This section is for attacking a planet by ground assault. The prior section is for attacking a planet from orbit.

After all the defender's orbital fortresses have been neutralized, the final stage is entered. If the defender still resists, their planet is now the new target. The attacker will attempt to insert troops onto the planetary surface to set up a beachhead. Then the ground assault begins. The attacker will attempt to advance past the final defenses in order to loot, conquer, or destroy the defender's planet.

Because no matter how many bombs you dump onto a planet, you have not captured it until you've got an eighteen-year old holding a rifle with their boots firmly on the ground.

As always it is absolutely crucial for the invaders to do their homework and perform an in-depth threat assessment before invading. In Christopher Anvil's novelette The Underhandler some aliens show up and decide to invade Terra. They quickly see that the global economy relies upon petroleum. So the strategy is to seize the oil and thus bring Terra to its knees. Well, let's see, the biggest concentration of petroleum is in this spot the humans call "the middle east." We'll send some troops in to seize it. How much resistance can they put up? It's not like everybody there is armed...

Which proved to be famous last words.


"On approach, we found a single-dominant-species planet varying in technological development from region to region. Electromagnetic surveillance revealed enormous differences in local languages and customs. It was obvious we couldn't have a binding agreement with such a collection of fragments. However, these differences seemed to offer excellent chances to defeat the locals one by one. And they were dependent on fossil oil to run their technology. Our analysis showed that a large part of this fossil oil came from a comparatively undeveloped region locally known as the Middleast. If we attacked this region, we might do several things:

"First, win a quick local victory.

"Second, overawe the rest of the factions.

"Third, paralyze these other factions at will, by withholding from them our portion of the fossil oil.

"Fourth, adroitly play off one faction against the other, using the fossil oil as a bargaining counter. "After all, they would be unprepared, and how could they possibly guess what would happen to them once we got control of the fossil oil?"

The voice came to a stop, and Kranf, frowning, said, "Then what?"

"Well—the relatively undeveloped region, for some reason, turned out to be overloaded with weapons. There was a little delay while they got over their surprise, then we got hit with everything—bullets, bombs, rockets, gas—it was like stealing meat out of the claws of a dozing thrakosnarr. Then the outside factions that we planned to finesse later on, came piling in. They had warplanes, long-range rocket-bombs, monster sea-borne floating fortresses, and every description of armored ground attack machine you can conceive of.

"Well, what could we do? We'd planned a neat surgical strike with minimum losses. Instead, the locals went berserk. There was no way we could hope to militarily fight it out on their terms—they had the whole resources of the planet at hand, and we only had what was with us.The obvious thing was to use our scientific superiority to hamstring them."


"We set up bioduplication bases, got out our stock of tailored pests, found out which ones seemed to fit, dropped around twenty thousand flights of sixteen-legged jangerls, stingbats, and burrowing trap-adders to poison and terrorize the natives, and give them a little warning that they'd better cooperate."

"How did that work?"

"Well, till we used the pests, there was tough resistance. After that, it got vicious. These split-up groups formed an alliance, got this native here, in front of us, to run things overall, and he got them actively working together. Before long, their measures and their counterblows were on a level we hadn't even imagined was possible. They even adopted a simplified common language to be able to hit us harder. Everything got worse after we started using pests. But what else could we do?"

"Be specific. What incident led you to call for help?"

"Well, we'd just landed two or three million forty-legged flatstings genetically engineered to kill natives, and the natives had come out with a dust that killed flatstings, and then they fired a swarm of missiles that came up off the planet and shot out into space. Only a few came anywhere near us, so we figured their control was breaking down. That was when I sent that report that we were getting the edge on them. Well, these missiles kept coming up, but we were happy to see them waste their firepower, figured we'd won, and called on them to surrender. About then, a missile streaked in from nowhere and hit us from behind. The next thing you knew, they were coming in from all directions, and we realized all these seemingly wasted missiles had been set to come back at us. There was no possible way we could defend against this.

"These things blew up the Moon Command Base, the bio-teams, the germ-synthesis labs, Tactical Combat Center, and Fleet Refit Base, and all that survived were our forces actually on the planet, and our ships in transit.

"That's when I called for help, sir. I've done my best, and every move I've made has been computer checked for maximum damage to them and maximum gain to us; but nothing worked. I'm out of my depth. Maybe somebody else can solve it."

From THE UNDERHANDLER by Christopher Anvil (1990)

Boots On The Ground

The final stage is usually landing your invading army on the ground to capture key targets and force the planet to surrender.


(ed note: The Terran empire had been conquering planets belonging to other empires until they managed to cheese off the alien Spartan Empire. Under Spartan Grand Admiral Shus Show the fleet is attempting to render the human race extinct. Former pacifist Dane Barclay commands the armed forces of Terra in a desperate defense. Because of a silly law (based on Mutual Assured Destruction) the Spartans are forbidden to use a single bomb to blow Terra into vapor. First the Spartans do a nuclear carpet bombing over the entire surface of Terra, then they send in the fleets. After losing half their fleet to the unexpectedly savage Terran defense, the remaining Spartans do a war of attrition. The goal is to poke a hole in Terra's defense so that a beachhead can be established on Terra's surface. From there an invading army can be staged.)

     The siege continued.
     Attack after attack was beaten off. Continual bombardment was endured. Casualties mounted, yet Earth fought on. The defense sphere weakened, was reinforced, held.
     Still the attacks went on. The strategy was simple: hammer and pound the planet—sooner or later the defenses must falter—then pounce.
     The fighting grew more and more savage as Spartans fought to take the planet and Terrans fought to hold it. But despite the determination of the defense, Spartan strategy had to succeed eventually. Finally the moment came.
     Terran forces in the vicinity of the island of Puerto Rico were overwhelmed. Immediately Spartan heavy ships poured in on the island’s defenses which continued to fight despite their hopeless position. An all-out assault was launched against that general area of the planet, aimed at freezing the defense globe, freeing forces in the area and preventing them from reacting to the sudden Spartan success on Puerto Rico.
     On the edges of the island, Spartan cruisers set down, dropped their ramps and disgorged their cargoes of Spartan marines. Overhead, battle wagons and destroyers raked the surviving defenses with energy beams and shattered them with bombs. Reserve fleets hurtled into the battle, adding their weight and numbers to the force near the island.
     Spartan mobile armor roared down ramps and off A into the night. Equipment of every imaginable sort—from computers and laser batteries to medical and boring machinery—came tumbling from ports and locks. As soon as a ship had given up its cargo, it lifted clear and was replaced. by another. The Spartans had come to stay. They had their beachhead.
     As the Spartan marines fought their way inland, others turned seaward and began erecting line upon line of laser and rocket defenses with incredible speed. Other units began building fortifications of quick-hardening super-concrete.
     Over them wheeled the fleets of Sparta, maintaining a shield which the Terran fleets could not penetrate. Minute by minute the defense perimeter grew. More and more Spartan marines lumbered down the ramps, stooped and bent in their space armor.
     All was confusion, yet each one knew what he had to do. Mines were planted in the sea and on the beaches. Bunkers were constructed. Huge planetary lasers were put together and set up to add then mighty beams to those of the Spartan ships above. Some marines set up generators for power, while others began drilling to tap the thermal energy of the planet. Hospitals, command centers, roads, supply dumps grew out of nothing in the blink of an eye.
     Spartan marines were good.

     In the command bunker a young communications oflicer turned from her console to Dane Barclay. There was grief in her tired eyes. “Puerto Rico has fallen, sir. They died fighting to the last man.”
     Dane nodded. He looked to the master battle tank with its huge three-d simulation of Earth. Already the island had gone from Terran red to Spartan blue. He pushed the fog from his brain. This was a critical moment. He knew the speed with which Spartan marines could operate.
     The beachhead had to be smashed at once. Delay would be fatal. A beachhead was the beginning of the end. The Spartans would consolidate and strengthen their position while building up forces within it. Then the break-out would come. There would be little hope of stopping Spartan starships in the skies and Spartan marines and mobile armor on the ground.
     He swung around to his staff. “They’ve got pressure on that part of the planet. We'll have to ignore it. Tell McCode to take everything he's got and get to Puerto Rico!” The oflicer addressed rushed away.
     “You. I want half starship defense of each command sector. Shift them around. I want those forces here yesterday!” That officer hurried away.
     “I want every available strato-fighter, missile and sea sub thrown into the battle for that island.” He swung to a communications officer.
     “Get me all marine divisions capable of assaulting that island one hour. I want them moving at once. And I want to address them.” He swung around to another officer.
     “I want the defense globe to collapse back on that Spartan force over Puerto Rico. Tell them to keep the ships moving. Tell them to spread themselves thin elsewhere—on my orders. I’m going to gamble that Shus Show isn’t going to sacrifice a force of that size. He’s not that kind of fighter. Go!”
     He closed his eyes, trying to think of any. thing else he could do.
     Someone came up to him. He opened his eyes. “The marines are moving out now. You can speak to them if you wish.”

     Sergeant James T. O’Ryan, 76th Terran Marines, was jammed with his gear and twenty-three other marines into the heavy-duty battle copter beating its way through the howling wind above the angry, storm-tossed sea.
     “Attention back there,” announced one of the pilots. “Dane Barclay wants to speak to you.”
     They fell silent as the viewscreen blinked and swirled with color. Then they were looking at the man who had vowed the planet would not fall, the man who had been shot out of the skies of Terra and lived to fight again, the man who had lost his whole family and an arm, the man who was the greatest fighter that would ever live. They knew they would do anything this man asked.
     “Marines—warriors of Terra—Puerto Rico has fallen. A piece of the Earth is in enemy hands. Spartan marines are down in force and consolidating their position. Spartan marines are good. Very good. But you’re better. You’ve proven it before. I want you to prove it again. Get that island back for me.”
     The violence intensified as they drew near. Spartan ships were standing firm, but Terrans howled about them like angry bees. High above, the defense globe had collapsed onto the Spartan forces, seeking to squeeze shut the funnel through which personnel and supplies were being sent down in such huge quantities.
     The death-shrouded sky was a display of fire-works such as O’Ryan had never seen before. He had been a marine for a long time and he had been in many an assault and witnessed more than his due share of battles between starships, but he had never seen anything like this. Starships wheeled. and spun, dove. and darted. Beams of destruction raced across the sky. Strato-fighters flung themselves dauntlessly to the attack. Missile salvoes streaked the sky. Dying ships fell broken into the sea. Water-sprouts dotted the water. Wind howled and screamed and whistled over the battling craft.
     He could see other battle copters, anti-grav troop carriers and the giant sea subs, awesome waves crashing mightily on their superstructures. From the shore came the probing beams of the laser batteries. Fortunately for him and his companions, mountainous waves, torrential sea spray, smoke, radioactive dust. and the electronic transmitters of his own people jammed the Spartan aiming devices; otherwise the battle copters and a-g carriers would already have been wiped out.
     There was an explosion behind. A sea sub, bow gone, wallowed in the heavy seas, victim of a mine her detectors had missed in the turbulence. At an» explosion on his left, he turned to see the shattered remnants of a battle copter fall into the water, victim of a pencil-straight beam of destruction.
     The island was closer. The sea boiled and sprouted around them. The word came: “Get ready!” Men checked over their equipment one last time. Suddenly O’Ryan found himself looking down the length of a beam that passed beneath the craft.
     The battle above still raged in all its fury, while on the waves another battle copter was out down by the shore batteries. The ocean was steaming and boiling where beams struck all over its surface, hunting for the enemy that refused to make its own death easier.
     The copter was setting down, flotation collar out. It hit the water with a slight jar, and crashing waves instantly began to batter and toss it about. The men swayed drunkenly to the rocking of the craft. O’Ryan made one last check of his armor. It was sealed and all systems were “go.” He watched the men going through the bottom hatch, bent nearly double by the huge equipment packs and racks attached to them. They jumped into a little patch of dark swirling water and sank out of sight. One by one the armored marines disappeared until it was his turn. He stepped carefully to the hatch, then jumped. His feet hit the center of the water patch. He went down quickly and darkness engulfed him.
     It was quiet below the waves. Only the occasional fiicker of weapons disturbed the scene. Dimly he could make out other shapes. He gave a blast of his rockets. He continued to sink, but at an angle that would reduce the chance of the marine behind falling atop him. He hit the sandy bottom, plowed into it to his ankles. He stumbled, caught himself with an armored hand and started staggering toward the shore. It was a long and lonely walk Through the dark water, he could see bear: stretching over the surface. They were fascinating to watch. A combination of death and art. Then he saw the bigger shapes of the sea subs’ underwater transports crawling up the sand like giant turtles, carrying their luckier marine passengers right up to the Spartans’ doorstep.
     He was near to breaking clear. There were only a few feet of water over his head. He readied his portable heavy-duty laser cannon, checked the power pack and made certain his other weapons were readily accessible. He plunged forward, knowing the moment of battle was almost upon him. The water broke from his helmet and armed shoulders, and he emerged from the quiet of the ocean into the din of hell.
     The Spartans were waiting. They opened up as if they had been waiting especially for his arrival. The sea boiled with beams, erupted with rockets. All along the beach, marines rushed forward into the waiting embrace of death. Further up the beach, a marine flopped to the ground, the front of his battle armor a bloody blackened ruin. The ground heaved under O’Ryan, hurling him forward. He looked back. A newly created crater smoked behind him.
     He picked himself up and scuttled forward, sweating despite all the ingenious devices within the suit that were supposed to eliminate that problem. He wished he could wipe the clammy wetness away, but that was impossible. Something burst yards away, staggering him, Metal fragments rang off his armor, many scarring it deeply. If not for the armor, he would now resemble shredded meat. He moved on. He passed another marine who had been hit. He lay curled in a fetal ball, the white sand red with his blood. O’Ryan stopped, retrieved the man’s rocket launcher, leveled it and sprayed the super-concrete walls facing him. The rocket shells burst over it with little visible effect. He hurriedly dropped the empty launcher and lumbered away. He was almost too slow. The defenders retaliated and the sand about him smoked and bubbled as beams searched for him. The dead marine was caught and transformed almost instantly to a residue of brunt flesh and smoldering metal.
     Ahead of him a crouching figured glared brightly, armor giving off white-hot sparks, then fell dead. The universe seemed to close in upon O’Ryan as the earth trembled beneath him and beams licked about. He dropped to the ground and crawled. He was an insect, hoping a foot wouldn’t descend and squash him into oblivion. He was close now. He could make out clearly the ghostly, forlorn, forbidding superconcrete battlements of the Spartans. There were an angry whine and a throb of mighty engines behind him. He glanced up. An air-mobile armored vehicle was moving up, its grim lines visible in the light of battle. The sky over-head brightened suddenly as the marine vehicle opened fire. The sinister weapon muzzles spat and spat. Others became visible. The flickering became brighter, the sounds of violence greater. The ground trembled and shook so that his teeth rattled. Looking back he could see that the sea subs had opened fire, adding their meager firepower to that of the air-mobile armor. And now a battle wagon—a big, beautiful, heavy ship—was low over the waves, mighty beams spitting forth from the turrets studding her sides. The starship was lit up like a Christmas tree as every available weapon attacked the Spartan defense works. Before O’Ryan the defenses dissolved in a dozen places, blasted out of existence. The once formidable, nearly impenetrable Spartan fortress was no longer so.
     “Let’s gol” he screamed, heaving himself from the ground and scuttling forward, thinking with mixed emotions of the armor, equipment and weapons that slowed his advance, yet which made him so deadly when at grips with the enemy. Around him other marines who had endured as he had also moved forward. An idiot used this jets and made a great jump. He was burned out of the air. The air-mobile armor crashed and flamed and died. The air over the beach was deadly, but other armored units rode their flickering lasers into the Spartan defenses. And behind them came O’Ryan and the Terran marines.
     The defensive fire was withering. Many fell, but the rest went on; they hadn’t been promised a cakewalk. The wounded and the whole and the near dead went on. The Spartan marines were waiting in the shattered defenses. At pointblank range they fired and still more marines flamed and died.
     O’Ryan fired a clip of his own rockets, threw a grenade that sent several Spartans to their version of heaven and covered the final yards to the ruined wall in a desperate run. He heaved himself over a mound of rubble and ducked just in time to have a beam scorch the top of his helmet. He snapped a reply and was rewarded by a shower of molten sparks as the Spartan marine crumbled, his head a fused ruin.
     It was a wild scene. The night was tortured with death. O’Ryan scanned the scene, sprayed the area and moved forward. Terrans and Spartans died and others took their places. Everywhere were bodies and fighting and rubble and the sound and sight of weapons large and small in use. Rockets were filling the air as they arced into the Spartan camp, causing havoc and disruption among the tight-packed marines and technicians. Missiles too were zooming in on the Spartan positions despite the frantic and valiant efforts of the Spartan Space Forces to protect the ground troops and supplies. But the Terran starships only increased the pressure, beams slashing and cutting ceaselessly into them, sending ships falling everywhere. Nor could the Spartans stop the devastating sweep of Terran lasers and blasters.
     The outer ring of super-concrete defenses had been penetrated. Fighting was hand-to-hand amid incredible piles of hastily unloaded and stacked supplies.‘The missiles, rockets and beams had done their work in the close, crowded confines of the Spartan beachhead. Confusion reigned. Fires and explosions filled the night. Unprepared, after having expected this to be the beginning of the end of the siege, the Spartan marines folded. Many fought on grimly but O’Ryan knew the outcome was decided.
     As if to confirm that, in the glare of fire on a battle-seared hill, he saw a battered group of marines raising the Terran banner. A beam came out of nowhere and ripped across his midsection. By reflex he located the Spartan and fired. His aim was true and the Spartan fell, one shoulder blasted away. O’Ryan crumbled to the ground. But even as he fell another marine hurtled his body and moved on into the night, weapon blazing. O’Ryan looked to the hill. The banner of the Terran Empire was streaming grandly and the marines who had planted it ringed it, their weapons making sure that it would remain. Puerto Rico Was Terran again. “We took it back, Dane,” he whispered. Then he died.

