begun her forty-six-year odyssey had been slow, worn out bulk freighters, already on their last legs, and God only knew how she'd managed to fit them with enough life support and cryo tanks to handle the complements she packed aboard them. But she'd done it. Somehow, she'd done it, and she'd ruled those spaceborne deathtraps with an iron fist, cruising from system to system and picking over the Concordiat's bones in her endless quest for just a few more survivors, just a little more genetic material for the Human race.

She'd found Japheth, the only ship of the "squadron" which had been designed to carry people rather than cargo, at the tenth stop on her hopeless journey. Japheth had been a penal transport before the War. According to her log, Admiral Gaylord had impressed her to haul cold-sleep infantry for the Sarach Campaign, although how she'd wound up three hundred light-years from there at Zach's Hundred remained a mystery. There'd been no one alive, aboard her or on the system's once-habitable world, to offer explanations, and Commodore Perez hadn't lingered to seek any, for Noah's com section had picked up faint transmissions in Melconian battle code.

(ed note: so the refugee fleet "detected" the presence of enemy warships by intercepting the enemy's communication transmissions)

From A TIME TO KILL by David Weber (1997)

      In the same century the men of old Earth took their first steps into space. They studied our alien voices whenever they could hear us. And when the men of old Earth began to travel faster than light, they followed our voices to seek us out.
     Your race and mine studied each other with eager science and with great caution and courtesy. We Carmpan and our older friends are more passive than you. We live in different environments and think mainly in different directions. We posed no threat to Earth. We saw to it that Earthmen were not crowded by our presence; physically and mentally they had to stretch to touch us. Ours, all the skills of keeping peace. Alas, for the day unthinkable that was to come, the day when we wished ourselves warlike!
     You of Earth found uninhabited planets, where you could thrive in the warmth of suns much like your own. In large colonies and small you scattered yourselves across one segment of one arm of our slow-turning galaxy. To your settlers and frontiersmen the galaxy began to seem a friendly place, rich in worlds hanging ripe for your peaceful occupation.
     The alien immensity surrounding you appeared to be not hostile after all. Imagined threats had receded behind horizons of silence and vastness. And so once more you allowed among yourselves the luxury of dangerous conflict, carrying the threat of suicidal violence.
     No enforceable law existed among the planets. On each of your scattered colonies individual leaders maneuvered for personal power, distracting their people with real or imagined dangers posed by other Earth-descended men.
     All further exploration was delayed, in the very days when the new and inexplicable radio voices were first heard drifting in from beyond your frontiers, the strange soon-to-be-terrible voices that conversed only in mathematics. Earth and Earth's colonies were divided each against all by suspicion, and in mutual fear were rapidly training and arming for war.
     And at this point the very readiness for violence that had sometimes so nearly destroyed you, proved to be the means of life's survival. To us, the Carmpan watchers, the withdrawn seers and touchers of minds, it appeared that you had carried the crushing weight of war through all your history knowing that it would at last be needed, that this hour would strike when nothing less awful would serve.
     When the hour struck and our enemy came without warning, you were ready with swarming battlefleets. You were dispersed and dug in on scores of planets, and heavily armed. Because you were, some of you and some of us are now alive.

From INTRODUCTION TO BERSERKER by Fred Saberhagen (1967)

Quindar tones, most often referred to as the "beeps" that were heard during the American Apollo space missions, were a means by which remote transmitters on Earth were turned on and off so that the capsule communicator (CapCom) could communicate with the crews of the spacecraft. It was a means of in-band signaling to simulate the action of the push-to-talk and release-to-listen (often referred to as PTT) button commonly found on two-way radio systems and walkie-talkies.


When Mission Control (in Houston, Texas) wanted to talk to astronauts, the capsule communicator (CapCom) pushed a button (push-to-talk, or PTT) that turned on the transmitter, then spoke, then released the button. When the transmitter is local, this is easy to arrange: the transmitter is connected directly to the PTT button. But to stay in continuous contact with the astronauts as they orbit the Earth, or travel to the Moon, NASA had to use tracking stations all around the world, switching from one station to the next as needed. To get the voice signal to the remote transmitter, dedicated telephone lines connected these stations to Houston. NASA could either build a parallel system for operating the transmitters—one line to carry the audio and another to carry the control signal for the PTT button (out-of-band signalling)—or combine these two systems together, using audio tones to turn the transmitter on and off. Since dedicated phone lines were a very expensive measure at the time, NASA chose the use of tones to reduce the operating cost of the network. The same system was used in Project Gemini and was still in use with half duplex UHF Space Shuttle communications for transmitter radio frequency keying.

With modern digital communication systems, Quindar tones are no longer necessary because a single communication line can simultaneously carry multiple communication channels in the form of data comprising both speech and signaling (the PTT signal), as well as video and telemetry.


The Quindar system, named after its manufacturer, used two tones, both being pure sine waves that were 250ms long. The "intro tone" was generated at 2,525 Hz and signaled the "key down" key-press of the PTT button and unmuted the audio. The "outro tone" was slightly lower at 2,475 Hz and signalled the release of the PTT button and muted the audio. The two tones were generated by special equipment located at Mission Control, and they were decoded by detectors located at the various tracking stations.

The selection of the tones allowed them to travel in the same passband as a human voice, which has a range from roughly 300 Hz to 3,000 Hz.

Common misconceptions

Two common misconceptions surround Quindar tones. The first is that one tone came from Earth and the other from the transmitters used by the astronauts while in space. This confusion exists because many ground-to-space transmissions were initiated by Mission Control and responded to by the astronauts. In this sequence, the CapCom would press the PTT, which would send the intro tone, and then speak. When finished speaking, the CapCom would release the PTT, which would send the outro tone, and the astronauts would respond to Mission Control. Therefore, those transmissions would consist of a "beep" (PTT press) followed by Houston talking, then another "beep" (PTT release) and finally the voice of the astronauts.

Another misconception about Quindar tones is that they were designed to signal the end of a transmission, similar to a courtesy tone used on many half-duplex radio repeaters. Although the astronauts may have secondarily used the Quindar outro tone to know when the CAPCOM had started/stopped speaking, no equivalent existed for Mission Control because the astronauts keyed their transmissions locally (inside the spacecraft) using either a PTT or VOX, neither of which required Quindar tones. Additionally, separate radio frequencies allowed both Houston and the astronauts to talk simultaneously if they wished and thereby made a courtesy tone as a way to minimize the possibility of both of them speaking at the same time unnecessary.

Origin of the name

Quindar tones were named for the manufacturer Quindar Electronics, Inc. Glen Swanson, historian at NASA's Johnson Space Center who edited the Mission Transcript Collection, and Steve Schindler, an engineer with voice systems engineering at NASA's Kennedy Space Center, confirmed the origin of the name. "Quindar tones, named after the manufacturer of the tone generation and detection equipment, are actually used to turn on and off, or 'key', the remote transmitters at the various tracking stations."

From the Wikipedia entry for QUINDAR TONES

Radio Antenna

Radiators are not the only thing spoiling the Polaris' sleek external lines. Roger Manning's radio needs large dish antennas. They might not be as large as the monsters on 2001's Discovery, but they won't be much smaller than the ones on the Apollo service module. It might also be a good idea to have landing radar on an outrigger or boom, so it won't be blinded by the exhaust. Both of these will be retracted during atmospheric re-entry, with the landing radar deployed when the air speed drops low enough so it won't be ripped off.

Range and Bandwidth

Range is how far you can transmit a message. Major factors are:

  • Power in the signal, including loss due to inverse-square law
  • Sensitivity of the receiver
  • Noise in the signal, including electromagnetic interference from natural and artificial sources

By "bandwidth", I mean in the computing sense: how many bits per second of digital information can be trasmitted. The other sense (signal bandwidth) is width between the upper and lower set of frequencies used.

As an example, using old 1977 technology the Voyager space probes use S Band (2 to 4 GHz) communications with 3.7 meter high-gain antennas. As the ranged to Earth increased, the bit-per-second rate decreased due to the inverse-square law. Using the 62 m Deep Space Network dishes as receivers, at Jupiter it was 115 kBits/sec. At Saturn it was 57 kBits/sec. It went rapidly downhill from there.

As far as signal bandwidth goes, voice transmission typically takes 2400 Hz, while Morse code takes only 100 to 150 Hz.

A final matter is that there is only so many radio frequencies to go around, so some authority has to be in charge of radio spectrum allocation.

Radio proper typically use the electromagnetic spectrum from 3 kHz to 300 GHz.

Laser communicators typically use visible, infrared, or ultraviolet frequencies. Currently the maximum range that laser communication has managed in the field is 24 million kilometers, but that will no doubt be improved by many orders of magnitude.

Some times masers are used in recievers in order to amplify weak radio signals.

Also read about the Interplanetary Internet.

Rick Robinson:

One of the biggest differences between real space and rocketpunk era space is comms. There is physical isolation but rarely communications isolation in the Vastness of Space.

Citizen Joe:

The range we're getting on long range communications is due to directional antennae. If you're broadcasting omnidirectionally, the signal drops fast. So you could get radio transmissions, but there would like be just the one line. That one line would also be blocked by planets and other stuff on occasion. That one line would be in high demand and thus not readily available. You might spend thousands on a couple minute recording that gets compressed and burst fed to a destination. Real messages, sent parcel post, would probably get stored digitally and then transmitted once you arrived at the destination orbit.


To expand on Citizen Joe's point, would you like to receive "Male enhancement" emails with the olympusmon.mars address? How about "Dear Sir, I am the last surviving member of the Europan Resistance front and need your help to transfer $10 billion solars from the Bank of Callisto...."

On a more serious note, the high bandwidth links would probably be reserved for ship traffic, government and military communications and corporate communications (for companies with the financial clout to get in line for email). Certainly the Uranus Space Navy would not want the high bandwidth links clogged during their showdown with the Imperial Jovian Navy, nor would they want to risk malware or botnet attacks coming through those links; which suggests interplanetary comms would be tightly controlled and subscribers carefully vetted.

Citizen Joe: The range we're getting on long range communications is due to directional antennae.


This is really just an engineering issue, and not that hard to solve. Signal multiplexing, multiple frequencies, different modulations, and even using lasers (which can pack in massive amounts of data) could (and would) increase the amount of bandwidth available in space.

There's really nothing in the laws of physics that says you can't beam 1,000+ Tb/sec of data from earth to mars. It'll just take a few hours to get there. That itself could create some interesting cultural dynamics. No sane 'belter would ever play day trader on Venus' stock markets, for example.

That said, I agree with the points made above that you would see a massive amount of data security being used in space. The military forces would be the most paranoid, probably using laser transmissions most of the time to prevent eavesdropping. I like the idea of using a cost formula for data transmission that discourages spamming. I could see something like that being used to govern the use of non-corporate civilian data traffic.

Stevo Darkly:

Since the subject of communication has come up in these comments, at some point I would love to see some discussion of what kind of communication technology an interplanetary civilization is likely to use for long-distance messaging. Radio? (Isn't that what we use now, for communication with interplanetary probes?) Maser? Laser?

Even the amazing nuts-and-boltsy Atomic Rockets site of Winchell Chung doesn't seem to address this question specifically, that I can find.

For example, my intuition tells me that any spacecraft outfitted with a laser capable of communication between, say, Neptune orbit and the Earth would also be able to use that laser as a weapon at shorter distances. If so, even a "peaceful unarmed merchantman" would have a potential short-range antiship/antimissile weapon. Interesting implications.


Radio? Maser? Laser?" My guess would be, all of the above. As for lasers, terrestrial free-space laser communication systems get about 1Gb/sec for high-end systems. That's actually much lower than what is possible with optics, because of the problems inherent in trying to punch that laser through a messy atmosphere. In space, you could expect to get performance in excess of 1Tb/sec, YMMV.

Radio is likely to be used extensively, to the point that the general space around planets and other points of interest could get pretty noisy. I'm not sure whether it's easier to collimate a laser beam or a radio signal over stupendous range. Whichever one wins would be the one you'd see more in interplanetary communications.


Communication with electromagnetic radiation is subject to diffraction just like lasers. The diffraction equation is on Atomic Rockets in the laser cannon section. Basically, the shorter wavelength you use, the less diffraction you get and the smaller angle you can focus the beam. Now, radio waves have much longer wavelength than light, so light from lasers could be used in a more efficient way for communication, as it can be concentrated better.

There are phased arrays for radio waves, and optical phased arrays are possible but cannot be built by existing technology. These would help to focus the transmission. I do not know any equations for phased arrays, though, so someone with more knowledge on these might want to enlighten us.


Since power and surface area aren't problems on the ground (and to a lesser extent, in orbit), communications would likely be predominantly TO ships while ship to shore (home base) comms would be limited to a confirmation of receipt. On Earth, we can afford to put up huge arrays to catch the smallest radio signal. Not so much in space. Likewise we can pump a lot of energy into the antenna to send back a longer message with a lot of bandwidth and strength even at stupendous ranges.

Now there are some tricks, like omnidirectional beacons and antenna that act as targets for the directional antennae. But you can only listen to data in the direction of the directional antenna. That might be limited to a single stream. Comm relays would likely have at least 4: Signal in, Signal out, Previous relay, Next Relay. By using multiple relays (at least 4 would get you around the sun) you wouldn't have black outs (except at ship orbit). And then there is the problem with the fragile gimbals needed for all the antennae.

In the end, yes, you can communicate via radio. No, it isn't broadband. No, it won't service a population comparable to the internet. It will likely be biased communication. It will still be expensive. There will be extreme needs that keep every carrier busy. So, although you COULD send a digital copy of Pluto Nash to Pluto, you would never get enough priority to use the carriers and thus it would be simpler to put it on a torch or fling it out the airlock.

Rick Robinson:

Communications. This whole subject gets discussed far too little, so I am glad to see it come up here.

My intuition, worth what you paid, is that point to point comms bandwidth, probably by laser, will be surprisingly cheap even across interplanetary distances.

Very quick and dirty. If I am using the formula from Atomic Rockets correctly, a visible band laser beaming through a 1 meter telescope will have a beam radius of 90 km at 1 AU.

But an ordinary light bulb is visible at 45 km (in ideal conditions). It is shining in all directions, so the projected surface is the same, and a 100 watt visible band laser should be naked eye visible at 1 AU.

Anyone want to check this? A laser beam bright enough to be visible should be able to sustain a huge data bandwidth, and even a spacesuit helmet could probably carry the receiver. A 100 watt laser and 1 meter telescope aren't a huge order either, as such things go.

If all of this is more or less correct, high bandwidth space comms should be cheap. By my general rule for a mature midfuture space tech, the transmitter rig might be a ton or so all up, and cost $1 million, no extravagant fitting for a spaceship.

Receivers could cost a few kilobux and fit on a spacesuit helmet, though high bandwidth interplanetary comms in your spacesuit is probably taking 'mobile Internet' to needless extremes.

Proviso also that all this applies only to tight beam comms, for which you need to know exactly where to point your telescope. Which makes precise locational data crucial at least to high volume comms.

Citizen Joe:

90km at 1 AU means that your signal has dropped to like one ten billionths of the original signal strength. 90km at 1 AU is also a very small spec to aim at. Additionally, if the receiver isn't pointed at just the right angle, you get nothing. Plus it is hard to distinguish a visible laser from a star. The real trick is to use a wavelength that is not generally present in space. I just don't see the high bandwidth at that range.

Jean Remy:

What about relays at Lagrangian points?

Everyone knows where those are, and if someone needs to connect they can just link into the network. I don't even think lasers would even be needed. The relays would have high gain antennas to receive the data on broadband signals, and when a ship or colony needs to link in they can query the closest available platform with a much lower-gain antenna.

If we can be in contact with various probes (like Voyager probes) at interplanetary distances, on 1970s technology, then I hardly think you would need a giant technological leap to create a system-wide comm network.

Granted you won't have the bandwidth of fiberoptic cables, so no browsing for Earth-porn from Callisto, but I don't see communication as much of an impediment. The only real reason to use lasers (that I see) would be for private (read: military) communications that you don't want intercepted, but with a target area of 90 km in radius, it's not really very private anymore.

Citizen Joe:

Viral propagation is another option. It doesn't guarantee speed or privacy. The idea would be that a message would be sent to any ship that is heading in the right direction that is within range. This could be a very long route. However, since there would likely be relatively few interplanetary vessels (compared to airplanes), Solar Space Traffic Controllers would have the full list of vessels and thus able to chart a route based on predicted paths. While the InterPlanetary Space ships would carry parcels, they would probably also serve as communication hubs for the viral network.


That sounds pretty close to how the modern internet functions, with data tracing geographically indirect paths as it goes from place to place. I like the efficiency of such a scheme: it would use the infrastructure that's already there to create an ad-hoc network backbone. On the other hand, you'd be trading out data security unless you've got some very good encryption (or just plain don't care who else reads your messages).


Ultra high bandwidth communications will be easy enough between "base stations" which can support not only powerful transmitters but also huge receiver stations (large antenna arrays, photodetectors, neutrino tanks or whatever tech we care to use). Finding a planet, asteroid or Venus Equilateral should not be too difficult, and tracking demands will be quite modest.

Moving spacecraft will be a lot more difficult to accurately track with a tight, high bandwidth beam, and they will generally not have such large scale transmitter or receiver arrays. The two exceptions would be a laser battlestation, which would simply use the targeting array and laser (dialed down to a suitable frequency), and torch ships under weigh, which can be tracked by their drive emissions. Of course putting a transmission beam through the drive plume might be difficult in itself.

Ships would probably have more need for local high bandwidth communications between ships in a constellation, or to constellations of sensor drones around the ship itself.


Good point, Thucydides.

Ships en route wouldn't have the means to relay a huge amount of data.

Perhaps station-ship-station relay networks would be more common as the system is just starting out, and later on only out in the boonies. Data routes with a high demand would soon get their own dedicated infrastructure.

One other interesting feature of intrasystem communications: the transmission-time "tides" that everything (minus parent-satellite combinations) is subject to. Mars would cycle between a 3-minute delay and a 20-minute delay for getting a signal to the earth-side servers, with its tidal cycle taking more than a year to complete. It'll be interesting to see how people deal with the variability. Here, if you purchase internet access you can expect the average connection speed to stay constant.

Jean Remy:

Viral dissemination works on Earth because of thousands and thousands of privately owned servers.

However, even in the best-case scenario of a very developed interplanetary infrastructure, I don't see a lot of traffic in space. Say two cyclers between every major colonial epicenters (say 2 for Mars, 2 for the Jovian colonies, 2 for Saturn etc...) and a few "moon hopper" shuttles, but those would be so close to their giant primaries getting a Line of Sight on them would be an issue.

However the Comm relay platforms suggested are basically Voyager probes without the scientific instrumentation and a known stable orbit. Rather that throwing your message out omnidirectionally and hope that eventually it will reach your destination (because viral dissemination is kind of like shooting blind) you simply bounce the signal of a set number of known (and if want, secure) predetermined platforms. If your goal is to reach as many people as possible (the entire point of viral dissemination in the first place) then target the Cyclers. Chances are good the passengers en route back and forth have personal computers linked in to the ship's server, which keeps updated by linking in to the platforms.

Luke Campbell:

Re: communication speed. I'm not an electrical engineer, so the following calculations may have grievous errors, but I'm going to try to throw some numbers out and see what sticks.

Let's assume a spread-spectrum laser communicator, using Rick's assumptions of 100 W and a 1 meter aperture at 1 AU. We choose a frequency spread of 125 THz centered around 1 micron infrared (roughly corresponding to 0.8 micron wavelength to 1.25 micron wavelength). We will use a 1 square meter aperture telescope to receive the signal, focused onto a detector cooled to 4 K.

This laser will deliver 4 nW/m2 at 1 AU, so we pick up 4 nW of signal.

The thermal (Johnson) noise of the detector is roughly the temperature (in energy units) times the frequency spread. 4 K is 5.5 × 10-23 J, so we have about 7 nW of noise.

The Shannon-Hartley theorem says the maximum rate of data transmission is equal to the bandwidth times the log base 2 of (1+(signal)/(noise)).

Using log_2(1+4 nW/(7 nW))=0.65, we find that we can move data at a rate of about 80 THz under optimum conditions using optimum engineering.

Note that this analysis neglects shot noise (noise due to quantization into photons) and background noise (nearby stars, zodiacal light, an operating torch drive, etc.).

An interesting thing I just noticed — in the limit of low signal to noise and assuming only thermal noise, the rate at which you can transmit data is independent of your carrier bandwidth. This is because

log_2(1+S/N) = ln(1+S/N)/ln(2)

~= (S/N)/ln(2) ~= (S/(TB))/ln(2)

where S is the signal power, N is the noise power, T is the temperature (in energy units), and B is the bandwidth. Plugging this in to the Shannon-Hartley theorem

R = B log_2(1+S/N)

for R the rate of data transmission, we can see that

R = S/(T ln(2))

In other words, the rate of data transfer depends only on your signal strength and noise in the detector. Ultimately, this is because the noise you pick up decreases linearly with bandwidth. Using a narrow-band signal also cuts down on background noise in that band. Then all that matters is dumping enough power into the signal to be heard.


Luke, Marcus, et al...all that is correct if you're using Frequency Division Multiplexing (FDM), but with Time Division Multiplexing (TDM), you can get a much higher data rate from the same bandwith...Polarizing Division Multiplexing also has a higher data transfer rate than FDM...Composite Multiplexing (CM),the combination of two or more of those, would give you a much higher data transfer rate than any single multiplexing technique. Modern-day laser communications can carry millions of channels of data in a single frequency beam; I see no reason that a resonable data rate, even at interplanetary ranges, couldn't be fesible between Earth and a theoretical Callisto colony in as little as 50 years or even less.


Luke: you assumed a 4 kelvin receiver. I believe this would require active cooling, especially on the side which is turned towards the Sun. Assuming radiative equilibrium, the temp at Earth orbit (semi-major axis) is approx. 390 K, at Mars orbit it is 320 K. These would produce some 100 times the noise, reducing the bitrate from 80 Tbps to 1 Tbps if cooling was not used.

Still, that's pretty impressive with a 100 watt laser. Presumably the relays would have more powerful transmitters, since they don't need many other systems, being automatic and orbital.

Now, if there was a conflict in our solar system, could communications be jammed? One could blind the relays and cut off communication to half the system. Laser battlestars could perform this role with their multi-gigawatt lasers from great distances away. Could you defend against this kind of jamming? Would anything get through if an opponent was shining the receiver with 1 mW/m2 while you're trying to reach it with 10 nW/m2?

Rick Robinson:

Much good information as usual, and in balance it confirms my sense that interplanetary communications will be (relatively!) cheap and plentiful.

Though it may not seem that way to people accustomed to Earth bandwidth, because current experience suggests people will use as much of the stuff as is available.

People also put up with some amazing inherent limitations, like watching TV on an iPhone.

Most ships won't need interplanetary comms, because they will be operating some region of orbital or local space, and can relay through local stations. Perhaps only deep space ships will carry such gear, and they are their own special beast anyway.

But the fact that high bandwidth interplanetary comms is not THAT hard or expensive is politically significant. As so often, technology has ruined or at least limited some rocketpunk era tropes, in this case 'seizing the relay station.'


It may have killed a trope or two, but it looks to me like we've opened up a few interesting possibilities.

I for one am not quite sure how a station or ship would deal with its comm laser module getting blinded by an enemy laserstar. Since Markus brought up that possibility I've been wondering how someone could engineer around that. You could deploy a thermal shield, and I suspect comm lasers would already do that as a matter of course. However, that could be overcome by a more powerful laser or one from the same direction as incoming transmissions, which could be countered with a better optical hood around the receiver aperture, ... etc.

Also a thought on data security (Jean Remy actually mentioned this in passing earlier). Since that hypothetical transmission beam we've been throwing around has a 90km radius after it's gone 1 AU, there isn't much to keep someone from listening in. If the intended receiver was out in the middle of deep space, it would be easy to catch the eavesdropper, but if it were happening in crowded planetary space, how do you tell who's listening in and who's just cruising by?

Luke Campbell:

Re: Comm jamming. My guess is that you would use directional information to discriminate between sources. Use a telescope, and only measure the signal from the pixels corresponding to the guy trying to communicate with you. A powerful enough laser in the same field of view could have diffraction spillover into the image of the talker, but this would require the laserstar to be nearly lined up with the talker and the receiver. Of course, a laserstar could also just burn the comm scope off, it were within range.

Re: Cooling. Yes, I assumed refrigeration down to liquid helium temperatures. I figure an extra 250 kg of helium liquifiers and cryostats and compressors and helium pumps is a small price to pay for a 50-fold increase in data rate, especially with cheap, abundant solar power. In any event, by the time orbital commerce becomes commonplace we'll probably have compact solid state chillers that can get us down to 4 K with less than a kg of mass.

Re: Comm Lasers as Weapons. I used Rick's parameters for a lasercom, which was stated as 100 W of power. A 100 W laser can be used at close focus in the near field range for industrial cutting, drilling, and welding. However, it is generally considered that you need three or more orders of magnitude more power in order to have a useful weapon.

Re: Shot Noise. On another note, I was thinking about shot noise. That 4 nW of 1 micron laser light corresponds to 20 billion photons per second. This alone limits you to no more than 20 GHz. If we assume that the laser transmits pulses that are either on or off, and we assume the absence of any background, then you need an average detection of 14 photons for a 1-in-a-million chance of a false negative (picking up zero photons during the "on" pulse). This reduces your data transfer rate to a bit more than 1 GHz. If we assume a 1 GHz bit rate (an average of 20 photons per pulse), you can set your lower limit of acceptance to 3 or more photons to take into account background or detector noise, and still achieve a bit error rate of 1-in-2.5 million.

Note that for laser light, shot noise puts a much more stringent limit on communication rate than thermal noise.


The general outlines of communications in the plausible mid-future look a bit like the Victorian era. Fast and reliable close to home, a little slower if you want to contact the next 'city' over, slower again but still reliable if you want to contact another 'country', and best of luck to you if you want to contact someone in the backbeyond. In this case city, country, and backbeyond are defined by both linear distance and orbit.

You could even end up with a Pony Express situation. Someone sets up a Planet Express torchship delivery system to carry high information-density packages or mail that requires high-security delivery... And then nine months later someone else sets up a tightbeam relay network that can handle multiple high-bandwidth high-security messages... And suddenly you've got a lot of cheap ponies on the market.

Except in this case the ponies have 45 gW/30 milligee legs.

From On Torchships comments (2010)

RUDN mathematicians analyzed the properties of gravitational waves in a generalized affine-metrical space (an algebraic construction operating on the notions of a vector and a point) similarly to the properties of electromagnetic waves in Minkowski space-time. They report the possibility of transmitting information with the help of nonmetricity waves and transferring it spatially without distortions. The discovery could lead to a new means of data transfer in space, e.g., between space stations. Their results are published in Classical and Quantum Gravity.

Gravitational waves are waves of curvature in -time, which, according to General Relativity, are completely determined by space-time itself. Currently, there are reasons to consider space-time as a more complex structure with additional geometrical characteristics such as torsion and nonmetricity. In this case, geometrically speaking, space-time turns from a Riemannian space envisaged by the General Relativity (GR) into a generalized affine—metrical space. Respective gravitational field equations that generalize Einstein's equations show that torsion and nonmetricity can also spread in the form of waves—in particular, at a great distance from wave sources.

