Orson Scott Card’s Ender’s Game SF novel is famous, and its ending well-known: the protagonist Ender Wiggin is tricked into commanding a pre-emptive invasion fleet which destroys all the enemy alien fleets, their homeplanet, and by a quirk of their biology, the entire alien race. The reasoning given is that they couldn’t risk waiting for a (third) invasion fleet.

If one wanted an analogy, conventional warfare is like martial arts sparring: multiple moves, each strike hurting but not fatal, with a back-and-forth, and winding up after a while; while nuclear warfare is like dueling with Desert Eagle s. And a curtain between the duelists.

A conventional war is intrinsically limited. A country can be invaded and lose territory without too much concern. Even the tiniest countries like Israel have space to fall back and regroup. The action happens on a human time-scale, with human levels of casualties. There can be ebb and flow; strikes can be probing, small-scale.

If a fleet of ballistic missile submarine s have anchored off one’s capital, the attack can begin and end within half an hour, and the missiles—which boomers carry in substantial quantities—can target anywhere in the nation. There is no practical missile defense more than half a century after nuclear warfare became possible, and little prospect of one that could withstand any more than a few missiles. Surviving nuclear explosions is an extremely difficult task, and cannot be guaranteed. A government attacked would crumble instantly, yielding immediate victory to the foe. Such decapitation attack s are strategically attractive.

In retrospect, such advocacy seems foolish to us who survived the Cold War; we are not quite so impressed by the argument that Communism is inimitable to the American Way of Life and that it is bound by its internal logic to subjugate the world and war on us. It did instigate various wars, prop us various regimes, and so on; but the USA has done quite as much of that in its history (and often for reasons only ostensibly related to ideals such as countering communism). Nor are we impressed by the arguments that such and such a achievement must be prevented by all-out war; the world did not end because Russia developed an atomic bomb, nor when it had constructed useful quantities of them, nor when it cracked the H-bomb.

This is Cold War reasoning, of course; version 1.0. The strategy is similar to early Cold War views, such as those who advocated that the USA strike Russia first, before it had developed A-bombs/lots-of-bombs/H-bombs.

It may be fun to read about “Warshawski sails” or “alpha and beta nodes”, or “wedges” and “sidewalls”; but these are all ad hoc bits of technology and arbitrary, most obviously in the case of the “wedges” . The mass of details may overload our critical thinking & fool us, but they ought to make us suspicious—in the real world, the principles are simple & elegant but the applications complex; both principles and applications shouldn’t be complex and detailed. We enjoy Honor Harrington only by suspending a great deal of disbelief.

But is it really? The Honorverse is a naked retelling of the old naval genre like Horatio Hornblower , right down to the names of the wicked French antagonists (eg. Oscar Saint-Just ). The ship designs and the universe is constructed to make ‘ crossing the T ’, ambushes, etc. plausible in outer space.

Consider the Honorverse of David Weber . His ships are described in loving detail; one can’t go a battle without figures like ‘10,000 KPS’ being thrown out; the history is described in tremendous detail in appendixes and secondary works; the planets number in the hundreds, and the complexities of their interactions feel authentic. Plot events often hinge on accelerations and vectors; books are powered by technical innovations. It feels awfully realistic.

We can easily criticize the realism of Battlestar Galactica (both of them) or that of Star Wars ( “probably the worst” ), but even the ones praised for their careful thought are vulnerable.

Even when the author throws many details into it, with hard numbers and equations and rules he must follow, the overall picture is sometimes still absurd. ( Robin Hanson warns us to be wary of being seduced by Near/Far thinking: it is not true that the more detail a projection or fiction has, the more true or likely it is! )

“No,” said Bean, his mind racing. “Not if we sent out a fleet immediately after the Second Bugger War. After Mazer Rackham’s strike force defeated them, it would take time for word of their defeat to come back to them. So we build a fleet as quickly as possible and launch it against their home world immediately. That way the news of their defeat reaches them at the same time as our devastating counterattack.”

Solution? What did Dimak think Bean was? I’m thinking about how to get control of the situation here in Battle School , not how to save the world! “I don’t think there is a solution,” said Bean, buying time again. But then, having said it, he began to believe it. “There’s no point in trying to defend Earth at all. In fact, unless they have some defensive device we don’t know about, like some way of putting an invisible shield around a planet or something, the enemy is just as vulnerable. So the only strategy that makes any sense at all is an all-out attack. To send our fleet against their home world and destroy it.”

Go on? That wasn’t enough to explain two hours of reading? “Well, so I thought that even that was a recipe for disaster, because the enemy is free to divide his forces. So even if we intercept and defeat ninety-nine of a hundred attacking squadrons, he only has to get one squadron through to cause terrible devastation on Earth. We saw how much territory a single ship could scour when they first showed up and started burning over China. Get ten ships to Earth for a single day – and if they spread us out enough, they’d have a lot more than a day! – and they could wipe out most of our main population centers. All our eggs are in that one basket.”

Bean was beginning to warm to the discussion. “The real problem is that unlike Vauban , we have only one strong point worth defending – Earth. And the enemy is not limited to a primary direction of approach. He could come from anywhere. From anywhere all at once. So we run into the classic problem of defense, cubed. The farther out you deploy your defenses, the more of them you have to have, and if your resources are limited, you soon have more fortifications than you can man. What good are bases on moons on Jupiter or Saturn or Neptune, when the enemy doesn’t even have to come in on the plane of the ecliptic ? He can bypass all our fortifications. The way Nimitz and MacArthur used two-dimensional island-hopping against the defense in depth of the Japanese in World War II. Only our enemy can work in three dimensions. Therefore we cannot possibly maintain defense in depth. Our only defense is early detection and a single massed force.”