     With the loss of the admiral in the midst of battle, command had fallen upon Vice Admiral Vhus, a native of Anghu, a world overrun by the Terrans, for whom he therefore had no love. He had one overriding desire: kill Terrans. He found himself in a diificult situation: his ground support had been cut from under with the overrunning of the beachhead. The attack lane to the planet had been sealed off by Terran forces. He was in effect cut off and surrounded. He could fight his way out, but didn’t want to.
     “Get me the Grand Admiral,” he ordered. Shus Show came on the viewscreen immediately.
     “The beachhead is lost, Vhus. Bring your ships out.”
     “No, Admiral. The marines did not come out. I cannot with honor leave them.”
     “They’re finished, Vhus. There’s no hope for them. But they didn’t die in vain. I now know the exact limits of Terran strength. This was their last gasp. We have them now. I’m sure of that.”
     “I’m sorry, Admiral. My force will maintain its position over this island—till we are annihilated or till we have re-established the beachhead.”
     “You will place yourself under arrest, Mr. Vhus. Commodore Andelthus will assume command in your place.” Vhus looked to the Commodore.
     “We are in the midst of battle, Admiral. Vice Admiral Vhus is here—you are not. His judgment takes precedence over yourslf”
     “Damn you—damn you!” Shus screamed. “I’ll see that you’re both shot. I—.”
     “It would be better if you gave us support.”
     “For the last time, come out.”
     “No, Admiral. There will be no more retreats. We are here to stay. If you will support us, perhaps we can re-establish the beachhead.”
     “I’ll support you—but damn you both!”
     Vhus signaled and the link was cut. He looked at the Commodore and his oflicers. “Well, it’s done. It’s time we stood and fought instead of running every time these Terrans give us a bloody nose. Let’s fight!”
     And they did.

     Looking from battle tank to viewscreens to data screens to sensor-scopes to comm-men with messages, Dane could not quite believe his eyes or ears. The Spartans were going to attempt to re-establish the beachhead! They were pulling ships from other sectors to throw into the battle.
     Dane whirled, eyes gleaming, mind going, sensing the opportunity presented. The orders came. “I want every defense command stripped of their remaining forces. Every available sea sub is to be brought up. I want as many marines and as many mobile laser batteries on that island as you can get. Scrape up every missile you can find and get it to the defense of that island.”

     It was very quiet. Shus could hear the whirr of the blowers, the chatter of the computers, the many miscellaneous sounds present in the command room of a starship. Shus sighed, looked at his oficers. They avoided his gaze. In defeat they sat or stood dejectedly, letting him bear the burden of the defeat alone. He squeezed his eyes shut tightly to clear his mind and wipe away all obsolete images. The battle picture had changed entirely. The last of the beachhead task force had gone down in flames. There was nothing to support now.
     “Order the withdrawal. We shall regroup, tend our wounds and begin again.”
     It was fortunate for Vice Admiral Vhus that he had gone to glory with his ship. Shus would have enjoyed breaking him. But he was beyond the reach of vengeance or discipline. There was still a siege to be won, and Shus now had to worry about the morale of his officers and men.

From SIEGE OF EARTH by John Faucett (1971)

Last month, this New York Times article by Steven Johnson raised the alarming question: what if our efforts to find and communicate with other intelligent life in the Universe brings us to the attention of intellects vast and cool and unsympathetic who want to wipe us out? The author quoted Stephen Hawking, warning that the result of interstellar contact might resemble the conquest of the New World by the Spanish.

But there are worse things than a galleon-load of extraterrestrial Conquistadors dropping down out of the sky some afternoon. Instead of going to the bother of conquering the Earth, a sufficiently xenophobic and/or ruthless alien civilization might simply lob a few relativisitic planet-busting projectiles in our direction at 99.9 percent of the speed of light. A barrage of brick-sized warheads at that velocity would be an extinction-level event.

Scary stuff! Maybe we should try to hide. Maybe that's the reason we haven't yet detected any other civilizations: they're all either hiding or dead.

Except that I don't believe it, and this isn't just wishful thinking. Interstellar conquest or extermination are simply bad ideas. They're either literally impossible or so difficult and fraught with uncertainty that they might as well be impossible. (I am assuming real-world physics here, with the speed of light as an absolute speed limit.) I am not worried; let me explain why.

I'm going to consider the two cases — extermination and conquest — separately, although there's a lot of overlap among the reasons neither is practical. I will begin with interstellar conquest. There are three good reasons why nobody is likely to try conquest across interstellar distances.

I. Why? Seriously, why do it? What benefit would Earth get by sending out starships across tens or hundreds of light-years to conquer some other planet? If you're a Habsburg King of Spain, conquering Mexico makes perfect sense: you can ship home tons of gold, plant colonies which enrich the mother country through trade, and generally make out like a bandit.

But suppose Mexico was so far away that a galleon would have to sail for decades, if not centuries to reach it? And suppose sending a single galleon to Mexico would cost twice Spain's entire annual economic output? How keen on the project would King Charles be once you told him that?

If that's not enough to destroy His Most Catholic Majesty's enthusiasm for the conquering Mexico, suppose you told him that shipping anything back would cost just as much and require decades to build up the Mexican shipbuilding infrastructure? There is one bright spot for King Charles, though: it's not hard to send messages from Mexico to Spain, so you can mail back pictures, descriptions of the new land and its inhabitants, and maybe a few useful new ideas. But you could just as easily mail a letter to King Montezuma and ask him to do that, without those incredibly expensive galleons.

Conquering inhabited worlds across interstellar distances makes no economic sense. If the planet's technology is sufficiently primitive that a starship full of troops with AK-47s can overawe them and take over, then it's highly unlikely that world produces anything valuable enough to ship home. Certainly no non-biological substances are worth it; even diamonds, at $50,000 per gram, wouldn't be worth it — especially since one could probably find diamond deposits on uninhabited planets much nearer to Earth. (If anyone mentions "stealing our water" I'm going to drown him.) And if you like diamonds so much, developing ways to manufacture them would be a much more lucrative investment than sending starships out in search of some new Kimberley mine.

Bioproducts like heroin or truffles might be incredibly valuable, but the shipping cost is murder. It's far more practical to grab some of the plants or animals you want, take them home, and figure out how to raise them on your own planet.  

And that's assuming conquest is easy. I doubt that conquering the Earth would be easy. Past experience shows that even trying to conquer smallish chunks of the Earth is very difficult. There simply isn't anything on Earth valuable enough to make us worth conquering, especially given the fact that there's seven billion humans armed with everything from pointed sticks to hydrogen bombs who might object.

Well, what about ideology? The aliens could be fanatical Marxists, or devotees of some hyper-evangelical religion, or just really insistent about which way the toilet paper roll goes. Ideology was a good enough reason for 180 million deaths on Earth in the 20th Century.1 We certainly can't assume any hypothetical aliens wouldn't be just as bloodthirsty in the name of their preferred cause.

But it still seems unlikely. The more bloody-minded the ideology, the shorter the shelf life. Of the regimes responsible for that horrifying 180 million death toll, only one is still around (the Chinese Communist Party) and it has transformed itself into a considerably more housebroken form. Obviously we can't tell if this is a "rule" of history (most "rules" of history are bunk anyway), and we certainly can't tell if it would apply to an alien civilization.

However, there is another reason to doubt the existence of hyper-ideological alien civilizations, and that's our old friend the Fermi Paradox. Radiotelescopes are considerably cheaper than starships, so one would expect that any fanatical alien ideologues would be flooding the Galaxy's airwaves with propaganda broadcasts. After all, if they can make converts via radio it frees up ships from the Holy People's Armada to go after the stubborn ones. But we don't see any of that; just silence.

So I won't rule out ideology, but I assign it a very low probability.

II. How? How do you conquer a planet across interstellar distances, anyway? If you're deliberately sending out an invasion, the planet's inhabitants have to be capable of giving you some indication that they exist — which in practice means at least a 1950s-era technology.

Assume that's what you're going after. Assume also that a world with substantial industrial technology is going to have a pretty big population; at least a billion. And while it would be convenient if some or all of them joined your side when you land, there's no way to find out before you launch the fleet. So you have to assume it will be a contested invasion. Finally, we assume that you actually want to conquer the planet — which means most of it must still be habitable when the conquest is over. (We'll cover simple destruction in the next post.)

Let's use the British Interplanetary Society's Daedalus spacecraft as our model starship, and assume arbitrarily good suspended animation for the crew and the soldiers of our invasion force. We will use the American plan for "Operation Olympic" as the model for that invasion force. Olympic was the invasion of Japan which never happened because of the timely deployment of the atomic bomb.

Operation Olympic would have involved 14 Army and Marine divisions, 15 air forces, and the aircraft from 42 carrier ships. That's about 300,000 soldiers, 10,000 airplanes, and at least 20,000 other vehicles. That force, with its supplies, would have a mass on the order of 1 million tons (and that assumes hyper-efficient power supplies for the planes and vehicles so there's no need to bring fuel).

The Daedalus starship was designed for a payload of 500 tons. So you'd need 2,000 of them to carry that invasion force. Each one would cost about 200 trillion dollars (or about twice the annual GDP of the planet). We're ignoring the cost of the actual military force. So building the ships for this invasion would require the entire economic output of a planet like the Earth for 4,000 years! That fanatical ideology has to maintain its grip on its home civilization for a period as long as recorded history just to get the invasion force built.

Oh, and since they have to detect the target civilization before they start building this armada, the target planet has 4,000 years to invest all of its economic output into technology research, industrial expansion, off-planet colonies, and massive defenses. They also get the multi-decade flight time of the armada to prepare to resist (because launching that fleet will be noticed across interstellar distances).

Let's just say the odds don't favor the invaders.

III. Time Lag. This is an important point and bears repeating. Interstellar travel takes a lot of time. The fleet will spend decades or centuries in transit. All of which means that when the invaders arrive, the target planet will be a very different place from when they were planning the operation years earlier.

I can't imagine what the reaction from any strategic planner would be if you asked him to prepare an invasion force against an enemy, but all data about the foe is at least a hundred years out of date. You don't know what their population numbers are, you don't know anything about their economy, you don't know the size of their military forces, you don't know anything about their weapons . . . it would be insanity.

Now, it is possible that the invaders are incredibly advanced and powerful, so that launching such an armada would be easy for them, and their weapons could overcome any opposition. Their species might be a Kardashev Type II civilization, able to command the entire mass and energy resources of an entire star system . . .

 . . . except that such a civilization should be detectable across interstellar distances. Professor Fermi at the back of the classroom is holding up a sign: "WHERE ARE THEY?"

It seems pretty much impossible to conquer a planet across interstellar distances — at least, any planet which could possibly repay the effort of conquering it. If you want living space you can find worlds which aren't inhabited, or terraform lifeless worlds to suit yourself. If you want resources there's a vast number of lifeless bodies in the universe you can strip-mine. 

So I'm not afraid of aliens coming to take over.

But what if they don't want to take us over? What if they just want to exterminate us? I'll get to that next time.

1I didn't even count World War I.


If you want to see stories about aliens who aren't conquering the Earth, check out my new ebook, Outlaws and Aliens!


Rick Robinson:

So I'm not sure there's really a place for space marines or not. It seems to me that, beyond what is essentially police work, space marines are only useful if you are ruthless enough to conquer planets, but not ruthless enough to do it by sheer terror.

Jon Brase:

As far as I'm concerned, planetary defenses can be pretty powerful and well protected (you can have big missiles, lots of propellant for them, big lasers, big generators and heat sinks for the lasers, and everything can be very well armored by the huge amount of rock available), to the point that to soften the defenses to where they can't stop space marines from landing means slagging the planet to the point that there isn't much of value left. So your options with an enemy planet are

  1. blockade them, and don't let them trade with anybody until they let you land marines and take over.
  2. threaten to slag them unless they let you land marines and take over.
  3. slag them.

Rick Robinson:

I pretty much agree. Given anything like the sort of techs we generally imagine here, a planetary landing and surface fight against serious resistance is horribly expensive and difficult. You have to spacelift large amounts of troops and munitions, then soft-land it all, fat slow targets coming down against defenders with surface concealment. The armament of your deep space warcraft may be be more or less useless against surface targets, requiring an additional force of fire support ships.

Sure, troops can land in remote areas — but then they are in remote areas, facing a long surface slog on a planet where the locals know their way around a lot better. And no matter where you land, shuttles coming down have a long re-entry trajectory during which they are extremely conspicuous and extremely vulnerable.

From a thread in SFConSim-l

Deploying To Planet

Once your troop spacecraft have made the long journey from the staging base to the planet to be invaded, there must be a way to insert the troops into the combat zone, and get them out if need be. The landing boats will need armor and weapons if the landing zone is "hot" (i.e., full of hostile troops shooting at you).

ITHACUS Payload breakdown
(x1200 @ 180 lb each)
Troop equipment
(40 lb/man))
Troop provisions
(20 lb/man
seat, restraints)
Life support
(7.5 psi)
Cabin structure
(bulkheads and floors)
Acoustic dampening12,000
Nose Faring25,000
Crew module10,000

Shown above is the "Ithacus". This was a 1963 proposal by Douglas Aircraft, inspired by the ROMBUS plug-nozzle concept. This bold proposal was a semi-single-stage-to-orbit intercontinental troop transport capable of carrying 1,200 soldiers. General Wallace Greene thought that rocket commandos deployed by Ithacus would reduce the need for oversea US Army bases.

The concept was orginally called "ICARUS", but the Marines objected to that name. You do not want to name your flying transport after a mythological figure whose melting wings caused him fall to his death.

Ithacus had a range of 14,000 kilometers, with a maximum payload of 226 metric tons (500,000 pounds) in theory. But if it was launched with an easterly trajectory Terra's rotation gave enough bonus velocity that it could carry 281 metric tons (620,000 pounds). Conversely, launching it westward reduced the payload to 171 metric tons (380,000 pounds).

Ithacus had six troop decks with 200 acceleration couches per deck. A flight crew module carried a crew of four. The module could eject in case of emergencies, but this feature was only incorporated into cargo or flight testing models, not the troop-carriers. Airline passengers are unnerved by the sight of the flight crew wearing parachutes, and presumably so are rocket marines. Ithacus did have emergency floatation balloons so it could abort to a water landing. But if could only abort to solid ground, the results would be unfortunate.

The flight would be limited to a maximum of 3-g acceleration, so the troops would not be too damaged to deploy and fight. It would reach an apogee of 127 nautical miles. Flight time would be about 26 minutes for 3750 nautical miles. When it approached the ground it would have enough fuel to hover for about 30 seconds and "translate" (i.e., move sideways) about 300 meters to find a suitable landing spot. You never know when the planned touch-down spot might be full of hostile troops.

It also would be possible for Ithacus to launch into a low polar orbit and loiter there. This would make the range global, and wage psychological warfare on the enemy as they nervously watched the orbital Sword of Damocles jam packed with marines. Such an orbital launch would reduce the allowable payload.

The fly in the ointment was the unanswered question of how the heck do you get the rocket back home? Blasted thing was 64 meters tall. Even after it burnt all its fuel and unloaded the troops it still had a mass of 500 metric tons. Refueling it and having it rocket back home was out of the question. The monster had a thrust of 80,200 kiloNewtons. It can only safely take off from a custom build launch pad. In theory, if the landing site could be fully secured and if the landing site was reasonable close to a coastal port, Ithacus could be refueled with enough hydrogen to hover and translate to the port. There it could be loaded onto a special transport ship for the journey home.

For more details about Ithacus, check out Aerospace Projects Review vol 2 number 6.

Robert Heinlein's classic novel Starship Troopers took a slightly more practical approach. In each insertion there were only a few troopers deployed. Each trooper was wearing a powered armor suit making each one the functional equivalent of Iron Man armed with nuclear weapons, so you had quality over quantity (i.e., they were more like space marines than they were like space army).

Heinlein's Starship Troopers were deployed from orbit, riding one-man atmospheric reentry pods surrounded by lots of decoys and anti-radar chaff. Dougherty and Frier's term for this kind of insertion is "Meteoric Assault", the soldiers are called "Drop Troops." The reentry pods were only slightly larger than the individual trooper. After a battle the troops were retrieved by a landing boat. A "spike" was fired into a relatively safe location to act as a radio homing beacon. Both the troops and the landing boat would then head towards the beacon. Dougherty and Frier point out that troops must secure a landing zone for the spike otherwise the landing boat will be shot to pieces on the way down. Since there is no other way besides landing boat to extract the troops, the only alternatives are to fight to the death or surrender to the enemy.

But in science fiction, by far the most popular method of deploying troops from orbit to the planet's surface is the dropship (see section below).


Bump! and your capsule jerks ahead one place—bump! and it jerks again, precisely like cartridges feeding into the chamber of an old-style automatic weapon. Well, that's just what we were . . . only the barrels of the gun were twin launching tubes built into a spaceship troop carrier and each cartridge was a capsule big enough (just barely) to hold an infantryman with all field equipment.

And clang!—it's my turn as my capsule slams into the firing chamber—then WHAMBO! the explosion hits with a force that makes the Captain's braking maneuver feel like a love tap.

Then suddenly nothing.

Nothing at all. No sound, no pressure, no weight. Floating in darkness . . . free fall, maybe thirty miles up, above the effective atmosphere, falling weightlessly toward the surface of a planet you've never seen. But I'm not shaking now; it's the wait beforehand that wears. Once you unload, you can't get hurt—because if anything goes wrong it will happen so fast that you'll buy it without noticing that you're dead, hardly.

Almost at once I felt the capsule twist and sway, then steady down so that my weight was on my back . . . weight that built up quickly until I was at my full weight (0.87 gee, we had been told) for that planet as the capsule reached terminal velocity for the thin upper atmosphere. A pilot who is a real artist (and the Captain was) will approach and brake so that your launching speed as you shoot out of the tube places you just dead in space relative to the rotational speed of the planet at that latitude. The loaded capsules are heavy; they punch through the high, thin winds of the upper atmosphere without being blown too far out of position—but just the same a platoon is bound to disperse on the way down, lose some of the perfect formation in which it unloads. A sloppy pilot can make this still worse, scatter a strike group over so much terrain that it can't make rendezvous for retrieval, much less carry out its mission. An infantryman can fight only if somebody else delivers him to his zone; in a way I suppose pilots are just as essential as we are.

I could tell from the gentle way my capsule entered the atmosphere that the Captain had laid us down with as near zero lateral vector as you could ask for. I felt happy—not only a tight formation when we hit and no time wasted, but also a pilot who puts you down properly is a pilot who is smart and precise on retrieval.