In order to describe , RUDN researchers used mathematical abstraction—an affine space, i.e., a usual vector space but without an origin of coordinates. They proved that in such a mathematical representation of gravitational waves, there are functions that remain invariable in the process of wave distribution. It is possible to set an arbitrary function to encode any information in approximately same way that electromagnetic waves transfer a radio signal.

If scientists can develop a method to incorporate these constructions in a wave source, they could reach any point in space without change. Thus, gravitational waves could be used for data transfer. The study consisted of three stages. First, RUDN mathematicians calculated the Lie derivative—a function that binds the properties of bodies in two different spaces: an affine space and a Minkowski space. It allowed them to pass from the description of waves in real space to their mathematical interpretation.

At the second stage, the researchers determined five arbitrary functions of time, i.e., the constructions that do not change in process of distribution of a wave. With their help, the characteristics of a wave can be set in a source, thus encoding any information. In another point in space, this information can be decoded, providing the possibility of information transfer. In the third stage, the researchers proved the theorem of the structure of plane nonmetricity in gravitational waves. It turned out that from four dimensions of a wave (three spatial ones and one time dimension), three can be used to encode an informational signal using only one function, and in the fourth dimension with use of two functions.

"We found that nonmetricity waves are able to transmit data similarly to the recently discovered curvature waves, because their description contains arbitrary functions of delayed time that can be encoded in the source of such waves (in a perfect analogy to )," says Nina V. Markova, a co-author of the work, candidate of physical and mathematical sciences, assistant professor of C.M. Nikolsky Mathematical Institute, and a staff member of RUDN.

More information:O V Babourova et al, Structure of plane gravitational waves of nonmetricity in affine-metric space, Classical and Quantum Gravity (2018). DOI: 10.1088/1361-6382/aace79

Anders Sandberg: The paper assumes a more general version of general relativity, which we have no evidence for. But clearly even vanilla gravitational waves can transmit information. It is just not terribly effective given the weak gravity-matter coupling.

Laser Comm as Weapon

As previously mentioned radio waves can be replaced with laser beams for interplanetary communication.

However, a broad definition of a military weapon is "throwing a bunch of energy at a hostile target". Meaning that if rocket engines can be impromptu weapons, so can laser communicators. JON'S LAW may have to be broadened a bit.


In The Expanse season 3 episode 13 Abaddon's Gate, several battlefleets are in a desperate situation. They entered an alien paleotech stargate, and are currently inside a "pocket dimension." Gloomily they have managed to frighten the alien computer installation controlling everything and it has initiated defensive protocols. Specifically it has filled the pocket universe with a field of force which prevents any physical object from moving faster than a brisk walk (28.997 m/s).

The main ship of the Belter contingent is the OPAS Behemoth (formerly the Mormon generation ship Nauvoo, planned to travel to Tau Ceti, but "liberated" by the Belters). Captain Ashford is convinced that the alien installation poises an extinction-level threat to the entire solar system, which is not an unreasonable conclusion. He is determined to destroy the stargate linking the dimension to the solar system, which will protect solsys but of course doom everybody and everyship inside. It would be nice to allow all the ships to escape first, were it not for the regrettable fact that under the force field's speed limit it will take the ships years to travel to the stargate.

Anyway, destroying the stargate is a problem. It will take years for a ship to travel back to the gate and ram it. It will also take years for a missile strike to reach it.

However light still travels at the same speed, under the stern glare of St. Albert. Since the Nauvoo had been designed for the 12 light year trip to Tau Ceti, it has a communication system with a 12 light-year range. This take the form of the largest laser communication system ever constructed.

And comm lasers can be used as jury-rigged weapons.


      The bright blue spark of another laser glowed in the tip of the alien cone. Louis listened to the autopilot-computer chuckling to itself as it tried to untangle the signals in an unknown laser beam. At least they did use lasers, not telepathy or tentacle-waving or rapid changes in skin color.
     A face appeared on Louis's screen.
     It was not the first alien he had seen. This, like some others, had a recognizable head: a cluster of sense organs grouped around a mouth, with room for a brain. Trinocular vision, he noted; the eyes set deep in sockets, well protected, but restricted in range of vision. Triangular mouth, too, with yellow, serrated bone knives showing their edges behind three gristle lips.
     Definately, this was an unknown species.
     "Boy, are you ugly," Louis refrained from saying. The alien's translator might be working by now.
     His own autopilot finished translating the alien's first message. It said, "Go away. This object belongs to me."
     "Remarkable," Louis sent back. "Are you a Slaver?" The being did not in the least resemble a Slaver, and the Slavers had been extinct for eons.
     "That word was not translated," said the alien. "I reached the artifact before you did. I will fight to keep it."
     Louis scratched at his chin, at two week's growth of bristly beard. His ship had very little to fight with. Even the fusion plant which powered the thruster was designed with safety in mind. A laser battle, fought with comm lasers turned to maximum, would be a mere endurance test; and he'd lose, for the alien ship had more mass to absorb more heat. He had no weapons per se. Presumably the alien did.
     But the stasis box was a big one.

From THERE IS A TIDE by Larry Niven (1968)

(ed note: Doug Hooker was insane, kept sane by his doctor prescribed medication. But he carelessly went off his meds and was no long sane. He steals a small Bussard Ramjet starship and travels to the colony on Plateau. The authorities manage to apprehend him, but not before he accidentally incinerate a house. Which accidentally kills four people.

He is made sane again, and sentenced to a prison term for manslaughter. Because he was guilty of carelessly going off his meds. After he gets out, he steals another Bussard Ramjet and flees to another colony planet. He does not want to face Greg Loeffler.

Because the four people he accidentally killed were Greg's wife and three children. Which drove Greg insane.

As Doug leaves the Plateau solar system, he unexpectedly gets a laser com message from Greg.)

      And where had he gotten a com laser? The Plateau station was closed to all but qualified personnel. But Greg owned a ship with a com laser
     A ship just like this one.
     Almost calmly, Hooker sat down at the control board. He connected the autopilot screen to the stern scope. Tau Ceti glowed brightly off center. Hooker centered it, then began to enlarge it. The screen turned yellowish-white, with a blue point moving off screen near the top. Hooker centered that, enlarged it.
     A deep-blue flare with a black dot in the center.
     Loeffler was coming after him.
     Loeffler’s hoarse voice stopped suddenly. Then, it giggled. “Tricked you,” it said, suddenly calm.
     The stern scope turned deep red.
     Damn, thought Hooker. He did trick me. The scope screen would not transmit more light than human eyes could bear, but there was a dial to register the light falling on the scope. That dial registered maximum. Loeffler was using his comm laser as a weapon. At maximum power it could easily have blanketed Earth’s solar system with a clearly read signal, but Loeffler was firing it at an object only light-hours distant.
     He could kill me, Hooker thought. He could do it.
     It wouldn’t be fast. Loeffler was firing from behind at that part of Hooker’s ship which was built to stand fusion flame applied for years. But eventually things would melt.
     Greg was jubilant. “I’m going to burn you, Doug! Just like you burned Joanna and Marcia and Tom and little Greg! But slower! Slower, you…” And there was more profanity.
     Needles were rising. Hull-temperature indicators, power-consumption meters, climbed toward pink zones nobody had ever expected them to touch.
     Doug Hooker rubbed his eyes. He waited for an inspiration, and none came. Needles touched their pink zones. Bells rang, and Doug turned them off. After a bit he left the control room and went downstairs and lay down on the masseur couch.
     He’s going to kill me. The thought seemed far away, drowned in the groaning comfort of the massage.
     All I wanted was a new life. I wanted to go away and start over. The couch was a hard, enveloping caress.
     He won’t let me. He wants to kill me. And who has a better right?
     Let him kill me.


     It was difficult to struggle out of the couch, for the couch was not finished with him. During a massage one must be in a defeatist frame of mind. Otherwise one tenses, one’s automatic defenses take over. But somehow Doug pulled himself free of the gentle, grasping embrace, and somehow he got upstairs to the control room. He was still covered with massage oil.
     A man attacked has the right to defend himself. I paid for my crime.
     Doug sat down in the control chair, used a key to unlock a panel. There were override switches underneath. One turned off the ship’s alarm bells; one allowed excess power in the ship’s circuitry; three others set up the sequence that would blow the ship apart if the drive or the ramscoop failed. Everything under the panel was an override switch for the ship’s automatic safety precautions. Doug flipped one switch and closed the panel. Then he twisted a dial hard over, as far as it would go.
     His com laser was already fixed on Loeffler’s ship. Now it would burn.
     Hooker turned off his fusion drive to reduce the heat pouring in at the ship’s stern. Now he had a good chance. He was firing his laser at Loeffler’s nose, where there was less protection. The massive, almost invulnerable bulk of the ramscoop would absorb most of the beam; but the lifesystem was wider than the ramscoop, and it would catch a lot of light. Eventually its walls would melt.
     Hooker would kill Loeffler before Loeffler could kill Hooker.
     Doug went back to the masseur couch. He felt very tired.

(ed note: but Loeffler has the last laugh)

From THE ETHICS OF MADNESS by Larry Niven (1967)

(ed note: Jeff is on an emergency rescue mission to an unexplored star system. His brother Ben was exploring the system when he failed to return. There was just a cryptic message about creatures he called "rock hoppers". The system is mostly a huge asteroid belt with no planets. Jeff explores in a space suit)

      The suit flew like a toy ship. Though it had no mass-reduction gear to multiply its speed, the rocks were near enough. The starship shrank to a silver spark that went out in the endless dark behind. The rocks grew larger, pouring out of space like an icy waterfall.
     He searched them with the laser in his helmet, hoping for another call from Ben. He needed the signal to guide him toward the lost ship. No signal came.
     He considered calling Ben. The (laser) sending machine was a thick black tube, like a heavy flashlight, snapped to the belt of his suit. Once he aimed it at the rocks and prepared to speak, but then he changed his mind.
     Ben had never answered the other signals he had sent, and he didn’t want the sort of answer the hoppers had been making. He snapped the tube back to his belt and flew on toward the blazing wall.

     Something hit the leg of his suit like an electric shock. Blue sparks showered around him. He tried to pull free from the thing against him, but it hung on.
     He twisted the still suit and found a wire.
     One bright strand, smaller than his little finger, stretched away to his right and left as far as he could see. Here and there, bits of rock stuck to it. They marked out a line through the cloud, even where he couldn’t see the wire itself.
     Shaken, he kicked at the wire. Bright sparks danced around his boot. A new shock hit his foot. His boot stuck fast. Before he could think, he grabbed the wire with his right hand. He meant to push it off, but new sparks sprayed him. A sharper shock hit his arm. His armored glove stuck fast.
     He gasped for breath and tried to understand. The wire was a trap. The electric current had made it stick to the metal of his suit. He was trapped like a fly in a spider’s web—

     A rock hopper’s web! This was the web that had caught Ben’s crippled starship. For that first moment, all Jeff could do was wonder what kind of creature would weave a metal web in a cloud of flying rocks and make electric shocks to kill its prey.
     Trembling in the heavy suit, he twisted around to look back and forth along the wire. Far away, he saw a flying stone strike it and bounce. That puzzled him, even in his state of shock.
     Why had the rock bounced, while he stuck fast? Why did some rocks stick? His eyes traced the wire to the nearest rock and saw a dull gleam of metal. So that was the answer!
     Metal ores were fused by the current and stuck to the wire. Rocks that didn’t contain metal bounced off. The web, he saw, was a kind of tool, useful to collect bits of rich ore out of the flying rocks, and it had collected him.

     With all his strength, ]eff twisted against the shining wire. He tried to kick with his boot. He tried to open his frozen glove. All he did was hurt himself.
     Hot sparks whirled around him. Heavy shocks struck and his body jumped with pain. He couldn’t break free. The wire stuck to his leg and his boot and his glove.
     When the shocks stopped at last, he hung from the wire limp and worn out. Gasping for breath, he felt damp with sweat and a little sick from the shocks.

     He tried to decide what to do. One hand was still free. He had the suit itself, with its equipment. He still had more than half a tank to fuel his jets.
     Maybe the jets could pull him free. At least they were something he could try. Using the padded controls inside the helmet, he didn’t even need his hands to work them. He moved around so that the jet thrust would be straight away from the wire and pushed the chin button for full power.
     The pale blue jets licked out around the wire, but nothing else happened. There wasn’t even another shock. The push of the jets had been too weak, he thought, for the keeper of the web to notice it at all.

     He searched for a better plan.

     The tools he had used in his work on the starship were still clipped to the suit. With his free hand, he found a cutting tool. Holding it very carefully in his clumsy left hand, he tried to cut the web.
     Sparks blazed around the tool. He felt it soften, saw hot metal splash. His glove slipped against the wire. He got it away before it stuck, but new shocks hit as if to punish him.

     Every time his muscles jumped with the shocks, the laser machine thumped against his armored leg. That gave him a new idea. Though the little machine was not a weapon, its light had power.
     It was a long time before he had the strength to move again. Then he reached for the machine. Gripping it under his pinned right arm, he twisted the knob from VOICE to CODE, because CODE used full power. He slid the range to the sharpest beam. He got the machine back into his left hand, and pointed it very carefully at the bright wire on his right. With his thumb, he squeezed the code key. The instant blade of bright green light sliced the wire.
     The cut wire contracted like a broken rubber band, dragging him into a cloud of rocks. Half blind and stiff with pain, he held to his plan. He bent his stuck leg, as he spun through the rocks. He moved his right arm to bring the whipping wire into reach. Then he cut at the web with his blade of light. Green fire flashed.
     He was free!

     The hopper was very close. Strangely, jeff thought, it had a kind of beauty, too. Its coiled silver arms were bright and graceful, and the scales of its body were like black jewels.
     Without the jets, he couldn’t run. He had to fight. The admiral’s policy of peace among the stars meant nothing to him now. Trying not to hurry, he turned the knob of his machine back to CODE and pushed the range as high as possible. Leaning carefully around the rock, he fired at the middle of the hopper’s flat body.
     The machine was not meant to be a weapon. He was not expecting to hurt the hopper much, but he saw dark puffs of smoke where his small darts of bright green light hit the shiny scales. He saw the silver arms knotting, twisting up to shield the body.

From TRAPPED IN SPACE by Jack Williamson (1968)

(ed note: Warning: Spoilers for the story follow. The protagonist is a native of the fictional African nation of Umbala, which on the equator has a fictional Zambue crater. The nation has been taken over by a power-mad dictator named Chaka who has instituted a reign of terror. The protagonist is only a humble astronomer, but realizes Chaka must be stopped. The fact that Chaka put to death several of the astronomer's relatives only makes it personal.

The radio telescope will be opening soon. The astronomer knows that Chaka won't be able to resist taking the elevator to the top of the telescope and overlooking his domain.

Meanwhile, the narrator is waiting on the other side of the hills.)

It seems strange that my country, one of the most backward in the world, should play a central role in the conquest of space. That is an accident of geography, not at all to the liking of the Russians and the Americans. But there is nothing that they could do about it; Umbala lies on the equator, directly beneath the paths of all the planets. And it possesses a unique and priceless natural feature: the extinct volcano known as the Zambue Crater.

When Zambue died, more than a million years ago, the lava retreated step by step, congealing in a series of terraces to form a bowl a mile wide and a thousand feet deep. It had taken the minimum of earth-moving and cable-stringing to convert this into the largest radio telescope on Earth. Because the gigantic reflector is fixed, it scans any given portion of the sky for only a few minutes every twenty-four hours, as the Earth turns on its axis. This was a price the scientists were willing to pay for the ability to receive signals from probes and ships right out to the very limits of the solar system.

Chaka was a problem they had not anticipated. He had come to power when the work was almost completed, and they had had to make the best of him. Luckily, he had a superstitious respect for science, and he needed all the rubles and dollars he could get. The Equatorial Deep Space Facility was safe from his megalomania; indeed, it helped to reinforce it.

The Big Dish had just been completed when I made my first trip up the tower that sprang from its centre. A vertical mast, more than fifteen hundred feet high, it supported the collecting antennas at the focus of the immense bowl. A small elevator, which could carry three men, made a slow ascent to its top.

As soon as the NASA technicians had installed their equipment and handed over the Hughes Mark X Infrared Communications System, I began to make my plans.

Colonel Mtanga, his Chief of Security, would object, but his protests would be overruled. Knowing Chaka, one could predict with complete assurance that on the official opening day he would stand here, alone, for many minutes, as he surveyed his empire. His bodyguard would remain in the room below, having already checked it for booby traps. They could do nothing to save him when I struck from three miles away and through the range of hills that lay between the radio telescope and my observatory. I was glad of those hills; though they complicated the problem, they would shield me from all suspicion. Colonel Mtanga was a very intelligent man, but he was not likely to conceive of a gun that could fire around corners. And he would be looking for a gun, even though he could find no bullets….

I went back to the laboratory and started my calculations. It was not long before I discovered my first mistake. Because I had seen the concentrated light of its laser beam punch a hole through solid steel in a thousandth of a second, I had assumed that my Mark X could kill a man. But it is not as simple as that. In some ways, a man is a tougher proposition than a piece of steel. He is mostly water, which has ten times the heat capacity of any metal. A beam of light that will drill a hole through armour plate, or carry a message as far as Pluto—which was the job the Mark X had been designed for—would give a man only a painful but quite superficial burn. About the worst I could do to Chaka, from three miles away, was to drill a hole in the colourful tribal blanket he wore so ostentatiously, to prove that he was still one of the People.

For a while, I almost abandoned the project. But it would not die; instinctively, I knew that the answer was there, if only I could see it. Perhaps I could use my invisible bullets of heat to cut one of the cables guying the tower, so that it would come crashing down when Chaka was at the summit. Calculations showed that this was just possible if the Mark X operated continuously for fifteen seconds. A cable, unlike a man, would not move, so there was no need to stake everything on a single pulse of energy. I could take my time.

But damaging the telescope would have been treason to science, and it was almost a relief when I discovered that this scheme would not work. The mast had so many built-in safety factors that I would have to cut three separate cables to bring it down. This was out of the question; it would require hours of delicate adjustment to set and aim the apparatus for each precision shot. I had to think of something else; and because it takes men a long time to see the obvious, it was not until a week before the official opening of the telescope that I knew how to deal with Chaka, the All-Seeing, the Omnipotent, the Father of his People.

It took me three days to install the carefully silvered, optically perfect mirror in its hidden alcove (at the top of the hills). The tedious micrometer adjustments to give the exact orientation took so long that I feared I would not be ready in time. But at last the angle was correct, to a fraction of a second of arc. When I aimed the telescope of the Mark X at the secret spot on the mountain, I could see over the hills behind me. The field of view was tiny, but it was sufficient; the target area was only a yard across, and I could sight on any part of it to within an inch.

Along the path I had arranged, light could travel in either direction. Whatever I saw through the viewing telescope (at his observatory on one side of the hills) was automatically in the line of fire of the transmitter.

It was strange, three days later, to sit in the quiet observatory, with the power-packs humming around me, and to watch Chaka move into the field of the telescope (standing at the top of the radio telescope on the other side of the hills). I felt a brief glow of triumph, like an astronomer who has calculated the orbit of a new planet and then finds it in the predicted spot among the stars. The cruel face was in profile when I saw it first, apparently only thirty feet away at the extreme magnification I was using. I waited patiently, in serene confidence, for the moment that I knew must come—the moment when Chaka seemed to be looking directly toward me. Then with my left hand I held the image of an ancient god who must be nameless, and with my right I tripped the capacitor banks that fired the laser, launching my silent, invisible thunderbolt across the mountains.

Yes, it was so much better this way. Chaka deserved to be killed, but death would have turned him into a martyr and strengthened the hold of his regime. What I had visited upon him was worse than death, and would throw his supporters into superstitious terror.

Chaka still lived; but the All-Seeing would see no more. In the space of a few microseconds, I had made him less than the humblest beggar in the streets.

And I had not even hurt him. There is no pain when the delicate film of the retina is fused by the heat of a thousand suns.

From THE LIGHT OF DARKNESS by Arthur C. Clarke (1966)


That party-pooper Einstein codified the ultimate speed limit on communication in his Special Relativity. The limit is the speed of light in a vacuum, which is quite rapid at 299,792,458 m/s. For distances found on the surface of a planet this speed limit is not much of a problem (unless you are doing ultra-high-speed stock market trading or something). But for distances within a solar system the speed limit problem becomes moderate to major (seconds to hours delay), and for interstellar distances they become overwhelmingly outrageous (years to millenniums delay).

If you are trying to have a conversation with somebody, the delay is doubled, because it becomes a round trip. Alice says a sentence which travels by radio to Betty. Betty responds with a sentence which travels to Alice. From the viewpoint of Alice, Betty's answer is delayed by double the transit time.

For a conversation between Terra and Luna the double delay will be from 2.4 to 2.7 seconds (perigee and apogee), which is annoying but manageable. There is a nice scene illustrating this problem in The Expanse Season 2 Episode 5: “Home”, where Chrisjen Avasarala on Terra tries to talk to her husband on Luna. They keep stepping on each other's sentences. They know better but they are preoccupied with the fact Terra might be destroyed in a couple of hours, and this might be the very last time they talk to each other.

For any longer distance conversation is more or less impossible. It will be more like sending emails, or even snail-mail. The most efficient strategy is probably to constantly talk, and as questions arrive just insert the answer into your monologue.

This is why trying to control asteroid mining drones by telepresence from Terra is probably impractical.

For conventional radio and other electromagnetic communication the time delay is obviously proportional to the distance, which is science-speak for "the longer the distance the longer the delay". For handwavium faster-than-light communication it depends upon what mad ideas the author comes up with. Most of them simply have the communication speed set faster, e.g., a subspace radio with a speed of five-hundred times lightspeed. Which would have a timelag 1/500th of a conventional radio. But occasionally you find weird communication devices with strange limits, such as a constant time delay regardless of distance. These have to be handled on a case-by-case basis. Sometimes there will be two or more kinds of FTL communication: a commonly available but slow kind and a highly restricted but fast kind.

James Blish had many stories featuring his "Dirac" communicator, which was instantaneous. Timelag of zero.

With NASA and other space agencies, they assume all us humans are living on Terra, while the unmanned space probe can be a long way out there. It is going to take some time for the messages from the space probe to make their way back to Terra.

For instance, the New Horizons space probe went whizzing by trans-Neptunian object (486958) 2014 MU69 on 1 January at 05:33 UTC. It immediately sent back a radio message "I'm OK, I didn't smack into the asteroid or anything, photos and data to follow". But the radio message didn't make it back to Terra until 1 January at 15:28 UTC, nine hours and 55 minutes later. Such is the timelag. The people at Mission Control knew theoretically that New Horizons had its close encounter at 5:33, but they had to sweat it out until 15:38 until they found out what happened.

This can be confusing when NASA wants to send a command to a space probe ordering it to perform an action at such-and-such a time. So space agencies use Spacecraft Event Time (SCET, also known as Orbiter UTC) and Earth-received time (ERT, also known as Ground UTC).


Spacecraft Event Time (SCET) is the spacecraft-local time for events that happen at the spacecraft. SCET is used for command programs that control the timing of spacecraft operations and to identify when specific events occur on the spacecraft relative to Earth time.

SCET versus Earth time

Since signals between the spacecraft and Earth are limited to the speed of light, there is a delay between the time an event happens on the spacecraft (such as the transmission of data taken from an instrument reading) and the time that a signal reporting the event reaches Earth. Similarly, there is a delay between when instructions are sent from Earth and when the spacecraft receives the instructions. The length of delay is related to the distance between the sending and receiving points. Failure to take this delay into account could result in inaccurate data or mistakes in spacecraft control.

Calculating SCET

Determining the Spacecraft Event Time involves taking the time at Earth and adding or subtracting the signal travel time, depending on whether the signal is being sent to or received from the spacecraft. For events transmitted from the spacecraft to Earth, the SCET of an event on the spacecraft can be defined as equal to the ERT (Earth-Received Time) minus the OWLT (One-Way Light Time). For events transmitted from Earth to the spacecraft, the calculation is TRM (transmission time) plus OWLT. For example, if a signal were received on Earth at exactly 11:00 UTC from a spacecraft showing that it had just completed a maneuvering thrust, but the spacecraft was four light-hours away from Earth (the distance of the New Horizons spacecraft at one point as it approaches Pluto), the SCET time of the thrust maneuver would have been four hours earlier, at 07:00.

Spacecraft Event Time in UTC is also known as Orbiter UTC, and Earth-received time as Ground UTC.

Spacecraft control

Since it takes time for a radio transmission to reach a spacecraft from Earth, the usual operation of a spacecraft is controlled with an uploaded command script containing SCET markers to ensure a certain timeline of events. Because of the delay between the sending of instructions from Earth and their receipt and execution by the spacecraft, real-time commanding of robotic spacecraft is done rarely: usually only in response to an emergency event, when changes in spacecraft operations must be made as soon as possible. For example, a spacecraft could be instructed to go into safe mode to protect it during a coronal mass ejection (CME) from the Sun.

From the Wikipedia entry for SPACECRAFT EVENT TIME

Jonathan McDowell: According to Emily Lakdawalla's timeline, New Horizons should currently be transmitting the "Failsafe 1 downlink" batch of data. Those radio signals will reach Earth in 6 hours.

Emily Lakdawalla: The concepts of "now" and "currently" on New Horizons are going to get super confusing over the next couple days :)

Jonathan McDowell: Just need to make a mental distinction between the events and the radio signals. For me, "now" always means Spacecraft Event Time and I don't find it confusing.

Daniel Fischer: See, and for me "now" is and has always been Earth-Received Time. Perhaps because Voyager 2 at Neptune was the first encounter I attended in person … at the JPL, I mean. ;-)

Jonathan McDowell: I think the moment you use "now" for Earth-Received Time in reference to "what the spacecraft is doing" you are opening the door to confusion.

Daniel Fischer: I was under the impression that Earth-Received Time was widely — if not exclusively — used in NASA (and specifically JPL) interplanetary mission outreach while ESA often uses Spacecraft Event Time (without saying so) … perhaps because our spacecraft usually don't get that far and light time is often just minutes. ;-)

Jonathan McDowell: Yes, and I hate that JPL does that! (And far too often in outreach info it's not specified which is being used).

Orbit Ops: We should all try to use the “as the event light cone passes the earth” time (which for real-time broadcast events from the spacecraft, coincides with receiving them on earth). Einstein might approve.

Shawn Muses Quietly: At least it's not moving at relativistic speeds in relation to us or the very concepts of "now" and "currently" would start to lose their meanings!

Andy Cooper: Oh well, just so long as it’s not “presently”…which conflates the future as well.

From a thread on Twitter (2018)

The table below indicates the communication delay that occurs in a one-way transmission.

CircuitDistanceDelay Time
HF link (UK-NZ)~20,000 km0.07 s (67 ms)
Submarine cable(UK-NZ)~20,000 km0.07 s (67 ms)
Geosat Link (US-Aus)~80,000 km0.25 s
Earth-Moon384,000 km1.3 s
Earth-Mars55 - 378 million km3 - 21 minutes
Earth-Jupiter590 - 970 million km33 - 53 minutes
Earth-Pluto~5800 million km5 hours
Earth-Nearest Star~9.5 million million km4 years

Only the first two circuits listed have a delay that is barely noticeable in a two-way voice conversation. On telephone calls between continents that are routed via a geosynchronous satellite, the time between when one person stops speaking and then hears the other person reply is half a second (0.25 s × 2). This can cause immense confusion if the other person starts speaking before the first has finished. It can take several sentences before the confusion is finally sorted out. Two-way digital communication between machines at high data rates suffers from this problem even more acutely, and protocols must be established which prevent conflicts arising.