“Well of course fortifications are impossible in space,” said Bean. “In the traditional sense, that is. But there are things you can do. Like his mini-fortresses, where you leave a sallying force outside the main fortification. You can station squads of ships to intercept raiders. And there are barriers you can put up. Mines. Fields of flotsam to cause collisions with fast-moving ships, holing them. That sort of thing.”

“Let me read you something,” said Dimak. “’There are no fortifications, no magazines, no strong points … In the enemy solar system, there can be no living off the land, since access to habitable planets will be possible only after complete victory … Supply lines are not a problem, since there are none to protect, but the cost of that is that all supplies and ordnance must be carried with the invading fleet … In effect, all interstellar invasion fleets are suicide attacks, because time dilation means that even if a fleet returns intact, almost no one they knew will still be alive. They can never return, and so must be sure that their force is sufficient to be decisive and therefore is worth the sacrifice…. Mixed-sex forces allow the possibility of the army becoming a permanent colony and/or occupying force on the captured enemy planet.”

After all, the humdrum alternative is something like Larry Niven’s Known Space universe, where travel between the stars takes years or millennia , where solar sail s and Bussard ramjet s are the best (and also nonfictional) methods of travel, where time dilation works its terrible magic, where trips are one-way and solar systems do not see incoming vessels (traveling at fractional c ) until it is too late.

Card’s analysis, while good as far as it goes, doesn’t go far enough. The situation is actually more unbalanced in favor of the attacker. Card seems to assume that combat will be conducted with ships, and that these ships while appropriately devastating are nevertheless rather short-ranged and must get close to a planet to attack.

But ships on a scale comparable to existing naval ships or spaceships have tremendous problems in plausible space warfare. (Space fighters analogous to airplanes have even more problems.) They can be seen coming from very far away (already private amateurs routinely spot spy satellites; academic telescopes can spot something like the Space Shuttle doing a little bit of maneuvering as far away as the asteroid belt, and spot its launch from past Pluto’s orbit—and the telescopes will only get better ); they can’t carry very much shielding and metal shielding can be an outright liability as far as defense goes ; being in a vacuum, they have great difficulty dumping heat generated by lasers; and require implausibly efficient engines just to carry any weapons at all! Nor do the SF stories we all think of when we think about space combat do justice to just how effective planetary defenses can be against any sort of space fleet. (The fleet can’t hide, can’t see its targets very well, can’t carry an occupation force, and the planet can build much bigger and longer-ranged weapons.)

What would one do if one wanted to destroy an opposing world’s civilian population, on the cheap, and also with great secrecy? One could send a fleet of powered warships straight in to smash the other fleet and then scour the world. If one can even do that, a premise I hope I cast a great deal of doubt upon with the preceding paragraph.

But there are many better alternatives. Geology teaches us that the 2 most devastating & life-destroying events are supervolcano eruptions, and asteroid impacts. Supervolcanos are rather hard to trigger, but asteroids? (And mass and velocity are interchangeable; an asteroid is heavy and slow, a relativistic kill vehicle light and fast. The power demands are extreme in either case, ruling out known feasible designs—we can’t build a Project Orion rocket which would kill a planet.)

If one is a interstellar power, asteroids are quite easy to come by. One could stealthily creep into the vast Kuiper Belt or Oort Cloud and send rocks flying into the inner system. The Kuiper and Oort are composed largely of lightweight bodies (composed of things like water or methane), but they are pretty sizable objects—Halley’s Comet is a typical Kuiper Belt inhabitant, and masses 2 to 3⋅1014 kg. That could still hurt, to put it lightly. Neptune’s Triton moon, which weighs a meaty 2.14⋅1022 kg, is a former Kuiper Belt object.

And such an attack would be impossible to prevent. The Kuiper Belt alone is a shell around the Solar System from 30 to 55 AU; or an area of ~1.9530 kilometers . This is a large volume of space to patrol. More feasible would be monitoring each and every body: after all, there are only ~70,000 bodies. Over 100 kilometers in diameter. Estimated.

And let’s not even talk about the Oort Cloud (5-50,000 AU)!

If that’s not bad enough, one could envision flinging appropriate small moons or large asteroids from other solar systems. Why not? Boosting a large body to fractional c velocities and aiming it at a far away planet isn’t inherently any more absurd than building a ship and boosting it to fractional c. The motion of stellar bodies is famously predictable out for many centuries. The body could guide itself: add some small motors, and it could even correct for small errors in prediction. It would be the ultimate fire-and-forget weapon of mass destruction.

Defense Stop it? How could one stop a small moon? If it is traveling at a good fraction of lightspeed, then its kinetic energy will have reached gargantuan proportions: no frantic last minute attacks will alter its path very much. Suppose one did shatter it with explosives; conservation of energy and mass will still apply—if a billion tons of ice are flying towards New York at 0.5 c, and a bomb shatters it, what’s flying towards New York at 0.5 c? A billion tons of ice. And remember the early points about the vastness of space and how enemies can attack from any degree—not just along the ecliptic. (Attacking from a Kuiper Belt, traveling through or just above or below the ecliptic may well be the most efficient path. But from another solar system located at some odd angle?) For every measure the defenders take, like extensive telescope arrays looking for fast-moving bodies, there are countermeasures like reducing albedo to pitch-blackness by applying soot.