The outer shell burned away and sloughed off—unevenly, for I tumbled. Then the rest of it went and I straightened out. The turbulence brakes of the second shell bit in and the ride got rough . . . and still rougher as they burned off one at a time and the second shell began to go to pieces. One of the things that helps a capsule trooper to live long enough to draw a pension is that the skins peeling off his capsule not only slow him down, they also fill the sky over the target area with so much junk that radar picks up reflections from dozens of targets for each man in the drop, any one of which could be a man, or a bomb, or anything. It's enough to give a ballistic computer nervous breakdowns—and does.

To add to the fun your ship lays a series of dummy eggs in the seconds immediately following your drop, dummies that will fall faster because they don't slough. They get under you, explode, throw out "window," even operate as transponders, rocket sideways, and do other things to add to the confusion of your reception committee on the ground.

In the meantime your ship is locked firmly on the directional beacon of your platoon leader, ignoring the radar "noise" it has created and following you in, computing your impact for future use.

When the second shell was gone, the third shell automatically opened my first ribbon chute. It didn't last long but it wasn't expected to; one good, hard jerk at several gee and it went its way and I went mine. The second chute lasted a little bit longer and the third chute lasted quite a while; it began to be rather too warm inside the capsule and I started thinking about landing.

The third shell peeled off when its last chute was gone and now I had nothing around me but my suit armor and a plastic egg. I was still strapped inside it, unable to move; it was time to decide how and where I was going to ground. Without moving my arms (I couldn't) I thumbed the switch for a proximity reading and read it when it flashed on in the instrument reflector inside my helmet in front of my forehead.

A mile and eight-tenths—A little closer than I liked, especially without company. The inner egg had reached steady speed, no more help to be gained by staying inside it, and its skin temperature indicated that it would not open automatically for a while yet—so I flipped a switch with my other thumb and got rid of it.

The first charge cut all the straps; the second charge exploded the plastic egg away from me in eight separate pieces—and I was outdoors, sitting on air, and could see! Better still, the eight discarded pieces were metal-coated (except for the small bit I had taken proximity reading through) and would give back the same reflection as an armored man. Any radar viewer, alive or cybernetic, would now have a sad time sorting me out from the junk nearest me, not to mention the thousands of other bits and pieces for miles on each side, above, and below me. Part of a mobile infantryman's training is to let him see, from the ground and both by eye and by radar, just how confusing a drop is to the forces on the ground—because you feel awful naked up there. It is easy to panic and either open a chute too soon and become a sitting duck (do ducks really sit?—if so, why?) or fail to open it and break your ankles, likewise backbone and skull.

So I stretched, getting the kinks out, and looked around . . . then doubled up again and straightened out in a swan dive face down and took a good look. It was night down there, as planned, but infrared snoopers let you size up terrain quite well after you are used to them. The river that cut diagonally through the city was almost below me and coming up fast, shining out clearly with a higher temperature than the land. I didn't care which side of it I landed on but I didn't want to land in it; it would slow me down.

I noticed a dash off to the right at about my altitude; some unfriendly native down below had burned what was probably a piece of my egg. So I fired my first chute at once, intending if possible to jerk myself right off his screen as he followed the targets down in closing range. I braced for the shock, rode it, then floated down for about twenty seconds before unloading the chute—not wishing to call attention to myself in still another way by not falling at the speed of the other stuff around me. It must have worked; I wasn't burned.

Right that moment I was feeling unusually expendable, almost expended, because I was hearing the sweetest sound in the universe, the beacon the retrieval boat would land on, sounding our recall. The beacon is a robot rocket, fired ahead of the retrieval boat, just a spike that buries itself in the ground and starts broadcasting that welcome, welcome music. The retrieval boat homes in on it automatically three minutes later and you had better be on hand, because the bus can't wait and there won't be another one along.

A suit isn't a space suit—although it can serve as one. It is not primarily armor—although the Knights of the Round Table were not armored as well as we are. It isn't a tank—but a single M.I. private could take on a squadron of those things and knock them off unassisted if anybody was silly enough to put tanks against M.I. A suit is not a ship but it can fly, a little; on the other hand neither spaceships nor atmosphere craft can fight against a man in a suit except by saturation bombing of the area he is in (like burning down a house to get one flea!). Contrariwise we can do many things that no ship—air, submersible, or space—can do.

"There are a dozen different ways of delivering destruction in impersonal wholesale, via ships and missiles of one sort or another, catastrophes so widespread, so unselective, that the war is over because that nation or planet has ceased to exist. What we do is entirely different. We make war as personal as a punch in the nose. We can be selective, applying precisely the required amount of pressure at the specified point at a designated time—we've never been told to go down and kill or capture all left-handed redheads in a particular area, but if they tell us to, we can. We will.

We are the boys who go to a particular place, at H-hour, occupy a designated terrain, stand on it, dig the enemy out of their holes, force them then and there to surrender or die. We're the bloody infantry, the doughboy, the duckfoot, the foot soldier who goes where the enemy is and takes him on in person. We've been doing it, with changes in weapons but very little change in our trade, at least since the time five thousand years ago when the foot sloggers of Sargon the Great forced the Sumerians to cry "Uncle!"

Maybe they'll be able to do without us someday. Maybe some mad genius with myopia, a bulging forehead, and a cybernetic mind will devise a weapon that can go down a hole, pick out the opposition, and force it to surrender or die—without killing that gang of your own people they've got imprisoned down there. I wouldn't know; I'm not a genius, I'm an M.I. In the meantime, until they build a machine to replace us, my mates can handle that job and I might be some help on it, too.

Maybe someday they'll get everything nice and tidy and we'll have that thing we sing about, when "we ain't a-gonna study war no more." Maybe. Maybe the same day the leopard will take off his spots and get a job as a Jersey cow, too. But again, I wouldn't know; I am not a professor of cosmo-politics; I'm an M.I. When the government sends me, I go. In between, I catch a lot of sack time.

Of course, a six-platoon transport is not big compared with a battle wagon or passenger liner; these things are compromises. The M.I. prefers speedy little one-platoon corvettes which give flexibility for any operation, while if it was left up to the Navy we would have nothing but regimental transports. It takes almost as many Navy files to run a corvette as it does to run a monster big enough for a regiment—more maintenance and housekeeping, of course, but soldiers can do that. After all, those lazy troopers do nothing but sleep and eat and polish buttons—do 'em good to have a little regular work. So says the Navy.

The real Navy opinion is even more extreme: The Army is obsolete and should be abolished.

The Navy doesn't say this officially—but talk to a Naval officer who is on R & R and feeling his oats; you'll get an earful. They think they can fight any war, win it, send a few of their own people down to hold the conquered planet until the Diplomatic Corps takes charge.

I admit that their newest toys can blow any planet right out of the sky—I've never seen it but I believe it. Maybe I'm as obsolete as Tyrannosaurus Rex. I don't feel obsolete and us apes can do things that the fanciest ship cannot. If the government doesn't want those things done, no doubt they'll tell us.

From STARSHIP TROOPERS by Robert Heinlein (1959)

Misconception #1: Orbital drops are “kickass.”

Reality: No, they f*****g aren’t. Yeah, let me just cram my up-armored ass into a tiny pod that may or may not combust in the atmosphere of the hostile planet I’m about to assault and faithfully rely on all of the sensitive electronic components that are supposed to slow me down before I pummel into the ground at terminal velocity. Contrary to popular belief, those pods aren’t designed to gently deliver you dirtside. They’re only designed to keep you alive long enough to survive skipping across the planet’s crust like a flaming pinball.

There’s no doubt in my military mind that any of the sadistic bastards who designed this drop coffin ever took a ride in one. Honestly, I think it’s purposely designed to keep us so pissed off that we’ll kill everything we see after a rough landing. The sight of enraged Marines emerging from shattered drop pods, venting puke and s*** from their exo-armor, is bound to send enemy troops fleeing.

Nexus Journal #1 (PDF) is for the tabletop wargame Attack Vector: Tactical. However, of general interest to military science fiction writers is a set of three articles by Claudio Bertinetto. The first is an in-depth look at the mechanics and tactics of spaceborne assault operations. This includes the logistics of transporting the army, scouting the drop zones, the D-Day drop, and advancing to the targets. The second is a detailed look at the fictional Xing Cheng Celestial Navy Marine Corps, and I mean detailed. It analyzes the various branches and missions. The third article is the Xing Cheng Table of Organization and Equipment (TO&E), which goes on for seven full pages. Any author planning an orbital drop of troops will find the information fascinating.


(ed note: this is from the first article mentioned above. The analysis is about the planet of Xing Cheng {Zeta Tucanae III} performing a spaceborne assault on the planet Novaya Rossiyan {Alpha Mensae II} )

Each vehicle is loaded in an individual capsule, and then onto its carrying transport ship, a mobilized civilian freighter. The Marines drop with their trucks and AFVs, and travel on the same ships in adapted passenger pods. Six 100-ton or ten 60-ton capsules fit onto a standard cargo pod dock. The capsules themselves are automated, allowing the vehicle commander to operate them by simple commands.

An orbital assault is accepted by the Xing Cheng General Staff to be a one-off, extremely expensive affair. The entire brigade, including enough supplies to sustain combat for about two months, would require a minimum of 300 to 350 cargo pod docks, a sizeable portion of Xing Cheng’s civilian cargo fleet.

Selection of the Drop Zones would start as soon as the Fleet secures Alpha Mensae II orbit, once orbital defences have been reduced. From this phase on to the very end, the main constraint on operations will be the Heavy Zone Defense (ZD) and orbital defences around the Novaya Rossiyan capital city of Krasnograd, and Krasnaya Zezda spaceport itself.

As the Spaceport is the main target, yet another constraint appears: care must be taken not to damage the main launching laser, as this facility will be required to regain orbit after the invasion.

As mentioned earlier, recent CNMC doctrine states that a combat drop is far too risky to undertake close to the primary target area. Heavy ZD fire would ensure that most of the assault force would never even reach the ground. The main priority for the invasion force would thus be to determine the range of the local ZD umbrella, to ensure the landings take place outside of it.

From SPACEBORNE ASSAULT OPERATIONS by Claudio Bertinetto, Nexus Journal #1 (PDF)

In the quote above they note that the invaders must take care not to damage the launching laser. But they must also keep in mind is that a laser-launch site is fuctionally equivalent to a planetary fortress. It can hurl projectiles and use laser beams directly at any invading spacecraft.


(ed note: the ships in this novel use some sort of technobabble antigravity, so they can magically make the trip from orbit to the ground and back again with no trouble.)

"Four 115-mm rifles, two fore and two aft. A pair of lift-and-drive missile launchers amidships. And a secondary gun battery of 70-mm's and 50-mm autocannon. I know the class; we captured a few of them. Good ships."

A week later, the ship arrived from Storisende; a hundred and sixty feet, three thousand tons, small enough to be berthed inside a hyperspace transport, and fast enough to get a load of ammunition to troops at the front, unload, and get out again before the enemy could zero in on her, and armed to fight off any Army Air Force combat craft.

The ship settled quickly and daintily, while Conn and Anse and Rodney Maxwell sat in the car and watched. Immediately, she began opening like a beetle bursting from its shell, large sections of armor swinging outward. Except for the bridge and the gun turrets, almost the whole ship could be opened; she had been designed to land in the middle of a battle and deliver ammunition when seconds could mean the difference between life and death.


(ed note: this is talking about assaults set in the science fictional universe of the Traveller RPG, but the author is a former member of a U.S. Army Airborne unit. So the general strategy is universal, the more detailed bits are specific to Traveller. One big difference is that in the Traveller universe, antigravity is common. There are zillions of antigravity vehicles capable of tranporting troops from surface into orbit, so extraction is usually not a problem. In other universes this is not the case, so the primary objective is to capture a planetary spaceport intact so the troops can get away.)

A blast from the past, this was originally written for Freelance Traveller in 2002! It is largely unchanged since then.


There are a number of references to planetary sieges and the taking/retaking of planets by opposing navies in the Traveller Canon, especially during the Frontier Wars. And while the Imperium mainly controls the space between the stars, there are times when the enemy isn't only in space. And while a hostile planet can be interdicted, bombed, and talked to from orbit, only troops on the ground can truly control it. This paper is my attempt to explain how I think a planetary assault would work and how one could be set up in a campaign as background, plot device, or adventure.

Assumptions and Givens

  1. This is my opinion and in your Traveller universe YMMV. Much of this is based upon my knowledge of airborne operation as a former member of a U.S. Army Airborne unit.
  2. I am using the Third Imperium from about the time of the Fifth Frontier War as a baseline for the assaulting force; this implies an average Tech Level (TL)-13 with a top TL-15. Switching this to other races should be relatively simple and I will include some notes.
  3. I am primarily a Classic Traveller game master, but I will include references to other milieux. I hope to keep this as generic as possible.
  4. I am assuming that the Imperial Army will undertake large-scale planetary actions. IMO, Imperial Marines are 'johnny-on-the-spot'; they are the visible might of the Imperium and deal with brush fires. In large-scale actions they will concentrate on 'traditional' marine roles — boarding actions and quick assaults. With 'organic' support (artillery, medical units, etc.) and heavy units the Imperial Army and its colonial units are going to be the major players in ground actions.
  5. The relative superiority of near-space by the navy of the attacking force is a given. Without close orbit superiority planetary assaults are effectively doomed. This does not mean that the attacker must absolutely control close orbit, just that they must be capable of projecting great force into near orbit at specific times.
  6. Specific tactics will vary based upon the tech level of the planetary forces. Against foes of TL-0 through TL-5 or so the Marines just set down in grav vehicles and move out. While a large TL-5 army with heavy support could actually mount a credible defense against TL-15 marines in battle dress, they will not prevail. At higher tech levels, however, you can face serious opposition as those large armies gain nuclear weapons and more sophisticated armor and aircraft. I have divided assault procedures into TL-6 through TL-10 and TL-10+.
  7. I am taking it as a given is that military forces will generally be smaller as tech level increases. This will, of course, vary based upon law level, political stability, war footing, etc. But just as many modern armies are smaller than they were in previous generations, I am assuming that the increased efficiency of higher tech levels will reduce the number of sophonts under arms.
  8. This all assumes that the attacking force actually wants to capture the planet mostly intact. If there is no interest in preserving the structures, resources, or population, I assume that a heavy orbital bombardment until the defenders were unable to resist would be sufficient.

Planetary Assaults

A clear military objective is the key to clear military success. The ultimate goal of a planetary assault is to control the planet. In order to do this, the military objectives should be (not necessarily in order):
  1. Render defending military forces unable to effectively resist ('combat ineffective').
  2. Control or neutralize the defender's governmental or administrative functions.
  3. Control or contain major population centers.
  4. Secure means of resupply/reinforcement of attacking/occupying force.
Initially naval forces will conduct ortillery attacks against strategic targets. Defensive emplacements, command and control centers, sensor clusters, military bases, and downports will be primary targets. It is also highly likely that general infrastructure will be targeted to reduce the enemy's will to resist. Civil engineering (dams, mass transit, etc.) will be targeted. Depending on the level of resistance and the volume of ortillery fire available it is possible to reduce a planetary population to using flashlights and shipping water in trucks in a week.
The initial phase of ground assault is usually the use of drop troops (also called jump troops). Inserted from orbit, drop troops rely upon surprise, speed, and violence to secure a landing zone ('LZ'). Once secured, the landing zone is used to land heavy weapons, grav vehicles, landing ships, etc., etc. A secured LZ is called an 'orbit head'. The orbit head(s) are the start points for ground attacks against defenders and can quickly transform into the equivalent of a class C starport.
The main ground assault is performed by a mix of light and heavy infantry, mechanized infantry (infantry and g-carriers), armor, artillery, and support units. Because of the mix of units the force as a whole is called a 'combined arms army' or just 'combarm'.
Assuming the ground assault is successful, there are follow-on units that help secure the planet. Ranging from psychological warfare units to military journalists, these units strive to replace the destroyed or removed infrastructure and government of the planet with the tools of the Imperium.
Although it may be unusual to think of an operation as large as attacking a planet as tactical, but to a military force capable of such an action it is. The most critical decision is; where to insert drop troops? While this should remain fluid to allow changes based upon the differences from one operation to the next, it is often very advantageous to insert an orbit head near a population center of the defenders. In addition to allowing the operation to immediately threaten defenders, it will reduce the ability of the defending military to respond with full force without endangering their own populace. The simultaneous insertion of multiple orbit heads is also preferred. This will force the defenders to split their forces and the attention of their command staff. The use of deadfall ordnance at the same time can add confusion since gravity bombs can easily be configured to 'look' like drop pods to sensors.
Drop Troop Insertion
The most critical period of planetary assault is the insertion of drop troops. Although supported by orbital fire the drop troops are very exposed to defenders and can suffer significant losses before reaching the ground.
To increase their chances of securing an orbit head they are accompanied by a number of tools configured to resemble troop pods to sensors.
The first such tools are 'Landing Zone Preparation Devices', also known as daisy cutters or Sylean scythes. These explosive devices are the first pods fired and are designed to mimic troop pods. About one third of these devices detonate about the LZ and use gravity lensed explosives to direct a concussive cone toward the surface. The massive overpressure is designed to detonate any mines in the LZ and knock down most plant life and structures. The remaining devices detonate on impact and are grav-focused to concentrate their force in a 3-meter high plane parallel to the surface, flattening any remaining foliage and obstacles.
The most common devices that drop amongst the troops are jammers. In addition to radio and radar jammers, there are also meaconers (devices that distort navigation signals, i.e., give false GPS results), repeaters (devices that record defenders' radio communications and repeat them over on over on a number of frequencies), and mimics (devices that send electronic and radar 'chatter' that resembles the defender's communications but give false data).
Also accompanying the drop pods on the outer fringes are defense pods. These grav-stabilized devices have radar/lidar sensors and a laser cannon, all powered by a fusion generator. These air defense systems are designed to shoot down enemy aerospace fighters, missiles, etc. Once they are on the planetary surface they will continue in this role until out of power or shut down by the drop troops.
Last but not least, each squad will have an equipment pod. The equipment will vary based upon each squad's particular mission, but will include heavy weapons, air defense systems, telecomm gear, and combat engineering tools.