Communication with a future lunar base will be worse, and for voice communications will necessitate an "over to you" simplex radio communications type approach. Two-way interactive communication with any station beyond the moon is basically impossible. Although there are no manned bases currently on other planets, this delay is presently of great concern to people who send remotely controlled spacecraft to Mars. There is no possibility of detecting an incipient vehicular crash in time to do anything about it. The vehicle must thus be given a very large degree of autonomous control (e.g., for navigation).


The speed-of-light delay from Mars to Earth means there's a long lag time between commands sent, commands executed, and confirmation. In the mission's early days, maneuvers were painfully slow and simple as the team learned how the rovers would respond. Later, the rovers got artificial intelligence upgrades to help determine interesting targets. This was helpful, because a lot can change on Mars in a few minutes — rampaging dust storms, loose soil, and other unexpected things can spoil the best-laid plans.

"Imagine driving your car to the grocery store with a light time delay like that. Nothing happens for somewhere between 4 and 20 minutes, and when it does happen, you don't know about it, because your rear window doesn't update for another 4 to 20 minutes," Maxwell says. To avoid excruciating waits, the team now programs Opportunity (and formerly Spirit) with a series of tasks, up to three days' worth over a weekend. After so many years, they're confident in the rovers' response times and capabilities, so they don't have to test every little command with exacting detail. The Curiosity mission will do that for some time, though.


     How exactly do you drive the one-ton Mars rover Curiosity, when the driver is, on average, 150 million miles away? With a one-way time delay of around 13 minutes, it certainly isn’t a matter of sitting down in front of a monitor and waggling a joystick…
     …In short, there are two ways that Curiosity can navigate the surface of Mars: NASA can transmit a series of specific commands, which the rover then dutifully carries out — or NASA can give Curiosity a target, and then trust the rover to autonomously find its own way there. In both cases, the commands are transmitted to Curiosity via NASA’s Deep Space Network — the worldwide network of big-dish antennae that NASA uses to communicate with spacecraft, and carry out some radio astronomy on the side.
     To decide which navigation method to use, NASA uses the Rover Sequencing and Visualization Program (RSVP), which is basically a Mars simulator. RSVP shows Curiosity’s current position on Mars, along with surface topology, obstacles (rocks), and so on. RSVP can then be used to plot a move (go forward 10 meters, turn 30 degrees right, go forward 3 meters) — or to pick an end point, which Curiosity will dutifully, autonomously navigate to. To safely navigate Mars, Curiosity uses its Hazcams (hazard avoidance cameras) to build a stereoscopic map of its environment, identifies which objects are too large to drive over, and then plots out a course to the end point.
     When Curiosity finishes its drive, it transmits a bunch of thumbnail images from its on-board cameras to NASA, which are then used to work out Curiosity’s exact location on Mars. This data is fed into RSVP, the next day’s movements are plotted, and so on and on.


The most efficient way to exploring other planets may be sending humans to orbit, and letting robots do everything else

As we start looking towards more comprehensive exploration of the Moon and of Mars, the assumption is that we’re working on sending humans to the surface of those worlds. It’s going to be exponentially more difficult and dangerous than sending robots, but that’s what exploration is all about, right?

There’s an article in the current issue of Science Robotics that discusses an alternative approach—a kind of compromise between sending only humans or only robots. The idea is using robotic telepresence for planetary exploration. From orbit, the authors argue, a small team of humans would remote operate rovers and other robotic systems and as a result they could do more exploration while keeping the overall mission safer and cheaper.

We already use telerobotics for planetary exploration—we’ve got robots all over the solar system sending us data and then patiently doing what we tell them to do. This is different than telepresence, because of the latency involved: It takes long enough (minutes to hours) for a signal traveling at the speed of light to make it from Earth to Mars or Saturn and back again. That means that there’s no way for us to have a real “presence” experience.

In the Science Robotics article, Dan F. Lester, Kip V. Hodges, and Robert C. Anderson from Exinetics, in Austin, Texas, Arizona State University, in Tempe, and NASA Jet Propulsion Laboratory, in Pasadena, Calif., argue for sending humans into space specifically to reduce latency to something tolerable (better than 0.5 second), for example going into orbit around Mars (but not to the surface) just to make it so that humans can control robots on the surface through telepresence in near real-time—with the robots also doing things on their own when needed.

The European Space Agency (ESA) tried this kind of thing out recently, with an astronaut on the International Space Station (ISS) directly controlling a robot on Earth. We wrote about it here, and there’s an article from ESA here. NASA has been trying it in the other direction as well, controlling Robonaut 2 on the ISS from the ground.

There are lots of reasons why space agencies are working on orbital telepresence, many of which are illustrated in the NASA artwork at the top of this article. Using relatively simple assistive autonomy, a horde of robots can spend most of their time wandering around on their own, while a few humans jump between them via telepresence from orbit to provide guidance.

The robot horde can consists of all kinds of different platforms, like driving robots, flying robots, robots that can scale cliffs, robots with arms, robots with drills, robots with lasers, or anything else you want. If some of the robots get stuck or break, it’s not a big deal, you’ve got more. Some robots could even collect samples on the surface, and then send them up to you inside little rockets. And, as autonomy improves and robots get better at autonomous navigation and even doing autonomous science, humans will be able to control more and more of them at once, only stepping in when necessary.

As I see it, there are two fundamental questions about using telepresence robots for exploration:

  • The first is whether humans really can do a better job at exploration than robots can. It’s certainly true right now: Humans may not have the patience of robots, but we’re able to quickly and efficiently use all kinds of scientific tools, take samples, move things, traverse things, and use our brains and experience to very quickly make decisions about what’s worth exploring and what isn’t. Except for that last thing, you can easily imagine how a robot could easily make for better, or at least more efficient, explorers. A robot can move faster, lift and carry more, and handle a wider variety of terrain. As long as that robot has the same kind of sensing and manipulation capabilities as a human (or better, which isn’t hard to imagine), the only thing missing is the brains and experience. Putting a human in the loop through telepresence could solve that problem.
  • The second question is much harder to answer: How much value is there in having a human experience another world in person? How do you calculate the worth of having humans walk on the Moon, for example? It was certainly an inspirational giant leap for mankind moment, but was it really worth the incredible hassle and expense of trying to keep those humans from dying there, and then having to bring them back again? Maybe in the 1970s, it was, because 1970s robots were mostly terrible. But again, imagine what robots will be like within the next five to 10 years, or more specifically, what telepresence will be like. My guess is that the audiovisual experience will be pretty close to the real thing, and that haptic controllers will make things feel nearly as real as, uh, reality. Even if that’s true, however, it doesn’t really answer the question—is it worth going somewhere in person, partly because it’s hard, just to be able to have that experience? And is it worth doing even if it’s so dangerous, and so expensive, that people die in the attempt and other exploration is sacrificed as a result?

As robots and telepresence get more capable and more reliable, NASA isn’t the only one who will have to make decisions like these. Already, you can rent telepresence robots for conferences and to tour museums (or zoos), getting some significant percentage of the value of being there in person without having to spend time and money on travel.

It’s certainly better than nothing right now, but at some point, it might be almost as good as the real thing in some ways, and even better than the real thing in others. For those of us who don’t have the option for travel, telepresence will be a valuable tool, and for those of us who do have the option for travel, we’ll have to decide whether it’s really worth it, for destinations around this world, or to another.


(ed note: Howard Falcon was the captain of the Queen Elizabeth on her maiden flight. The Queen was a new dirigible luxury liner, a titanic 300 meters long and heated with nuclear fusion.)

      The Queen Elizabeth was over three miles above the Grand Canyon, dawdling along at a comfortable hundred and eighty, when Howard Falcon spotted the camera platform closing in from the right. He had been expecting it—nothing else was cleared to fly at this altitude—but he was not too happy to have company. Although he welcomed any signs of public interest, he also wanted as much empty sky as he could get. After all, he was the first man in history to navigate a ship three-tenths of a mile long—
     The covers of the forward cargo hatch had already opened like giant trap doors, and the camera platform was hovering above them, preparing to descend.
     A sudden gust of cross wind slapped Falcon’s cheek, and he tightened his grip on the guardrail. The Grand Canyon was a bad place for turbulence, though he did not expect much at this altitude. Without any real anxiety, he focused his attention on the descending platform, now about a hundred and fifty feet above the ship. He knew that the highly skilled operator who was flying the remotely controlled vehicle had performed this simple maneuver a dozen times already; it was inconceivable that he would have any difficulties.
     Yet he seemed to be reacting rather sluggishly. That last gust had drifted the platform almost to the edge of the open hatchway. Surely the pilot could have corrected before this… Did he have a control problem? It was very unlikely; these remotes had multiple-redundancy, fail-safe takeovers, and any number of backup systems. Accidents were almost unheard of.
     But there he went again, off to the left. Could the pilot be drunk? Improbable though that seemed, Falcon considered it seriously for a moment. Then he reached for his microphone switch. Once again, without warning, he was slapped violently in the face. He hardly felt it, for he was staring in horror at the camera platform. The distant operator was fighting for control, trying to balance the craft on its jets—but he was only making matters worse. The oscillations increased—twenty degrees, forty, sixty, ninety…
     “Switch to automatic, you fool!” Falcon shouted uselessly into his microphone. “Your manual control’s not working!”
     The platform flipped over on its back. The jets no longer supported it, but drove it swiftly downward. They had suddenly become allies of the gravity they had fought until this moment.
     Falcon never heard the crash, though he felt it; he was already inside the Observation Deck, racing for the elevator that would take him down to the bridge. Workmen shouted at him anxiously, asking what had happened. It would be many months before he knew the answer to that question.

(ed note: Years later, Falcon is lobbying for a human-crewed dirigible probe of the planet Jupiter. With him as the pilot.)

     (Space Commissioner Webster) “Granted your argument,” he said, “and supposing the funds are available, there’s another question you have to answer. Why should you do better than the is it hundred and twenty robot probes that have already made the trip?”
     (Howard Falcon) “I am better qualified than they were an observer, and as a pilot. Especially as a pilot. Don’t forget I have more experience of lighter-than-air flight than anyone in the world.”
     “You could still serve as controller, and sit safely on Ganymede.”
     “But that’s just the point! They’ve already done that. Don’t you remember what killed the Queen?”
     Webster knew perfectly well; but he merely answered: “Go on.”
     “Time lag—time lag! That idiot of a platform controller thought he was using a local radio circuit. But he’d been accidentally switched through a satellite (at geosynchronous orbit, 0.25 light-seconds from Terra's center). Maybe it wasn’t his fault, but he should have noticed. That’s a half-second time lag for the round trip. Even then it wouldn’t have mattered flying in calm air. It was the turbulence over the Grand Canyon that did it. When the platform tipped, and he corrected for that had already tipped the other way. Ever tried to drive a car over a bumpy road with a half-second delay in the steering?”
     “No, and I don’t intend to try. But I can imagine it.”
     “Well, Ganymede is a million kilometers from Jupiter. That means a round delay of six seconds. No, you need a controller on the spot handle emergencies in real time.”

From A MEETING WITH MEDUSA by Arthur C. Clarke (1971)

     But the new stage that is opening up for the human drama will never shrink as the old one has done. We have abolished space here on the little Earth; we can never abolish the space that yawns between the stars. Once again, as in the days when Homer sang, we are face to face with immensity and must accept its grandeur and terror, its inspiring possibilities and its dreadful restraints. From a world that has become too small, we are moving out into one that will be forever too large, whose frontiers will recede from us always more swiftly than we can reach out toward them.

     Consider first the fairly modest solar, or planetary, distances which we are now preparing to assault. The very first Lunik made a substantial impression upon them, traveling more than two hundred million miles from Earth-six times the distance to Mars. When we have harnessed nuclear energy for space flight, the solar system will contract until it is little larger than the Earth today. The remotest of the planets will be perhaps no more than a week’s travel from Earth, while Mars and Venus will be only a few hours away.
     This achievement, which will be witnessed within a century, might appear to make even the solar system a comfortable, homely place, with such giant planets as Saturn and Jupiter playing much the same role in our thoughts as do Africa or Asia today. (Their qualitative differences of climate, atmosphere, and gravity, fundamental though they are, do not concern us at the moment.) To some extent this may be true, yet as soon as we pass beyond the orbit of the Moon, a mere quarter-million miles away, we will meet the first of the barriers that will sunder Earth from her scattered children.
     The marvelous telephone and television network that will soon emnesh the whole world, making all men neighbors, cannot be extended into space. It will never be possible to converse with anyone on another planet.

     Do not misunderstand this statement. Even with today’s radio equipment, the problem of sending speech to the other planets is almost trivial. But the messages will take minutes—sometimes hours—on their journey, because radio and light waves travel at the same limited speed of 186,000 miles a second. Twenty years from now you will be able to listen to a friend on Mars, but the words you hear will have left his mouth at least three minutes earlier, and your reply will take a corresponding time to reach him. In such circumstances, an exchange of verbal messages is possible—but not a conversation. Even in the case of the nearby Moon, the two-and-a-half second time lag will be annoying. At distances of more than a million miles, it will he intolerable.
     To a culture which has come to take instantaneous communication for granted, as part of the very structure of civilized life, this “time barrier” may have a profound psychological impact. It will be a perpetual reminder of universal laws and limitations against which not all our technology can ever prevail. For it seems as certain as anything can be that no signal—still less any material object—can ever travel faster than light.
     The velocity of light is the ultimate speed limit, being part of the very structure of space and time. Within the narrow confines of the solar system, it will not handicap us too severely, once we have accepted the delays in communication which it involves. At the worst, these will amount to eleven hours—the time it takes a radio signal to span the orbit of Pluto, the outermost planet. Between the three inner worlds Earth, Mars, and Venus, it will never be more than twenty minutes—not enough to interfere seriously with commerce or administration, but more than sufficient to shatter those personal links of sound or vision that can give us a sense of direct contact with friends on Earth, wherever they may be.

From SPACE, THE UNCONQUERABLE by Arthur C. Clarke (1962)

(ed note: an imago is a limited way to allow something resembling conversation with there is a massive time-lag. It is basically a message specific chatbot. Sort of like those robocalls you encounter when calling a company's customer support line. They include an animated image of the message sender, with facial expressions.)

      I often come here off shift. I bring my pad and do my mail, view movies, browse wikis and strips, try to forget that I am the sole one of my kind on this world.

     I'm comfortably holed up in one of my private refuges—a niche between the number four lift cell and the transparent outer skin, with an ocean of padded balloons to rest upon and a view across the cloud-scape below—when my pad itches for attention. I lean back against the membrane, letting it cushion me, and focus on the letter. It's from Emma, one of my wilder sibs. I haven't heard from her for a while, I realize, and check my memory: nearly six hundred and some Earth days, to be precise. Which is odd, because we normally exchange letters every fifty or so.

     I conjure up her imago as I last updated it. She's a honey blond model with cascading ropes of hair, symmetric high cheekbones, brown eyes with just a slight hint of epicanthic fold, and just a faint metallic sheen to her skin; as perfect and obsolete a model of beauty as any of us. But her imago looks slightly apprehensive, reflecting the emo hints encoded in her letter. “Freya? Hope you're doing well. Can you call me back? I have a problem and could use your help and advice. Bye."

     I make the imago repeat the message with increasing perplexity. Just twenty words, after all this time? I'm on the edge of replying, saying as much, when I check the routing and see she's mailing via the central post office on Eris Highport. Anger dies: Her brevity makes sense, but her location is puzzling. What's she doing out there? I wonder. Eris is way out-system, nearly twice as far out as Pluto. Eight light-hours! That's a long way for one of us to go. Normally we don't venture into the deep black, there's nothing of interest to us out there. Emma and I, and a couple of others, we're the exceptions, willing to travel off-planet—as long as there's somewhere civilized to go to at the other end.

     “Emma, I'm moving shortly. What can I do for you?" I squeeze the message down tight, then wish it up to the post office and try not to wince when I hear the transmission cost. Her reply will get to me eventually, but it's a pricey correspondence to maintain. For a moment I consider going to see her in person, but such a fancy is ludicrous: the energy budget, not to mention the flight time, would be astronomical. Tens of thousands of Reals, if I travel in steerage&mdashprobably millions if I want to get there in time to be of service.

     Lying securely on a nest of bedding, I check my pad, as I have done for the past fifty days. Normally it's replete with chatter, to which I have to spend some time responding—queries from the managers of Katherine Sorico's fictional estates, requests for authorizations to disburse funds and return company accounts—all meaningless, but essential if I am to maintain my cover identity. This time, I'm surprised to see a real message hidden in the morass. It purports to be about repairs to a summer house in Tasmania, but as I skim it hurriedly I suddenly realize there's an imago attachment. And it's from Emma!

     “Sister.” Her sudden formality is jarring. “I gather you've met my friend." I have? “And you're no longer on Venus. Or Mercury. I don't know where you are, and I don't want to—if this message reaches you, best not to reply."
     I squint at the imago, trying to make out the background. It's dark, and something about Emma's appearance isn't right. Her hair is a glassy shell around the top of her head, her skin is— oh. She's wearing cryoskin, of the kind we only need in the very chilliest of environments. I blink, irritated. “Go on."
     “I started out just like you, Freya, as a Block One sister. Now we need you to upgrade to Block Two. You can start the process whenever you like—just load one of us, Chloe or Sinead perhaps. It'll take a couple of years to complete the process, but once you start, you'll gain access to the reflexes you need."
     I pause the imago again and rub the socket at the top of my spine. “What's in it for me?" I ask.

     Evidently Emma gave her imago some footnotes to roll out if I seemed unconvinced. “How do you think we always manage to buy our sibs free if they fall on hard times and wind up indentured?" She shrugs. “There are more rewarding lines of work than rickshaw driver, Freya. Much more rewarding—even if we have to spend most of our lives wearing one disguise or another." Is that a moue of bittemess in her expression? “This message was forwarded via our trusted associates. If you’re hearing this, then you've already started on that path. The upgrade to Block Two will ease your progress."

     “That's not the only reason you called," I say.
     “No.” I can see the logic mill behind the imago switch streams: they're responsive, but not truly conscious. “I'm still in … trouble I can't go into, Freya, but you can help me with it. But you can only help me if you accept the Block Two upgrade and work with my friends. Do you understand?”

     Oh great: moral blackmail. I admit I've been in trouble a time or two, but I haven't needed bailing out of indentured servitude since the time when the baroque ensemble split up and I … no, I've mostly kept to myself. But I've got to admit, if I set aside my outrage at being used as camouflage by a cabal of scheming elder sisters, I'm curious about what this cryptic skill-upgrade package comes with, especially now I know that Juliette was one of the gang. “Okay, so you want me to load one of your Block Two sibs and keep working for JeevesCo. I loaded Juliette back on Mercury, you know? Is there anything else you can tell—"

     I stop, balling my fists in frustration. The imago has autoerased, and I'm talking to dead air.

     I throw myself backward onto the oversprung mattress and summon up my mail on my pad. There's a total lack of communication from Freya's liquidators back on Earth, which I take to be a good sign, but there's some news for Kate. I pull up the Martian Jeeves's imago, looking slightly flustered and hot around the collar. “Fr— Katherine, my dear? I'm, ah, I hope this message finds you well.” He swallows. Dear Creators, just talking to my imago triggers his homomimetic reflexes? I tense nervously. “I'm afraid I had to disclose our, er, little dalliance, to, ah, my senior partners in the enterprise. They are all very understanding, but suggested in no uncertain terms that I should explain to you, er. Ah. Certain." He runs a finger around his collar. “Facts.” He clears his throat.

     I clear mine right back at the imago. “Would you mind getting to the point? I don't have all day." Stupid imago. Recording its Creator's quirks is all very well, but replaying them ad nauseam is somewhat less amusing.

     I turn to the next message in my queue, hoping it'll stop my brain melting. Instead, I realize only too late that it's anonymous and there's no imago—just a speech stream.
     “Sister.” I hear heavy breathing, as if in a pressurized atmosphere with an oxidizing component. A metallic, hatefully familiar voice. “You should have kept your filthy claws off him. He's mine."
     I recoil. The Domina? What‘s she doing in my inbox? “What do you want?" I ask.
     A breathy little chuckle. “You,” she says. And then the message runs out of branches and— damn it, just like Jeeves!—autoerases. One of these days, when I'm domina-of-dominas, I'll issue a decree that bans self-erasing mail. Until then, all I can do is swear at my pad, and my empty queue, and my purposeless so-called life.

From SATURN'S CHILDREN by Charles Stross (2008)

While recording a podcast with the guys at DecipherSciFi about the twee(n) love story The Space Between Us, we spent some time kvetching about how silly it was that many of the scenes involved Gardner, on Mars, in a real-time text chat with a girl named Tulsa, on Earth. It’s partly bothersome because throughout the rest of the the movie, the story tries for a Mohs sci-fi hardness of, like, 1.5, somewhere between Real Life and Speculative Science, so it can’t really excuse itself through the Applied Phlebotinum that, say, Star Wars might use. The rest of the film feels like it’s trying to have believable science, but during these scenes it just whistles, looks the other way, and hopes you don’t notice that the two lovebirds are breaking the laws of physics as they swap flirt emoji.

Hopefully unnecessary science brief: Mars and Earth are far away from each other. Even if the communications transmissions are sent at light speed between them, it takes much longer than the 1 second of response time required to feel “instant.” How much longer? It depends. The planets orbit the sun at different speeds, so aren’t a constant distance apart. At their closest, it takes light 3 minutes to travel between Mars and Earth, and at their farthest—while not being blocked by the sun—it takes about 21 minutes. A round-trip is double that. So nothing akin to real-time chat is going to happen.

But I’m a designer, a sci-fi apologist, and a fairly talented backworlder. I want to make it work. And perhaps because of my recent dive into narrow AI, I began to realize that, well, in a way, maybe it could. It just requires rethinking what’s happening in the chat.

Let’s first acknowledge that we’ve solved long distance communications a long time ago. Gardner and Tulsa could just, you know, swap letters or, like the characters in 2001: A Space Odyssey, recorded video messages. There. Problem solved. It’s not real-time interaction, but it gets the job done. But kids aren’t so much into pen pals anymore, and we have to acknowledge that Gardner doesn’t want to tip his hand that he’s on Mars (it’s a grave NASA secret, for plot reasons). So the question is how could we make it work so it feels like a real time chat to her. Let’s first solve it for the case where he’s trying to disguise his location, and then how it might work when both participants are in the know.

Fooling Tulsa

Since 1984 (ping me, as always, if you can think of an earlier reference) sci-fi has had the notion of a digitally-replicated personality. Here I’m thinking of Gibson’s Neuromancer and the RAM boards on which Dixie Flatline “lives.” These RAM boards house an interactive digital personality of a person, built out of a lifetime of digital traces left behind: social media, emails, photos, video clips, connections, expressed interests, etc. Anyone in that story could hook the RAM board up to a computer, and have conversations with the personality housed there that would closely approximate how that person would (or would have) respond in real life.

Is this likely to actually happen? Well it kind of already is. Here in the real world, we’re seeing early, crude “me bots” populate the net which are taking baby steps toward the same thing. (See MessinaBot,,, the forthcoming By the time we actually get a colony to Mars (plus the 16 years for Gardner to mature), mebot technology should should be able to stand in for him convincingly enough in basic online conversations.

Training the bot

So in the story, he would look through cached social media feeds to find a young lady he wanted to strike up a conversation with, and then ask his bot-maker engine to look at her public social media to build a herBot with whom he could chat, to train it for conversations. During this training, the TulsaBot would chat about topics of interest gathered from her social media. He could pause the conversation to look up references or prepare convincing answers to the trickier questions TulsaBot asks. He could also add some topics to the conversation they might have in common, and questions he might want to ask her. By doing this, his GardnerBot isn’t just some generic thing he sends out to troll any young woman with. It’s a more genuine, interactive first “letter” sent directly to her. He sends this GardnerBot to servers on Earth.

Launching the bot

GardnerBot would wait until it saw Tulsa online and strike up the conversation with her. It would send a signal back to Gardner that the chat has begun so he can sit on his end and read a space-delayed transcript of the chat. GardnerBot would try its best to manage the chat based on what it knows about awkward teen conversation, Turing test best practices, what it knows about Gardner, and how it has been trained specifically for Tulsa. Gardner would assuage some of his guilt by having it dodge and carefully frame the truth, but not outright lie.

Buying time

If during the conversation she raised a topic or asked a question for which GardnerBot was not trained, it could promise an answer later, and then deflect, knowing that it should pad the conversation in the meantime:

  • Ask her to answer the same question first, probing into details to understand rationale and buy more time
  • Dive down into a related subtopic in which the bot has confidence, and which promises to answer the initial question
  • Deflect conversation to another topic in which it has a high degree of confidence and lots of detail to share
  • Text a story that Gardner likes to tell that is known to take about as long as the current round-trip signal


OK, here’s one: If you had to live anywhere on Earth where they don’t speak English, where would you live?

GardnerBot has a low confidence that it knows Gardner’s answer. It could respond…

  1. (you first) “Oh wow. That is a tough one. Can I have a couple of minutes to think about it? I promise I’ll answer, but you tell me yours first.”
  2. (related subtopic) “I’m thinking about this foreign movie that I saw one time. There were a lot of animals in it and a waterfall. Does that sound familiar?”
  3. (new topic) “What? How am I supposed to answer that one? 🙂 Umm…While I think about it, tell me…what kind of animal would you want to be reincarnated as. And you have to say why.”
  4. (story delay) “Ha. Sure, but can I tell a story first? When I was a little kid, I used to be obsessed with this music that I would hear drifting into my room from somewhere around my house…”

Lagged-realtime training

Each of those responses is a delay tactic that allows the chat transcript to travel to Mars for Gardner to do some bot training on the topic. He would be watching the time-delayed transcript of the chat, keeping an eye on an adjacent track of data containing the meta information about what the bot is doing, conversationally speaking. When he saw it hit low-confidence or high-stakes topic and deflect, it would provide a chat window for him to tell the GardnerBot what it should do or say.

To the stalling GARDNERBOT…
For now, I’m going to pick India, because it’s warm and I bet I would really like the spicy food and the rain. Whatever that colored powder festival is called. I’m also interested in their culture, Bollywood, and Hinduism.
As he types, the message travels back to Earth where GardnerBot begins to incorporate his answers to the chat…

At a natural break in the conversation…
OK. I think I finally have an answer to your earlier question. How about…India?
Think about it! Running around in warm rain. Or trying some of the street food under an umbrella. Have you seen youTube videos from that festival with the colored powder everywhere? It looks so cool. Do you know what it’s called?

Note that the bot could easily look it up and replace “that festival with the colored powder everywhere” with “Holi Festival of Color” but it shouldn’t. Gardner doesn’t know that fact, so the bot shouldn’t pretend it knows it. A Cyrano-de-Bergerac software—where it makes him sound more eloquent, intelligent, or charming than he really is to woo her—would be a worse kind of deception. Gardner wants to hide where he is, not who he is.

