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Sunday, June 6, 2010

Stealth in Space

“Fixed fortifications are monuments to the stupidity of man.”

General George S. Patton

Stealth has been an important aspect of military campaigns since prehistoric times. Two trends have continued since then. First, military assets have become easier to hide. Second, targets with known position have become easier to destroy.

Weapons have become incredibly accurate and reliable. If your enemy knows where you are, you’re dead.

Populations are especially vulnerable to modern weapons, because they are hard to hide. Hence the era of strategic nuclear warfare. Generally, the term “tactical warfare” refers to the battlefield, or limited theater of war, in which military units try to destroy each other, and “strategic warfare” involves each side using its military to destroy the population and civilian assets of the other. If you think the Cold War was MAD, just wait until you see it in space!

As we have seen with various missile defense programs, defending a fixed position is much more difficult than offense. You have to be able to outspend your enemy at least 100 to 1, to have any chance of defending a population. In space, that factor of 100 may increase to 1000 or more.

Future populations will have to become stealthy, in order to survive. That means no colonies on planets or moons, or anything too massive to be moved out of a predictable orbit. It means living in hollowed-out asteroids, or ships painted black, or ships disguised to look like natural objects. Above all, it means radio silence.

Detecting a threat at a distance of several millions of kilometers would be virtually impossible. To put this in perspective, consider our efforts at detecting near-Earth asteroids. A similar object large enough to carry thousands of H-bombs would go undetected until it entered our atmosphere. As another example, consider Voyager 1. It’s not even trying to be stealthy – in fact, it periodically sends out radio signals to Earth. But trying to find it, if you didn’t know where to look, would be very difficult with a radio telescope, and impossible with visual or infrared. Now imagine it is coming toward us at 170 km/sec, instead of away from us at 17 km/sec, and imagine that instead of 722 kg of scientific instruments, it carries 7000 kg of H-bombs. And it’s radio silent. We would never know what hit us!

A military ship, or any spaceship, would be more visible in the optical and infrared, when thrusting. More visible from the stern (more precisely, the direction of thrust) than the bow. The visibility depends on the type of engine used, but as long as it isn’t emitting a lot of radio energy, it won’t be detectable at great distance. As an example to illustrate this, recall that the volcanic activity on Io, one of Jupiter’s moons, was not discovered until Voyager 1 flew by it in March, 1979. I don’t know if they have been detected since then by any Earth-based or LEO-based optical or infrared telescopes, but the point is that they are very difficult to detect from the typical Earth-Jupiter distance. And they are very hot, and much bigger than any rocket engine yet proposed.

Radar may not play as important a role in space warfare as it has in Earth warfare. Radar returns are attenuated by a factor of 1/r^4. In other words, it takes 16 times as much power to detect something twice as far away. So you quickly reach a distance beyond which you can’t detect something as large as a ship. For modern military radars, that maximum range is classified, but a ballpark figure might be a few hundred, or maybe a thousand, kilometers. You might imagine a much more powerful radar, with 16 times the power, able to reach out to 2000 km, or with 256 times the power, at 4000 km. You can see that you are going to need an “astronomical” amout of power to get out to a few million km. That’s why radar is used in astronomy only for near Earth objects, or by sensors that get close to their targets.

If warfare becomes large-scale enough that light delay becomes a factor, then radar pays another penalty, for relying on a there-and-back light time.

From the stealth point of view, the bad thing about radio waves is that they travel great distances, through atmospheres and interplanetary dust, needing very little power. But on the positive side, they are hard to pin down, in terms of direction. It's difficult to tell exactly where a radio source is located. There are many ways around this, involving advanced antenna design, multiple sensors, etc, but the fundamental difficulty remains. The offense will probably have to send a missile in the general direction of a radio source, and then use radar or optical/infrared sensors mounted on the missile, when it gets close.

A population center might also deploy decoys (imagine a decoy city in space!) or other countermeasures. But the best policy is to turn off any source of radio. And hide under a very dark or camoflaged covering.

Even though radar won’t be as important in space warfare (at least at long range), the passive detection of radio signals will be crucial. The military will use radio telescopes to identify targets, and populations must avoid any radio emissions. Developing a civilization without radio emissions will be difficult, but the alternative is extinction.

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