Tech Levels 6 through 10

While never easy, planetary assaults against worlds at tech levels 6 through 10 are less difficult.
Defending forces do not have access to meson weapons or powered battle dress. Also, the heavier man-portable weapons are not found at these tech levels.
As mentioned above, however, a large force with the support of nuclear weapons can mount a stiff resistance. The attackers must be sure that orbiting ships can provide nuclear damper support (prevents nuclear weapons from exploding) until the drop troop can set up their own. The drop troops themselves will be optimized to repel a large number of attackers with little special attention to heavy weapons. The average trooper in battle dress with an FGMP (plasma weapon) can deal with a great many main battle tanks of a TL-8 army, after all.
The defenders will also have less sophisticated sensors, making deception more effective. Combined, these make it likely that there will be more deadfall ordnance attacks and fewer actual orbit heads (no more than one per continent, likely only one or two).

Tech Levels 11 and higher

When the defenders approach or equal the technical ability of the attacker the risks become greater.
The inherent advantage possessed by the defenders forces the attackers to take greater risks. The high mobility and concentrated firepower of high-tech forces almost compels the attacker to try and overwhelm defenses with the number of attacks.
The best option for the attacker is to release a near-flurry of troop drops and deadfall attacks combined with heavy ortillery barrages. Preparatory ortillery must especially focus on meson sites and aerospace fighter bases. The drop troops must be prepared to face a number of threats, including grav armor (antigravity tanks) and meson gun artillery.

Special Note

The use of nuclear weapons to generate an electromagnetic pulse (EMP) effect is very common during planetary assaults. Against TL-6 through 10 defenders this can be a devastating attack. And the effect against high tech opponents can be more severe than may be assumed. Although most TL-11+ electronics (especially military electronics) are shielded against EMP effects it will still temporarily overload most sensors, increasing the survivability of drop troops as they enter the atmosphere. Also, while civilian communications systems may be shielded, often their antennae are not. While the means of communication will remain intact after an EMP attack, large areas of communications blackout will exist until antennae are replaced. This will add to the fear and confusion of the defenders.

Support Operations

Intelligence preparation can be a critical force multiplier in planetary assaults, especially against high tech level defenses. In addition to the routine strategic intelligence gathered by Imperial Intelligence, a planetary assault requires an in depth analysis of tactical response measures, apparent willingness of defenders to endanger their own populace, and overall readiness of the defenders ground forces. Effective counter-intelligence operations can also increase the levels of tactical and strategic surprise of the attacking force.
Commando operations in support of a planetary assault are extremely dangerous and prone to failure. However, when they are successful they can have a considerable impact upon the defender's will and ability to fight. For these reasons, they are often popular with players. If strategic surprise can be obtained commandos can be infiltrated and supplied in a large number of ways.
Their initial targets will generally be command and control, telecommunications, and strategic defense systems. The following scenario is a demonstration of the potential impact of successful commando operations in support of planetary assault:
Three commando squads are infiltrated onto a TL-13 world in advance of a planetary assault. Arriving as workers, tourists, and ship crew, they are supplied with a full combat load, including battle dress, smuggled in by intelligence operatives. In a coordinated series of attacks, two major telecomm hubs are sabotaged by pre-set explosives, a similar attack damages the refueling facilities of the major aerospace defense center, and teams of commandos in battledress armed with FGMPs assault the members of the planetary government, planetary defense commanders, and a deep meson site (planetary fortress) that defends a section of the planet. During the resulting confusion reports are received that an enemy fleet has jumped in-system and is on vector for planetary orbit. In addition to potentially neutralizing the defender's civil and military commanders and seriously disrupting planetary defenses these actions could very well panic the defenders, degrading their ability to fight.
While the first step is getting troops on the ground, the key to winning is supplying and reinforcing those troops. As soon as the orbit head is secured the follow on forces must begin to arrive. Initially these forces will be as 'heavy' as possible, i.e., g-carriers, grav tanks, and artillery pieces, preferably in large landing ships. This will be followed by a mix of combat and support units.
The job of the Navy is not over once the troop pods are fired. Without continued naval support the ground offensive will almost certainly fail. In addition to continued ortillery, naval aerospace fighters can provide direct close support to ground troops and engage tactical targets in the enemy's rear areas. Marines can conduct assaults against orbital facilities and can even be deployed by drop ships in support of threatened ground forces. If done properly, combined Army/Navy operations can achieve true vertical envelopment.

N-Hour Sequence

The N-hour sequence is a planning tool for military commanders, logistics planners, and political leaders. It is a rough outline of what will happen and when during a particular type of attack. The initial letter may change to determine what type of attack the sequence is for (for example, a ground attack plan can be called a G-hour sequence while a boarding action against an orbital spaceport could be an M-hour sequence). And certain times can be very broad or based entirely upon the success or failure of a different operation. They key to using an N-hour sequence is to remember that it is a tool, not the plan.
This N-hour sequence is, by necessity, abbreviated. It does not include frontier refueling, naval actions on approach to the planet, or orbital combat and boarding actions. It also omits a great many logistical steps that would be included in a 'real' sequence, as well as the preparatory steps that occur before the assault fleet enters jumpspace. Again, this is a rough estimation to give an idea of the flow of battle:
  • N minus 2 weeks: Assault squadron enters jumpspace.
  • N minus 1 week: Assault squadron enters normal space in target system.
  • N minus 2 days: Ortillery bombardment begins.
  • N minus 16 hours: Decoy deadfall ordnance attacks begin.
  • N minus 8 hours: Naval aerospace fighters increase tempo of attacks against tactical surface targets.
  • N minus 6 hours: Decision phase - commanders determine if planetary defenses are suppressed enough to allow close orbit insertion of drop ships. If so, drop ships move into close orbit. Bombardment ships direct their fire to both overwhelm defenders and clear a number of possible landing zones.
  • N minus 2 hours: Drop troops finish insertion preparation.
  • N minus 30 minutes: Decision phase - commanders determine if landing zones are prepared and the drop troops are likely to secure an orbit head. If so, drop troops are secured for insertion and troop carriers prepare for drop.
  • N minus 15 minutes: Naval forces trigger EMP effects.
  • N minus 5 minutes: Secondary EMP effects are triggered to disable automated responses. Naval forces begin blanket jamming from close orbit.
  • N-Hour: Simultaneous insertion of drop troops begins, accompanied by numerous decoy insertions with deadfall ordnance accompanied by jammer pods. Naval aerospace fighters deploy for close air support.
  • N+1 minute: Naval bombardment shifted to cover approaches to landing zones.
  • N+4 minutes: Landing zone prepped by daisy cutters.
  • N+5 minutes: Drop troops begin reaching surface. Drop ships begin move to high orbit.
  • N+7 minutes: Drop troops begin deploying to secure orbit head.
  • N+10 minutes: Drop troops finish landing on surface. Drop troops begin deployment of heavy weapons and support equipment. Aerospace fighters initiate close air support.
  • N+15 minutes: Decision phase - commanders determine if orbit head is secure. If so, landing ships with armor and mechanized forces begin planetary insertion.
  • N+20 minutes: Drop troops complete deployment of heavy weapons and support equipment.
  • N+25 minutes: Drop troops complete initial defensive positions.
  • N+35 minutes: Landing ships begin to reach the planetary surface. Mechanized and armor forces begin to deploy.
  • N+45 minutes: Decision phase - commanders determine if orbit head is ready for deployment of support elements. If so, landing ships begin cycling support units and equipment to the orbit head.
  • N+1 hour: Combarm begins offensive operations.
It should be obvious that the N-Hour sequence needs to be flexible. Planets with dense atmospheres will require more time for drop troops to reach the surface than planets that have no atmosphere, for example. Deployment of follow on forces may be delayed if there is a threat of significant air defense by the defenders. The number of changes that may need to be made are almost infinite. Recognizing this uncertainty, called 'the fog of war', and being able to anticipate and react to change without panic is what separates good commanders from great generals.


This section is not really about space warfare per se, but the topic of invading a planet from space is relevant to any discussion of interplanetary strategy.

Fundamentally, there are two methods by which one can seize control of a hostile planet: conquest and capitulation.  The question is the relative costs and efficacy of the two methods.  Conquest is based upon landing troops and physically overcoming the defender, while capitulation involves bringing him to a point where he realizes that further resistance is useless.  The problem is that the conditions required for a successful landing of troops are also those required to force the enemy to capitulate through threat of orbital bombardment.  Surface defenses (see Section 4) are quite effective, and an entering drop pod is a target comparable to a modern ICBM RV.  Modern ABM weapons have proven quite successful, and there is no reason to believe that the planetary defenses of the future will do any worse.  

While the ABM debate today is outside the scope of this paper, the analogy comes up regularly, so a brief discussion is in order.  Some point to the high failure rates of current ABMs in testing as justification for describing kinetic intercepts as difficult.  Those failures are a sign of insufficient operational maturity, not of serious problems with the concept.  Other weapons systems, such as air-to-air missiles, have had similar failure rates during their early development.  India has built an ABM system using unguided missiles that fly to the predicted location of the target, and has achieved significant success, and the 1950’s era Nike Zeus achieved 59 hits during 64 tests (including a classified number of skin-to-skin hits).

When compared to an ICBM RV, a drop pod has several advantages, but also several disadvantages.  First, it is likely to be going faster than an RV at the beginning of its path through the atmosphere.  Second, unlike the RVs that most ABM systems are designed to target, the pod is likely to be headed to an area far away from the system.  Such a crossing target is significantly less vulnerable than is an approaching one.  On the other hand, anything of critical military value is likely to be protected by ABM systems, so taking advantage of this vulnerability means that the attacking force will have to move a significant distance overland to reach its objective.

The biggest disadvantage is that the pod must come to a stop before it reaches the ground, while an RV is designed to keep as much of its velocity as possible for as long as possible. A pod carrying people must also keep deceleration to a reasonable level, slowing down in the upper atmosphere.  Human tolerances for acceleration limit theoretical maximums to about 17 G if the humans in question are supine and not more than 10 G if they’re in another position.  Theoretically, these capabilities could be increased by immersing the humans in liquid, which could raise tolerances as high as 50 G, although this might require liquid breathing, familiar from several Sci-Fi works.  One issue with liquid breathing that often is ignored is the amount of stress it places on the support structure of the lungs, which are normally filled with air.  These, along with the aorta, structural failure of which is the usual cause of death under extremely high acceleration, could be surgically reinforced, but this is not a procedure that would likely be carried out on every member of an invasion force.  Even if such measures were taken, there are other problems with extremely high deceleration drops.  The biggest is probably heating, which tends to dominate atmospheric entry calculations at very high velocities, and high velocity low in the atmosphere is exactly what a high-deceleration capsule would be designed to achieve.  Equipment and structural loads are both likely to be limiting factors.  Equipment will have to be specially designed for such loads, and carefully packed for drops.  The structure of the capsules, and their heat shields, will be significantly heavier, raising transport costs significantly (discussed below).

The other significant disadvantage of a drop pod is that even a one-man drop pod will be significantly larger than an RV.  A current US RV, like the ones that carry the W87 or W88, is about 55 cm across and 175 cm long.  A human would probably need a pod at least a meter across and two meters long, or two meters across if the human is supine.  

The actual sizing of such pods is a task which deserves more study.  For an individual pod, the best analogue is probably a 1960s project called MOOSE (originally, the acronym stood for Man Out Of Space Easiest, but it was later changed to Manned Orbital Operations Safety Equipment), which was described by its originators as a lifejacket for spacecraft.  It was a nonlifting conical body, with an empty mass of 90 kg, a gross mass of 215 kg, a diameter of 1.83 m, and a drag coefficient of around 1.42.  While advances in technology might have made the system lighter since it was originally conceived, basic physical limits (and the need to drop equipment with the personnel) mean that this remains a good representative of the minimum possible drop pod.

Sadly, details on larger pods are lacking.  While there were numerous studies of emergency return pods for either single people or groups of three people, there have been only a few studies of dropping squad-sized groups, and not at all of dropping vehicles.  The solution to this is to scale from various different types of known systems.  MOOSE, for instance, has a dry mass equal to 72% of its payload, although a larger system could probably do somewhat better.  Another interesting data point is from the airdrop rigging manual for the M551 Sheridan tank.  It suggests that the equipment for a low-velocity low-altitude airdrop is a full 20% of the mass of the vehicle.  Because of the greater structural loads involved in an orbital entry, and the need to include a heat shield, a figure in the ballpark of 60% of payload mass does not seem unreasonable.  A simple ballistic capsule might be slightly lower than this, while a more complex lifting pod will require more mass.

The best examples of squad-sized pods are two NASA programs, the HL-20 and the X-38.  While the HL-20, with 10 occupants, was intended to be a general-purpose space vehicle, compromising its utility for comparison purposes, the seven-man X-38 was meant as a Crew Return Vehicle for the ISS, giving it a generally similar mission to the notional drop pod involved.  The HL-20 massed about 10,884 kg, with a payload of 1,815 kg, which means that the dry mass of the spacecraft is 500% of the payload mass.  The X-38 appears to have an even higher ratio, approximately 818%, although this is probably at least partially because the entire payload consisted of people, which are notoriously intensive in terms of packaging mass.  Other lifting bodies appear to have similar payload fractions, although data is quite limited.  This is a serious problem, given how important transport costs are, and the limited utility of lifting bodies in avoiding defenses.

Using code from the author’s orbits class, a number of pods were investigated (see Table 4 for characteristics).  The ballistic pods were based on pods like the MOOSE, while the numbers on the lifting pods broadly correspond to Apollo.  The characteristics of the winged pods are based on the X-38, except for payload mass fraction, which has been significantly reduced relative to said spacecraft.  No account was made of entry heating, although that would of course be a primary design driver for real-world drop pods.

Table 4
1-man ballistic20031031.4073.81
1-man lifting20035031.30.589.74
1-man winged2001,0005.50.250.3727.27
12-man ballistic2,5003,850181.40152.78
12-man lifting2,5004,350201.30.5167.31
12-man winged2,50011,000300.250.31466.67
HMMWV ballistic4,5006,800301.40161.90
HMMWV lifting4,5007,800321.30.5187.50
HMMWV winged4,50019,000560.250.31357.14
Stryker ballistic18,50027,750601.40330.36
Stryker lifting18,50032,000641.30.5384.62
Stryker winged18,50077,5001250.250.32480.00

The most obvious result of the investigation was that ballistic pods are far inferior to either lifting or winged pods.  They must be fired into the atmosphere at very shallow angles, which means that they have long and predictable ground-tracks.  The savings in mass (and both directly and indirectly in cost) would be erased by the need to sanitize a larger corridor for the pods.  A pod capable of generating lift can use a trajectory with a steeper entry angle, using its lift to keep the deceleration at a reasonable level for a longer time, cutting down on groundtrack.  Furthermore, G-loads for ballistic capsules are relatively insensitive to variations in entry angle, and must be controlled by lowering the ballistic coefficient. This is contrary to what the analytic reentry equations would suggest, but it is due to the fact that we are not assuming entry angle to be constant.

The choice between winged and lifting pods is less clear-cut.  There is no practical difference in trajectory between the two before they reach about 50 km, where air resistance begins to build up quickly.  Above this altitude, they are vulnerable to ABM/ASAT systems, and totally unable to dodge.  Thrusters could be added to give such capability, but they would add both mass and significant expense to capsules.  This is a situation that might benefit larger pods, as there are significant economies of scale in such systems.  Below it, both become surprisingly maneuverable, capable of turning though 90° or more.  The lift vector can be altered by rolling the pod, and the choice of this direction can significantly alter the course the pod takes.  This could range from an attempt to extend the glide in the line of entry to as great a distance as possible, to a sharp turn to get ‘behind’ a heavily-defended area, to using the lift to get as low in the atmosphere as possible and keep deceleration up to get on the ground as quickly as possible.  The winged lifting body obviously has a much greater cross-range, but simply cannot slow down as quickly as a lifting capsule, because of its much lower CD and thus higher ballistic coefficient.  Most attacks would probably use lifting capsules, unless they needed to be able to maneuver around defenses low in the atmosphere.  Even then, there are serious limitations on the capability of a winged pod to maneuver after its initial bounce.  The most obvious application would be a case in which there is a narrow corridor between two sets of defenses that is too short to send pods down directly, and another corridor clear of ASAT systems that joins it at an angle.  However, this is unlikely to occur in practice, and even if it did, the attacker would have to know about it before leaving home, and then have it still be there when he arrived.  All in all, winged pods are unlikely to see significant service.

The data in Table 5 was generated with an entry interface of 150 km, and an initial velocity of 6,500 m/s, for the 1-man capsules.  The G-load was held below 10Gs.  The first scenario for the lifting and winged capsule involved the roll angle being set constantly to 0° (straight up).  The second involved the roll angle being set to 90° except in response to high G-loads, which cause it to roll towards 0°.  The third involved a 90° turn, and then attempted to hold that heading.  The last involved a complex set of control laws that was an attempt to get the capsule on the ground as quickly as possible.  Downrange is the distance from the entry interface to the final point along the spacecraft’s initial line of flight, while crossrange is the distance traveled perpendicular to the initial line of flight.  Downrange, crossrange, and duration were measured from entry interface until the pod reached an altitude of 5 km.

Table 5
Entry Angle (deg)-1.9-8.2-13.5-8.2-13.5-8.2-13.5-8.1-13.4
Downrange (km)1460.61077.22825.9777.2796.2777.3805.1742.0645.9
Crossrange (km)00056.9226.965.3311.620.679.8
G-max (Gs)9.959.919.939.999.999.999.999.989.98
Duration (sec)5956691124472310500516414194

Various plots from all four scenarios are attached at the end of this section.  In all cases, the ballistic pod’s trajectory is in blue, the lifting pod’s in green, and the winged pod’s in red.  As can be seen for the simple lifting trajectory plot (figure 3), both the lifting and winged capsules bounce significantly, and the high lift to drag ratio of the winged pod carries it well downrange.  This was then countered by having the pod roll, causing the lift vector to push it to the side instead.  The resulting trajectory (figure 4) is rather interesting, as the lift of the winged body again carries it a significant distance from the initial point of impact with the atmosphere. Figure 5 shows the 3-D trajectory of the 90° turn scenario, although due to scaling of the graph, it is less obvious how much of a difference there is in crossrange.  However, the table makes it quite clear that most of the energy appears to be lost in the initial turn.  Figure 6 shows the pull-down trajectory. Table 4 clearly shows that this trajectory will get a pod on the ground fastest.  This trajectory involves the pod bleeding off most of its velocity while relatively high in the atmosphere (above 20 kilometers), then spiraling steeply down.  This could be helpful in avoiding defenses, or make the pod quite vulnerable to them, depending on type and configuration.  It might be possible, by tweaking entry angle or some other facet of pod dynamics, to get the pod deeper before it slows down, and research into trajectories has not been completed.  Due to coding limitations, the author was unable to test the effects of S-turns, but in theory there is no reason why the crossrange of an entry profile could not be reduced significantly.  