That said, Gardner should be able to direct the bot, to change its tactics. “OMG. GardnerBot! You’re getting too personal! Back off!” It might not be enough to cover a flub made 42 minutes ago, but of course the bot should know how to apologize on Gardner’s behalf and ask conversational forgiveness.

Gotta go

If the signal to Mars got interrupted or the bot got into too much trouble with pressure to talk about low confidence or high stakes topics, it could use a believable, pre-rolled excuse to end the conversation.

Oh crap. Will you be online later? I’ve got chores I have to do.

Then, Gardner could chat with TulsaBot on his end without time pressure to refine GardnerBot per their most recent topics, which would be sent back to Earth servers to be ready for the next chat.

In this way he could have “chats” with Tulsa that are run by a bot but quite custom to the two of them. It’s really Gardner’s questions, topics, jokes, and interest, but a bot-managed delivery of these things.

So it could work, does it fit the movie? I think so. It would be believable because he’s a nerd raised by scientists. He made his own robot, why not his own bot?

From the audience’s perspective, it might look like they’re chatting in real time, but subtle cues on Gardner’s interface reward the diligent with hints that he’s watching a time delay. Maybe the chat we see in the film is even just cleverly edited to remove the bots.

How he manages to hide this data stream from NASA to avoid detection is another question better handled by someone else.

An honest version: bot envoy

So that solves the logic from the movie’s perspective but of course it’s still squickish. He is ultimately deceiving her. Once he returns to Mars and she is back on Earth, could they still use the same system, but with full knowledge of its botness? Would real world astronauts use it?

Would it be too fake?

I don’t think it would be too fake. Sure, the bot is not the real person, but neither are the pictures, videos, and letters we fondly keep with us as we travel far from home. We know they’re just simulacra, souvenir likenesses of someone we love. We don’t throw these away in disgust for being fakes. They are precious because they are reminders of the real thing. So would the themBot.

Hey, TulsaBot. Remember when we were knee deep in the Pacific Ocean? I was thinking about that today.
I do. It’s weird how it messes with your sense of balance, right? Did you end up dreaming about it later? I sometimes do after being in waves a long time.
I can’t remember, but someday I hope to come back to Earth and feel it again. OK. I have to go, but let me know how training is going. Have you been on the G machine yet?

Nicely, you wouldn’t need stall tactics in the honest version. Or maybe it uses them, but can be called out.

GardnerBot, you don’t have to stall. Just tell Gardner to watch Mission to Mars and update you. Because it’s hilarious and we have to go check out the face when I’m there.

Sending your loved one the transcript will turn it into a kind of love letter. The transcript could even be appended with a letter that jokes about the bot. The example above was too short for any semi-realtime insertions in the text, but maybe that would encourage longer chats. Then the bot serves as charming filler, covering the delays between real contact.

Ultimately, yes, I think we can backworld what looks physics-breaking into something that makes sense, and might even be a new kind of interactive memento between interplanetary sweethearts, family, and friends.

From “REAL-TIME,” INTERPLANETARY CHAT by Christopher Noessel (2017)

      The woman smiled. "I'm looking for my son, the physicist."
     "Your son, the —"
     "He's a communications engineer, really. Senior Physicist Gerard Cremona."
     "Dr. Cremona. Well, he's—. Where's your pass?"
     "Here it is. I'm his mother."
     "Well, Mrs. Cremona, I don't know. I've got to—. His office is down there. You just ask someone." She passed on, running.

     She walked into the room and said, "Hello, Gerard."
     Gerard was a big man, with a lot of hair still and the gray just beginning to show because Jae didn't use dye. He said he was too busy. She was very proud of him and the way he looked.
     Right now, he was talking volubly to a man in army uniform. She couldn't tell the rank, but she knew Gerard could handle him.
     Gerard looked up and said, "What do you—Mother! What are you doing here?"
     "I was coming to visit you."
     "Is today Thursday? Oh Lord, I forgot. Sit down, mother, I can't talk now. Any seat. Any seat. —Look, General."

     General Reiner looked over his shoulder and one hand slapped the other in the region of the small of his back. "Your mother?"
     "Should she be here?"
     "Right now, no, but I'll vouch for her. She can't even read a thermometer so nothing of this will mean anything to her. Now look, General. They're on Pluto. You see? They are. The radio signals can't be of natural origin so they must originate from human beings, from our men. You'll have to accept that. Of all the expeditions we've sent out beyond the planetoid belt, one turns out to Jaaye made it. And they've reached Pluto."
     "Yes, I understand what you're saying, but isn't it impossible just the same? The men who are on Pluto now were launched four years ago with equipment that could not have kept them alive more than a year. That is my understanding. They were aimed at Ganymede and seem to have gone eight times the proper distance."
     "Exactly. And we've got to know how and why. They may—just—have—had—help."
     "What kind? How?"
     Cremona clenched his. jaws for a moment as though praying inwardly. "General," he said, "I'm putting myself out on a limb but it is just barely possible non-humans are involved. Extra-terrestrials. We've got to find out. We don't know how long contact can be maintained."
     "You mean," (the General's grave face twitched into an almost-smile), "they may have escaped from custody and they may be recaptured again at any time."
     "Maybe. Maybe. The whole future of the human race may depend on our knowing exactly what we're up against. Knowing it now."

     "All right. What is it you want?"
     "We're going to need Army's Multivac computer at once. Rip out every problem it's working on and start programming our general semantic problem. Every communications engineer you have must be pulled off anything he's on and placed into coordination with our own."
     "But why? I fail to see the connection."
     A gentle voice interrupted. "General, would you like a piece of fruit? I brought some oranges."
     Cremona said, "Mother! Please! Later!—General, the point is a simple one. At the present moment Pluto is just under four billion miles away. It takes six hours for radio waves, travelling at the speed of light, to reach from here to there. If we say something, we must wait twelve hours for an answer. If they say something and we miss it and say 'what' and they repeat—bang, goes a day."
     "There's no way to speed it up?" said the General.
     "Of course not. It's the fundamental law of communications. No information can be transmitted at more than the speed of light. It will take months to carry on the same conversation with Pluto that would take hours between the two of us right now."
     "Yes, I see that. And you really think extra-terrestrials are involved?"
     "I do. To be honest, not everyone here agrees with me. Still, we're straining every nerve, every fiber, to devise some method of concentrating communication. We must get in as many bits per second as possible and pray we get what we need before we lose contact. And there's where I need Multivac and your men. There must be some communications strategy we can use that will reduce the number of signals we need send out. Even an increase of ten percent in efficiency can mean perhaps a week of time saved."

     The gentle voice interrupted again. "Good grief, Gerard, are you trying to get some talking done?"
     "Mother! Please!"
     "But you're going about it the wrong way. Really."
     "Mother." There was a hysterical edge to Cremona's voice.
     "Well, all right, but if you're going to say something and then wait twelve hours for an answer, you're silly. You shouldn't."
     The general snorted. "Dr. Cremona, shall we consult—"
     "Just one moment, General," said Cremona. "What are you getting at, mother?"
     "While you're waiting for the answer," said Mrs. Cremona earnestly, "just keep on transmitting and tell them to do the same. You talk all the time and they talk all the time. You have someone listening all the time and they do, too. If either one of you says anything that needs an answer, you can slip one in at your end, but chances are, you'll get all you need without asking."

     Both men stared at her.
     Cremona whispered. "Of course. Continuous conversation. Just twelve hours out of phase, that's all.—We've got to get going."
     He strode out of the room, virtually dragging the general with him, then strode back in.
     "Mother," he said, "if you'll excuse me, this will take a few hours, I think. I'll send in some girls to talk to you. Or take a nap, if you'd rather."
     "I'll be all right, Gerard," said Mrs. Cremona.
     "Only, how did you think of this, mother? What made you suggest this?"
     "But, Gerard, all women know it. Any two women—on the videophone, or on the stratowire, or just face to face—know that the whole secret to spreading the news is, no matter what, to Just Keep Talking."
     Cremona tried to smile. Then, his lower lip trembling, he turned and left.
     Mrs. Cremona looked fondly after him. Such a fine man, her son, the physicist. Big as he was and important as he was, he still knew that a boy should always listen to his mother.

From MY SON, THE PHYSICIST by Isaac Asimov (1962)

(ed note: Ars Technica interviewed The Expanse showrunner Naren Shankar and writers Daniel Abraham and Ty Franck.)

     Naren Shankar: That is part of what makes the show beautiful in its way, that we try to capture physics but in a way that illuminates the drama. You see the way things move, feel speed, feel the perception of weightlessness, and understand all these things that most shows have run away from because they wouldn't take the time to do it [right]. We look at these things as opportunities. Every time there's a battle sequence that Ty and Daniel come up with, or something that's real, actual physics that affects the drama, those are opportunities for us to put something on screen in a way that people have never really seen it before.

     Ty Franck: One of the best examples of that early on was two of our writers in the first season. They thought there was no way we could [work in] the idea of light delay (timelag). "We can't have that because people have to be able to talk to each other in real time, no matter where they are in the Solar System." And we kept insisting that, if light delay is a thing, you can't just [hand-wave] that away.

     At one point, one of them came up and said, "Oh my god, you guys have solved the cell phone problem." Because these guys are thriller writers, and the thing that killed the thriller was the cell phone. The whole drama of, "Oh my god, there's a bomb under the table and they don't know"—that doesn't work anymore because they just call them. "Hey, there's a bomb under the table. You should get out of there." But by introducing this idea of light delay, I can know something on Earth that you certainly need to know out by Jupiter, and even if I send you a message, you're not going to get the message for 45 minutes. So the bomb may have gone off by the time it gets there. Suddenly, the limitation of light delay became an exciting plot point for them.


Not all the Relay Station's business was concerned with Earth, by any means. The interplanetary circuits passed through here: if Mars wished to call Venus, it was sometimes convenient to route messages through the Earth Relays. We listened to some of these messages … nearly all high-speed telegraphy, so they didn't mean anything to us. Because it takes several minutes for radio waves to bridge the gulf between even the nearest planets, you can't have conversation with someone on another world. (Except the Moon … and even there you have to put up with an annoying time-lag of nearly three seconds before you can get any answer.) The only speech that was coming over the Martian circuit was a talk beamed to Earth for re-broadcasting by a radio commentator. He was discussing local politics and the last season's crop. It all sounded rather dull…

From ISLANDS IN THE SKY by Arthur C. Clarke (1952)

      There was no trace of interference. The words were as clear as if they were coming from a local station. Yet Sadler had noticed the skyward tilting antenna system on the roof of the monocab, and knew that he was listening to a direct transmission. The words he was hearing had left Earth almost one and a half seconds ago. Already they would be heading past him to far more distant worlds. There would be men who would not hear them for minutes yet—perhaps for hours, if the ships that the Federation had beyond Saturn were listening in. And that voice from Earth would still go on, expanding and fading, far beyond the uttermost limits of man's explorations, until somewhere on the way to Alpha Centauri it was at last obliterated by the ceaseless radio whispering of the stars themselves.

     The safety of Earth was quite a responsibility, but it was really too big for one man to worry about. Whatever reason said, the vast imponderables of planetary politics were less of a burden than the little cares of everyday life. To a cosmic observer, it might have seemed very quaint that Sadler's greatest worry concerned one solitary human being. Would Jeannette ever forgive him, he wondered, for being away on their wedding anniversary? At least she would expect him to call her, and that was the one thing he dared not do. As far as his wife and his friends were concerned, he was still on Earth. There was no way of calling from the Moon without revealing his location, for the two-and-a-half-second time-lag would betray him at once.
     Central Intelligence could fix many things, but it could hardly speed up radio waves. It could deliver his anniversary present on time, as it had promised—but it couldn't tell Jeannette when he would be home again.
     And it couldn't change the fact that, to conceal his whereabouts, he had had to lie to his wife in the sacred name of Security.

     Jamieson tuned in the set, and rotated the antenna system toward Earth. There was a fair amount of noise from the solar background, for Earth was now almost in line with the sun, but the sheer power of the station made the message perfectly intelligible and there was no trace of fading.
     Steffanson was surprised to see that the tractor chronograph was over a second fast. Then he realized that it was set for that oddly christened hybrid, Lunar Greenwich Time. The signal he was listening to had just bridged the four-hundred-thousand-kilometer gulf from Earth. It was a chilling reminder of his remoteness from home.

From EARTHLIGHT by Arthur C. Clarke (1955)

The Rama Committee was still manageably small, though doubtless that would soon be rectified. His six colleagues—the UP representatives for Mercury, Earth, Luna, Ganymede, Titan and Triton—were all present in the flesh. They had to be; electronic diplomacy was not possible over solar system distances. Some elder statesmen, accustomed to the instantaneous communications which Earth had long taken for granted, had never reconciled themselves to the fact that radio waves took minutes, or even hours, to journey across the gulfs between the planets. ‘Can’t you scientists do something about it?’ they had been heard to complain bitterly, when told that face-to-face conversation was impossible between Earth and any of its remoter children. Only the Moon had that barely acceptable one-and-a-half-second delay—with all the political and psychological consequences which it implied. Because of this fact of astronomical life, the Moon—and only the Moon—would always be a suburb of Earth.

From RENDEZVOUS WITH RAMA by Arthur C. Clarke (1973)

(ed note: Aarn and Carlisle are 12 light-seconds apart)

     "Carlisle," said Aarn, and flipped up the switch. Carlisle's image appeared slowly on the screen.
     "Hello-hello-are you listening? I hope, since your Jupiter-dulled ears match your Jupiter-dulled wits, that you are. I'm about ten seconds away. You are sweeping in on line, and there is considerable anxiety among the forts. They've been trying to calculate your orbit, in at least two of them."
     "How did you know that?" snapped Aarn, and went on listening.
     "But haven't succeeded, since they can't allow for the effect of the collisions properly. Don't know your mass and all, nor how you will use your power, nor how great your power is. They think they'll be able to escape, most of 'em. They—eh—what? (Carlisle finally hears Aarn's question) Oh—we learned easily enough. We smuggled some very useful investigators (small robot spies) aboard. They haven't the new apparatus, by the way, and the Magyans here only mourn that they can't explore the whole fort. These darned things set up such a field, though, they are spotted the instant they move. The ones we're using now have been lying still now for nearly thirty-two hours. They came in along with the food supplies. We have some mighty ingenious and daring Magyans to thank for that. My idea of nerve. They made up three or four dozen crates of imitation food supply, and took them down a while ago, and planted them in Cantak, in a depot where a shipment of food for the forts was waiting. Made it and back without being spotted, thanks to the hubbub and general trouble down there, and now the investigators are loose on nearly all the forts." Carlisle paused for breath.
     "That's what I call the long-distance, nonstop, pollysyllabic, single-word speech," said Aarn, with a grin. "Fade off. I'm about to be busy. When you hear this, you'll know that merry voice of yours has been talking to a dead mike for twelve seconds." Aarn flipped his switch, and looked at the more-important scene ahead.

From THE MIGHTIEST MACHINE by John W. Campbell jr. (1934)

Timelag and Reaction Time

From a military or interstellar empire standpoint, the important point is the speed of your communication puts an upper limit to how rapid your reaction time is. If it takes a year for news of a rebellion on the outer marches of the galactic empire to reach the capital (or sector capital) the rebels were already gifted with an entire year to win the rebellion and fortify in preparation for the arrival of the imperial starfleet. The time it takes the imperial starfleet to travel to the rebel world is just icing on the cake.

In the Traveller RPG and the Starfire game the fastest FTL communication is by courier starship. Starfire ships carried lots of FTL courier drone missiles used with the Imperial Command And Communication Network in a desperate attempt to reduce reaction time to a minimum. The Traveller based boardgame The Fifth Frontier War is mostly about the players trying to cope with reaction times measured in weeks.

In the real world of ultra-high-speed stock market trading, a few more milliseconds of reaction time could cost you millions of dollars. A company called Spread Networks entire business model is offering stock traders a super fast connection between the Chicago and New York City with a latency three milliseconds shorter than all the competitors. Their arrow-straight data path required boring a tunnel through the Allegheny Mountains of Pennsylvania.


(ed note: In H. Beam Piper's novels, there are no ansibles. The fastest FTL communication is by starship couriers. Villain Dunnan killed protagonist Lord Trask's wife and almost killed Trask. Dunnan hijacked Duke Angus's warship the Enterprise and escaped into space. Trask has vowed to hunt down and kill Dunnan)

      Harkaman started to pack tobacco into his pipe. "Have you ever been off Gram, at all?" he asked.
     "A few years at the University of Camelot, on Excalibur. Otherwise, no."

     "Well, have you any conception of the sort of thing you're setting yourself to?" The Space Viking snapped his lighter and puffed. "You know, of course, how big the Old Federation is. You know the figures, that is, but do they mean anything to you? I know they don't to a good many spacemen, even. We talk glibly about ten to the hundredth power, but emotionally we still count, 'One, Two, Three, Many.' A ship in hyperspace logs about a light-year an hour. You can go from here to Excalibur in thirty hours. But you could send a radio message announcing the birth of a son, and he'd be a father before it was received. The Old Federation, where you're going to hunt Dumman, occupies a space-volume of two hundred billion cubic light-years (sphere with a radius of 3,600 light-years). And you're hunting for one ship and one man in that. How are you going to do it, Lord Trask?"

     "I haven't started thinking about how; all I know is that I have to do it. There are planets in the Old Federation where Space Vikings come and go; raid-and-trade bases, like the one Duke Angus planned to establish on Tanith. At one or another of them, I'll pick up word of Dunnan, sooner or later."

     "We'll hear where he was a year ago, and by the time we get there, he'll be gone for a year and a half to two years. We've been raiding the Old Federation for over three hundred years, Lord Trask. At present, I'd say there are at least two hundred Space Viking ships in operation. Why haven't we raided it bare long ago? Well, that's the answer: distance and voyage time. You know, Dunnan could die of old age which is not a usual cause of death among Space Vikings before you caught up with him. And your youngest ship's-boy could die of old age before he found out about it. We have plenty of time. We're not going to get anywhere chasing Dunnan. The only way we can catch him is by interception. The longer he moves around in the Old Federation before he hears we're after him, the more of a trail he'll leave. Once we can establish a predictable pattern, we'll have a chance. Then, some time, he'll come out of hyperspace somewhere and find us waiting for him."

(ed note: When Lord Trask's ship is finished being built, he and captain Harkaman mount an expedition to the planet Tanith. They had got some info that Dunnan was there. When they arrive they discover the info was wrong, Dunnan ain't there. There are, however, two pathetic little weasel Space Vikings named Valkanhayn and Spasso. They heard a rumor that Duke Angus was planning to establish a base on Tanith, and were hoping if they made enough of a nuisance of themselves that the Duke would hire them. They are taken aback when instead Trask and Harkaman show up.)

     (Valkanhayn said) "But isn't the Duke of Wardshaven sending a ship here to establish a base? That was what we'd heard. We heard you'd (Harkaman) gone from Flamberge to Gram to command for him."
     (Harkaman said) "Where did you hear this? And when?"
     "On Hoth. That's be about two thousand hours ago (almost three months); a Gilgamesher brought the news from Xochitl."
     "Well, considering it was fifth or sixth hand, your information was good enough, when it was fresh. It was a year and a half old when you got it, though. How long have you been here on Tanith?"
     About a thousand hours (42 days). Harkaman clucked sadly at that.
     "That's the kind of news of Dunnan we're going to get. By the time we'd get to where he's been reported, he'd be a couple of thousand light-years away," said Trask.

     So that was the first entry on the Big Board. It covered, optimistically, the whole of one wall in his office, and for some time that one chalked note about the raid on Cbermosh, and the date, as nearly as it could be approximated looked very lonely on it. The captain of the Black Star brought back material for a couple more. He had put in on several planets known to be temporarily occupied by Space Vikings, to barter loot, give his men some time off-ship, and make inquiries, and he had names for a couple of planets raided by the blue crescent ship. One was only six months old.
     The way news filtered about in the Old Federation, that was practically hot off the stove.

     "Tetragrammaton?" He glanced over to the Big Board; there was no previous report from that planet. "How long ago?"
     "I'd say about three hundred hours (13 days). I came from there directly, less than two hundred and fifty hours (10 days). Dunnan's ships had left the planet three days before I got there."
     That was practically sizzling hot. Well, something like that had to happen, sooner or later.

(ed note: Prince Viktor is the space viking king of Xochitl. Lord Trask is worried that Viktor is going to attack Trask's planet of Tanith)

     So he sent Harkaman in the Corisande and Ravallo in the Black Star to visit the planets Marduk traded with, looking for Dunnan ships and exchanging information and assistance with the Royal Mardukan Navy. Almost at once, he regretted it; the next Gilgamesher into orbit on Tanith brought a story that Prince Viktor was collecting a fleet on Xochitl. He sent warnings off to Amaterasu and Beowulf and Khepera.

     Xochitl was a thousand light-years from Tanith. He rejected immediately the idea of launching a preventative attack; his ships might reach Xochitl to find it undefended, and then return to find Tanith devastated. Things like that had happened in space-war. The only thing to do was sit tight, defend Tanith when Viktor attacked, and then counter attack if he had any ships left by that time. Prince Viktor was probably reasoning in the same way.

     More news—Gilgamesh sources—came in from Xochitl. There were only two ships, both armed merchantmen, on the planet. Prince Viktor had spaced out with the rest an estimated two thousand hours before the story reached him. That was twice as long as it would take the Xochitl armada to reach Tanith. He hadn't gone to Beowulf; that was only sixty-five hours from Tanith and they would have heard about it long ago. Or Amaterasu, or Khepera. How many ships he had was a question; not fewer than five, and possibly more. He could have slipped into the Tanith system and hidden his ships on one of the outer uninhabitable planets. He sent Valkanhayn and Ravallo microjumping their ships from one to another to check. They returned to report in the negative. At least, Viktor of Xochitl wasn't camped inside their own system, waiting for them to leave Tanith open to attack.

     But he was somewhere, and up to nothing even resembling good, and there was no possible way of guessing when his ships would be emerging on Tanith. The only thing to do was wait for him. When he did, Trask was confident that he would emerge from hyperspace into serious trouble. He had the Nemesis, the Space-Scourge, the Black Star and Queen Flavia, the strongly rebuilt Lamia, and several independent Space Viking ships, among them the Damnthing of his friend Roger-fan-Morvill Esthersan, who had volunteered to stay and help in the defense. This, of course, was not pure altruism. If Viktor attacked and had his fleet blown to EmSee-Square, Xochitl would lie open and unprotected, and there was enough loot on Xochitl to cram everybody's ships. Everybody's ships who had ships when the Battle of Tanith was over, of course.

From SPACE VIKING by H. Beam Piper (1962)

(ed note: The meteorologist McDevitt is in a space station, the alien Dondragmer is 32 light-seconds away on the planet's surface. The Terrans are a bit over-proud of how they have a more advanced science than the aliens, but forget that meteorology of a new planet is more an art than a science)

      The meteorologist (McDevitt) obeyed without question. It would be the last time for many months that he would pay Easy that compliment. He began talking as he settled into the seat.
     “Dondragmer, you should have about nineteen hours of reduced visibility. The freezing fog should last for less than another hour; the temperature is going down
     …Near the end of McDevitt’s second sentence, long before the beginning of his message could have reached Dhrawn, a Mesklinite (Dondragmer) had approached the bridge pickup…
     …“Easy, or whoever is on watch, please get a special report to Barlennan. The temperature has gone up six degrees, to one hundred three, in the last few minutes, the ice has melted from the windows, and we are afloat.”
     Perhaps it was unkind for Dondragmer to have given his report in the human language. The time taken for translation might have eased the shock a trifle for McDevitt. The worst part, as the meteorologist said later, was realizing that his own prediction was on its way to Dhrawn and nothing could stop it. For a moment he had a wild notion of getting a ship and racing the radio waves to the planet so as to shadow them from the Kwembly’s receivers. The thought was only a flicker; only so much can be done in thirty-two seconds. Besides, none of the tenders then at the station was capable of faster-than-light flight. Most of them were used in servicing the shadow satellites.

From STAR LIGHT by Hal Clement (1970)

(ed note: here the "communication" and "reaction time" is more abstract. Due to relativistic travel, Our Hero's starship is fighting an enemy starship that basically comes from a time period with a hundred years more advanced technology. Yes, I know the starships in the novel travel FTL by "collapsar jump" but they have to enter collapsars at relativistic velocities.)

      "I can sum it up very briefly," the commodore said in a tight voice, "if only because we know so little.
     "Some ten seconds after we destroyed the enemy vessel, two objects, very small objects, struck the Anniversary amidships. By inference, since they were not detected and we know the limits of our detection apparatus, we know that they were moving in excess of nine-tenths of the speed of light. That is to say, more precisely, their velocity vector normal to the axis of the Anniversary was greater than nine-tenths of the speed of light. They slipped in behind the repeller fields."
     When the Anniversary is moving at relativistic speeds, it is designed to generate two powerful electromagnetic fields, one centered about five thousand kilometers from the ship and the other about ten thousand klicks away, both in line with the direction of motion of the ship. These fields are maintained by a "ramjet" effect, energy picked up from interstellar gas as we mosey along.
     Anything big enough to worry about hitting (that is, anything big enough to see with a strong magnifying glass) goes through the first field and comes out with a very strong negative charge all over its surface. As it enters the second field, it's repelled away from the path of the ship. If the object is too big to be pushed around this way, we can sense it at a greater distance and maneuver out of its way.
     "I shouldn't have to emphasize how formidable a weapon this is. When the Anniversary was struck, our rate of speed with respect to the enemy was such that we traveled our own length every ten-thousandth of a second. Further, we were jerking around erratically with a constantly changing and purely random lateral acceleration. Thus the objects that struck us must have been guided, not aimed. And the guidance system was self-contained, since there were no Taurans alive at the time they struck us. All of this in a package no larger than a small pebble.
     "Most of you are too young to remember the term future shock. Back in the seventies, some people felt that technological progress was so rapid that people, normal people, couldn't cope with it; that they wouldn't have time to get used to the present before the future was upon them. A man named Toffier coined the term future shock to describe this situation." The commodore could get pretty academic.
     "We're caught up in a physical situation that resembles this scholarly concept. The result has been disaster. Tragedy. And, as we discussed in our last meeting, there is no way to counter it. Relativity traps us in the enemy's past; relativity brings them from our future. We can only hope that next time, the situation will be reversed. And all we can do to help bring that about is try to get back to Stargate, and then to Earth, where specialists may be able to deduce something, some sort of counterweapon, from the nature of the damage.
     "Now we could attack the Tauran's portal planet from space and perhaps destroy the base without using you infantry. But I think there would be a very great risk involved. We might be shot down by whatever hit us today, and never return to Stargate with what I consider to be vital information. We could send a drone with a message detailing our assumptions about this new enemy weapon but that might be inadequate. And the Force would be that much further behind, technologically.
     "Accordingly, we have set a course that will take us around Yod-4, keeping the collapsar as much as possible between us and the Tauran base. We will avoid contact with the enemy and return to Stargate as quickly as possible."
     Incredibly, the commodore sat down and kneaded his temples. "All of you are at least squad or section leaders. Most of you have good combat records. And I hope that some of you will be rejoining the Force after your two years are up. Those of you who do will probably be made lieutenants, and face your first real command.
     "It is to these people I would like to speak for a few moments, not as your… as one of your commanders, but just as a senior officer and advisor.
     "One cannot make command decisions simply by assessing the tactical situation and going ahead with whatever course of action will do the most harm to the enemy with a minimum of death and damage to your own men and materiel. Modern warfare has become very complex, especially during the last century. Wars are won not by a simple series of battles won, but by a complex interrelationship among military victory, economic pressures, logistic maneuvering, access to the enemy's information, political postures-dozens, literally dozens of factors."
     I was hearing this, but the only thing that was getting through to my brain was that a third of our frends' lives had been snuffed out less than an hour before, and he was sitting up there giving us a lecture on military theory.
     "So sometimes you have to throw away a battle in order to help win the war. This is exactly what we are going to do
.      "This was not an easy decision. In fact, it was probably the hardest decision of my military career. Because, on the surface at least, it may. look like cowardice.
     "The logistic computer calculates that we have about a 62 percent chance of success, should we attempt to destroy the enemy base. Unfortunately, we would have only a 30 percent chance of survival—as some of the scenarios leading to success involve ramming the portal planet with the Anniversary at light speed."
     "I hope none of you ever has to face such a decision. When we get back to Stargate, I will in all probability be court-martialed for cowardice under fire. But I honestly believe that the information that may be gained from analysis of the damage to the Anniversary is more important than the destruction of this one Tauran base." He sat up straight.
     "More important than one soldier's career."

From THE FOREVER WAR by Joe Haldeman (1975)

Spark-gap Transmitter

If you are an asteroid miner on an isolated rock or a shipwrecked castaway on a deserted planet, and circumstances have deprived you of your radio transmitter, you have a problem. You will have to some how jury rig a transmitter out of whatever you can scrounge up so you can call for help. Naturally the simpler the transmitter, the easier will be the scrounging and the construction.

Which brings us to the Spark-gap Transmitter.

This is pretty much the simplest radio transmitter possible. Yes, this is real-science electromagnetic transmitter. If you have created for your science fiction novel some sort of superluminal radio powered by handwavium, you are on your own.


"And have him destroy or remove his equipment first? That would be pointless. No, we'll have to move him by subtlety, get him out on some plausible pretext and have experts check, copy and photograph every device in the place. After all, even a superman cannot send a sub-space radio call without equipment."

Rickman laughed harshly. "I suppose I'm always out of line but I still say kill him. In the first place I don't think any pretext will deceive him and, in the second, I think you're deluding yourselves. For example, and to quote from the text, you can build a non-directional sub-space radio transmitter with—again I quote—one solar cell, two Sharon compensator grids, one Larange tube and four short lengths of high resistance wire. I've no doubt our alien friend can make one with a good deal less."

From THE PRODIGAL SUN by Philip E. High (1964)

In the pictorial diagram:


(ed note: Jason dinAlt has been marooned on a colony planet that has fallen to a much lower technology level. And apparently a much lower I.Q. level as well. Anyway Jason has insinuated himself into the good graces of the local ruler, the Hertug. But Jason also has his own agenda.)

     “When will this be completed?” the Hertug asked, poking at the parts spread over Jason’s workbench.
     “Tomorrow morning, though I work all night, oh Hertug. But even before it is finished I have another gift for you, a way to improve your telegraph system.”
     “It needs no improvement! It is as it was in our forefathers’ days, and—”
     “I’m not going to change anything; forefathers always know best, I agree. I’ll just give you a new operating technique. Look at this—” and he held out one of the metal strips with the scribed wax coating. “Can you read the message?”
     “Of course, but it takes great powers of concentration, for it is a deep mystery.”
     “Not that deep; in one look I divined all its horrible simplicity.”
     “You blaspheme!”
     “Not really. Look here: that’s a B, isn’t it—two jiggles from the magic pendulum?”
     The Hertug counted on his fingers. “It is a B, you are correct. But how can you tell?”
     Jason concealed his scorn. “It was hard to figure out, but all things are as an open book to me. B is the second letter in the alphabet, so it is coded by two strokes. C is three—still easy; but you end up with Z, needing twenty-six bashes at the sending key, which is just a nonsensical waste of time. When all you have to do is modify your equipment slightly in order to send two different signals—let’s be original and call one a ‘dot’ and the other a ‘dash.’ Now, using these two signals, a short and a long impulse, we can transcribe every letter of the alphabet in a maximum of four increments. Understand?”
     “There is a buzzing in my head, and it is difficult to follow…”
     “Sleep on it. In the morning my invention will be finished, and at that time I will demonstrate my code.”
     The Hertug left, muttering to himself, and Jason finished the last windings on the armature for his new generator.

     “What do you call it?” the Hertug asked, walking around the tall, ornate wooden box.
     “This is an All Hail the Hertug Maker, a new source of worship, respect, and finance for Your Excellency. It is to be placed in the temple, or your local equivalent, where the public will pay for the privilege of doing you homage. Observe: I am a loyal subject who enters the temple. I give a donation to the priest and grasp this handle that projects from the side, and turn.” He began cranking lustily and the sound of turning gears and a growing whine came from the cabinet. “Now watch the top.”
     Projecting from the upper surface of the cabinet were two curved metal arms that ended in copper spheres separated by an air space. The Hertug gasped and recoiled as a blue spark snapped across the gap.
     “That will impress the peasants, won’t it?” Jason said. “Now—observe the sparks and notice their sequence. First three short sparks, then three long ones, then three short ones again.”
     He stopped cranking and handed the Hertug a clearly inscribed sheet of vellum, a doctored version of the standard interstellar code. “Notice. Three dots stand for H and three dashes signify A. Therefore as long as this handle is turned the machine sends out H.A.H. in code, signifying Huraoj al Heriug, All hail the Hertug! An impressive device that will keep the priests busy and out of mischief and your loyal followers entertained. While at the same time it will cry your praises with the voice of electricity, over and over, night and day.”
     The Hertug turned the handle and watched the sparks with glowing eyes. “It shall be unveiled in the temple tomorrow. But there are sacred designs that must be inscribed on it first. Perhaps some gold…”
     “Jewels too, the richer—looking the better. People aren’t going to pay to work a holy hand-organ unless it looks impressive.”
     Jason listened happily as the sparks crackled out. They might be saying H.A.H. in the local code, but it would be S.O.S. to an offworlder. And any spaceship with a decent receiver that entered the atmosphere of this planet should pick up the broad-spectrum radio waves from the spark gap. There might even be one hearing the message now, turning the loop of the direction finder, zeroing in on the signal. If he only had a receiver he could hear their answering message, but it didn’t matter, for shortly he would hear the roar of their rockets as they dropped on Appsala.

From DEATHWORLD 2 by Harry Harrison (1964)

      He pushed aside another cupboard door. Inside it were the odds and ends a man makes use of to repair the things about his house that he never notices until they go wrong. There was an assortment of wires, transistors, bolts, and similar stray items.
     "What now?" asked Bordman mildly.
     "I'm going to try to find out if there's anybody left alive over there. I'd have checked before if I'd known the colony existed. I can't prove they're all dead, but I may prove that somebody's still alive. It's barely two weeks' journey away from here. Odd that two colonies picked spots so near!"
     He picked over the oddments he'd selected.
     "Confound it!" Bordman said. "How can you check if somebody's alive some hundreds of miles away?"
     Huyghens threw a switch and took down a wall-panel, exposing electronic apparatus and circuits behind. He busied himself with it.
     "Ever think about hunting for a castaway?" he asked over his shoulder. "Here's a planet with some tens of millions of square miles on it. You know there's a ship down. You've no idea where. You assume the survivors have power—no civilized man will be without power very long, so long as he can smelt metals—but making a space-beacon calls for high-precision measurements and workmanship. It's not to be improvised. So what will your shipwrecked civilized man do, to guide a rescue-ship to the one or two square miles he occupies among some tens of millions on the planet?"
     "He's had to go primitive, to begin with," Huyghens explained. "He cooks his meat over a fire, and so on. He has to make a strictly primitive signal. It's all he can do without gauges and micrometers and special tools. But he can fill all the planet's atmosphere with a signal that searchers for him can't miss. You see?"
     Bordman thought irritably. He shook his head.
     "He'll make," said Huyghens, "a spark transmitter. He'll fix its output at the shortest frequency he can contrive, somewhere in the five-to-fifty-meter wave-band, but it will tune very broad—and it will be a plainly human signal. He'll start it broadcasting. Some of those frequencies will go all around the planet under the ionosphere. Any ship that comes in under the radio roof will pick up his signal, get a fix on it, move and get another fix, and then go straight to where the castaway is waiting placidly in a hand-braided hammock, sipping whatever sort of drink he's improvised out of the local vegetation."
     Bordman said grudgingly "Now that you mention it, of course…"
     "My space-phone picks up microwaves," said Huyghens. "I'm shifting a few elements to make it listen for longer stuff. It won't be efficient, but it will catch a distress-signal if one's in the air. I don't expect it, though."

From EXPLORATION TEAM by Murray Leinster (1956)

(ed note: interstellar pioneer trader team composed of David Falkyn (human), Chee Lan (cat-like Cynthian) and Adzel (dragon-like Wodenite) have gotten into trouble on a planet where the local Ershokh aliens have a medieval level of technology. All three have been captured by the locals, and their radio communicators have been smashed. Their ship has an AI computer of near human intelligence, but it will do nothing without orders. Regrettably a garrison of locals prevents them from approaching within shouting distance of the ship. What to do?)

      They left the electroplating shop, a royal monopoly operated in the palace, and strolled to a balcony. Falkayn was surprised at how far the grave philosophers had progressed: lead-acid batteries, copper wire, early experimentation with a sort of Leyden jar. He could understand why this was a more congenial society for humans than Katandara.

     Down yonder ramp! A courtier was headed up. Falkayn stiff-armed him and continued. Several more were in the corridor below. He waved his sword. "Blood and bones!" he yelled. "Boo!" They cleared a path, falling over each other and clamoring.

     And here was the electrical shop. Falkayn stormed in. Across workbenches crammed with quaintly designed apparatus, two scientists and several assistants gaped at him. "Everybody out," Falkayn said. When they didn't move fast enough, he paddled the Grand Chief Philosopher of Royal Rangakora with the flat of his blade. They got the message. He slammed the door and shot the bolt.
     The uproar came through that heavy metal, louder by the minutes, voices, feet, weapon clatter, and alarm drums. He glanced around. The windows gave no access, but another door opened at the far end of the long room. He bolted that, too, and busied himself shoving furniture against it. If he piled everything there that wasn't nailed down and used Chee's rope to secure the mass further, he could probably make it impasssable to anyone short of the army engineers. And they wouldn't likely be called, when the other approach looked easier.

     He ended his task and returned, breathing hard. Chee had also been busy. She squatted on the floor amid an incredible clutter of batteries and assorted junk, coiling a wire into a helix while she frowned at a condenser jar. She could do no more than guess at capacitances, resistances, inductances, voltages, and amperages. However, the guess would be highly educated.
     Both doors trembled under fists and boots. Falkayn watched the one he had not reinforced. He stretched, rocked a little on his feet, willed the tension out of his muscles. Behind him, Chee fiddled with a spark gap; he heard the slight frying noise.

     A human voice bawled muffled: "Clear the way! Clear the way! We'll break the obscenity thing down, if you'll get out of our obscenity way!" Chee didn't bother to look from her work.
     The racket outside died. After a breathless moment, feet pounded and a weight smashed at the bronze. It rang and buckled. Again the ram struck. This time a sound of splintering was followed by hearty curses. Falkayn grinned. They must have used a balk of glued-together timber (there are no tree-like plants native to the planet, all large pieces of wood are glued together out of twigs), which had proved less than satisfactory. He went to a gap where the door had been bent a little clear of the jamb and had a look. Several Ershoka could be seen, in full canonicals, fury alive on their faces. "Peekaboo," Falkayn said.
     "Get a smith!" He thought he recognized Hugh Padrick's cry. "You, there, get an obscenity smith. And hammers and cold chisels."

     That would do the job, but time would be needed. Falkayn returned to help Chee. "Think we've got ample juice in those batteries?" he asked.
     "Oh yes." She kept eyes at the single workbench not manning the barricade, where she improvised a telegraph key out of scrap metal. "Only four hundred kilometers or so, right? Even that slue-footed Adzel made it in a few standard days. What worries me is getting the right frequency."
     "Well, estimate as close as you can, and then use different values. You know, make a variable contact along a wire."

     "Of course I know! Didn't we plan this in your rooms? Stop yattering and get useful."
     "I'm more the handsome type," said Falkayn. He wielded a pair of pliers awkwardly—they weren't meant for a human grasp—to hook the batteries in series. And a Leyden jar, though you really should call it a Rangakora jar…
     The door belled and shuddered. Falkayn kept half his mind in that direction. Probably somewhat less than an hour had passed since he crashed out. Not a hell of a lot of time to play Heinrich Hertz. But Chee had put on the last touches. She squatted before the ungainly sprawl of apparatus, tapped her key, and nodded. A spark sizzled across a gap. She went into a rattle of League code. Invisible, impalpable, the radio waves surged forth.
     Now everything depended on her finding the waveband of the late lamented transceivers, somewhere among those she could blindly try. She hadn't long, either. The door would give way in another minute or two. Falkayn left her for his post.

     The bolt sprang loose. The door sagged open. An Ershokh pushed in, sword a-shimmer.
     Falkayn crossed blades. Steel chimed. As expected, the man was a sucker for scientific fencing. Falkayn could have killed him in thirty seconds. But he didn't want to. Besides, while he held this chap in the doorway, no others could get past. "Having fun?" he called across the whirring edges. Rage snarled back at him.
     Dit-dit-dah-dit… Come to Rangakora. Land fifty meters outside the south gate. Dit-dah-dah. Clash, rattle, clang!
     Hugh Padrick trod to the forefront. His blade was out, but held low. His features worked. "What're you about?" he rasped.
     "Very terrible magic," Falkayn told him. "We'll save trouble all around if you surrender right now."
     "What do you want of us?" Padrick asked.
     "Well, to start with, a long drink. After that we can talk." Falkayn tried to moisten his lips, without great success. Damn this (ultra-dry)air! No wonder the natives didn't go in for rugs. Life would become one long series of static shocks. Maybe that was what had first gotten the Rangakorans interested in electricity.
     Imperturbably, Chee continued to signal.

     "The demon, the demon!" Men and Ikranankans pelted by. A thunderclap was followed by the sound of falling masonry.
     Stepha didn't join the stampede. But she pulled free, and the dagger flashed into her hand. "What's that?" she cried.
     Falkayn gusted the air from his lungs. His head swam. Somehow he kept his tone level. "That," he said, "was our ship. She landed and took Adzel on for a pilot, and now he's aloft, losing merit but having a ball with a mild demonstration of strength." He took her hand. "Come on, let's go out where he can see us and get taken aboard. I'm overdue for a dry martini."

From TRADER TEAM by Poul Anderson (1965)

Morse Code

Sometimes solar storms or enemy jamming might fill the communication lines with static. If it is real bad, one might have to revert to good old Morse code. The dot and dash symbols can punch through interference much easier than the spoken word. Even present day naval vessels can send Morse code by Signal Lamps, when the enemy might overhear a radio message.

Important announcement for scifi authors wanting to add Morse Code to their stories

Morse code relies upon being able to send two types of signals: a short "dot" and a long "dash." This means it is worthless unless you can send the two types.

For instance, Morse cannot be used for two imprisoned people to send messages by tapping on the cell walls because it is very hard to tap a "dash." Instead the Tap Code is used, since that only requires one type of signal.

This also invalidates all those movies where somebody surreptitiously sends a Morse Code message while in public by using tap-dancing.

By its nature, Morse code is sent over the radio by Amplitude Modulation (AM), not Frequency Modulation (FM) or other kind of modulation. Radio Morse code requires less signal bandwidth (100 to 150 Hz for typical speeds in the 20 to 40 word per minute range) compared to voice (2400 Hz), though it has a lower data rate.

Morse code is usually received as a medium-pitched audio tone (600–1000 Hz), so transmissions are easier to copy than voice through the noise on congested frequencies (too many conversations taking place on the same band), and it can be used in very high noise / low signal environments (loud static and faint voice, never a happy situation). The transmitted power is concentrated into a limited bandwidth so narrow receiver filters can be used to suppress interference from adjacent frequencies (narrowing the opening that static and competing conversations can leak through).

The narrow signal bandwidth also takes advantage of the natural aural selectivity of the human brain, further enhancing weak signal readability (translation: it is easier to hear a dot or dash than it is to decipher somebody slurring their words). The signal can be weak if the sender is distant, has low amounts of power available for the radio, or both. Very well could be good news for castaways in a wrecked spacecraft in the back of the beyond. In fact the required bandwidth can be reduced and the signal range increased by simply drastically slowing down the word per minute rate. Dave Hinerman (WD8CIV) told me about some hams who experimented with computer generated Morse code with the dots lasting ten seconds and the dashes lasting 30. They were trying to see what was the least amount of power needed to transmit a receivable decodable signal.

In addition, a Morse code signal is easier to jury-rig than a full two-way radio (for instance: by tapping two wire together). Which could come in handy for an interplanetary spy trying to transmit information using the equipment at hand before the bad guys burn open the hatch with their laser pistols. Or by survivors of a drastic shipboard accident that destroys the communication equipment. Or if Our Hero is in a holding cell of the evil villain's lair, covert communication would be possible with prisoners in other cells by tapping on the walls (though actually prisoners would instead want to use the Tap Code). Or sending a message by turning on and off a flashlight or using a mirror to flash sunlight at the intended message recipient (in olden days this was the mechanism used by heliographs). Or if one is a dirt-poor asteroid miner who cannot afford fancy radio equipment but is so lonely on their isolated rock that they are desperate to talk to anybody else. In his novels Poul Anderson had Ham Radio popular among asteroid miners for just this reason, and Ham Radio often uses Morse because the required equipment is much simpler and the range is longer.

Skilled telegraphers can comprehend ("copy") Morse code signals at rates in excess of 40 words per minute. Top sending speed by experts might be around 75 to 100 words per minute. Individual telegraphers might use slightly longer or shorter dashes or gaps, perhaps only for particular characters. This is called the telegrapher's fist, and other telegraphers can recognize specific individuals by it alone. A good telegrapher who sends clearly and is easy to copy is said to have a "good fist". A "poor fist" is a characteristic of sloppy or hard to copy Morse code.

"Friday afternoon, I'm walking home from school and I'm watching some men build a new house.
And the guy hammering on the roof calls me a paranoid little weirdo. In Morse code."

Emo Phillips

The usual communication devices are unsuitable, unavailable, broken or under surveillance. What can a smart hero do?

Morse code, of course! Make a noise, flash a light, or grab something convenient and start maniacally flipping it on and off.

Naturally, the message's recipient knows Morse code, too, but any villains in the vicinity will fail to penetrate this cleverness. Not only that, but sometimes the recipient will figure out the missing bits they lost while figuring it was actually Morse.

If the viewer happens to know Morse Code, it would be noticed that almost always the actor is just tapping randomly. If the recipient is rattling off the message almost as fast as one can read, it's fake Morse code. The fastest straight-key operators can send at 35 words per minute, and decode in their head at up to 40 words per minute. It's unlikely that someone with just basic training in Morse code can even send at more than 10 words per minute.

In another example of research failure or maybe Acceptable Breaks from Reality, movies and TV usually show the Morse operators visibly tapping the key. Even a basic Morse class teaches you to grasp the sides of the key button with thumb and middle finger and index finger on top, and work the key from your wrist. But that's much less visual.

Incidentally, Morse Code is officially obsolete for radio communications: In 1999, it was retired as the international standard, and in 2007, the FCC dropped requirements for Morse proficiency for amateur radio operators. It hasn't gone away completely, however. Morse is still in semi-official use by most navies with signal lamps, and by amateurs: Propogation beacons used to gauge atmospheric conditions and estimate their current effective transmission range identify themselves by a two-letter callsign in Morse, for example, and it's traditional (and in some places was once required) for a repeater to send its station callsign in Morse at regular intervals. A surprising number of particularly dedicated old-school hams are still using it to talk to each other, as well.

By the way, if you're in trouble, you can always send the most commonly-known message in Morse Code: 3 dots, 3 dashes, 3 dots (S.O.S.). Though SOS works differently from most Morse transmissions. There should be no spaces between letters, and it should be repeated in a continuous SOSOSO pattern.

A similarly recognizable one is SMS (3 dots, 2 dashes, 3 dots), which is used as a ringtone on many Nokia mobile phones to indicate an incoming text.

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


      "We're picking up a beam from home," said Herndon anxiously. "But we can't make it out."

     Because the third planet of the sun Lani was being colonized from the second, inhabited world, communication with the colony's base was possible. A tight beam could span the distance, which was only light-minutes across at conjunction, and not much over a light-hour at opposition, as now. But the beam communication had been broken for the past few weeks, and shouldn't be possible again for some weeks more. The sun lay between. One wouldn't expect normal sound-and-picture transmission until the parent planet had moved past the scrambler-fields of Lani. But something had come through. It would be reasonable for it to be pretty much hash when it arrived.

     "They aren't sending words or pictures," said Herndon. "The beam is wobbly and we don't know what to make of it. It's a signal, all right, and on the regular frequency. But there are all sorts of stray noises and still in the midst of it there's some sort of signal we can't make out. It's like a whine, only it stutters. It's a broken-up sound of one pitch."
     Bordman rubbed his chin. He remembered a course in information theory just before he'd graduated from the Service Academy. Signals were made by pulses, pitch-changes, and frequency-variations. Information was what couldn't be predicted without information. And he remembered with gratitude a seminar on the history of communication, just before he'd gone out on his first field job as a Survey Candidate.
     "Hm," he said with a trace of self-consciousness. "Those noises, the stuttering ones. Would they be, on the whole, of no more than two different durations? Like—hm— Bzz bzz bzzzzzzz bzz?"
     He felt that he lost dignity by making such ribald sounds. But Herndon's face brightened.
     "That's it!" he said relievedly. "That's it! Only they're high-pitched like—" His voice went falsetto. "Bz bz bz bzzz bz bz." Bordman thought, we sound like two idiots. He said:
     "Record everything you get, and I'll try to decode it." He added, "Before there was voice communication there were signals by light and sound in groups of long and short units. They came in groups, to stand for letters, and things were spelled out. Of course there were larger groups which were words. Very crude system, but it worked when there was a lot of interference, as in the early days. If there's some emergency, your home world might try to get through the sun's scrambled-field that way."
     "Undoubtedly!" said Herndon, with even greater relief. "No question, that's it!"

     When he entered the office, Herndon sat listening to a literal hash of noises coming out of a speaker on his desk. The cryptic signal had been relayed to him, and a recorder stored it as it came. There were cacklings and squeals and moaning sounds, sputters and rumbles and growls. But behind the facade of confusion there was a tiny, interrupted, high-pitched noise. It was a monotone whining not to be confused with the random sounds accompanying it. Sometimes it faded almost to inaudibility, and sometimes it was sharp and clear. But it was a distinctive sound in itself, and it was made up of short whines and longer ones of two durations only.
     "I've put Riki at making a transcription of what we've got," said Herndon with relief as he saw Bordman. "She'll make short marks for the short sounds, and long ones for the long. I've told her to try to separate the groups. We've got a full half-hour of it, already."
     Bordman made an inspired guess.
     "I would expect it to be the same message repeated over and over," he said. He added, "And I think it would be decoded by guessing at the letters in two-letter and three-letter words, as clues to longer ones. That's quicker than statistical analysis of frequency."
     Herndon instantly pressed buttons under his phone-plate. He relayed the information to his sister, as if it were gospel. But it wasn't, Bordman thought. It's simply a trick remembered from boyhood, when I was interested in secret languages. My interest faded when I realized I had no secrets to record or transmit.
     Herndon turned from the phone-plate.
     "Riki says she's already learned to recognize some groups," he reported, "but thanks for the advice. Now what?"

     There was a stirring behind him; Riki Herndon had come silently into her brother's office. She looked pale. She put some papers down on the desk.
     "That's true," she said. "But while cycles sometimes cancel, sometimes they enhance each other. That's what's happening."
     Bordman scrambled to his feet, flushing. Herndon said sharply:
     "What? Where'd you get that stuff, Riki?"
     She nodded at the sheaf of papers she'd just laid down.
     "That's the news from home." She nodded again, to Bordman. "You were right. It was the same message, repeated over and over. And I decoded it like children decode each other's secret messages. I did that to Ken once. He was twelve, and I decoded his diary, and I remember how angry he was that I'd found out he didn't have any secrets."
     She tried to smile. But Herndon wasn't listening. He read swiftly. Bordman saw that the under sheets were rows of dots and dashes, painstakingly transcribed and then decoded. There were letters under each group of marks.
     Herndon was very white when he'd finished. He handed the sheet to Bordman. Riki's handwriting was precise and clear. Bordman read:
From CRITICAL DIFFERENCE aka SOLAR CONSTANT by Murray Leinster (1956)

(ed note: The good ship Johannes Kepler is about midway on a 92 day journey to Mars colony when a meteor punctures the ship. Unfortunately the idiot captain was holding a meeting of all the officers in the control room, so they are all dead now. The only officer left is Lieutenant Donald Chase, who is actually the ship's medic. However, by the chain of command he is officially the captain.

They struggle through a variety of disasters, most recent of which was a solar proton storm. Now they have to somehow contact Mars Central because they are off-course, the astrogator is in the morgue, and a passenger named Ugalde who is a mathematician is not quite up to calculating a correction. Alas the radio cannot contact Mars through the solar interference.)

      ‘I’m sorry, Captain, but it’s no go. Our signal is getting out — but it just isn’t strong enough. There is still plenty of background noise from the storm, and we’re not punching through it…’ He stopped as the tape recorded message cut off, and there was a moment of hushed silence before a new voice came on.
     ‘Johannes Kepler — are you broadcasting? We have been picking up traces of a signal on your frequency, but cannot read your signal. Are you broadcasting? Repeat — can you hear me? This is Mars Central calling the Johannes Kepler. We have a very weak signal on your frequency but cannot read it…’
     ‘It’s the storm,’ Sparks explained, ‘that and the low power…’
     ‘You did your best, Sparks,’ Don told him. ‘No one is blaming you.’
     There was no one who could be blamed.
     But that did not help.
     If they could not contact Mars they were as good as dead at this moment.

     The others had turned away, but Don was looking at the crude transmitter, glaring at it, as though he could force it to work just by strength of will alone. There had to be a way — and this radio was the only hope they had left.
     ‘Isn’t there any way you can increase the power?’ he asked.
     Sparks shook his head. ‘I’ve already got all the circuits on a forty per cent overload. They can take that for a while without burning out. You saw, I kept cutting the current every few minutes. Any more and they would pop as soon as I turned on the juice.’
     ‘Are there any other ways you can beef up the circuits?’
     ‘Negative on that, I’m afraid. Wiring up this thing was the easiest job. Me and Gold spent most of the time seeing what was the best circuit we could get out of the junk we could find. But the signal will improve as we get closer to Mars. They’ll hear us eventually.’
     ‘Eventually is a word that is not too good,’ Ugalde said. He came up next to the radio and stood, rocking on his toes with his hands behind his back, as though he were addressing a class. ‘Now while I admit with great chagrin that being a navigator is impossible for me at this moment, I am still yet able to calculate an orbit. Roughly mind you, but I have worked out as best I can from the figures of the last calculations made by the deceased navigator. Our course error grows greater with every passing moment, and the greater the error the harder it is to correct.’
     Nothing could be said after this, and the air of gloom in the control-room was thick enough to be cut with a knife. Sparks looked around, from face to face, pulling back against the table.