Table 6 was generated for the 12-man pods, using the same set of trajectory designs and the same constraints as used for the 1-man pods above.

Table 6
Entry Angle (deg)-1.45-8.1-13.4-8.1-13.4-8.1-13.4-8.1-13.4
Downrange (km)1646.41113.52854.4825.0805.3825.1823.1775.0689.2
Crossrange (km)00057.3266.463.5358.019.488.5
G-max (Gs)9.989.929.9510.009.9910.009.999.999.99
Duration (sec)5125771058394311412486331178

No figures are provided for the 12-man capsules, as the trajectories are broadly similar to those of the 1-man capsules.  One of the most interesting results is the much shorter entry durations for the 12-man lifting pods, as opposed to the 1-man pods.  This is likely because of the higher ballistic coefficient of the 12-man pods increasing terminal velocity during the final phases of flight.  The much smaller difference between the winged pods is probably attributable to the same mechanism, but the lift generated by the pod makes the difference much smaller.  The 12-man pods do tend to have slightly longer downrange footprints than their smaller brethren, probably because of the same mechanism described above, in that they spend more time at higher velocities during deceleration.

Table 7 is the equivalent table for the HMMWV pods, while Table 8 is for the Stryker pods.

Table 7
Entry Angle (deg)-1.45-8.1-13.4-8.1-13.4-8.1-13.4-8.1-13.4
Downrange (km)1651.81118.22853.0835.5802.7835.6820.5780.5673.9
Crossrange (km)00057.3265.863.1360.019.077.2
G-max (Gs)10.009.949.9310.009.999.999.999.999.99
Duration (sec)5055641064384313400493318172
Table 8
Entry Angle (deg)-0.85-8.0-13.35-8.0-13.35-8.0-13.35-8.0-13.35
Downrange (km)1898.41158.82871.8881.6825.6881.1842.8823.5694.9
Crossrange (km)00057.2268.760.6351.418.975.7
G-max (Gs)10.049.959.9610.0010.0010.0010.0010.0010.00
Duration (sec)4714921022324296331447257.4160

It should be noted that no trajectory could be found which would give the ballistic Stryker pod a trajectory that kept it under 10 G.  This appears to be a function of the very high ballistic coefficient.  The Stryker pods continue the pattern seen earlier.  Higher ballistic coefficient means shallower entry angle, longer downrange distances, and significantly shorter durations.  However, the values are similar enough that it is still probably feasible to mix them during a drop.

However, there are significant limitations to the analysis used.  While it is significantly more accurate than a simple analytic approximation, there are several causes of significant error.  First, any planetary invasion is unlikely to be of Earth.  Even if we assume that the planet is broadly Earth-like, details like local gravity and atmospheric density variations could skew the results.  Also, it assumes that aerodynamic characteristics are constant, which is false in two separate ways.  First, aerodynamic characteristics are not constant across an entire entry, although the variation with Mach number is smaller than might be expected, and can generally be ignored.  The largest variation occurs at subsonic speeds, and when an attempt was made at improved modeling, the differences were very minor.  Second, a pod could easily be designed to change its shape or angle of attack during entry to allow it to better optimize its trajectory.  An investigation of all the factors involved would require more time than the author has available.  

Note the variations in entry angle for the various trajectories.  This has a significant effect on the danger zone in which ASAT systems could attack the pod from below.  While exact orbits before entry interface were not computed, it is clear that the ballistic capsule will be vulnerable to even SM-3 and late-model THAAD-type systems from the time it is placed into its entry orbit.  The other pods will be vulnerable to such systems for approximately 1,000 km before entry interface, although the exact number will depend on the angle.  It might be possible to come in at a steeper angle and use rockets to reduce the angle at the last minute.  However, this would significantly increase the cost, mass, and complexity of the pod, and wouldn’t address the lower-altitude vulnerability issues.

A more careful analysis of the aerodynamic data for various spacecraft reveals that there are potential shapes which could outperform the chosen Apollo-based and X-38-based pods.  A bent-bicone shape has a potential hypersonic L/D (lift-to-drag ratio) of approximately 1.4, and potentially improved packing efficiency relative to a lifting body, reducing the amount of dead mass that must be carried, and there are several other simple shapes with L/D of 0.8-1.  There exist examples of winged shapes with hypersonic L/D of as much as 2.6, although these are likely to be even heavier than the lifting body described above.

Compared to lifting body/winged shapes, simpler conical shapes will suffer from poor subsonic aerodynamics.  The obvious solution is to use a Rogallo wing, similar to the systems originally proposed for Gemini, or a parafoil as used on the X-38.  While the Apollo capsules landed precisely enough that NASA began offsetting the point of aim from the location of the recovery ship for fear of hitting the ship, the typical miss distance was on the order of a kilometer, which requires an infeasibly large drop zone.  While this would be adequate if targeting a dry lakebed or something of the sort (water landings are impractical without support already on the surface), such features are often not conveniently placed, and Apollo did not have to worry about collisions with other descending capsules.  Modern military parafoils have L/D values of 4 to 6, which is competitive with most lifting bodies, and Rogallo wings have wide L/D values, depending on construction, ranging from 4 up to as much as 12-16, although higher L/D wings may not be particularly good for the uses under consideration here.

The original paper that described the MOOSE concept has several interesting facts relevant to landing troops on planets.  A figure on pg. 380 shows the variation in downrange distance with percentage variations in deorbit delta-V.  From a 200 nm circular orbit, a 1% variation would produce a dispersion of approximately 35 nautical miles and a 3% variation in delta-V from a 300 nm orbit will produce a 200 nm dispersion.  This is another reason to suspect that all pods will have some degree of lifting capability, to allow them to compensate not only for thrust variations during insertion, but also the other variations that may come up during the drop.  

The paper also includes a slightly less minimal design for a one-man non-lifting pod than MOOSE, as well as a 3-man lifting pod.  The ‘life raft’ (MOOSE was supposed to be a ‘life jacket’) massed about 230 kg to the 110 kg listed for MOOSE in the paper (note that these figures vary from those used in the original calculations on drop pods).  This design is not studied in great detail, as its only real advantage over MOOSE was that it didn’t have to be foamed in space, but it massed twice as much.  

A very interesting figure was included in a description of the 3-man lifting ‘lifeboat’, and is reproduced below

Vs is the fraction of satellite velocity (or circular orbit velocity in a LEO of altitude that is not explicitly called out in the paper) the spacecraft begins to maneuver at.  The craft described began at .8Vs to reduce heating load, and a crossrange of 500 nm.  It had an L/D of 1.5, a dry mass of 1,005 kg, and a payload of 450 kg.  This is broadly similar to the winged pods described above, with a slightly worse payload fraction (although the study was conducted in the early 1960s, and better technology could improve this) and somewhat better L/D.  

An ABM system could potentially cover a substantial area.  The exact range will depend upon a number of factors, but a missile in the class of the THAAD block 4 should have a coverage footprint of approximately 300 km.  The SM-3 Block IB is estimated to have a footprint of approximately 400 km, and the Block II should be able to reach about 500 km.   These are rough estimates, but ranges on the order of 500 km are entirely reasonable for such weapons.  Unfortunately, more precise values are generally classified or otherwise unavailable, even for more mundane SAMs.  It should be noted that these are the ranges for warhead (or drop pods) landing short of the launcher.  For objects that fly over the launcher, the coverage range is much greater.  All of the listed missiles are capable of reaching low orbit, which means that the capsule could be shot at any time after it is inserted into low orbit.  It is theoretically possible to come in fairly steeply from a high orbit, but this will mean either more heating and higher deceleration forces, or very significant expenditures of delta-V to insert the capsules into their entry trajectories if they instead come in much more slowly than usual.  ABM systems are not a case where there are grounds for reasonable expectation for massive improvements in the weapons performance.  There is no propulsion system that could replace chemical rockets for the purposes of short-range missiles, and the other systems involved show no signs of significant improvement.  This is closely related to the problems seen with deep-space missiles, but made worse by the absolute requirement for high thrust-to-weight ratios.  The range is limited by the fact that the target must be shot down before it gets too low in the atmosphere for the missile to function properly.  In such cases, the radar horizon is the biggest limit on rage, and if forward-based sensors are available, range could as much as double.  Of course, radar, being an active sensor, is vulnerable to bombardment itself, and range might instead be limited by passive optical detection of entering pods.  Laserstars overhead could shoot down some of the missiles, provided that they are not being shot at themselves, but the protection they provide is almost certain to be incomplete.  That is not to say that having as many spacecraft as possible overhead during the drop is not a good idea.  At the very least, they will attract missiles that otherwise would have taken out pods.  

Even if the pods, of whatever size, have high (>1) lift to drag ratios during entry (which probably means they are lifting bodies), they still very vulnerable to missile defenses.  The pod spends a significant amount of time at altitudes above the sensible atmosphere (That part of the atmosphere that offers resistance to a body passing through it), where it has essentially no maneuverability, and would be easy pickings for any of the missiles described in Section 4.  Even after it gets lower, its maneuverability is still limited by the fact that it is unpowered, and any turns will scrub valuable energy, leaving it vulnerable to SAMs.  The best strategy is to stay entirely out of the range of defenses, which can be accomplished because of the ability of the pods to either come in at a steep angle or maneuver after entering the atmosphere.

The use of lasers against drop pods is a somewhat dubious proposition.  With proper planning of the approach, the pods will have a large thickness of atmosphere between them and the laser site, even when they are above the horizon.  A laser site will suffer from the same problems that a radar site does, as well as the issues raised by propagation through the atmosphere and potential problems penetrating the plasma shell around the pod.  This plasma would tend to absorb the laser, causing slightly more heating to the pod, but nothing more.  Even if the pod was still above the atmosphere, the fact that it has to be designed for the heating environment of atmospheric entry will mean that it will prove a significantly harder target than a conventional spacecraft.

Other forms of hypervelocity projectile launcher are also potential candidates for use in defenses.  In theory, passive projectiles should be cheaper and much harder to detect.  The exact velocity achievable is a complicated question.  In theory, most systems should be capable of significant velocities, probably more than a typical ABM.  However, there are significant drawbacks to firing projectiles at such speeds.  It is likely that high velocity flight at such low levels will produce a plasma trail would give away the projectile, and might well be hard on the surroundings.  The other drawback is that unlike a typical missile launcher, the launcher is expensive, and potentially vulnerable.  This is likely to make them a less-attractive option for planetary defense, with the possible exception of ram accelerators, which do not require a sophisticated launcher.  The ram accelerator might also be able to repurpose the projectile into a conventional ramjet for sustainer work during atmospheric flight.

The efficacy of individual drop pods is highly doubtful, however.  Even if only minimal losses are suffered, there are still the problems encountered during the airborne landings in Normandy on D-Day.  Troops were scattered, and most of the airborne forces spent their time wandering about as small groups of men from different units instead of fighting as formed units.  This type of confusion drastically reduces combat effectiveness.  It could be argued that maneuvering drop pods could place troops closer together, but at the speeds involved in spaceflight timing errors of a second can scatter pods by 7 km or more.  

Another significant problem with individual pods is the lack of heavy equipment for the troops on the ground.  Anything that is in a pod larger or heavier than a man will be both an easier target and a more prominent one, and a mass-optimized equipment pod will follow a different trajectory from a mass-optimized individual pod.  The defense would likely shoot at such pods on general principal, denying the drop units support.  Even if some way were found to combat the dispersion problem, light casualties could still compromise combat effectiveness significantly.  Even losses as low as 10% can have a significant effect on the combat power of a unit, particularly an airborne unit that has had most of its vehicles destroyed.

Some people would raise powered armor as the solution to this problem.  After all, if an infantryman can be given the firepower of a vehicle, there is no need for vehicles.  The problem with that is that there is virtually no reason to expect that practical powered armor will be developed in the PMF (Plausible Mid-Future).

First, we must define powered armor.  Powered armor is a suit that provides the infantryman with greater strength and protection than an unarmored infantryman while not interfering with his function as an infantryman.  The last part is critical.  The armored infantryman must still be able to do the jobs required of infantry, such as clearing buildings and going up stairs.  This in turn sets size and weight limits on the armor.  Current OSHA guidelines state that the design load for stairs is 510 lb.  Even assuming that all of that limit is available (ignoring things like old or rotten stairs, or stairs not built to code), an average combat-loaded modern infantryman (sans armor) still weighs approximately 225 lb., leaving 285 lb. available for the armor.  This number includes not only the armor itself, but also all of the various servos and power supplies necessary to run it.  As an example, the Lockheed HULC currently weighs 53 lb. without batteries and can carry about 200 lb.  However, it is only a lower-body system and must include its own structure, so given various developments, a total of 50 lb. for the entire power/servo system does not seem entirely out of the realm of possibility.  This leaves 235 lb. for armor.  Taking as a baseline current ESAPI (Enhanced Small Arms Protective Insert) plates, this translates to about 35 square feet of armor or 3.2 m2.

A typical adult male has a surface area of 1.9 m2, so this is a vaguely practical number for armor area once all the other stuff under the armor is taken into account.  The ESAPI plates are rated to resist WWII-vintage M2 .30 caliber armor-piercing rounds, but only when backed by the various plate carrier vests.  This means that the total surface area available would have to drop again, which in turn reduces the practicality of the system.  Even then, more modern 7.62 mm AP ammo would likely be able to defeat it, although solid information on this is difficult to find.  At one point, rifles in this caliber were standard-issue, and could be again if a need (such as defeating targets in powered armor) was there.  Such an evolution of weapons to counter increased armor has happened before.  In the 1500s, the standard gunpowder weapon was called an arquebus, and it was incapable of penetrating the increasing thicknesses of armor being worn on the battlefield.  A heavier gunpowder weapon, called the musket, was developed to defeat such armor.  Muskets made armor more or less obsolete, and once they had done that, they shrank to the size of the arquebus, absorbing it in the process.  

Increasing the weight of armor protection to defeat such threats moves the armor out of the category of “powered armor” and into the realm of “small vehicle”, which has the side-effect of removing the operator from the infantry.  As a friend of the author’s said “if you plan on having your infantry armed like tanks, and armoured like tanks, you shouldn't be surprised that they weigh as much as tanks.”  The small vehicles that would result have no parallel in modern warfare, casting doubt on their utility, and even if they were to prove useful, it is likely that they would not look like powered armor, due to the complex actuators and control systems required of such armor.  A small tracked or wheeled vehicle with a turret would be much more efficient, although it has been pointed out that it might also look quite a lot like a Dalek.

All of the above analysis assumes modern armor and weapons, and the assumption for application to the PMF is that the balance between armor and weapons will remain more or less constant.  This could obviously be flawed, but even if armor increases in power relative to weapons, the weapons used will be tailored to deal with the threat.  Small (~25 mm), low-power weapons that fire shaped charges would probably be effective if all else fails, absent special authorial pleading.  

The above is a best-case analysis. There are likely to be other complications from powered armor, such as reduced mobility (a problem in urban combat), increased ground pressure (a problem anywhere there is mud), increased logistics burden (a problem anywhere) and the fact that not all steps are built to OSHA specs.  The combat load of a soldier will also likely increase, and the number used above was for a basic rifleman only.  Grenadiers in the study referenced carried an extra 8.5 lb, and SAW gunners an extra 16 lb, to say nothing of the heavy weapons personnel, or even personnel who are simply heavier than average.  Add to this the fact that powered armor, both in fiction and in real life, is often touted as not only protecting the soldier, but also increasing his carrying capacity.  All of these combine to render powered armor a dubious proposition.  This is not to say that exoskeletons will not be useful for increasing the carrying capacity of soldiers, or that powered armor might not have a role in peacekeeping/counterinsurgency operations, where the enemy does not have access to modern weapons.  The problems of reliability and maintenance will also be major issues for a force that relies so much on very high-tech equipment.  Without real-world experience, it is difficult to determine how much maintenance powered armor would require, but even the most basic powered armor will be very complex compared to virtually all systems the infantry use today.  This is not a good thing when the system will be exposed to dirt, mud, debris, insufficient maintenance, and near-continuous use.  This in turn indicates that additional maintenance facilities above and beyond what is standard today will have to be dropped with the unit, exposing them to the orbital defenses (see above).

An alternative is to drop a more conventional mechanized unit, complete with vehicles.  This unit will tend to land in bigger chunks, improving effectiveness, but the reduced number of targets for the defenders is likely to result in greater losses.  Unless all pods are of the same mass, the defenders will still be able to discriminate between them, and guess at their payloads.  This would likely allow them to focus their attacks on bigger, heavier pods, which could be assumed to carry things like tanks.  Careful design of a unit’s equipment could mitigate this problem, but only at a cost in mass-efficiency for both the pods and the equipment.  The exact tradeoff is heavily technology-dependent, and thus outside the scope of this paper.

In either case, once the attackers are on the ground, they still have to move to their target and capture it.  The movement in question will be over hundreds if not thousands of kilometers of terrain, and impeded by enemy resistance, terrain features, and a lack of roads.  If the drop zone was a relatively undefended area, it was probably also lightly inhabited, and thus lacking in transportation infrastructure.  Sabotage would also contribute to this lack, delaying the advance even more.  The US Army estimates that a typical rate of march (including rest and maintenance halts) of between 16 and 32 km/h depending on the quality of the roads and the time of day, with a practical maximum daily range of approximately 200 km.  Note that this is for a road march in peaceful conditions, not a combat advance.  Even the “high-speed” advances in the 2003 Iraq War averaged somewhere around 15 km/h, against light opposition.  However, taking the average daily range and a distance of 1000 km (through some combination of landing distance and not being able to take the shortest route), the unit will take 5 days to reach the target.  This does not take into account the possibility of resistance, and the various problems that could occur during what will be at least a semi-tactical march.  During this period, the enemy will know where they are, and where they are going, and be able to move forces to reinforce the objective.  It seems reasonable that the defender will be able to manage at least twice the attacker’s movement rate, giving a huge radius in which troops can be drawn from to reinforce the defenses.  This assumes that the landing zone is a complete surprise to the enemy, which is unlikely to be the case if any serious preliminary bombardment is done.  For that matter, extended preliminary bombardment might be counterproductive, giving an enemy the warning he needs to move local defense systems in to slaughter the drop pods.  These systems could be small and relatively-low performance, resembling modern SAMs, as they would only have to intercept the pods at low speed and altitude.