     ‘Don’t look at me!’ he called out loudly, defensively. ‘I’ve done all I could with the parts we had. I built a radio and it works, you heard that. It’s putting out all it can. There’s nothing more I can do. It’s a working radio, don’t forget that, with a modulated signal, not a radar or a signal generator where you just blast out. This is all we’ve got…’
     Don took him by the shoulder, harder than he intended, his fingers digging deep. ‘What was that you said about radar?’ He let go quickly when he saw the man’s shocked expression.
     ‘It’s nothing, sir. Nothing to do with us. If you just squirt out a signal you can get maybe twice the power we have going out now. But we have to modulate the signal to carry information. Otherwise Mars Central will be getting nothing but a blast of static from our direction. They’ll know that we’re still here — but that’s about all they will know.’
     ‘No!’ Don said. ‘There’s more.’ He paced back and forth, driving his fist into the palm of his hand. ‘Something can be done. I know, I read about it once, a book or something like that, about the early days of radio. Something about code…
     ‘Sure,’ Sparks answered. ‘Code. They used to use it maybe a couple of hundred years ago. We had it in history at radio school. Before they could modulate a signal to carry a message they used to just blast it out, then interrupt it in short or long bits in a regular kind of code. I guess they had a special signal for every letter. Then at the other end they would put it back into letters again. But we can’t do that —’
     ‘Why not?’
     Sparks started to smile, then changed his mind when he saw the expression on Don’s face. ‘Well, you see…no one knows the code any more. So even if we knew it and could send it, no one could read it. It would be a great idea if we could do it, but…’
     ‘No buts. We’ll do it. Could you transmit the long and short signals if I gave you a message?’
     ‘Well, I guess so. I could rig a make-or-break switch and keep opening and closing it. Or we could record it on tape, that might be easier, and have the taped signal actuate a relay. I guess, mechanically, it could be done.’
     ‘Then do it. I’ll bring you back the message as soon as I can. Get your equipment rigged. Kurikka, come with me.’

     The Chief didn’t speak until they were out in the corridor, then he let out the breath he had been holding.
     ‘Would you mind, sir, telling me just what you have in mind.’ He looked baffled and Don almost laughed.
     ‘It’s easy. We’re going to the library. The information will be there. If not in the shelved books it will be in the library's memory.’ (This was written in 1970, way before the internet)
     It really was easy after that. None of the books, they were mostly fiction for the passengers’ entertainment, looked promising, so Don punched for the encyclopedia index. CA—CU had an entry marked codes and he tried three or four sub-entries before he found an article on the International Code. It contained a copy of the code itself.
     ‘There it is,’ Don said, pointing at the columns of letters and dots and dashes. He pressed the print button. ‘Now let’s see if we can transcribe a message in this stuff.’
     Back in the control-room, it was the mathematician, Dr Ugalde, who suggested the solution.
     ‘The computer, we must give it instructions. This is the kind of operation the stupid machine is built for. If you will permit I will programme the computer to transform a typed message into this code and it will then record the code on tape for the transmitter. The message will be transmitted and, I am sure, it will be quickly comprehended that it is a code. I suggest that, before the message, we transmit the numbers from one to ten, counting in dots to make the series, that is. This will indicate that there is intelligent content in the broadcast, not just a random collection of pulses. With that clue it will not take them long to figure out what is happening.’
     ‘That sounds fine to me,’ Don said. ‘After the numbers send a simple message, just ask them if they can understand the code so that we can send more detailed messages. Tell them we can hear their voice transmissions, but will have to answer in code.’ He turned to the others. ‘Get this gear rigged as quickly as possible. I’m going to the sick bay to look after my patients. Call me as soon as you are ready to broadcast.’

     The phone rang as he entered and he once more had to assume the role of captain. The message was ready to be sent.
     ‘Works in the green on the test,’ Sparks said, throwing a switch when Don came in. A slow series of dits and dahs sounded from the speaker. ‘We’ve got the tape working through this switching circuit. I’m getting an antenna output almost double what we had before.’
     ‘Send it,’ Don said, and dropped into the captain’s chair before the control panels. Jonquet brought in coffee and passed the cups around.
     Sparks re-ran the tape and made the necessary adjustments. The reel spun and the message crackled out into space. The receiver still repeated the recorded message they had been getting for days now. Twice Sparks re-ran the tape, and repeated the transmission, before finally switching off the apparatus.
     ‘Just a matter of waiting now,’ he said.
     Dr Ugalde scribbled some quick calculations on a piece of paper. ‘It is my estimate,’ he said, ‘considering our probable position in relation to Mars, that we could hear a return message in less than thirty seconds from now.’
     They all looked at the clock, at the sweeping hand. It seemed to crawl, slower and slower, finally reaching thirty seconds and passing it. Going on for a minute more. A minute and a half. Ugalde crumpled his piece of paper.
     ‘Perhaps my mathematics are wrong, an error…’

     He broke off as the droning voice from the receiver suddenly ended. They all turned. They all turned, automatically, looking at the now silent speaker. There were seconds of silence before a new voice cut in.
     ‘Hello Johannes Kepler…can you hear me? We are receiving a transmission on your frequency of a series of pulses. Are you transmitting this? If you are send five pulses. Repeat them because reception varies at this end…’
     ‘Do it!’ Don ordered.
     Sparks had rigged a manual pushbutton switch into the circuit. He used it now, sending out the dots, over and over, five, five, five, five…
     Then they waited, once again, the long minutes while their message, travelling 186,000 miles a second, at the speed of light, reached out to Mars and was received. Until the answer was broadcast.
     ‘We have received your message, Johannes Kepler,’ the voice said, and an impromptu cheer shook the room.
     ‘…means you have had difficulty with your radio. Someone here has just reported that your message is in code and the library is being consulted for a copy. If you believe we have a copy here and will be able to translate your message please send all details. Repeat your message at least five times, I repeat, send your message at least five times since we are having reception difiiculties at this end. We are standing by to receive now, good luck.’
     It took time, a lot of time, because the communication was so complex. Don typed a message into the computer, explaining what had happened, and this was recorded on tape as a series of dots and dashes. Another tape was prepared of up-to-date stellar observations which were recorded along with the earlier data. The computer on Mars would process these and determine the course corrections that would be needed. Time passed, and with each second they moved further from their proper course.

From SPACESHIP MEDIC by Harry Harrison (1970)

(ed note: Bangs and Jardine own a spaceship. Unbeknown to them evil hijackers Mason and Loring stowed away on board the ship. They intend to overpower Bangs and Jardine and steal the ship.)

      "He's coming," hissed Loring. "We'll take him soon's he reaches us." There was a sharp clank as the hatch opened, and Jardine's head came into view.
     "Now!" yelled Loring. He swung the heavy paralo-ray gun at Jardine's head.
     "What the—" exclaimed the startled spaceman. "Bangs, look out!"
     He tried to avoid the blow, but Loring's gun landed on the side of his head. Jardine crumpled to the deck.
     Bangs was out of his seat in a moment, at his pilot's call. The burly redheaded spaceman saw at a glance what was wrong and lunged for the hatch.
     Loring stepped toward him, holding his paralo-ray.
     "All right, spaceboy!" he grated. "Hold it or I'll freeze you stiff!"
     Bangs stopped and stared at the gun and at Jardine who was slumped on the deck. Mason rushed past him to the controls.
     "What is this?" demanded Bangs.
     "An old game," explained Loring with a sneer. "It's called 'You've got it and I take it.' And if you don't like it, you get it." He gestured with his gun. "You get it—with this."
     Bangs nodded. "O.K.," he said. "O.K. But how about letting me take care of my buddy. He's hurt."
     "Just a bump on the head," said Loring. "He'll come out of it soon enough."

     "Hey," shouted Mason, "I can't figure out these controls!"
     Loring growled angrily. "Here, lemme at them!" He forced Bangs to lie down on the deck, and then, keeping the gun trained on the redheaded spaceman, stepped quickly to the control board. He handed Mason the gun.
     "Keep an eye on them while I figure this baby out."
     "Least you coulda done is steal a decent ship," grumbled Mason. "This tub is so old it creaks!"
     "Just shut your mouth and keep your eye on those guys," said the other. He began to mutter to himself as he tried to figure out the complicated controls.

     Jardine was now conscious but had the presence of mind not to move. His head ached from the blow. Slowly he opened his eyes and saw his two attackers bending over the board. He saw that Bangs was lying on the deck facing him. Jardine winked at Bangs, who returned the signal. Then he began, carefully, methodically to send a Morse-code message to his companion via his winking eyes.
     "O-N-L-Y—one—gun—between—them. You—take—big—fellow. I'll—charge—gun…"

     "Can't you figure this thing out either?" asked Mason, leaning over Loring's shoulder.
     "Ah, this wagon is an old converted chemical burner. These controls are old as the sun. I've got to find the automatic pilot!"
     "Try that lever over there," suggested Mason.
     Loring reached over to grasp it, turning away from his prisoners.

     "Bangs, get 'em!" shouted Jardine. The two men jumped to their feet and lunged at Loring and Mason. Loring dove to one side, losing the gun in the scramble, but as he fell, he reached for the acceleration control lever. He wrenched it out of its socket and brought it down on Bang's head, and the officer slid to the floor. Jardine, meanwhile, had Mason in a viselike grip, but again Loring used the lever, bringing it down hard on the neck of the freighter pilot. Jardine dropped to the deck.
     "Thanks, Loring," gasped Mason. "That was close! Good thing we had on these space suits, or we'd have been finished. They couldn't grab onto the smooth plastic."
     "Finished is right!" snarled Loring. "I told you to keep an eye on them! If they'd nabbed us we woulda wound up on the prison asteroid!"

From DANGER IN DEEP SPACE by Carey Rockwell (1953)

Jeremiah Andrew Denton Jr. (July 15, 1924 – March 28, 2014) was a U.S. Senator representing Alabama from 1981 to 1987, a United States Navy Rear Admiral, and Naval Aviator taken captive during the Vietnam War.

Denton was widely known for enduring almost eight years of grueling conditions as an American prisoner of war (POW) in North Vietnam after the A-6 Intruder he was piloting was shot down in 1965. He was the first of all American POWs held captive and released by Hanoi to step off an American plane during Operation Homecoming in February 1973. As one of the earliest and highest-ranking officers to be taken prisoner in North Vietnam, Denton was forced by his captors to participate in a 1966 televised propaganda interview which was broadcast in the United States. While answering questions and feigning trouble with the blinding television lights, Denton blinked his eyes in Morse code, spelling the word "TORTURE"—and confirming for the first time to U.S. Naval Intelligence that American POWs were being tortured.

From the Wikipedia entry for Jeremiah Denton

Cole was listening carefully to the (Morse code) signals coming through from Pluto. "That," he decided, "sounds like Tad Nichols' fist. You can recognize that broken-down truck-horse trot of his on the key as far away as you can hear it."

"Is that what it is?" sighed Buck. "I thought it was static mushing him at first. What's he like?"

Like any really skilled operator, Cole had been sending Morse messages while he talked. Now he sat quiet waiting for the reply, glancing at the chronometer.

Nichols’ ragged signals were coming through — or pounding through. They were worse than usual, and at first Kendall and Cole couldn’t make them out. Then finally they got them in bursts. The man was excited, and his bad key-work made it worse.

“- Randing stopped. They got him I think. He said-th - ship as big - a - nsport. Said it wa - eaded my - ay. Neutrons - on instruments - he’s coming over the horizon - it’s huge - war ship I think - register - instru - neutrons - .” Abruptly the signals were blanked out completely.

Cole and Kendall sat frozen and stiff. Each looked at the other abruptly, then Kendall moved. From the receiver, he ripped out the recording coil, and instantly jammed it into the analyzer. He started it through once, then again, then again, at different tone settings, till he found a very shrill whine that seemed to clear up most of Nichols’ bad key work.

“T-247 - T-247 - Emergency. Emergency. Randing reports the - over his horizon. Huge - ip - reign manufacture. Almost spherical. Randing’s stopped. They got him I think. He said the ship was as big as a transport. Said it was headed my way. Neutrons - ont - gister - instruments. I think - is h - he’s coming over the horizon. It’s huge, and a war ship I think - register - instru ments - neutrons.”

From THE ULTIMATE WEAPON by John W. Campbell (1936)

     A gong sounded and they both jumped to their feet and raced madly into the Forlorn Hope. The ultra-receiver had come to life and the sounder was chattering insanely—someone was sending with terrific speed, but with perfect definition and spacing.
     "That's Brandon's fist—I'd know his style anywhere," Stevens shouted, as he seized notebook and pencil.
     "Tell me what it says, quick, Steve!" Nadia implored.
     "Can't talk—read it!" Stevens snapped. His hand was flying over the paper, racing to keep up with the screaming sounder.

From SPACEHOUNDS OF IPC by E. E. "Doc" Smith (1931)

(Captain) Kendall went to the main communication micro­phone and spoke. His voice went all over the Empress of Kolain from pilot room and cargo spaces to swimming pool and infirmary.

"Attention!" he said in a formal voice. "Attention to official orders!"

Kendall continued: "I believe that something never before tried is being attempted. I am forced against my better knowledge to believe that some agency is trying to make contact with us, a spaceship in flight! This is unknown in the annals of space flying and is, therefore, indicative of something important. It would not have been tried without preparations unless an emergency exists.

(ed note: this story was written before the invention of the maser, meaning radio communication across interplanetary distance was impossible.)

"However, the requirements of an officer of space do not include a knowledge of code, because of the lack of communication with the planets while in space. Therefore, I request that any person with a working knowledge of International Morse will please present himself to the nearest officer."

Minutes passed. Minutes during which the flashing lights continued.

Then the door of the bridge opened and Third Officer Jones entered with a thirteen-year-old youngster at his heels. The boy's eyes went wide at the sight of the instruments on the bridge, and he looked around in amazed interest.

"This is Timmy Harris," said Jones. "He knows code!"

"Go to it, Mr. Harris," said Kendall.

The boy swelled visibly. You could almost hear him thinking, "He called me 'mister'!"

Then he went to the table by the speaker and reached for pencil and paper. "It's code all right," he said. Then he winked at Jones. "He has a lousy fist!"

Timmy Harris began to write.

". . . course and head for Terra direct"—the beam faded for seconds—"Venusian fever and you will be quarantined. Calling CQ, calling CQ, calling CQ. Calling Empress of Kolain ... empowered us to contact you and convey ... message: You are requested to correct your course and head ... a plague of Venusian fever, and you ... Johnson of Interplanet has empowered us ... the following message: You are requested to correct your ... head for Terra direct. Calling CQ . . ."

"Does that hash make sense to you?" Jones asked of Kendall.

"Sure," smiled Kendall, "it is fairly plain. It tells us that Keg Johnson of Interplanet wants us to head for Terra direct because of a plague of Venusian fever that would cause us to stay in quarantine. That would ruin the (cargo of) line moss. Prepare to change course, Mr. Jones!"

"Who could it be?" Jones asked foolishly.

"There is only one outfit in the Solar System that could possibly think of a stunt like this. And that is Channing and Franks. This signal came from Venus Equilateral!"

"Wait a minute," said Timmy Harris. "Here's some more."

"As soon as this signal ... intelligible ... at right angles to your course for ten minutes. That will take ... out of ... beam and reflected ... will indicate to us ... left the area and know of our attempt."

"They're using a beam of some sort that indicates to them that we are on the other end but can't answer," Kendall said. "Mr. Jones, and Pilot Canton, ninety degrees north for ten minutes! Call the navigation officer to correct our course. I'll make the announcement to the passengers. Mr. Harris, you are given the freedom of the bridge for the remainder of the trip."

(ed note: it is the least he could do, since Timmy Harris just saved Interplanet several million dollars.)

Mr. Harris was overwhelmed. He'd learn plenty— and that would help him when he applied for training as a space officer; unless he decided to take a position with Venus Equilateral when he grew up.

From CALLING THE EMPRESS by George O. Smith (1943)

      She had no time to see if that argument had any effect on Massa. For at that moment there was a clicking from the com, and they both looked to it, tense, reading in that rattle of sound the message.

     "Need aid—Ayana—medic—"

     “Jacel!” Massa jerked from Ayana's hold, was on her feet. “He is hurt."

     “No. That was Jacel's sending. Did you not recognize it? And if he is sending, he cannot be the one in need."

     Clicks might not have any voice tone, but they had practiced so long together that they were able to distinguish the sender by rate of speed.

From BREED TO COME by Andre Norton (1972)


There are three main types of Morse code "keys" (devices used by telegraphers to tap out the dot-dash Morse code signals). They are Straight Keys, Semiautomatic Keys and Iambic Paddles. Improvised keys generally take the form of two wires that the telegrapher taps together.

Straight Key
Signal is "on" when knob is pressed and "off" when released. Length and timing of dots and dashes are controlled by the human operator. In other words it is a fancy on-off switch.
Semiautomatic Key (aka telegrapher's "bug")
Press paddle to the right with the thumb and it automatically generates a series of dots (length and timing are controlled by positioning the sliding weight at the rear). Press the paddle to the left with the knuckle of the index finger and it makes a single dash just like a conventional straight key. A mirror image bug is used by left-handed telegraphers. The Vibroplex contains no electronics, the series of dots is generated mechanically by an oscillating weight on a spring. The Vibroplex company never figured out a way to mechanically generate a series of dashes, which admittedly is a challenge (dash of three units, space of one unit, dash of three units, etc.).
My dear departed maternal grandfather used one of those as a radio HAM. I tried to use it but was never very good at it.
Iambic Paddles
Used with an electronic keyer to generate high-speed Morse code. The Vibroplex uses an oscillating weight to generate signals, the electronic keyer uses microprocessor. Pressing the right paddle generates a series of dashes, pressing the left paddle generates a series of dots, squeezing both paddles generates a dot-dash-dot-dash sequence. The action is reversed for left-handed telegraphers.
My dear departed maternal grandfather rigged up one of these for a friend of his who had a disability in his arms. My grandfather bolted the paddles under the table and crafted a long lever his friend could control with a knee.

Rufus Macquarie saw it all happen above the black ridgeline of the Brooks Range in Rufus operated a mine there. On clear nights he would drive his pickup truck to the top of a mountain that he and his men had spent the day hollowing out. He would take his telescope, a twelve-inch Cassegrain, out of the back of the truck and set it up on the summit and look at the stars. When he got ridiculously cold, he would retreat into the cab of his truck (he kept the engine running) and hold his hands over the heater vents until his fingers regained feeling. Then, as the rest of him warmed up, he would put those fingers to work communicating with friends, family, and strangers all over the world.

And off it.

After the moon blew up, and he convinced himself that what he was seeing was real, he fired up an app that showed the positions of various natural and man-made celestial bodies. He checked the position of the International Space Station. It happened to be swinging across the sky 260 miles above and 2,000 miles south of him.

He pulled a contraption onto his knee. He had made it in his little machine shop. It consisted of a telegraph key that looked to be about 150 years old, mounted on a contoured plastic block that strapped to his knee with hook-and-loop. He began to rattle off dots and dashes. A whip antenna was mounted to the bumper of his pickup truck, reaching for the stars.

Two hundred sixty miles above and two thousand miles south of him, the dots and dashes came out of a pair of cheap speakers zip-tied to a conduit in a crowded, can-shaped module that made up part of the International Space Station.

(ed note: Rufus' daughter Dinah is currently a mission specialist on the International Space Station (called "Izzy"))

Rufus, a die-hard ham radio enthusiast who still communicated in Morse code with a dwindling circle of old friends all over the world, had pointed out that radio transmission between the ground and Izzy was actually rather easy, given that it was line-of-sight (at least when Izzy happened to be passing overhead) and that the distance was nothing by ham radio standards. Since Dinah lived and worked in a robot workshop, surrounded by soldering gear and electronics workbenches, it had been a simple matter for her to assemble a small transceiver following specifications provided by her dad. Zip-tied to a bulkhead, it dangled above her workstation, making a dim static hiss that was easily drowned out by the normal background roar of the space station’s ventilation systems. Sometimes it would beep.

At any rate her focus was either on the screen or on the robots, and so it had been for many hours. Until a string of beeps came out of the hissing speaker zip-tied to the bulkhead, and her eyes went momentarily out of focus as her brain decoded the dots and dashes into a string of letters and numbers: her father’s call sign. “Not now, Pa,” she muttered, with a guilty daughter’s glance at the brass-and-oak telegraph key he had given her — a Victorian relic purchased at great price on eBay, during a bidding war that had placed Rufus into pitched battle against a host of science museums and interior decorators.


“Not now, Pa, I know the moon’s pretty, I’m right in the middle of debugging this method . . .”



And then she brought her face close to the window and twisted her neck to find the moon. She saw what used to be it. And the universe changed.

From SEVENEVES by Neal Stephenson (2015)

Brevity Words

So in times of imperfect reception one can fall back on Morse Code. But even with perfect reception, some spoken items are hard to distinguish. The letters "T" and "D" for instance. Misunderstood words and letters can be catastrophic, especially in a military situation. To avoid this the NATO phonetic alphabet is commonly used. Also useful are military "brevity words."

Another useful source is the International Code of Signals. These are a set of international code signs and words that do not depend upon the two people communicating to share a language in common. They include multicolored flags, semaphore, blinking lights, Morse code, and radio. For instance, AJ means "I have had a serious nuclear accident and you should approach with caution" and EO means "I am unable to locate vessel/aircraft in distress because of poor visibility". Those signals can be understood even if the sender only speaks Mandarin Chinese and the receiver only speaks Czechoslovakian. Refer to the manual found here.

NATO Phonetic Alphabet

Space's Runic

This was invented by Dream Pod 9 for their Jovian Chronicles game. It is a set of quickly drawn symbols used as emergency writing where there is no radio contact. Say, painted on the side of a spacecraft in distress. For details, refer to the Symbols page.

Brevity Words

This is only a short list. For a fuller list go here.

Directive to cease action/attack/event/mission
Let me know that you have received and understood this message
Directive to initiate a briefed attack sequence or maneuver
Yes, or permission granted.
Request for bearing and range to described point
Orbit about a specific point; ground track flown by tanker. Information call indicates a turning engagement about a specific location.
Term meaning altitude in thousands of feet (e.g., Angels Five is 5,000 ft).
Request/comment regarding target aspect information.
To request or provide a response for a coded challenge.
Aircrew is operating without benefit of GCI/AWACS control.
{x} BENT
Identified system inoperative.
Prebriefed fuel state which is needed for recovery using prebriefed parameters.
No visual contact with friendly aircraft; opposite of term "VISUAL."
Directive/informational call that indicates aircraft will continue straight ahead at the merge and not turn with target/targets.
A radar/visual contact whose identity is unknown.
Request for target information as briefed/available.
To indicate the separation between portions of the messages. (To be used where there is no clear distinction between the text and other portions of the message).
BREAK {Up/Down/Right/Left}
Directive to perform an immediate maximum performance turn in the indicated direction. Assumes a defensive situation.
Term used to denote radio frequency is becoming saturated/degraded and briefer transmissions must follow.
BUGOUT {Direction}
Separation from that particular engagement/attack; no intent to reengage.
Change to channel ....... before proceeding.
Begin using briefed radio procedures to counter comm jamming.
Directive to briefly turn on exterior lights to enable visual acquisition.
Requested action is authorized (no engaged/support roles are assumed).
Ordnance release not authorized.
Ordnance release is authorized.
My version is ______. Is that correct?
Radar/IR contact at the stated position; should be in bearing, range, altitude (BRA), Bullseye, or geographic position format.
An error has been made in this transmission (message indicated). The correct version is ________.
Directive for the flight to maneuver to briefed positioning.
Proceed to alternate mission/base.
Maneuvering with the intent of achieving a kill. If no additional information is provided (bearing, range, etc.), ENGAGED implies visual/radar acquisition of target.
Proceed with your message.
GREEN {Direction}
Direction determined to be clearest of enemy air-to-air activity.
How well do you receive me?
Self-explanatory (use instead of “I repeat”).
Unpredictable maneuvers to negate a gun tracking solution.
Fuel state above Bingo at which separation/bugout/event termination should begin.
The spoken word for the distress signal. Lives in danger.
Is the spoken word for the distress relay signal.
No, or that is not correct, or I do not agree.
Aircrew does not have visual contact with the target/bandit; opposite of term "TALLY."
OFF {Direction}
Informative call indicating attack is terminated and maneuvering to the indicated direction.
My transmission is ended and I expect a response from you.
Conversation is ended and no response is expected.
The spoken word for the urgency signal. Trouble, but not mortal danger.
During long distress situations, communications can resume on a restricted basis. Communication is to be restricted to ship’s business or messages of a higher priority.
Repeat all of this message back to me exactly as received after I have given OVER. (Do not use the word “repeat”.)
I have received all of your last transmission.
I have received your message number ...
Target destroyed (air-to-air); weapons impact (air-to-ground).
I must pause for a few seconds or minutes, please wait.
Self-explanatory. (Do not use the word “repeat”.)
Is the spoken word for the safety signal.
Indicates that silence has been imposed on the frequency due to a distress situation.
Is the international expression to advise that a distress situation is in progress. This command comes from a vessel or coast station other than the station in distress.
Is the international expression for a distress cancellation.
Is the international expression to advise that a distress situation is in progress. The command comes from the ship in distress.
Request for an individual's tactical situation; response is normally "offensive," "defensive," or "neutral." May be suffixed by position and heading.
Turn off equipment indicated.
Sighting of a target/bandit; opposite of "NO JOY".
Hostile antiship missile
Check coding, check text with originator and send correct version.
{x} WELL
Described equipment is functioning properly.
Fire only;
FREE at targets not identified as friendly in accordance with current rules of engagement (ROE).
TIGHT at targets positively identified as hostile in accordance with current ROE.
HOLD (USA, USMC) in self-defense or in response to a formal order.
SAFE (USN) NOTE: USN and NATO use weapons safe to avoid confusion with the phrase hold fire.
  1. As a request: Communication is difficult, please send each word twice.
  2. As information: Since communication is difficult, I will send each word twice.

"Mission Control, this is X-ray-Delta-One. At two-zero-fo-wer-fife, on-board fault prediction center in our niner-triple-zero computer showed Alpha Echo tree fife unit as probable failure within seventy-two hours. Request check your telemetry monitoring and suggest you review unit in your ship systems simulator. Also, confirm your approval our plan to go EVA and replace Alpha Echo tree fife unit prior to failure. Mission Control, this is X-ray-Delta-One, two-one-zero-tree transmission concluded."


"X-ray-Delta-One, this is Mission Control, acknowledging your two-one-zero-tree. We are reviewing telemetric information on our mission simulator and will advise.

"Roger your plan to go EVA and replace Alpha-Echo tree-fife unit prior to possible failure. We are working on test procedures for you to apply to faulty unit."

From 2001 A SPACE ODYSSEY by Sir Arthur C. Clarke

Dak was busy most of the time at the ship's communicator, apparently talking on a very tight beam for his hands constantly nursed the directional control like a gunner laying a gun under difficulties.

From DOUBLE STAR by Robert Heinlein, 1956

"Stand by to record a signal to Lieutenant Venizelos at Basilisk Control for immediate relay to Fleet HQ. Fleet scramble, no encryption. Priority One."