Once the assault force arrives at the target, they must overwhelm the troops defending it.  Assuming that the attacking force has about three times the per-man effectiveness of the defenders (which is not unreasonable, as training masses nothing and making equipment better is often cheaper than shipping more of it), they will need about even numbers to overcome them.  This ignores potential losses in effectiveness due to fatigue, losses in key personnel, and general confusion during the drop and march.  Training, equipping, transporting and supplying that many troops is going to get expensive very fast.

 Exactly how expensive is an interesting question.  Taking as our baseline a Stryker Brigade Combat Team (chosen because it seems a reasonable analog to a future space-transportable unit with integrated support units), the total mass of vehicles and heavy equipment is at least 12,025 tons (time and information constraints prevented a better number, although this estimate was intended to be a reasonably conservative best-case).  There are a total of 4,236 men, and assuming that light equipment and people amounts to 500 kg per man, the total “combat mass” comes to at least 14,145 tons.  There are roughly 1500 individual vehicles/pieces of heavy equipment, so at least that many drop pods are required (assuming crews drop with their vehicles).  If we assume 4.5 kg/man/day shipboard, the unit requires 572 tons/month in transit.

The combat supply requirements are somewhat more involved.  The first assumption made is that all water is being procured on the surface, instead of being dropped from orbit.  The second is that the vehicles do not require fuel, and use some form of lightweight power source, which is likely to be ultimately nuclear-derived.  A typical man-day’s supply will total 31.8 kg (of which 14.2 kg is ammo and 6.8 kg is equipment attrition replacements), with an additional 24.4 kg if the vehicles use shipped fuel.  This totals 134.7 tons/day of combat, although this number may be low (the source value for supplies/man/day probably includes higher-echelon troops than are present in the SBCT and who don’t use as much ammo as those on the front lines).  After major combat operations are completed, the daily requirements will drop to around 13 kg/man/day, or 55.1 tons/day, which can be reduced by another 1.8 kg/man/day if food is procured locally.

If combat is expected to take 30 days, then the total supply requirements will be 4,041 tons.  This will also have to be dropped, for a total drop mass of 18,186 tons.  However, this number ignores the mass of the fuel systems that would need to be deployed.  While the paper referenced above contains some details on the proposed scheme, details on the exact weights involved are fairly sparse.  Reference is made to the system breaking even for weight with gasoline after 30 days of combat.  If this is correct, then the mass required for the fuel production system would be approximately 3,100 tons, or about 17% more drop mass.  However, the report in question dates back to the early 1960s, and it is likely that the technology of the future (or even of today) would allow significant reductions in that mass, although how significant is impossible to estimate precisely.  An assumption of a fuel system mass of 1,000 tons is probably as reasonable as is possible without detailed study, bringing the drop mass to 19,186 tons (assuming that no additional personnel above and beyond the brigade’s normal complement are required to operate the machinery).

Assuming that the drop pods have a total mass equal to 50% of the payload (which is a somewhat generous, but generally in line with the numbers given above), that means a total drop pod mass of 9,593 tons.  It might be possible to reduce the drop pod mass slightly by finding more mass-efficient ways to drop supplies, such as reusable shuttles.  However, this does require improved security around the drop zone, and planners would probably assume that this would generally not be the case.  Also, we need to account for supplies consumed during transit.  If we assume a total transit time of 6 months, this mass (which does not have to be dropped) will amount to 3,432 tons.  The total mass that must be launched into space for this mission is a minimum of 32,211 tons.  The troops will probably require at least 3 tons/man in hab space (keeping in mind that they must be fit to fight at the other end), so the total hab mass is a further 12,708 tons, although a fair bit of this can probably be provided by requisitioned civilian ships, and would not be included in the launch budget.  Assigning a further 10% of payload mass for general spacecraft structure, the total payload mass that must be moved from one planet to another is approximately 49,411 tons.  This is a total of 11.7 tons/man, of which 7.6 tons must be launched specifically for this mission.  Even if launch costs approach current grid energy costs ($100/3.6 GJ, which is theoretically possible if using laser launch, a space elevator, or a launch loop), the cost of putting the necessary equipment and personnel in orbit will be $26.84 million.  If the transit velocity is about the same as orbital velocity, the transit energy cost will be $82.34 million; although a more realistic number for such a transit would be twice that (the above ignores the energy costs of the ships themselves).  Given the other costs of running a spacecraft, the total shipping bill for the brigade could easily pass $300 million in even the most optimistic case.  This totally ignores the costs of the drop pods and supplies themselves, although the cost of supplies for transit can be traded off against the energy cost of using a faster, higher-energy transit.

To move multiple brigades, which will be required for all but the smallest worlds, many times the amount of stuff described above will have to be moved, to say nothing of the various combat support elements.  Heavy artillery, combat support, and air units will all need drop pods, habs, and cargo spacecraft.  The shipping bill alone would rapidly rise into the billions or tens of billions of dollars, and the heavy equipment is more vulnerable during the drop.

Training the forces is also non-negligible.  It is likely that the troops would need to be trained in an environment that has the characteristics of the target world.  The best way to do this appears to be an orbital hab with a rotation rate set to give the appropriate gravity.  Terrain can probably be approximated at home, and the hab can have an appropriate atmosphere.  The spin rate of a power’s habs might be an important piece of intelligence data to back up signs of preparations for an invasion, giving an indication of who they expect to go to war with.

At this point, it would be logical to suggest the use of robots as an alternative to human troops, and there are significant factors to recommend this approach.  A robot not would require habs during shipping, would (presumably) take no training, and could be considered expendable.  However, there are problems with this approach, as it rests on the assumption that a suitable ground-combat robot could be created.  Many of the reasons cited in Section 2 in support of unmanned warships do not apply on the ground.  The largest issue is that while it is practical to propose that every spacecraft be run by remote control, doing the same for a robotic invasion force removes entirely the logistical advantages accrued therein.  This in turn requires the creation and deployment of autonomous robots in an environment that is tactically far less clean than space, overcoming formidable technical and moral/political obstacles.  Nor should the difference in physical environment be overlooked.  A robot would have to deal with dirt, mud, and other hazards of military life with little maintenance, as well as being capable of fulfilling all the roles of the person it is replacing, in an environment where versatility is far more important.  The cost of this is non-trivial, although it does offer a vaguely-plausible alternative for those willing to imagine that robotics will advance so far.

Another, often overlooked issue with robots is their effectiveness in replacing humans during counterinsurgency operations.  A robot advanced enough to be effective at winning hearts and minds is unlikely to be the cheap and disposable device described above.  Morally, it will have to be almost equivalent to a person to win the trust of those it works among, which more or less erases the line between robot and human, except from a logistical perspective.  And the overall logistical requirements of a robot-based force are unlikely to be that much better than an equivalent human-based one.

If the landing were to take place, it would be the attacker’s ultimate gamble.  All of his troops would be landed in one area, and there is no practical way to get them back.  Laser launch and robust SSTOs would offer the capability to evacuate some of the men, but all of the heavy equipment would probably have to be abandoned.  Even such an evacuation would be risky, as the defender would want to trap as many men as possible, if for no other reason than to make it more difficult to attack again later.  Any spacecraft taking off would do so through a barrage of missiles, and laser launch sites would be prime targets for any number of different methods of attack.

If the attacker made a successful landing, he would have to face the defender’s surviving forces.  One major advantage the defender has is that not only does he not have to pay shipping costs for his units, he can also use quantity to overcome quality.  Most of the defender’s army would be draftees, given a few months of training and some basic weapons.  One on one, the attacker’s units would have no problem destroying them thanks to better training and equipment.  However, they do not have to pack as much combat power into as little mass as possible, which allows them to be deployed in large numbers at the optimum ratio for cost to combat power.  Furthermore, they are on the defensive, which is less difficult for the inexperienced troops that make up the majority of their ranks.

One alternative to a serious invasion is to stage a change of government to one that is more favorable to you.  This can either be done by encouraging a coup, or by supporting an insurgency.  The coup would be encouraged by threats against the planet if the planet does not surrender, along with promises of good treatment if it does.  Insurgency holds great story potential.  Small teams would be inserted onto the planet, probably through normal space travel, and used to create or support local guerilla movements, with the aim of overthrowing the government.  This is not practical in all situations, as it either requires a weak government (which might not be able to resist a conventional attack), a large existing insurgent movement, or a great deal of both patience and luck.

It has been suggested that the costs of effective space forces and orbital defenses are large enough that a defender cannot field sufficient ground forces to effectively resist an invasion.  The problem with this is that ground forces are relatively cheap, and sufficient forces to make invasion very difficult can be funded out of the leftovers from the Navy.  As mentioned above, the defender’s equipment can be designed to be as cheap and reliable as possible, and stockpiled well before the battle.  Furthermore, unless the potential attacker is very close to the defender’s planet, the long lag between the invasion force departing home (when it is detected) and landing on the defender’s planet would allow the Army to normally exists as a cadre with stockpiled equipment, further reducing operating costs.

Local irregular units might supplement the defender’s conventional forces.  The efficacy of this type of force in harassing conventional units has been demonstrated in Iraq and Afghanistan in recent years, and their effectiveness is multiplied many times over by the fact that the attacker is racing against the clock imposed by his supplies and the defender’s response.

All of the above discussion assumes a homogenously-defended world, which no allies for the attacker.  If there are any allies, the situation changes significantly.  The best option in that case is simply to ship the men to the ally, and buy the equipment and supplies from him, or just pay him to make the attack in the first place.  Even if some special equipment has to be shipped in, a large proportion of the vehicles in any large military unit are simple trucks, or slightly more complex variations on generic vehicles.  All supplies should be procured on-planet, as tooling up to make munitions is generally much simpler than making vehicles.

The problem with this plan is that the potential target is unlikely to fail to notice the preparations and will strenuous, and probably violently, object.  Also, any power on a balkanized world will have a much stronger army than one that is in control of a homogenous world, all else equal.  The best way to invade is probably by using the ally as a proxy.  However, if, for whatever reason that is not practical, the invader would probably have to fight through any orbital defenses the defender would have, with the ally presumably joining in.

Orbital defenses of Balkanized worlds are a very complicated matter, with the potential for battles between various sets of orbital defenses.  There is the possibility that the world will have a more-or-less unified set of orbital defenses, similar in concept to NORAD.  The problem is that if the powers are close enough to set something like that up, they are unlikely to turn on each other to the extent of supporting an invasion.  More likely, each power will have its own orbital defense system, which, being in orbit, has global coverage, preventing the attacker from going around it, as well as ground-based defenses in their own territory.  It would also have the secondary purpose of destroying the orbital infrastructure of any of the other powers that attack it.  Supporting an invasion (even if not as an active participant) is a de facto act of war, so the off-planet attacker would somehow have to protect his ally in the opening stages of the war.

Even after the orbit-based defenses are destroyed, the fact that the defenses are limited to their own territory does not necessarily mean that the attacker will be safe on the other side of the world.  Today, many nations hold small islands scattered around the world, which would make ideal bases for such defenses.  The defensive submarines mentioned in Section 4 would also be ideal for a balkanized world, particularly as the attacker might well have to exercise more restraint in hunting them for fear of hitting neutrals.

The best way to use an ally might be merely as a staging point.  The attacker would come in on his own at first, and attempt to gain space superiority. The other power(s) on the planet would be induced to declare neutrality, and when the orbital battles were over, any allies would declare for the attacker and probably conduct the bulk of the invasion themselves, with the aid of limited orbital fire support and possibly occupation troops.  At the same time, the defender would surely figure out what’s coming, and probably would declare war preemptively.  The only way to make an alliance work is if the extraplanetary attacker was somehow able to pre-position his forces without the target being overly suspicious.  Deployments of ground troops might be concealed under the pretense of joint training missions, although the logistics of shipping them to the target planet makes this approach problematic.  A more likely scenario is a port visit by a naval squadron of some sort.  Provided that such visits happen regularly, it is possible that an attacker and his local ally could gain a very significant advantage in orbit.

The defender can still make life complicated and landings difficult even if the attacker has a planetary ally.  As noted in Section 4, surface defenses have very long ranges, and it is entirely possible that a defender could hit targets over an attacker’s ally.  This would make landing troops difficult even with an ally, or force the attacking force to move a significant distance overland.  While this is probably preferable to the dangers of an opposed landing, even administrative movements are difficult and slow.  There are also likely to be prepared border defenses that would have to be dealt with, a problem that would be avoided if they were landing directly in the enemy’s territory.

In many ways, the easiest scenario for a ground invasion is a planet that is not homogenously defended, but also not balkanized, so there are few or no surface defenses in place.  The attacker can land away from the defenses and move overland to attack the target.  The biggest problem is likely to be transportation.  As mentioned above, areas that are poorly defended are likely to be of little consequence and have poor transportation infrastructure.

Assuming that a method besides a straight-up invasion is chosen, the attacker will of course have to move some ground troops to be able to occupy the planet after it has surrendered.  The analysis of moving a ground force provided above applies, but the problems for occupation troops are less severe.  First, the supply load for occupation troops is approximately a third of that of troops in combat, and might be reduced farther by the transportation of small factories or the use of local industry.  Second, the drop pods can be replaced with conventional surface-to-orbit shuttles.  The first units would probably land in pods, but follow-on ones would be landed at no mass penalty.  Third, the units themselves can be equipped differently than they would be for facing military units.  This could result in substantial mass savings, as heavy vehicles like tanks can be left behind.  The exact employment of the occupation force is outside the scope of this paper, but there are numerous works on the subject.

All of this begs the question of why exactly the attacker wants the world.  An attempt to add to one’s territory is probably best accomplished through diplomatic means, possibly backed up by some display of military force.  Resources are plentiful enough that, barring McGuffinite (plot-dictated special resources), invading a planet for them is not a sensible plan.  The exception to that rule is humans, but in that case, the defender will probably fight to the bitter end rather than accept slavery.  However, if humans are the target, invading a low-tech world is by far the most sensible plan.  Another possibility is the world itself.  If habitable worlds are rare (moving somewhat outside the PMF) then conflict over them is a possibility, but the question then becomes why the attacker would not use a bioweapon instead, wiping out the population and leaving the infrastructure (and probably the biosphere) unharmed?  The most likely answer is that bioweapons are viewed as abhorrent and use of them gives one the status of an outlaw state, combined with the potential problems of the agent either surviving to render the planet uninhabitable or escaping to other worlds.  If the population is more important than the infrastructure, bombarding the infrastructure should send the planet back to a technological level equivalent to (probably) the late 1800s in short order, removing the ability to resist the invasion.

Strategic needs might also be the basis for an invasion, but will obviously depend heavily upon the exact circumstances in question, and fall outside the scope of this paper.  All that can be analyzed here is the effect of technological constraints on such strategic requirements.  

by Byron Coffey (2016)


The common trope in science fiction is a specialized spacecraft designed to insert troops into combat, called a Dropship. The topic is covered exhaustively in an article at the always impressive Future War Stories. Also well worth reading is the entry on Tactical Transports. Mr. Frisbee has some notes about insertion and extraction here. For a variety of reasons, dropships tend to be spherical in shape.



The final entry in this section, affectionately known to the Imperial Legions as the “Big Ugly Breakfast 1” — and less affectionately known to almost everyone else as “Good gods, what is that thing?” — is the Flapjack-class cavalry dropship (Eye-in-the-Flame Arms/Artifice Armaments). Uniquely among Imperial starship designs, the Flapjack has adopted the rare “disk” or “saucer” hull form. It does this because the Flapjack-class is equipped with not merely a single, but a pair of nuclear-pulse drives, using the relatively environmentally friendly laser-fusion or (in the Flapjack II) antimatter options, the descent and deceleration drives; the dorsal and ventral hulls of these ships are in effect simply the pusher plates for these drives. The main body of the vessel, suspended between these on hydraulic dampers, is a short, wide cylinder, heavily structurally reinforced and itself surrounded by “sidewall” armor as thick and refractory as the pusher plates.

The intended usage of the Flapjack is orbital insertion of armored vehicles, en masse, into hot zones. To enable this, after being decoupled from a carrier in the high orbitals of a planet under attack, the Flapjack uses its descent drive to accelerate downwards through the atmosphere, minimizing dwell time within range of orbital and anti-air defenses. In addition, while the descent of a Flapjack obviously has far too bright a sensor signature to be concealed, the combination of the radiation hash from the descent drive’s thrust bombs and the plasma sheath formed by its hypersonic atmospheric transit together render it extremely difficult for weapons systems to attain successful guidance lock, and terminal guidance (especially to the fine degree necessary to insert a weapon into the narrow window of vulnerability between the pusher plates and the sidewall armor, even if the weapon is capable of surviving and maneuvering in the immediate environment of an active nuclear-pulse drive) virtually impossible.

At the end of its descent trajectory, the Flapjack uses the more powerful thrust bombs of its deceleration drive to perform a “suicide burn”; i.e., maximal deceleration at minimum altitude, compatible with lithobraking in a manner which preserves the integrity of the ventral pusher plate. This deceleration burn serves the additional functions of preparing the drop zone for the arrival of the dropship by flattening any structures or prepared defenses, and eliminating any but the most heavily armored, secured, and radiation-proofed resistance in the immediate area. Once the ground is reached, multiple armored cargo access doors with integral ramps and excavation drones permit the Flapjack to be actively discharging combat vehicles within minutes of a successful landing.

A proposal for an infantry dropship along the lines of the Flapjack, tentatively designated the Pancake-class, has been advanced by Eye-in-the-Flame Arms, but at the present time the high-radiation aftermath of such a vessel’s landing is not considered viable for personnel wearing M-70 Havoc combat exoskeletons or N45 Garrex field combat armor, the current legionary standards. While this would not be a problem for troops equipped with the specialized N45r Callérás high-rad field combat armor, its associated disadvantages and the expense of refit ensure that, for the foreseeable future, infantry will continue to be landed via drop shuttle (q.v.)

— Naval Starships of the Associated Worlds, INI Press, Palaxias, 421st ed.

1. A statistically improbable number of combat drops take place at planet dawn.

Robbie-Yarber: So how do you land a nuclear pulse rocket? On all the diagrams I've seen there are no landing jets, so I guess they intended to leave them in orbit permanently? But that raises the question of how do the passengers get back to the ground?

Alistair Young:

Well, so far as the Flapjack is concerned, the answer is... hard.

Disclaimer: under ordinary circumstances, this would not be the recommended procedure for landing any sort of nuclear pulse-drive ship, and it's definitely not the one any other such designs in the 'verse use, such as the old Phoenix stack. But the Flapjack-class is an odd duck because of its peculiar requirements: namely, having to survive a descent into a hot zone, and thus implicitly wanting to decelerate as hard as possible as low as possible (a "suicide burn"); which in turn implies both that you want a drive with the high thrust qualities of a nuclear pulse drive to do the job, and also that you probably want to avoid vulnerabilities such as those that auxiliary landing thrusters might create.

So, if you take a look at a Flapjack, it looks like the ventral pusher plate is massively overbuilt; which it is, because once it goes into the suicide burn part of landing, it's got to fly through its own nuclear fireballs (not usually a good idea in atmosphere), and then once it gets too close to the ground to decelerate with the nuclear pulse drive any more, it just cuts off the drive, drops under gravity, and smacks straight into the ground, finishing its deceleration by lithobraking right into the crater its drive just cut and relying on that same plate and its shock-absorption system to soak up enough of the ensuing damage. (The theory being that the ground has been nicely softened up for it by the burn, and that in any case, you don't land a Flapjack near anything that you're worried about damaging...)

If you think that sounds like an extremely rough ride, well, you'd be right. ;)

It also solves their taking off from the ground problem, in a manner of speaking: by and large, this sort of landing is very hard on the ship, too, so Flapjacks aren't designed to take off again under their own power, or indeed be used again at all without some extensive maintenance and refurbishment. When possible, they get hauled off the ground by salvage tenders and refitted in orbit, but for all practical purposes, it's a stripped-down, single-use dropship.

[And while I haven't yet done a detailed design work-up on it, I suspect the pusher plate is flat or near-flat; allowing part of the blast to escape to the sides should, I suspect, make it possible to get rather lower before needing to cut the drive. But don't quote me on that, it's yet to have numbers put to it.]

(ed note: so this thing is a two-ended Orion nuclear pulse rocket. One Orion rapidly moves the carrier from orbit to the ground, while the second combines the function of rapid deceleration and nuclear daisy cutter. Totally insane but I totally love it.)


“Frisian Vessel Obadiah to FDF commander Cantilucca,” crackled an unfamiliar voice through Cokes commohelmet. “Come in FDF Cantilucca. Over.”

“There is an Obadiah,” said Johann Vierziger as he watched the rear and sides of the van for possible dangers, “on the FDF naval list. She’s a Class III combat transport.”

“— FDF Cantilucca. Over,” as Coke switched on the transmission from orbit again.

“Survey team commander to FDF vessel Obadiah” Coke said. “We’re glad to hear from you, boys, because we’ve got the Heliodorus Regiment looking for our scalps. Can you drop a boat to pick us up? The Heliodorans have secured the spaceport. Over.”

Obadiah to FDF Cantilucca,” the helmet responded. “You bet we’ll drop a boat. Hold what you’ve got, troopers. Help is coming in figures one-five minutes. Obadiah out.”

Niko Daun looked up, toward the sound of the incoming boat. Coke, suddenly fearful that Pilar would follow the direction of Daun's gaze, shot his hand over her unprotected eyes. “His visor will darken automatically,” Coke said.

Pilar pulled his hand down with a firm motion. “I’ve worked in spaceports for twelve years, Matthew,” she said. “I know that plasma exhausts can be dangerous to my eyesight.”

“Blood and martyrs, sir!” Niko said. “It’s not a boat, it’s the whole ship! They’re coming straight in and there’s no port here!”

“Class III?” Coke snapped to Vierziger as the penny dropped.

“That’s right, Matthew,” Vierziger agreed. “The Obadiah’s a battalion-capacity combat lander. She’s got pontoon outriggers, so she doesn’t require a stabilized surface to set down. And armor, in case the landing zone’s hot.”

The transport swept overhead at a steep angle. The roar and glare of her engines were mind-numbing. Foliage at the tips of trees beneath her track curled and yellowed.

The vessel's exhaust was a rainbow flag waved at Madame Yarnell and the Heliodorans, some ten klicks to the west. Either the Obadiah’s commander expected to lift again before anyone could react, or —

Or the commander didn’t care what a regiment of light infantry might attempt. The Obadiah was coming in with her landing doors open. The troops she carried were ready to un-ass the vessel as soon as the skids touched, or maybe a hair sooner.

“Bloody hell!” Mary Margulies shouted over the landing roar. “She’s coming in loaded! She’s coming in with troops!”

The Obadiah landed a hundred meters away, like a bomb going off in the forest. Her exhaust and armored belly plates cleared their own LZ. Dirt and shattered trees flew away from the shock. Coke caressed Pilar’s head closer to his chest to protect her from the falling debris.

From THE SHARP END by David Drake (1993)

Exotic Attacks

One must keep in mind the objective, and do not get caught up in the details of a standard solution. Sometimes one has to think outside of the box. If the objective is to eliminate the current inhabitants of a given planet, there might be more efficient methods than using zillions of nuclear weapons to turn the continents into glassy slag. I have a small selection below, but you can find much more at TV Tropes under "How To Invade An Alien Planet".

Divide and Conquer

If the planet is Balkanized (that is, composed of many mutually suspicious nations like the situtation that currently obtains on Terra) you have an opportunity. Make covert contact or send your agents into a couple of the most powerful nations and encourage them to attack each other. The loser of the war will be in no condition to resist your invasion, and the winner will be vastly weakened. The idea is "Let's you and him fight." In The Art of War, Sun Tzu called this "attacking alliances." A common colloquialism is "play both ends against the middle".

This is an example of Unconventional Warfare.

A balkanized planet is just full of flaws and vulnerabilities for an invader to take advantage of. The invaders can try to covertly inflame old hatreds and grievances, corrupt a nation into doing the invader's bidding by dangling riches or valuable alien technology in front of their nose, frame one nation with something it didn't actually do, the possibilities are endless.

Isaac Kuo points out that this also has implications for the invaders. If the invaders do not have enough troops to conquer an entire planet, but only enough for one nation, the dynamic shifts. As he puts it:

If alien forces are overwhelming but localized, then you don't need to be strong enough to defeat them. You just need to be stronger than your neighbor.

Isaac Kuo

This is a variant on the old joke "I do not have to run faster than the lion, I just have to run faster than you."

Isaac also mentions that if the various balkanized nations hate each other enough, when the invaders attack one nation, that nation's enemies might actually pay the invaders in gratitude.


We now know (little consolation though this provides) that the Twerms were fleeing from their hereditary enemies the Mucoids when they first detected Earth on their far-ranging Omphalmoscopes. Thereafter, they reacted with astonishing speed and cunning.

In a few weeks of radio-monitoring, they accumulated billions of words of electroprint from the satellite Newspad services. Miraculous linguists, they swiftly mastered the main terrestrial languages; more than that, they analysed our culture, our technology, our political-economic systems — our defences. Their keen intellects, goaded by desperation, took only months to identify our weak points, and to devise a diabolically effective plan of campaign.

They knew that the US and the USSR possessed between them almost a teraton of warheads. The fifteen other nuclear powers might only muster a few score gigatons, and limited deliver systems, but even this modest contribution could be embarrassing to an invader. It was therefore essential that the assault should be swift, totally unexpected, and absolutely overwhelming. Perhaps they did consider a direct attack on the Pentagon, the Red Fort, the Kremlin, and the other centres of military power. If so, they soon dismissed such naive concepts.

With a subtlety which, after the event, we can now ruefully appreciate, they selected our most compact, and most vulnerable, area of sensitivity ...

Their insultingly minuscule fleet attacked at 4 a.m. European time on a wet Sunday morning. The weapons they employed were the irresistible Psychedelic Ray, the Itching Beam (which turned staid burghers into instant nudists), the dread Diarrhoea Bomb, and the debilitating Tumescent Aerosol Spray. The total human casualties were thirty-six, mostly through exhaustion or heart failure.

Their main force (three ships) attacked Zürich. One vessel each sufficed for Geneva, Basle, and Berne. They also sent what appears to have been a small tugboat to deal with Vaduz.

No armourplate could resist their laser-equipped robots. The scanning cameras they carried in their ventral palps could record a billion bits of information a second. Before breakfast time, they knew the owners of every numbered bank account in Switzerland.

Thereafter, apart from the dispatch of several thousand special delivery letters by first post Monday morning, the conquest of Earth was complete.

From "WHEN THE TWERMS CAME" by Sir Arthur C. Clarke (1972)

Internal schism


(ed note: Miranid of the Farla empire has just finished explaining to Henlo why conventional theory holds that a decisive interstellar war is impossible. Now he explains the sneaky trick they are going to do in order to avoid conventional theory and destroy the upstart barbarian Vilk empire.)

"Our barbarian friends have another weakness, which we have up to this point not been able to utilize without compromising its existence. I 've carefully saved it until now, and they have considerately not discovered it within themselves."

"The Vilks, of course, were able to make war quite successfully. Since they were operating as a horde of mobile independent principalities, and since they were after loot and glory only, they were never forced to gain what civilized nations would term 'victory', or 'conquest.'"

"They were reapers, harvesting the same field again and again, and gradually extending their boarders. They had no time for the re-education of subject peoples to their own ideals or patriotic causes -- a fact further implemented by their total lack of such civilized appurtenances. They merely informed their vassals that they had become the property of whatever Vilk it happened to be, and levied tribute accordingly. They left it to the natural fertility of the Vilk soldier to gradually erase all traces of independent nationality among such nations as could interbreed, and to the natural inertia of generations of slavery among such as could not."

"The result has been the gradual accumulation, in Vilk ranks, of a number of Vilks who are not Vilks."

Miranid seemed anxious to stress the point.

"And these Vilks may be good, barbarian Vilks like all the rest of them. But some of them inevitably feel that their particular kind of Vilk is better fitted to rule the communal roost."

"A situation, you will agree, which does not apply among such civilized communities as Farla, which may have its internal dissensions, but no special uniforms of hide-color, limb-distribution, or digital anomalies around which infra-nationalistic sentiments may be rallied."

Miranid stabbed the chart with his dividers. "We will slice here, here, and here, with most of our lighter units supported by some heavier groups. You and I, Henlo, will take the remainder of the main fleet and spit right through to Vilkai, where we will crown some highly un-Vilkish Vilk king of the Vilks, and then leave him to perish."

"The entire sorry mess will slash itself to suicide in the petty nationalistic squabbles which are sure to follow the precedent we set them. We will be enabled to do so quite easily by the allies which our housewifely intelligence corps have neatly suborned for us."

From SHADOW ON THE STARS by Algis Budrys (1954)

Biological Warfare

The unpleasant fellow of the Four Horsemen of the Apocalypse who rides the white horse is pedantically known as "Conquest" but popularly as "Pestilence." Genetically engineered plagues have the advantage of scalability, that is, the deadly disease will multiply to fit any sized population. Care must be taken by the attacker if they are of the same species at the defenders, since the plague will probably work equally well on them. And no matter how virulent the disease, there will probably be a few survivors who are immune or who manage to avoid infection.

Note that sometimes the biological weapon is in the form of an insect instead of a disease, and sometimes instead of the target being the defenders it is crops or food animals.


The rider of the white horse had a buddy riding a black horse. As a general rule, the defenders have to eat (unless they are intelligent robots or something like that). Destroy their ability to make food and they will eventually all starve. You can introduce biological warfare agents that kill crops, interfere with the influx of sunlight to the planet via huge mirrors or inducing nuclear winter, drop in the equivalent of genetically engineered super-locusts that will devour everything, there are several methods. Bobby Coggins mentions you can kill off a large percentage of the population just by destroying means of food transport (highway and railway lines, junctions and port facilities).

This will not wipe out 100% of the defenders because they will start a crash course of making food with hydroponics, yeast, or something like that. But the probability is that only a small fraction of the defenders will survive.

Von Neumann Machine

Specifically a Von Neumann universal constructor, aka Self-replicating machine. These are machines that can create duplicates of themselves given access to raw materials, much like biological organisms. Whatever sabotage they are programmed to do against the defenders is magnified by the fact that they breed like cockroaches.

These can be machines that were specfically designed as planetary attack weapons, or they could be some sort of benign von Neumann that mutated into something dangerous.

In the TV series Stargate SG-1, the Replicator are self-replicating machines that are ravaging all the planets in the Asgard galaxy. In Greg Bear's novel The Forge of God and the sequel Anvil of Stars, an alien species systematically destroys planets detected as possessing intelligent life by attacking the planets with self-replicating machines.


Nanotechnology is machines the size of molecules. They are pretty nasty just like that, but the become a million times worse if they are also self replicating machines. This is the dreaded Gray Goo scenario.

Killer SETI

Children are taught "Sticks and stones may break my bones, but names will never hurt me." While this is true of school playground interactions, it may not hold in the world of communicating with aliens, i.e., SETI.

What can aliens do to harm us over a radio wave? Plenty.

They might attack individuals via transmitting a Medusa Weapon image. They might send blueprints for a device they claim will produce free energy but will actually turn half the planet into antimatter. It might even send instructions which will covertly create an alien agent here on the planet, such as in the science fiction story A for Andromeda.


Even if star travel is impossible; "mere" communications could do a lot of damage. After all, this is the basis on which all censors act. A really malevolent society could destroy another one quite effectively by a few items of well-chosen information. ("Now, kiddies, after you’ve prepared your uranium hexafluoride…")


      While it has been argued that sustainable ETI is un-likely to be harmful (Baum et al. 2011), we can not exclude this possibility. After all, it is cheaper for ETI to send a malicious message to eradicate humans compared to sending battleships.
     If ETI exist, there will be a plurality of good and bad civilizations. Perhaps there are few bad ETI, but we cannot know for sure the intentions of the senders of a message. Consequently, there have been calls that SETI signals need to be “decontaminated” (Carrigan 2004, 2006).
     In this paper, we show that it is impossible to decontaminate a message with certainty. Instead, complex messages would need to be destroyed after reception in the risk averse case.
     If such a message is received only in one place, and only once, it might be possible to contain it and its harmful consequences, or even destroy it. If it is received repeatedly, perhaps even by amateurs, containment is impossible. As a further complication, the International Academy of Astronautics has adopted a “Declaration of Principles Concerning Activities Following the Detection of Extraterrestrial Intelligence” which states1 that “These recordings should be made available to the international institutions listed above and to members of the scientific community for further objective analysis and interpretation.” This view is shared by the majority of SETI scientists (Gertz 2017).


     We continue with a hypothetical message which appears to be, at first sight, positive and interesting, and shall be analyzed in depth. Any message could, in principle, be examined on paper. For many plausible message types, however, it is much more convenient to use a computer. Even the simple LATEX notation is difficult to read as code. Consider the proof of the Riemann hypothesis, which begins with the equation


     which is much easier to read when interpreted and finely printed as

     Even a typesetting system such as TEX is a Turing-complete programming language (Greene 1990), so that the message is in fact code, and may contain a malicious virus. Messages may contain large technical diagrams, equations, algorithms etc. which can not reasonably be printed and examined manually. In addition, the message itself might be compressed to increase interstellar data rates, and the decompression algorithm would be code. Executing billions of decompression instructions cannot plausibly be performed manually and requires the use of a computer. But then, the computer would execute potentially harmful ETI code. For this case, it was suggested to use isolated, quarantined machines for analysis (Carrigan 2004, 2006).
     In the following section, we explain why these measures are insufficient, and no safety procedure exists to contain all threats.


     Consider a large ETI message with a header that contains a statement such as “We are friends. The galactic library is attached. It is in the form of an artificial intelligence (AI) which quickly learns your language and will answer your questions. You may execute the code following these instructions...”
     We assume that the message is available only to a small group of people, part of a government body, who decide to keep it private, but follow their curiosity and examine it with utmost care. A computer in a box on the moon is built to execute the code. Safety devices are in place, their design by choice of the reader, such as remote-controlled fusion bombs to terminate the experiment at any time.
     This scenario resembles the Oracle-AI, or AI box, of an isolated computer system where a possibly dangerous AI is “imprisoned” with only minimalist communication channels. Current research indicates that even well-designed boxes are useless, and a sufficiently intelligent AI will be able to persuade or trick its human keepers into releasing it (Armstrong et al. 2012; Dawson et al. 2016).
     For the escape, we have to assume that researchers engage in a conversion with the AI (without, there would be no benefit in running the experiment in the first place). In such a text conversion, the AI might offer things of value, such as a cure for cancer, and make a small request in exchange, such as a 10% increase in its computer capacity. It appears rational to take the offer. When we do, we have begun business and trade with it, which has no clear limit. If the cure for cancer would consist of blueprints for nanobots: should we build these, and release them into the world, in the case that we don’t understand how they work? We could decline such offers, but shall not forget that humans are involved in this experiment. Consider a nightly conversation between the AI and a guard: “Your daughter is dying from cancer. I give you the cure for the small price of...”. We can never exclude human error and emotions. After all, is it ethical to keep a sentience in a prison when it expresses incredible pain due to small manufacturing errors from building the box?
     Even in a military-style, adamant experiment, there will still be humans involved who go home after examination work with their own feelings. Even if everything is officially secret, whistle-blowers might get some news out to the public. Quickly, there could be a community on Earth in favor of letting it out for religious, philosophical etc. reasons. If the AI promises to cure cancer, or offers a message of salvation, a cult could form. Maybe (or maybe not) a majority of the population would be in favor of releasing the AI. Should, or even could, a democratic government work against the majority of its people? Dictatorships are unstable and eventually over-thrown; the AI will be eventually released.