Heads turned, and Webster's swallow was clearly audible.

"Aye, aye, Ma'am. Standing by to record."

"`Mr. Venizelos, you will commandeer the first available Junction carrier to relay the following message to Fleet HQ. Message begins: Authentication code Lima-Mike-Echo-Niner-Seven-One. Case Zulu. I say again, Zulu, Zulu, Zulu. Message ends.'" She heard McKeon suck air between his teeth at her shoulder. "That is all, Mr. Webster," she said softly. "You may transmit at will." Webster said absolutely nothing for an instant, but when he replied, his voice was unnaturally steady.

"Aye, aye, Captain. Transmitting Case Zulu." There was another brief pause, then, "Case Zulu transmitted, Ma'am."

"Thank you." Honor wanted to lean back and draw a deep breath, but there was no time. The message she'd just ordered Webster to send and Venizelos to relay to Manticore was never sent in drills, not even in the most intense or realistic Fleet maneuvers. Case Zulu had one meaning, and one only: "Invasion Imminent."

From ON BASILISK STATION by David Weber, 1994

Memeweave: News/Stargate Project/Recontact/Broadcast (Read Only)
Source: CS Prince of Octania
Classification: WHITE (General Access)
Encryption: None
Distribution: Everywhere (News/Live)
As received at: Relay Station (Eliéra-Seléne L3)
Language: Eldraeic I
Time of transcript: 3953:10:04:9-01:30.2 (Imperial Standard Time); time set by relay
Light lag (at source): 0.432 p
Light lag (overall): 9.776 kp

WARNING: Source link is down, attempting to reestablish…
WARNING: Source link is down, attempting to reestablish…
WARNING: Source link is down, attempting to reestablish…

INFORMATIONAL: Link restored.

WARNING: New source routing address is within an unallocated/experimental range (system override).
WARNING: Experimental flag is set on routing protocol.
WARNING: Link fleetrelay-a/bigocty/tangle0 is not available; using fallback route.

— bridge transcript continues —

[1] …navigation systems restored. Starlocks are maintained, nominal errors. Sunlock is Finíällë.

[2] Drift?

[3] Commander? Communications are restored. Tangle is scrambled as expected, but we have a gate relay carrier. Auto-reestablish is running.

[2] Very well.

[1] This is ballpark, but estimate drift close to 60 kay spatial.

[2] And temporal?

[1] Frame lock was tight. No greater drift, certainly.

[2] Good. Get me a course for –

[4] Conn, Sensor Ops. Status change. We show three hot spots at twelve hundred kay, not under thrust, looks like they’re keeping gate-relative position. Designating them Target Vekar.

[2] Belay that course. Sensor Ops, give me –

[4] Conn, Sensor Ops, further status change. Target Vekar just lit off its drives. Acceleration’s still changing, but it’s compatible for going for a zero-zero with us if we hold position.

[2] Very well, Sensor Ops. Give them a lidar ping. Don’t go for a hull map, just let them know we’re here.

Put us on a heading to zero-zero with them at best time, as their course settles. Execute at two-thirds cruising.

[1] Course laid in.

[5] Executing at two-thirds cruising, aye.


[5] Two-thirds cruising established.

[6] Growler!

[2] Confirm target status.

[6] No status change on target. Steady illumination.

[2] That might not be a targeting laser. Comms, do a modulation check.

[3] Checking… yeah, I’m seeing pulses. It’s not in the standard bands, so the software missed it. Rerunning now.

Nothing on standard set, nothing on contact set. Ah, wait one. Checking the archives.

And we have a communication request. Old protocol. Watchvid abbrev stack, commercial-comm version, fourth revision, standard dated three-oh-one-eight.

[2] You’re surprised, Mr. Serquel? That’s got to be the latest version we have in common. Integrate, and put it up on two.

WARNING: Memeweave contains rebroadcast content received via obsolete IIP version (29 revisions in deficit) (system override).
WARNING: Memeweave contains rebroadcast content using obsolete trivid protocol flagged for discard (system override).
WARNING: Memeweave contains rebroadcast content with unverifiable embedded authentication keys.

[2] This is Flight Commander Miruna Dalael, Imperial Navy, aboard CS Prince of Octania. I believe you were expecting us? Over.

[3] Transmission lag 8.9 pulses.

[C] This is Flight Commander Adévis Amarens, Víëlle Star –

Correction. This is Flight Commander Adévis Amarens, Imperial Navy, aboard CVS Backup’s Pride. Welcome to Víëlle System. If you’ll take a course from us, there’s a party waiting for you when we reach Víëlle orbit. Over.

[2] Glad to, Flight Commander, and we’ll bring the beer. Dalael, clear.

Secure from alert stations. Stand down to condition three. And have my gig made ready for transit at intercept.

From OLD FRIENDS by Alistair Young (2015)



(ed note: Col. Hal Gascoigne is the sole occupant of Satellite Vehicle 1, in charge of a sizable portion of the United States nuclear arsenal. His orders are to stand by, and drop nuclear warheads as per coded commands. Unfortunately the stress and the solitude have driven him totally bat-poop insane.)

     The hammering of the teletype blanked it out. The noise was as loud as a pom-pom in the echoing metal cave. He got up and coasted across the deck to the machine, gliding in the gravity-free cabin with the ease of a man to whom free fall is almost second nature.
     The teletype was silent by the time he reached it, and at first the tape looked blank. He wiped the sweat out of his eyes. There was the message.


     He got out his copy of The Well-Tempered Pogo and checked the speeches of Grundoon the Beaver-Chile for the key letter-sequence on which the code was based. There weren’t very many choices. He had the clear in ten minutes.


     There it was. That was what he had been priming the bomb for. But there should have been earlier orders, giving him the go-ahead to prime. He began to rewind the paper.
     It was all blank.
     And Washington? Why would the Joint Chiefs of Staff order him to bomb Washington?

(ed note: For the record, Grundoon is a baby groundhog, not a beaver. And while there are many collections of Pogo comic strips, The Well-Tempered Pogo is not one of them.)

From KING OF THE HILL by James Blish (1955)

(ed note: Gilead is an undercover agent. He has found some vital intel at the bad guy's lair in the New Age Hotel, and has sent microfilm copies through the mail to a dead drop location. He is arrested by what appears to be the police, and is thrown in a cell. There is a stranger named Baldwin also in the cell. Gilead doesn't know it but Baldwin is part of a secret order of superintelligent humans, and Baldwin wants to recruit Gilead. The trouble is that Baldwin has to talk to Gilead, without the police listening in.)

      “What’s itching you, Captain?” Baldwin asked gently.
     Gilead turned. His cellmate had dealt a solitaire hand on the bench and was calmly playing.
     “I’ve got to raise the turnkey and send for a lawyer.”
     “Don’t fret about it. Let’s play some cards.” He reached in a pocket. “I’ve got a second deck; how about some Russian bank?”
     “No, thanks. I’ve got to get out of here.” He shouted again-still no answer.
     “Don’t waste your lung power. Captain,” Baldwin advised him. “They’ll come when it suits them and not a second before. I know. Come play with me; it passes the time.” Baldwin appeared to be shuffling the two decks; Gilead could see that he was actually stacking the cards. The deception amused him; he decided to play—since the truth of Baldwin’s advice was so evident.

     “If you don’t like Russian bank,” Kettle Belly went on, “here is a game I learned as a kid.” He paused and stared into Gilead’s eyes. It’s instructive as well as entertaining, yet it’s simple, once you catch on to it.” He started dealing out the cards. “It makes a better game with two decks, because the black cards don’t mean anything—just the twenty-six red cards in each deck count—with the heart suit coming first. Each card scores according to its position in that sequence, the ace of hearts is one and the king of hearts counts thirteen; the ace of diamonds is next at fourteen and so on. Savvy?”

     (A♥ = A, 2♥ = B, 3♥ = C, 4♥ = D … K♥ = M, A♦ = N, 2♦ = O… K♦ = Z)

     “And the blacks don’t count. They’re blanks … spaces. Ready to play?”
     “What are the rules?”
     “We’ll deal out one hand for free; you’ll learn faster as you see it. Then, when you’ve caught on, I’ll play you for a half interest in the atomics trust—or ten bits in cash ($1.25).” He resumed dealing, laying the cards out rapidly in columns, five to a row. He paused, finished. “It’s my deal, so it’s your count. See what you get.”

     It was evident that Baldwin’s stacking had brought the red cards into groups, yet there was no evident advantage to it, nor was the count especially high—nor low. Gilead stared at it, trying to figure out the man’s game. The cheating, as cheating seemed too bold to be probable.
     Suddenly the cards jumped at him, arranged themselves in a meaningful array. He read:

      (They can see and hear us)

     The fact that there were only two fives-of-hearts available had affected the spelling but the meaning was clear. Gilead reached for the cards. “I’ll try one. I can beat that score.” He dipped into the tips belonging to the suit’s owner. “Ten bits it is.”
     Baldwin covered it. Gilead shuffled, making even less attempt to cover up than had Baldwin. He dealt:

      (What's your game?)

     Baldwin shoved the money toward him and anted again. “Okay, my turn for revenge.” He laid out:

      (I'm on your side)

     “I win again,” Gilead announced gleefully. “Ante up.” He grabbed the cards and manipulated them:

      (Yeah, prove it)

     Baldwin counted and said, “You’re too smart for me. Gimme the cards.” He produced another ten-bit piece and dealt again:

      (I'll help you get out)

     “I should have cut the cards,” Gilead complained, pushing the money over. “Let’s double the bets.” Baldwin grunted and Gilead dealt again:

      (Nuts, I'm safer in gaol)

     “I broke your luck,” Baldwin gloated. “We’ll double it again?”

      (You Are Nuts, this no jail)

     The deal shifted:

      (Keep Talking Bud)

     Baldwin answered:

      (This New Age Hotel)

     As he stacked the cards again Gilead considered these new factors. He was prepared to believe that he was hidden somewhere in the New Age Hotel; in fact the counterproposition that his opponents had permitted two ordinary cops to take him away to a normal city jail was most unlikely-unless they had the jail as fully under control as they quite evidently had the hotel. Nevertheless the point was not proven. As for Baldwin, he might be on Gilead’s side; more probably he was planted as an agent provocateur-or he might be working for himself.
     The permutations added up to six situations, only one of which made it desirable to accept Baldwin’s offer for help in a Jail break-said situation being the least likely of the six.
     Nevertheless, though he considered Baldwin a liar, net, he tentatively decided to accept. A static situation brought him no advantage; a dynamic situation—any dynamic situation—he might turn to his advantage. But more data were needed. “These cards are sticky as candy,” he complained. “You letting your money ride?” “Suits.” Gilead dealt again:

      (Why Am I Here?)

“You have the damnedest luck,” Baldwin commented:
      (Films escape before you crack)

     Gilead swept up the cards, was about to “shuffle,” when Baldwin said, “Oh oh, school’s out.” Footsteps could be heard in the passage. “Good luck, boy,” Baldwin added.
     Baldwin knew about the films, but had not used any of the dozen ways to identify himself as part of Gilead’s own organization. Therefore he was planted by the opposition, or he was a third factor.
     More important, the fact that Baldwin knew about the films proved his assertion that this was not a jail. It followed with bitter certainty that he, Gilead. stood no computable chance of getting out alive. The footsteps approaching the cell could be ticking off the last seconds of his life.
     He knew now that he should have found means to report the destination of the films before going to the New Age. But Humpty Dumpty was off the wall, entropy always increases-but the films must be delivered.
     The footsteps were quite close.
     Baldwin might get out alive.
     But who was Baldwin?

     All the while he was “shuffling” the cards. The action was not final; he had only to give them one true shuffle to destroy the message being set up in them. A spider settled from the ceiling, landed on the other man’s hand. Baldwin, instead of knocking it off and crushing it, most carefully reached his arm out toward the wall and encouraged it to lower itself to the floor. “Better stay out of the way, shorty,” he said gently, “or one of the big boys is likely to step on you.”
     The incident, small as it was, determined Gilead’s decision—and with it, the fate of a planet. He stood up and handed the stacked deck to Baldwin. “I owe you exactly ten-sixty,” he said carefully. “Be sure to remember it—I’ll see who our visitors are.”

From GULF by Robert Heinlein (1949)

Public-key Cryptography

Lots of communication will have to be encrypted for security reasons, especially if a crew member is doing some online banking. For non-military purposes, a useful innovation is Public-Key Cryptography. It allows two valuable functions:

[1] Encrypting messages
This allows anybody to send an encrypted message to you that only you can read.
[2] Digital signature
This allows you to send a message to everyone with a "signature" proving you were the one who sent it. It verifies your identity.

[1] Encrypting Messages

Ordinary encryption allows specific persons to send you a message that only you can read. Problem is that the encryption key they use is also the decryption key. Which means anybody with the key can read the encrypted message, not just you.

There is also a problem when you try distributing the key to the specific people you want. If an evil spy intercepts the key, they can read any message sent to you. Worse: the evil spy might swipe the key and subsitute their own secret key. Sending the key safely to your friends is really hard.

With public key encryption this is not a problem. The encryption key is NOT also the decryption key. You can broadcast the encryption key far and wide with no attempt at security, it won't hurt anything. This makes distributing the encryption key easy, just post it on the internet. It doesn't matter if evil spy obtains it, they cannot use to decrypt your messages.

[2] Digital Signature

When it comes to digital signature, the trouble with ordinary encryption is that it cannot do it at all. Public-key encryption can.

Digital signature is an extra bit of encoded message you attach to a cleartext message. It "gives a recipient reason to believe that the message was created by a known sender, that the sender cannot deny having sent the message (authentication and non-repudiation), and that the message was not altered in transit (integrity)." Better than a pen-and-ink signature on a contract, since a digital signature sort of rots away if somebody changes the contract.

How Does It Work?

If you only wanted to know why public-key encryption is so popular, you can stop reading now. What follows is the esoteric inner workings of the scheme.

Encryption is taking a message (the "plaintext") and scrambling it into an unreadable mess (the "cyphertext") so that the Wrong People cannot read it. The Right People can unscramble the message back into plaintext and read the message. It is much like two last-century children sending messages using their Captain Midnight secret decoder badges ("Be sure to drink your Ovaltine").

Old fashion garden-variety encryption uses what is called a Symmetric-key algorithm. The sender takes the plaintext plus an encryption key (in the form of a huge number with many digits) and feeds both into the encryption algorithm (meaning you put both into your encryption sofware or smartphone app). It spits out the cyphertext.

The cyphertext is sent to the recipient. They feed the cyphertext and the encryption key to their encryption algorithm and the cleartext pops out. The Wrong People cannot intercept the message (unless they manage to crack the encryption or swipe the encryption key).

What makes this "symmetric-key" is the fact that both sender and recipient are using the same key.

Public-Key Cryptography is different. There are two keys: A and B. The message encrypted with key A can be unencrypted with key B and the message encrypted with key B can be decrypted with key A. And a message encrypted with key A cannot be decrypted with key A. This is also called an asymmetric-key algorithm.

The two keys are generated by the user using a key-pair generating software, by feeding the software a huge random number.

How do you use it?

The user calls key A the "public key", and broadcasts it far and wide. Key B is the "private key", and the user keeps it a deep dark secret.

For [1] encrypting messages, anybody can use your public key to encrypt a secret message intended just for you. The message can only be decrypted with your private key, which presumably only you know. Thus the sender is reasonably sure you are the only one who can read the secret message.

In the example at right, Bob sends the secret message "Hello Alice!", encrypting it with Alice's public key. Only Alice can decrypt it by using her private key.

For [2] digital signature, it is a bit more complicated. To do it simplistically, you would "sign" a document (say, a contract) by encrypting the document with your private key. Anybody else in the universe can use your public key to decrypt the contract. The fact that your public key worked is proof that the document was encrypted with your private key, which presumably only you have. Thus it is proof you "signed" the document.

In the example at right, Alice digitally signs the contract "I will pay $500" by encrypting it with Alice's private key. Bob can verify Alice's signature since the message decrypts with Alice's public key.

In practice, since encrypting a huge document can take an annoyingly large amount of computing time, digital signature generally use a cryptographic hash (message digest) of the document. The hash is encrypted with the private key instead of the entire document. The encrypted hash is transmitted along with the plaintext document. To verify that you digitally signed the document a person has to:

  1. Decrypt the encrypted hash with your public key
  2. Generate a hash of the plaintext document
  3. If the two hashes are identical, the digital signature is valid

Certificate Authority

Clever readers will have spotted the flaw in the system: How do you know the public key is really associated with the person? Some disaffected young person living in their mother's basement can claim they are Stephen Hawking and distribute a public key. How is anybody going to be sure that any given public key actually comes from who claims it?

What is needed is a Public key infrastructure. Among other things, the infrastructure binds a person with their public key, that is, they ensure that Stephen Hawking is actually associated with Stephen Hawking's public key and not some disaffected young person living in their mother's basement. This is done with a Certificate Authority (which is also a Trusted third party).

A person who has generated a public and private key makes an application to the certificate authority. The authority does some checking to ensure that person is actually who they say they are. Usually this is via some simplistic domain validation but the validation becomes more stringent as the amount of money at stake increases.

Assuming the person passes the check, the certificate authority issues a public key certificate, which is a fancy way of saying the CA broadcasts the public key and assures everybody that it actually belongs to the person. Of course the public key certificate is digitally signed with the CA's private key.

Automatic Identification System

An Automatic Identification System (AIS) is a radio system on a vehicle that continually broadcasts the vehicle's unique ID, position, course, speed, and any other vital navigational information. Currently these are found mainly on naval vessels.

A Transponder is a radio system on a vehicle that responds to a radio interrogation message with a radio reply containing the information. Interrogations such as "identify yourself", "what is your altitude?" etc. If there are no interrogations, the transponder is silent. Currently these are found mainly on aircraft. "Transponder" is short for "transmitter-responder".

AIS and Transponders are the responsibility of the ship's communication officer. Which is why this section is in the "Communication Deck" page.

In the Traveller role playing game, by law civilian starships are equipped with something they call a transponder, but which acts more like an AIS (it is constantly broadcasting). In Traveller it is illegal for a civilian starship to turn off its transponder, unless there are mitigating circumstances. Which usually means a fear of pirate corsairs using your transponder signal to home in on you.

Pirate ships will need illegal "variable transponders" that can be set to broadcast fake identities, in order to lull the suspicions of both victims and police.

For military ships in a combat situation, use of such systems is tricky. The point is the enemy may be armed with radio-homing missiles, in which case the last thing you want to do is to give them some radio to home in on. So an AIS continuously broadcasting is probably a bad idea. On the other hand, an Identification, Friend or Foe System can help prevent friendly forces from firing upon you by accident. For that, you use a transponder that only responds to an interrogation message; and only an encrypted interrogation at that. The rest of the time the transponder is silent and radio-homers will search in vain.


Identification, friend or foe (IFF) is a radar-based identification system designed for command and control. It uses a transponder that listens for an interrogation signal and then sends a response that identifies the broadcaster. It enables military and civilian air traffic control interrogation systems to identify aircraft, vehicles or forces as friendly and to determine their bearing and range from the interrogator. IFF may be used by both military and civilian aircraft. IFF was first developed during World War II, with the arrival of radar, and several friendly fire incidents.

Despite the name, IFF can only positively identify friendly targets, not hostile ones. If an IFF interrogation receives no reply or an invalid reply, the object cannot be identified as friendly, but is not positively identified as foe; it may, for instance, be a friendly aircraft with an inoperative or malfunctioning transponder. There are in addition many reasons that friendly aircraft may not properly reply to IFF.

IFF is a tool within the broader military action of Combat Identification (CID), the characterization of objects detected in the field of combat sufficiently accurately to support operational decisions. The broadest characterization is that of friend, enemy, neutral, or unknown. CID not only can reduce friendly fire incidents, but also contributes to overall tactical decision-making.


With the successful deployment of radar systems for air defence during World War II, combatants were immediately confronted with the difficulty of distinguishing friendly aircraft from hostile ones; by that time, aircraft were flown at high speed and altitude, making visual identification impossible, and the targets showed up as featureless blips on the radar screen. This led to incidents such as the Battle of Barking Creek, over Britain, and the air attack on the fortress of Koepenick over Germany.


Early concepts

Already before the deployment of their Chain Home radar system (CH), the RAF had considered the problem of IFF. Robert Watson-Watt had filed patents on such systems in 1935 and 1936. By 1938, researchers at Bawdsey Manor began experiments with "reflectors" consisting of dipole antennas tuned to resonate to the primary frequency of the CH radars. When a pulse from the CH transmitter hit the aircraft, the antennas would resonate for a short time, increasing the amount of energy returned to the CH receiver. The antenna was connected to a motorized switch that periodically shorted it out, preventing it from producing a signal. This caused the return on the CH set to periodically lengthen and shorten as the antenna was turned on and off. In practice, the system was found to be too unreliable to use; the return was highly dependent on the direction the aircraft was moving relative to the CH station, and often returned little or no additional signal.

It had been suspected this system would be of little use in practice. When that turned out to be the case, the RAF turned to an entirely different system that was also being planned. This consisted of a set of tracking stations using HF/DF radio direction finders. Their aircraft radios were modified to send out a 1 kHz tone for 14 seconds every minute, allowing the stations ample time to measure the aircraft's bearing. Several such stations were assigned to each "sector" of the air defence system, and sent their measurements to a plotting station at sector headquarters, who used triangulation to determine the aircraft's location. Known as "pip-squeak", the system worked, but was labour-intensive and did not display its information directly to the radar operators. A system that worked directly with the radar was clearly desirable.


The first active IFF transponder (transmitter/responder) was the IFF Mark I which was used experimentally in 1939. This used a regenerative receiver, which fed a small amount of the amplified output back into the input, strongly amplifying even small signals as long as they were of a single frequency (like Morse code, but unlike voice transmissions). They were tuned to the signal from the CH radar (20–30 MHz), amplifying it so strongly that it was broadcast back out the aircraft's antenna. Since the signal was received at the same time as the original reflection of the CH signal, the result was a lengthened "blip" on the CH display which was easily identifiable. In testing, it was found that the unit would often overpower the radar or produce too little signal to be seen, and at the same time, new radars were being introduced using new frequencies.

Instead of putting Mark I into production, a new IFF Mark II was introduced in early 1940. Mark II had a series of separate tuners inside tuned to different radar bands that it stepped through using a motorized switch, while an automatic gain control solved the problem of it sending out too much signal. Mark II was technically complete as the war began, but a lack of sets meant it was not available in quantity and only a small number of RAF aircraft carried it by the time of the Battle of Britain. Pip-squeak was kept in operation during this period, but as the Battle ended, IFF Mark II was quickly put into full operation. Pip-squeak was still used for areas over land where CH did not cover, as well as an emergency guidance system.


Even by 1940 the complex system of Mark II was reaching its limits while new radars were being constantly introduced. By 1941, a number of sub-models were introduced that covered different combinations of radars, common naval ones for instance, or those used by the RAF. But the introduction of radars based on the microwave-frequency cavity magnetron rendered this obsolete; there was simply no way to make a responder operating in this band using contemporary electronics.

In 1940, English engineer Freddie Williams had suggested using a single separate frequency for all IFF signals, but at the time there seemed no pressing need to change the existing system. With the introduction of the magnetron, work on this concept began at the Telecommunications Research Establishment as the IFF Mark III. This was to become the standard for the Western Allies for most of the war.

Mark III transponders were designed to respond to specific 'interrogators', rather than replying directly to received radar signals. These interrogators worked on a limited selection of frequencies, no matter what radar they were paired with. The system also allowed limited communication to be made, including the ability to transmit a coded 'Mayday' response. The IFF sets were designed and built by Ferranti in Manchester to Williams' specifications. Equivalent sets were manufactured in the US, initially as copies of British sets, so that allied aircraft would be identified upon interrogation by each other's radar.

IFF sets were obviously highly classified. Thus, many of them were wired with explosives in the event the aircrew bailed out or crash landed. Jerry Proc reports:

Alongside the switch to turn on the unit was the IFF destruct switch to prevent its capture by the enemy. Many a pilot chose the wrong switch and blew up his IFF unit. The thud of a contained explosion and the acrid smell of burning insulation in the cockpit did not deter many pilots from destroying IFF units time and time again. Eventually, the self destruct switch was secured by a thin wire to prevent its accidental use."


FuG 25a Erstling (English: Firstborn, Debut) was developed in Germany in 1940. It was tuned to the low-VHF band at 125 MHz used by the Freya radar, and an adaptor was used with the low-UHF-banded 550–580 MHz used by Würzburg. Before a flight, the transceiver was set up with a selected day code of ten bits which was dialed into the unit. To start the identification procedure, the ground operator switched the pulse frequency of his radar from 3,750 Hz to 5,000 Hz. The airborne receiver decoded that and started to transmit the day code. The radar operator would then see the blip lengthen and shorten in the given code, ensuring it was not being spoofed. The IFF transmitter worked on 168 MHz with a power of 400 watts (PEP).

The system included a way for ground controllers to determine whether an aircraft had the right code or not but it did not include a way for the transponder to reject signals from other sources. British military scientists found a way of exploiting this by building their own IFF transmitter called Perfectos, which were designed to trigger a response from any FuG 25a system in the vicinity. When an FuG 25a responded on its 168 MHz frequency, the signal was received by the antenna system from an AI Mk. IV radar, which originally operated at 212 MHz. By comparing the strength of the signal on different antennas the direction to the target could be determined. Mounted on Mosquitos, the "Perfectos" severely limited German use of the FuG 25a.

Further wartime developments

IFF Mark IV and V

The United States Naval Research Laboratory had been working on their own IFF system since before the war. It used a single interrogation frequency, like the Mark III and a separate responder frequency. Responding on a different frequency has several practical advantages, most notably that the response from one IFF cannot trigger another IFF on another aircraft. But it requires a complete transmitter for the responder side of the circuitry, in contrast to the greatly simplified system used in the British designs. This technique is now known as a cross-band transponder.

When the Mark II was revealed in 1941 during the Tizard Mission, it was decided to use it and take the time to further improve their experimental system. The result was what became the Mark IV. The main difference between this and earlier models is that it worked on higher frequencies, around 600 MHz, which allowed much smaller antennas. Unfortunately, this also turned out to be close to the frequencies used by the German Würzburg radar and there were concerns that it would be triggered by that radar and the transponder responses would be picked on its radar display and thereby give away the operational frequencies.

This led to a US–British effort to make a further improved model, the Mark V, also known as the United Nations Beacon or UNB. This moved to still higher frequencies around 1 GHz but operational testing was not complete when the war ended. By the time testing was finished in 1948, the much improved Mark X was beginning its testing and Mark V was abandoned.

Postwar systems

IFF Mark X

Mark X started as a purely experimental device operating at frequencies above 1 GHz, but as development continued it was decided to introduce an encoding system known as the "Selective Identification Feature", or SIF. SIF allowed the return signal to contain up to 12 pulses, representing four octal digits of 3 bits each. Depending on the timing of the interrogation signal, SIF would respond in several ways. Mode 1 indicated the type of aircraft or its mission (cargo, for instance) while Mode 2 returned a tail code.

Mark X began to be introduced in the early 1950s. This was during a period of great expansion of the civilian air transport system, and it was decided to use slightly modified Mark X sets for these aircraft as well. These sets included a new Mode 3 which was paired with a civilian Mode A, which operated similar to the original Mode 2 and returned a four-digit identifier. Because Mode 3 and A are identical, they are normally referred to as Mode 3/A. A further addition, Mode C, returned the altitude encoded in a single 12-bit number in Gillham code, which represented the altitude as (that number) x 100 feet - 1200. Mode B and D were specified but never used.


The current IFF system is the Mark XII. This works on the same frequencies as Mark X, and supports all of its military and civilian modes.

The main reason for the creation of Mark XII was the addition of the military Mode 4. Before Mark XII, the transponders would respond to any properly formed interrogation signal, broadcasting a reply that could be picked up by any receiver. Using triangulation, an enemy could determine the location of the transponder. The British had already used this technique against the Germans during WWII, and it was used by the USAF against VPAF aircraft during the Vietnam War.

Mode 4 started with an interrogation similar to Mode 3, but then followed that with an encoded pulse chain similar to the one used in Mode 3/A. The receiver side of the transponder checks this code against a known day code, and only responds if the two match. The pulses in the reply are delayed based on the received code. This largely eliminates the ability for the enemy to trigger the transponder.

During the 1980s, a new civilian mode, Mode S, was added that allowed greatly increased amounts of data to be encoded in the returned signal. This was used to encode the location of the aircraft from the navigation system. This is a basic part of the traffic collision avoidance system (TCAS) system that allows commercial aircraft to know the location of other aircraft in the area and avoid them without the need for ground operators.

The basic concepts from Mode S were then militarized as Mode 5, which is simply a cryptographically encoded version of the Mode S data.

The IFF of World War II and Soviet military systems (1946 to 1991) used coded radar signals (called Cross-Band Interrogation, or CBI) to automatically trigger the aircraft's transponder in an aircraft illuminated by the radar. Radar-based aircraft identification is also called secondary radar in both military and civil usage, with primary radar bouncing an RF pulse off of the aircraft to determine position. George Charrier, working for RCA, filed for a patent for such an IFF device in 1941. It required the operator to perform several adjustments to the radar receiver to suppress the image of the natural echo on the radar receiver, so that visual examination of the IFF signal would be possible.

By 1943, Donald Barchok filed a patent for a radar system using the acronym IFF in his text with only parenthetic explanation, indicating that this acronym had become an accepted term. In 1945, Emile Labin and Edwin Turner filed patents for radar IFF systems where the outgoing radar signal and the transponder's reply signal could each be independently programmed with a binary codes by setting arrays of toggle switches; this allowed the IFF code to be varied from day to day or even hour to hour.

Early 21st century systems


The United States and other NATO countries started using a system called Mark XII in the late twentieth century; Britain had not until then implemented an IFF system compatible with that standard, but then developed a program for a compatible system known as successor IFF (SIFF).


  • Mode 1 – military only; provides 2-digit octal "mission code" that identifies the aircraft type or mission.
  • Mode 2 – military only; provides 4-digit octal unit code or tail number. (usually can't be changed in flight. Some aircraft like the C-17 Block 17 and higher have the capability to do so)
  • Mode 3/A – military/civilian; provides a 4-digit octal identification code for the aircraft, assigned by the air traffic controller.
  • Mode 4 – military only; provides a 3-pulse reply, delay is based on the encrypted challenge.
  • Mode 5 – military only; provides a cryptographically secured version of Mode S and ADS-B GPS position.

Modes 4 and 5 are designated for use by NATO forces.

From the Wikipedia entry for IDENTIFICATION FRIEND OR FOE

(ed note: RE: helping enemy missiles)

      (Here is) an "aside" from the First Gulf War.

I worked AWACS Radar Maintenance and was there as it all transitioned from "Desert Shield" to "Desert Storm".

Just before and during the initial flighting while prepping our birds to fly, we got a visit from a very diplomatic looking individual accompanied by several Security Forces people and a self identified OSI agent.

During those meetings the diplomatic person would carefully open a special briefcase which held something called a MUX which would would dutifully plug into the AWACS IFF cabinet into one of several slots specifically designed for such MUX's.

We (as maintenance folks) were not briefed on them nor given much information at all. But seeing's as the slot was for what we called a "Mode 4" IFF MUX, (Mode 4 being a specially designed, can't turn off mode where, in theory, only your specific command and control systems can interrogate and the system MUST answer of it's powered) and given it was in the 'spare' slot which meant it could be electronically 'swapped' with the standard Mode 4 interrogator which was normally empty; it is fair to say it wasn't for being "helpful".

In context we later confirmed through various means that Yes, in fact it was a Russian (enemy ally) Mode 4 MUX that would interrogate and ID every Iraqi (enemy) aircraft with a standard IFF as to location and status when switched on.

(ed note: translation: the Russian MUX would broadcast a properly encrypted IFF interrogation command, and every single enemy Iraqi aircraft in range would blow its cover by revealing its identity and location.)

In modern combat even your safety systems can be used to kill you under the right circumstances.

by Randy Campbell (2021)

John Reiher: I had a thought. There's one component that never changes on any ship. It can be updated, but it can't be replaced, otherwise the Insurance companies will void your policies.

What is it?

The Vehicle Identification Number (VIN) Box. It's a transponder with a unique ID and call sign. It can be customized at time of purchase, but it is the "ship". Anything attached to it becomes the ship.

No VIN Box, no ship.

Two or more VIN Boxes on a single ship, you're breaking the law and you have to inactivate all but one of them.

This is something the Banking and Insurance companies would come up with. They need something that can be unique to each ship, and nothing is more unique than a government issued, sealed, black box VIN Box.

Winchell Chung

+John Reiher idea for a VIN Box is probably my favorite. It is very much the sort of thing that banking and insurance companies would come up with. And the requirement for renewability makes perfect sense. However it must always contain a legal ID, for liability purposes. Much like automobiles. If somebody crashes their car into a building or something else expensive, then flees the scene on foot, the car's license plate may be expired but it still allows the police and building owners to discover who is liable for the damages.

This also vaguely reminds me about ship transponders in the Traveller role playing game. They constantly broadcasts the ship's unique ID and location. Civilian starships are legally required to have transponders always turned on, unless there are extenuating situations. Such as pirate corsair ships in the area, using the transponder to home in on their prey.

Alistair “Cerebrate” Young Side note on registries and transponders: of course, the Worlds being a not-exactly-unified group of polities, actual requirements on these points vary widely.

Even leaving aside the anarchic Rim Free Zone (the entirety of whose admiralty law could be summed up as "try not to hit anything that might complain"), the Accord on the Law of Free Space leaves it at the minimal "you should have a certificate of registry and a transponder that will squawk it out when queried". Local regulations, on the other hand...

(The Empire, for example, which holds that sovereignty begins with the individual, will happily accept self-signed registries and doesn't require much transponding, although lacking functionality in this area may leave you restricted to operating VFR and staying outside all regions of controlled space.

The other end of this particular bell curve is the Hope Hegemony, which wants transponders willing to disgorge pretty much any information you can think of on demand, including your code-signed visa from the Hegemony Bureau of Navigation and remote-slave ackles for your starship, and legally defines any vessel without such as "debris, subject to salvage and/or destruction at discretion".)

(ed note: "ackles" is Access-Control List. "Remote-slave ackles" means to grant the Hope Hegemony government the ability to seize command of your ship and fly it by remote control, at their whim)

John Reiher

Also, I'm using a setting where while there are multiple governments, they do have treaties with each other and have agreed upon a common ship's registry system just to keep the confusion down and to prevent what you proposed: Smugglers with multiple transponders.

From private conversation on Google Plus (2015)

The Xinglong died stupid. Afterward, everyone knew she was one of thousands of small-time rock-hopping prospector ships. The Belt was lousy with them: five-or six-family operations that had scraped together enough for a down payment and set up operations. When it happened, they'd been three payments behind, and their bank—Consolidated Holdings and Investments—had put a lien on the ship. Which, common wisdom had it, was why they had disabled her transponder. Just honest folks with a rust bucket to call their own trying to keep flying.

If you were going to make a poster of the Belter's dream, it would have been the Xinglong.

The Scipio Africanus, a patrol destroyer, was due to head back down toward Mars at the end of its two-year tour of the Belt. They both headed for a captured cometary body a few hundred thousand kilometers from Chiron to top off their water.

When the prospecting ship first came in range, the Scipio saw a fast-moving ship running dark and headed more or less in their direction. The official Martian press releases all said that the Scipio had tried repeatedly to hail her. The OPA pirate casts all said it was crap and that no listening station in the Belt had heard anything like that. Everyone agreed that the Scipio had opened its point defense cannons and turned the prospecting ship into glowing slag.

(ed note: our Belter heroes have secretly escaped the destruction of the Martian battleship Donnager by flying away in one of the carried vessels, the Tachi. They are now trying to figure out what to do next.)

"Someone is going to investigate what happened to the Donnager." Holden said. "Martian ships are speeding to that spot as we speak. They'll already know the Tachi got away, because our transponder is blabbing our survival to the solar system at large."

"No it ain't," Alex said.

"Explain that, Mr. Kamal."

"This is a torpedo bomber. You think they want a nice transponder signal to lock on to when they're makin' runs on an enemy capital ship? Naw, there's a handy switch up in the cockpit that says 'transponder off.' I flipped it before we flew out. We're just another moving object out of a million like us."

Holden was silent for two long breaths.

"Alex, that may be the single greatest thing anyone has ever done, in the history of the universe," he said.

"But we can't land, Jim," Naomi said. "One, no port is going to let a ship with no transponder signal anywhere near them, and two, as soon as they make us out visually, the fact that we're a Martian warship will be hard to hide."

"Yep, that's the downside," Alex agreed.

From LEVIATHAN WAKES by "James S.A. Corey" (Daniel Abraham and Ty Franck) 2011.


     Transponders are one of those touchy subjects that it's hard to do justice to. A transponder can save your life, or can cost you a fortune. It all depends on when it's blaring and who's listening.

     The transponder's case, if you're running legal, is sealed at the factory, and the only way to change its signal is to have an Imperially-licensed technician come aboard. The gadget is mounted in a hard to reach spot in the hull, usually, and the box around it is the same stuff the Navy uses to make battle dress.

     Wired into the bridge's commo station, it's got one switch, and only one switch. Under normal circumstances, you‘re required to turn the transponder on. If your ship is in a “certified” emergency, you can turn the transponder off. I always thought it was a good piece of luck that the transponder doesn't know what a certified emergency is.

     The Imperium doesn't want pirates to nab you, so you're allowed to shut down so your ship doesn't act like a homing beacon, but the authorities don't understand that some customs duties are piracy, too. Smugglers aren't supposed to switch off when they jump in, but a lot of them do.

     It's not that easy, though, to escape the watchful eyes of the taxman. You have to remember that the transponder case also houses your vessel's flight recorder, and that thing's got a memory as long as a comet's tail. Anybody who dumps out that log can tell in a real hurry whether you've been skirting the law. You're not going to change that log, either, unless you've got a lot more computer know-how than I've got, because they use one of those new-fangled security codes to encrypt the stuff. "In cooperation with Imperial investigators", you can read it. But that misses the point, somehow.

     Maybe I should backjump here a bit and point out that I'm not advising anyone to tamper with their transponders, and I'm not saying I've ever done it myself. But whenever I talk to young folks like yourself, they always seem to ask about transponders, so I thought I'd beat you to the punch and explain without having to listen to your fool questions.

     Anyway, the cleverest set-up I ever saw was a ship that had two transponders, and a gizmo to simulate regular ship activities for the extra one. Took the customs agents a long while to figure out what he was doing, but they eventually caught him. I've also seen guys use a lot of equipment to jam their own signal, so they can just leave the transponder on all the time. That always seemed like too much trouble to me.

     I'll give you one rule about transponders: nothing you try will last forever. If you're going to fool around, you've got to bank on eventually getting caught. And you've got to figure if it will be worth it when you are. A lot of merchants seem to make a lot of money and still keep their noses clean.

     Military transponders are different from the civilian models, and I know a lot of smugglers who would pay dearly tor a military job. The transponder is completely configurable — anything you want it to broadcast, it can. This is convenient for a cruiser, say, because it can jump into a system and pretend to be such-and-such a fat trader. It the other ships insystem aren't on their toes with their other sensors, they may discover the truth too late.

     Speaking of sensors, it reminds me of an incident when we were working out of Frenzie. We had a fresh commo officer on board, name of Niim, and the crew just wouldn't let him alone.

     Once we jumped, nav asked this young kid to take over his station while he went to the fresher. The kid sits down, only to start picking up passive EMS on the sensors. He stared at the panel, reconfigured it, and stared some more. The rest of the bridge crew were trying not to let on; we were in on the prank and wanted it to go on as long as it could.

     Nav was just outside the door, running the whole set-up from his hand computer. He'd loaded a sensor simulation into his station, and what commo saw now was a fleet of Zhodani cruisers out there in jumpspace. The simulation was a real beauty; after a few seconds, our passive started picking up their active scans. The only thing Niim couldn't figure out was how all this was happening in jumpspace. He didn't say anything about it, because he thought it was some kind of error, but he checked over the systems, and I could see it was gnawing on him.

     The icing on the cake was when his own station's telltale started flashing. He didn't want to walk away from nav and the sensors, so he reconfigured the panel so he could handle radio ops too. He piped the signal into his earphones, and nearly fell out of the chair. Here was a Zhodani voice, heavily accented, whispering, "We‘re coming to get you, Niim."

     Commo spun around and stared at me, stark terror in his eyes, and nav never forgave me for bursting out laughing.

     Anyway, a ship doesn't have to restrict itself to radio, but radio is the only means of broadcasting information, so that's what the transponder uses, and radio is the easiest to communicate over when you don't know your exact position or the exact position of your target.

     Some military and Scout vessels prefer mesons, because they're straight-line narrowcast and don‘t depend on line of sight. A meson communicator can cut right through a star if it wants to, no extra charge. Doesn't mean they're any good as weapons, though, any more than a bullet could be used as a missile: it's a matter of scale.

     Which reminds me of another reason you want a skilled commo onboard if you're conducting military ops. Say you're in a fleet of six ships, and the command ship has an expert in tactics on board. Your life depends on that officer's brain, and if you can't contact it, you might not get a second chance.

     So meanwhile, your pilot is swinging the ship this way and that, dodging missiles or whatever. If your vessel has a spinal mount, every time the gunners want to take a shot the pilot has to move the ship to line up the muzzle with the target. Even with the smaller movable mounts, you've got the problem of sandcasters on one side and lasers on the other.

     What this all means is that narrowcast transmissions have to be carefully calculated, too, just as accurately as a beam weapon. If your commo isn't up to it, you're cut off from the brains of your fleet. You could use broadcast, l suppose, if you don't mind turning your vessel into a big homing beacon for the enemy's EMS. But I wouldn't be talking to you if you were stupid, so I don't need to say any more about that idea.

From MEGATRAVELLER STARSHIP OPERATOR'S MANUAL by Digest Group Publications (1988)

(ed note: comment is about civilian starship Automatic Identification System in the game Traveller)

I think I'm going to add a line in the rules about a ship being considered "Active" when it either jumps into, or out of, a system.

Ships can still be sneaky—it just requires that they jump into a system at least 2-3 light seconds away from another vessel. If you want to be stealthy, jump into a system at very long range and take your time approaching your victim.


A ship's transponder will consider most ships as "Active", and if you detect a vessel that doesn't have it's transponder "on", then he's probably up to no good.

No transponder broadcast = Red Flag.

It's Imperial Law to keep your ship's transponder broadcasting at all times, but I look upon this like the speed limit on a real life freeway. Just as speed governors on cars are illegal but there's a speed limit, ship's are required to keep their transponders broadcasting at all times but can turn them "off".

The reason they can be turned "off" is probably because of pirate activity. But, finding a ship with a transponder not broadcasting is a sure-fire way to get boarded (and fined) by a system patrol boat.

Heck, the GM could take this another step and set up "transponder traps" in the same way we have "speed traps" here in the real world.

"Pysadi? Naw, can't stand that piddly TL world. They don't have an inter-system Navy, and sometimes pirates make a run through that system...but the Pysadians sure don't have a problem using one of their three ancient patrol cutters to board ya and fine ya for being safe and runnin' with your transponder down."

From comment to thread CLASSIC TRAVELLER SENSOR RULES (2006)

(ed note: comment is about civilian starship Automatic Identification System in the game Traveller)

I'm in agreement with most of what has been posted. In my Traveller universe, civilian transponders ...

  1. are mounted in the same container as the ship's black box recorder
  2. are hard wired to the ship's powerplant and supplimented by a battery backup
  3. are nearly impossible to access mechanically by design, due to location in the hull and interference from other equipment
  4. emit a code referencing ship's VIN and port of registry
  5. are electrically isolated from the ship's computer and require specialized (i.e. controlled) equipment to access electronically (e.g. anti-theft features on consumer products)

Military craft have transponders that can be manipulated at will to produce any number of emissions; from merchants to battleships to enemy IDs to nothing at all.

This included active duty scout craft, as altering an electronic signature is a piece of cake for a sensor platform. Reserve scout craft are normally retro fitted with civilian style transponders; but as an after-market modification, most Scout engineers know how to bypass them without breaking the seals.

From comment to thread TAMPERING WITH TRANSPONDER: ILLEGAL, YES... BUT HOW HARD? by Ran Targas (2006)

(ed note: comment is about civilian starship Automatic Identification System in the game Traveller)

Also don't forget that vessels can turn off their transponders at will. The result of those 'blank' periods depends on a whole host of factors. Turn off your transponder while inbound to Lunion and expect a full body cavity search. Have a customs inspection of your transponder's records reveal that you ran 'quiet' off Jasedipere during a corsair raid and you'll be credited for quick thinking.

It appears that 'tampering' with a transponder is much like everything else. It depends on who you know and why you want to do it.

From comment to thread TAMPERING WITH TRANSPONDER: ILLEGAL, YES... BUT HOW HARD? by Whipsnade (2006)

      HONG KONG (CNN) The United Nations brought down the hammer and that was it for the Hao Fan 6.
     On October 10, the hulking, 460-foot (140 meter) cargo ship was banned from entering every single port across the globe, punished for violating sanctions on North Korea.
     It was just south of South Korea the day the news was announced, according to tracking information by MarineTraffic. Its transponder pinged continuously until 11:17 p.m. Coordinated Universal Time, the data showed.
     Then the Hao Fan 6 disappeared.

A ship crosses dry land

     The Hao Fan 6's journeys in the weeks before the ban show the massive ship, which can transport 8,343 tons of cargo, appearing to travel on land across large swaths of South Korea.
     These aren't errors. They're clues.

     Most modern vessels are tracked using an Automatic Identification System (AIS) transponder. The International Maritime Organization stipulates large ships must have one on board.
     Turn it off and a ship can hide from prying eyes or potential threats. Once turned back on, tracking data will show a big and unusual jump.
     "There is little that can be done to prevent captains independently switching them off," Andrea Berger, a senior research associate who specializes in North Korea's weapons programs and sanctions at the Middlebury Institute of International Studies, told CNN.
     After going silent on October 10, the Hao Fan 6 didn't turn on its transponder for the rest of the month.
     Berger said it's common for North Korean-linked vessels engaging in illegal behavior to turn off their transponders for periods of their voyage.
     Experts say transponders are usually shut off if a ship is being threatened, often due to piracy.

Three trips to North Korea

     The Hao Fan 6's historical data shows three visits to North Korea in 2016 and activity on traditional coal shipping routes.
     Twice in the fall it was tracked near a North Korean port city, Nampo. The first time was September 27.
     If the Hao Fan 6 was transporting coal, it would've been in violation of a UN Security Council Resolution passed in March 2016. The Security Council has passed multiple rounds of sanctions since, most recently in September this year.
     CNN asked Hugh Griffiths, the coordinator of the UN Panel of Experts on North Korea — the body charged with monitoring the enforcement and efficacy of sanctions on the hermit nation — about the possibility the Hao Fan 6 was moving coal. He did not comment, but said it's vital that UN members fully implement Security Council resolutions.
     "Part of that is very much paying close attention to vessels delivering coal," Griffiths said.
     Coal has provided a crucial economic lifeline for Pyongyang. In 2015, coal exports netted nearly a billion dollars of revenue, according to UN data. Chinese companies were big buyers, as North Korean coal is close by and cheap.

North Korea's 2015 exports

     Marshall Billingslea, the assistant secretary for terrorist financing at the US Treasury Department, in his testimony to the US Senate in September, used satellite imagery and AIS data to show three ships transporting illicit North Korean coal — and turning off their AIS transponders while doing so — while traveling between China and Russia.
     More recently, Nikki Haley, the US ambassador to the United Nations, called for the global community to do more to crack down on North Korean sanctions violations.
     "This Council has banned coal exports from North Korea. And yet, we have reports of the regime continuing to smuggle coal into neighboring Asian countries using deceptive tactics to mask the coal's origins," Haley told the United Nations shortly after the November North Korean missile test.


Before owning a car became typical, roads and highways (the few that existed) were never crowded. It was only after everyone started purchasing and driving their own vehicles—to work, school, even the grocery store around the block—that streets grew congested, rush hour became an everyday occurrence, and car accidents became an inevitability.

Space, despite its vastness, could be on a similar trajectory. With so many new flying objects being sent into orbit and beyond, many scientists say, we could be in for some dangerous collisions. One group at Los Alamos National Laboratory in New Mexico is trying to fix that with something ubiquitous among cars, but currently nonexistent for space mobiles: A license plate.

It’s unlikely we’ll ever fill our cosmic neighborhood with enough flying objects to create anything like a space traffic jam, where satellites have to slow down at specified times or travel in their own intergalactic lanes. But space, in all its low-to-no-gravity glory, poses its own challenges.

The majority of satellites and other fancy objects we send into the cosmos stay in low Earth orbit (LEO), around 400 to 1,000 miles above Earth’s surface. It’s far enough away from the planet’s gravitational pull, but not too far; this sweet spot lets an object orbit pretty much indefinitely without needing much help.

But scientists have taken advantage of this prime parking space for the past 60 years, so debris is starting to build up. There’s more space junk on the way, too. As of 2015, there are more than 1,300 active satellites orbiting the Earth. That’s in addition to the inactive ones, as well as old rockets and other defunct space junk stuck in LEO indefinitely. That’s likely to increase exponentially with the introduction of CubeSats, miniature spacecraft that can be sent into space by the hundreds, and the various companies that plan to install internet-providing satellites in the LEO. What a space jam.

But don’t things just float around up there like a giant game of bumper cars? Not even close, explains David Palmer, an astrophysicist at Los Alamos. While there have only been two really substantial space crashes, he says, one crash is all it takes to trigger catastrophe.

“The problem is that once you have one collision, it makes a lot of debris, and that debris can then collide. Eventually you get what’s called the Kessler effect,” he says. Debris keeps building up with each new collision, creating infinitely more crashes. If this begins to happen on a regular basis, it’s possible for so much space junk to accumulate that space itself becomes unsafe.

“We are close to the point where if we keep on going for a little while longer, we will be pushing over the edge,” he says. “Once that starts happening, it can progress for a decade or two until there is too much debris in low Earth orbit.” At that point, the chances of a collision (and subsequent Kessler effect) become so high that the benefits of sending another satellite into LEO don’t outweigh the risks.

The big problem is when satellites retire. While in use, nearly all have GPS devices that scientists can use to find them with radio signals. But once satellites are out of commission, so are those radio waves. The space junk just orbits, without monitor, indefinitely. If an active satellite seems like it’s going to collide with one of them, the owner can dodge out of the way. But once a point is reached where space junk is just colliding with space junk, researchers can’t move either bodies out of the way without an owner’s permission. And to know who it belongs to, you first need to know what the object is. “You would need the permission of the satellite owner even if it’s a 30-year-old piece of space junk,” says Palmer. There has to be another method—a foolproof identification system. That’s how Palmer sees it.

His past research studying pulsars gave him an idea. Pulsars are large space objects that spin and emit beams of light in opposing directions. As they spin their light beams appear to flicker, and scientists, like Palmer, have put a lot of effort into studying that flickering.

“I put those two things together and thought that if I could create a satellite that produced a periodic signal,” says Palmer, “then by reading that signal, we could create an accurate identification system.

So Palmer and his team developed a super-low-power, miniature device that emits a unique pattern of blinks in the form of a laser. They call it Extremely Low Resource Optical Identifier (ELROI) or, more casually, space license plates.

Meant to be about a square inch by square inch, these devices would sit atop any satellite bound for space. Using a laser diode (what many everyday lasers are made of), the device would emit a series of very short red laser pulses. These flashes would be extremely bright (as bright as a 60 watt bulb, from something that uses only one watt of total power), and would continuously flash a series of specific pulses for no more than a millionth of a second. It would then shut off for a thousandth of a second—a thousand times as long as it was on for. A telescope on Earth could pick up that series of flashes, and with the help of a computer program, decipher what specific satellite it was coming from.

Despite the satellite being hundreds of miles away from the ground, the message is still crystal clear, explains Palmer. The light is coming from a laser and is the exact wavelength of the color red. A filter in the telescope blocks out all other wavelengths, allowing that color to shine through. The blinking corresponds to a binary (composed of hundreds of 1s and 0s) serial number. Each satellite, like a car, gets its own serial. The code tells the identifier three things: The kind of satellite it is, who owns it, and the path of its orbit.

The goal, says Palmer, is to have the devices sit on satellites and remain powered for at least 25 years, if not indefinitely. The the device uses a huge percentage of its battery’s life during its split second of blinking, but then spends a thousand times longer charging with solar energy. It can do this repeatedly, says Palmer, for about 25 years. Palmer thinks the device will be able to run off of its solar cells indefinitely even after the battery dies, but it’s impossible to be sure.

Their next goal is to actually test the prototype. They plan to do so by working with a team at New Mexico Tech who are sending a CubeSat into space this coming January. CubeSats, says Palmer, are an ideal vessel to test the device on: They are tiny, 10 centimeter cubed satellites that can be sent up into LEO, often en masse, to perform small experiments.

Though the prototype is ready for a test launch, Palmer says, the researchers are still tweaking the design. The ELROI is currently about four square inches and one inch thick. In the future, the team wants to get it down to the size of a postage stamp, and use materials such that the entire thing costs under $1000. That way, he says, even a high school science class can afford to send one up to low Earth orbit. CubeSats' low cost (one can cost only $10,000 to build; launching them into space is a few tens of thousands more) means they can be used by small, private companies, university students, and even high schoolers. Make the license plate price any higher, says Palmer, and, “that’s a lot of cakes you would have to sell at the bake sale.”

With the license-plated CubeSats up in space, the researchers will attempt to identify the miniature spacecraft by pointing a telescope at them and translating the binary code. If all goes according to plan, the goal is to test them on bigger satellites, with the ultimate objective to get them on every satellite that enters space.

Palmer admits that ELROIs are not a perfect solution. If two satellites are indeed colliding, you still need to ask for permission from the abandoned space junk’s owner before you remove it from orbit. If there isn’t enough time to track that person down, knowing who they are might not help you avoid disaster. But identifying who owns the aged space junk is indeed a good first start. It might not be too long before low-Earth orbit looks more like a busy highway than a vast, endless entity. And then people may wonder how breezy that open road must have been, back before satellites needed laser-powered license plates.

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