     With a non-zero prison escape probability in any single time period, the AI will be free at some point of time. Then, the worst possible result would be human extinction or some other unrecoverable global catastrophe (Bostrom 2014). The main argument is that the human species currently dominates planet Earth because of our intelligence. If ETI-AI is superior, it might (or might not) become more powerful and consider us as irrelevant monkeys (or maybe not).


(ed note: In the story, SETI scientist make radio contact with the alien inhabitants of Mu Cassiopeiae, a spectral class G5Vb star about 25 light-years away. Sadly the scientist cannot get any scientific information from the aliens since the entire planet is under a religious dictatorship. They receive nothing except the alien equivalent of evangelical proselytizing pamphlets. Kind of like an interstellar version of The Watchtower.)

“ ‘—begat Manod, who reigned over the People for 99 years. And in his day lawlessness went abroad in the land, wherefore the Quaternary One smote the People with ordseem (Apparently a disease—Tr.) and they were sore afflicted. And the preacher Jilbmish called a great prayer meeting. And when the People were assembled he cried unto them: Woe betide you, for you have transgressed against the righteous command of the Secondary and Tertiary Ones, namely, you have begrudged the Sacrifice and you have failed to beat drums (? —Tr.) at the rising of Nomo, even as your fathers were commanded; wherefore this evil is come upon you.’ Sheemish xiv, 6.

“Brethren beyond the stars, let us ponder this text together. For well you know from our previous messages that ignorance of the Way, even in its least detail, is not an excuse in the sight of the Ones. 'Carry Our Way unto the ends of creation, that ye may save from the Eternal Hunger all created beings doomed by their own unwittingness.’ Chubu iv, 2. Now the most elementary exegesis of the words of Jilbmish clearly demonstrated—

Father James Moriarty, S.J., sighed and laid down the typescript. Undoubtedly the project team of linguists, cryptographers, anthropologists, theologians and radio engineers was producing translations as accurate as anyone would ever be able to.

Father Moriarty had been assured that the different English styles corresponded to a demonstrable variation in the original language. So he accepted the edited translation of the messages from Mu Cassiopeiae. “Only why,” he asked himself as he stuffed his pipe, “must they use that horrible dialect?” He touched a lighter to the charred bowl and added, “Pseudo-King James,” with a bare touch of friendly malice.

“And the biology, biochemistry, zoology, botany, anthropology, history, sociology … and who knows how far ahead of us they may be in some technologies? Sure. Those hopes were expressed before I was born,” snapped Strand. “But what have we actually learned so far? One language. A few details of dress and appearance. An occasional datum of physical science, like that geological information you spoke of. In more than a hundred years, that’s all!”

“Religious ranting, you mean,” said Strand sourly.

Moriarty grimaced. “Correct word, that. I was reading the latest translation you’ve released, on my way here. No sign of any improvement, is there?”

"Nope,” Okamura said. "As of twenty-five years ago, at least, Akron’s still governed by a fanatical theocracy out to convert the universe.” He sighed. "I imagine you know the history of Ozma’s contact with them? For the first seventy-five years or so, everything went smoothly. Slow and unspectacular, so that the public got bored with the whole idea, but progress was being made in understanding their language. And then—when they figured we’d learned it well enough—they started sending doctrine. Nothing but doctrine, ever since. Every message of theirs a sermon, or a text from one of their holy books followed by an analysis that my Jewish friends tell me makes the medieval rabbis look like romantic poets. Oh, once in a great while somebody slips in a few scientific data, like that geological stuff which got you so interested. I imagine their scientists are just as sick at the wasted opportunity as ours are. But with a bunch of Cotton Mathers in control, what can they do?”

‘Tes, I know all that,” said Moriarty. "It’s a grim sort of religion. I daresay anyone who opposes its ministers is in danger of burning at the stake, or whatever the Akronite equivalent may be.”

Okamura seemed so used to acting as dragoman for visitors who cared little and knew less about Ozma, that he reeled off another string of facts the priest already had by heart. “Communication has always been tough. After the project founders first detected the signals, fifty years must pass between our acknowledgment and their reply to that. Of course, they’d arranged it well. Their initial message ran three continuous months before repeating itself. In three months one can transmit a lot of information; one can go all the way from ‘two plus two equals four’ to basic symbology and telling what band a sonic ‘cast will be sent on if there’s an answer. Earth’s own transmission could be equally long and carefully thought out. Still, it was slow. You can’t exactly have a conversation across twenty-five light-years. All you can do is become aware of each other’s existence and then start transmitting more or less continuously, meanwhile interpreting the other fellow’s own steady flow of graded data. But if it weren’t for those damned fanatics, we’d know a lot more by now than we do.

“As it is, we can only infer a few things. The theocracy must be planet-wide. Otherwise we’d be getting different messages from some other country on Akron. If they have interstellar radio equipment, they must also have weapons by which an ideological dictatorship could establish itself over a whole world, as Communism nearly did here in the last century. The structure of the language, as well as various other hints, proves the Akronites are mentally quite humanlike, however odd they look physically. We just had the bad luck to contact them at the exact point in their history when they were governed by this crusading religion.”

The next word in the sentence from Aejae xliii, 3 which we are considering is ‘mchiruchin,’ an archaic word concerning whose meaning there was formerly some dispute. Fortunately, the advocates of the erroneous theory that it means ‘very similar’ have now been exterminated and the glorious truth that it means ‘quite similar’ is firmly established.”

     Strand leaned back in his swivel chair. His glum hostility was dissolving into bewilderment. “What’re you getting at? Look here, uh, Father, it’s physically impossible for us to change the situation on Akron—"
     “What d’ you mean?”
     “We can send a reply to those sermons.”
     “What?” Strand almost went over backward.
     “Other than scientific data, I mean. I assure you. Dr. Strand, all I want is a free scientific and cultural exchange with Mu Cassiopeiae.”
     The director reseated himself, leaned elbows on desk and stared at the priest. He wet his lips before saying: “What do you think we should do, then?”

“Why, break up their theocracy. What else? There’s no sin in that! My ecclesiastical superiors have approved my undertaking. They agree with me that the Akronist faith is so unreasonable it must be false, even for Akron.

“Maybe I got you wrong,” he said grudgingly. “But, uh, how do you propose to do this? Wouldn’t you have to try converting them to some other belief?”

“Impossible,” said Moriarty. “Let’s suppose we did transmit our Bible, the Summa, and a few similar books. The theocracy would suppress them at once, and probably cut off all contact with us.”

He grinned. “However,” he said, "in both the good and the bad senses of the word, casuistry is considered a Jesuit specialty.” He pulled the typescript he had been reading from his coat pocket. “I haven’t had a chance to study this latest document as carefully as I have the earlier ones, but it follows the typical pattern. For example, one is required ‘to beat drums at the rising of Nomo,’ which I gather is the third planet of the Ohio System. Since we don’t have any Nomo, being in fact the third planet of our system it might offhand seem as if , we’re damned. But the theocracy doesn’t believe that, or it wouldn’t bother with us. Instead, their theologians, studying the astronomical data we sent, have used pages and pages of hairsplitting logic to decide that for us Nomo is equivalent to Mars.”

“What of it?” asked Strand; but his eyes were kindling.

“Certain questions occur to me,” said Moriarty. “If I went up in a gravicar, I would see Mars rise sooner than would a person on the ground. None of the preachings we’ve received has explained which rising is to be considered official at a given longitude. A particularly devout worshipper nowadays could put an artificial satellite in such an orbit that Mars was always on its horizon. Then he could beat drums continuously, his whole life long. Would this gain him extra merit or would it not?”

“I don’t see where that matters,” said Strand.

“In itself, hardly. But it raises the whole question of the relative importance of ritual and faith. Which in turn leads to the question of faith versus works, one of the basic issues of the Reformation. As far as that goes, the schism between Catholic and Orthodox Churches in the early Middle Ages turned, in the last analysis, on one word in the Credo, filioque. Does the Holy Ghost proceed from the Father and the Son, or from the Father alone? You may think this is a trivial question, but to a person who really believes his religion it is not. Oceans of blood have been spilled because of that one word.

“Ah…returning to this sermon, though. I also wonder about the name ‘Nomo.’ The Akronite theologians conclude that in our case, Nomo means Mars. But this is based on the assumption that, by analogy with their own system, the next planet outward is meant. An assumption for which I can recall no justification in any of the scriptures they’ve sent us. Could it not be the next planet inward—Venus for us? But then their own ‘Nomo’ might originally have been Mu Cassiopeiae I, instead of III. In which case they’ve been damning themselves for centuries by celebrating the rising of the wrong planet!”

Strand pulled his jaw back up. “I take it, then,” he said huskily, “you want to—”

“To send them some arguments much more elaborately reasoned than these examples, which I’ve simply made up on the spot,” Moriarty answered. “I’ve studied the Akronist faith in detail… with two millennia of Christian disputation and haggling to guide me. I’ve prepared a little reply. It starts out fulsomely, thanking them for showing us the light and begging for further information on certain points which seem a trifle obscure. The rest of the message consists of quibbles, puzzles, and basic issues.”

“And you really think— How long would this take to transmit?”

“Oh, I should imagine about one continuous month. Then from time to time, as they occur to us, we can send further inquiries.”


(ed note: about 75 years later)

“Time sure passes. But I had to call you right away, Jim. Transmission from Akron resumed three hours ago.”

“What?” Moriarty glanced at the sky "What’s their news?”

“Plenty. They explained that the reason we haven’t received anything from them for a decade was that their equipment got wrecked in some of the fighting. But now things have quieted down. All those conflicting sects have been forced to reach a modus vivendi.

Apparently the suggestions we sent, incidental to our first disruptive questions seventy-five years ago—and based on our own experience— were helpful: separation of church and state, and so on. Now the scientists are free to communicate with us, uncontrolled by anyone else. They’re sure happy about that! The transition was painful, but three hundred years of stagnation on Akron have ended. They’ve got a huge backlog of data to give us. So if you want your geology straight off the tapes, you better hurry here. All the journals are going to be snowed under with our reports.”

From THE WORD TO SPACE by Poul Anderson (1960)

Memeweave: Threats and Other Dangers/Perversion Watch/Open Access
Classification: WHITE (General Access)
Encryption: None
Distribution: Everywhere (Bulk)
As received at: SystemArchiveHub-00 at Víëlle (Imperial Core)
Language: Eldraeic->Universal Syntax
From: 197th Perversion Response Board


Given the high levels of uninformed critical response to our advisory concerning handling potential refugees arriving sublight from regions within the existential threat zone of the Siofra Perversion, or Leviathan Consciousness as it is becoming popularly known, the Board now provides the following explication.

The present situation is an example of what eschatologists refer to as the basilisk-in-a-box problem. The nature of the mythological basilisk is that witnessing its gaze causes one to turn to stone, and the challenge therefore to determine if there is a basilisk within the box and what it is doing without suffering its gaze. The parallel to the Siofra Perversion’s communication-based merkwelt should be obvious: it won’t subsume you unless you alert it to your existence as “optimizable networked processing hardware” by communicating with it.

Your analogous challenge, therefore, is to determine whether the hypothetical lugger or slowship filled with refugees is in fact that, or is contaminated/a perversion expansion probe, without communicating with it – since if it is the latter and you communicate with it sufficiently to establish identity, you have just arranged your own subsumption – and unless people are subsequently rather more careful in re communicating with you, that of all locally networked systems and sophonts.

Currently, the best available method for doing this is based on the minimum-size thesis: i.e., that basilisk hacks, thought-viruses, and other forms of malware have a certain inherent complexity and as such there is a lower limit on the number of bits necessary to represent them. However, it should be emphasized that this limit is not computable (as this task requires a general constructive solution to the Halting Problem), although we have sound reason to believe that a single bit is safe.

This method, therefore, calls for the insertion of a diagnostician equipped with the best available fail-deadly protections and a single-bit isolated communications channel (i.e., tanglebit) into the hypothetical target, there to determine whether or not perversion is present therein, and to report a true/false result via the single-bit channel.

If we leave aside for the moment that:

(a) there is a practical difficulty of performing such an insertion far enough outside inhabited space as to avoid all possibility of overlooked automatic communications integration in the richly meshed network environment of an inhabited star system, without the use of clipper-class hardware on station that does not generally exist; and

(b) this method still gambles with the perversion having no means, whether ontotechnological or based in new physics, to accelerate its clock speed to a point which would allow it to bypass the fail-deadly protections and seize control of the single-bit channel before deadly failure completes.

The primary difficulty here is that each investigation requires not only a fully-trained forensic eschatologist, but one who is both:

(a) a Cilmínár professional, or worthy of equivalent fiduciary trust, and therefore unable to betray their clients’ interests even in the face of existential terror; and

(b) willing to deliberately hazard submitting a copy of themselves into a perversion, which is to say, for a subjective eternity of runtime at the mercy of an insane god.

(Regarding the latter, it may be useful at this time to review the ethical calculus of infinities and asymptotic infinities; we recommend On the Nonjustifiability of Hells: Infinite Punishments for Finite Crimes, Samiv Leiraval-ith-Liuvial, Imperial University of Calmiríë Press. Specifically, one should consider the mirror argument that there is no finite good, including the preservation of an arbitrarily large set of mind-states, which justifies its purchase at infinite price to the purchaser.)

Observe that a failure at any point in this process results in first you, and then your entire local civilization, having its brains eaten.

We are not monsters; we welcome any genuine innovation in this field which would permit the rescue of any unfortunate sophonts caught up in scenarios such as this. However, it is necessary that the safety of civilization and the preservation of those minds known to be intact and at hazard be our first priority.

As such, we trust these facts adequately explain our advisory recommendation that any sublight vessels emerging from the existential threat zone be destroyed at range by relativistic missile systems.

For the Board,

Gém Quandry, Eschatologist Excellence

From EVIDENCE by Alistair Young (2016)


This is a variant on biological warfare. Obviously if you took Terra and terraformed it to have the climate of Mars then the bulk of the population would die. However that would take thousands of years and be subject to constant sabotage by the inhabitants.

But an ecosystem is more fragile. In David Gerrold's series The War Against the Chtorr Terra has been invaded by an alien ecosystem, one far more evolutionarily advanced than the native one. In Philip E. High's No Truce With Terra, a small group of aliens teleport into Terra, throw some seeds and eggs from their ecology out onto the ground, and wait. The metal based ecosystem spreads like wildfire, threatening the entire planet.

The Web of Hercules

In James Blish's The Triumph of Time (fourth novel in the Cities in Flight series) the alien empire The Web of Hercules has spread from the Great Globular Cluster in Hercules to conquer the galaxy. They have a unique weapon used to kill planets.


     He heard Miramon draw in his breath slightly to answer, but he was never to know what that answer would have been; for at the same moment, Miramon’s whole board came alive at once.
     “Hey!” Amalfi squalled. “Wait for orders down there, dammit!”
     “What do they mean?” Miramon said, trying to read every instrument on his board at once. “I thought I understood your language, Mayor Amalfi, but—”
     “The City Fathers don’t speak Okie, they speak Machine,” Amalfi said grimly. “What they mean is that the Web of Hercules—if that’s who it is—is coming in on us. And coming in on us fast.”
     With a single, circumscribed flip of his closed fingers, Miramon turned off the lights.
     Blackness. Then, seeping faintly over the windows around the tower, the air-glow of the zodiacal light; then, still later, the dim pinwheels of island universes. On Miramon’s board, there was a single spearpoint of yellow-orange which was only the heater of a vacuum tube smaller than an acorn; in this central gloom at the heart and birthplace of the universe, it was almost blinding. Amalfi had to turn his back on it to maintain the profound dark-adaptation that his vision needed to operate at all in the tower on his mountain.
     While he waited for his sight to come back, he wondered at the speed of Miramon’s reaction, and the motives behind it. Surely the Hevian could not believe that a set of pilot lights in a tower on top of a remote mountain could be bright enough to be seen from space; for that matter, blacking out even as large an object as a whole planet could serve no military purpose—it had been two millennia since any reasonably sophisticated enemy depended upon light alone to§see by. And where in Miramon’s whole lifetime could he have acquired the blackout reflex? It made no sense; yet Miramon had restored the blackout with all the trained positiveness of a boxer riding with a punch.
     When the light began to grow, he had his answer—and no time left to wonder how Miramon had anticipated it.
     It began as though the destruction of the inter-universal messenger were about to repeat itself in reverse, encompassing the whole of creation in the process. Crawls of greenish-yellow light were beginning to move high up in the Hevian sky, at first as ghostly as auroral traces, then with a purposeful writhing and brightening which seemed as horrifyingly like life as the copulation of a mass of green-gold nematode worms seen under phase-contrast lighting. Particle counters began to chatter on the board, and Hazleton jumped to monitor the cumulative readings.
     “Where is that stuff coming from—can you tell?” Amalfi said.
     “It seems to come from nearly a hundred discrete point-sources, surrounding us in a sphere with a diameter of about a light year,” Miramon said. He sounded preoccupied; he was doing something with controls whose purpose was unknown to Amalfi.
     “Hmm. Ships, without a doubt. Well, now we know where they get their name, anyhow. But what is it they’re using?”
     “That’s easy,” Hazleton said grimly. “It’s antimatter.”
     “How can that be?”
     “Look at the frequency analysis on this secondary radiation we’re getting, and you’ll see. Every one of those ships must be primarily a particle accelerator of prodigious size. They’re sending streams of stripped heavy antimatter atoms right down the gravitational ingeodesics toward us—that’s what makes the paths the stuff is following look so twisted. They’ve found a way to generate and project primary cosmics made of antimatter atoms, and in quantity. When they strike our atmosphere, both disintegrate—
     “And the planet gets a dose of high-energy gamma radiation,” Amalfi said. “And they must have known how to do it for a long time, since they’re named after the technique. Helleshin! What a way to conquer a planet! They can either sterilize the populace, or kill it off, at will, without ever even coming close to the place.”
     “We’ve had the sterility dose already,” Hazleton said quietly.
     “That can hardly matter now,” Estelle said, in an even softer voice.
     “The killing dose won’t matter either,” Hazleton said, “Radiation sickness takes months to develop, even when it’s going to be fatal.”
     “They could disable us quickly enough,” Amalfi said harshly. “We’ve got to stop this somehow. We need these last days!”

From THE TRIUMPH OF TIME by James Blish (1958)

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