Tech Talk

The Chinese anti-satellite shot on 12 January produced fireworks that are now branching into the non-celestial spheres of politics, national security, and space technology. I covered the issue for the general reader a few years ago (see "Taikonauts Prepare for Liftoff") for IEEE Spectrum,—where I foresaw some of these developments—and yesterday for MSNBC online (see "Bold move escalates space war debate"). This blog gives me a chance to elaborate on the more technical parts of the story.

Fifty years ago this May, Russia test-fired its first intercontinental ballistic missile, the R-7, and some time later used it to launch the Sputnik satellite into orbit. The R-7 was lousy as a weapon, and the Soviets soon scrapped it. It was potent as a symbol, though, because it began what became known as the Space Race.

The question now is whether China's ASAT missile is a serious weapon or merely a symbol, meant to put pressure on other countries, particularly the United States. To answer it, we must examine the gap separating the satellite-killing demonstration and the needs of a real weapon—one that would be a genuine threat to other countries' satellites.

The missile's kill mechanism is that of a bullet: It crashes head-on into a target moving at 28 000 km/hr, adding its own speed to the total impact velocity. Such an impact creates a hypersonic shock wave that propagates from the inside of the target outward and, at the outer edge, shreds the target into metallic confetti that moves away at up to hundreds of meters per second. That's why the mechanism is called "kinetic kill."

Now it's important to keep in mind that the Chinese carefully timed the launch of their kinetic kill vehicle so that it would intercept the known position and orbit of the satellite it was aiming for—intercepting a target in an arbitrary orbit is a much more difficult proposition.

The Chinese targeted a low-orbiting, obsolete, weather satellite, where the kinetic kill energy was very great. However, the really strategic satellites fly much higher—the navigation network is 20 000 km up, and the communications constellations are in a geosynchronous arc at 40 000 km. At geosynchronous altitudes, the orbital velocities are so much lower that the impact energy would be only about a tenth as high as in last week's test.

Distance introduces a second burden: terminal navigation. When a target satellite is close to the Earth, ground radars can track it and relay final course corrections, both to the rocket during its ascent and to the kill vehicle, once it has been deployed on its hoped-for collision course. Radar operates at an inverse fourth power law, which means that for the Chinese system to aim many times farther than low Earth orbit—as it would have to do to track objects geosynchronously—the demands on a ground-based radar would be simply impossible. The engineering challenges don't need much description for this audience.

The Chinese weapons system has so far demonstrated only that it can pose a threat to low-orbiting objects, of which the most important are reconnaissance satellites. But these satellites have backup. If anyone interferes with them, countries can dispatch aircraft to conduct the reconnaissance. Although this might entail trespassing on other nations' airspace, an act forbidden by international law, such restrictions no longer apply once war has broken out.

Nor are space targets helpless victims to such kinetic kill attacks, especially at higher altitudes. In such cases, the best defense is usually a good pretense—the intended victim has options to degrade the accuracy of the attacker's critical terminal guidance.

During the final moments of a high-speed intercept, the attacking missile is able to make increasingly accurate sightings of the target, and therefore the uncertainty as to the target's relative position shrinks. The missile's navigation must be robust enough so that the uncertainty doesn't shrink so fast that the onboard control jets can't alter the expected impact point (or the attacker will get locked into a trajectory that will zoom by the target), and in the end the uncertainty has to be smaller than the physical size of the target. When it isn't, the attacker misses, and in this game, "close" doesn't count. To an even greater extent than with the anti-ballistic missile solution problem, a target satellite can take steps to interfere with the attacker obtaining a workable targeting solution, and the farther from Earth the attack occurs, the more the odds favor the target.

Objects can hide in space, to a greater or lesser degree, by lowering their radar reflectivity or optical brightness along the attacker's expected line of approach. This makes terminal navigation and guidance more difficult. That effect can be augmented with decoys, which can either be deployed when an attack is detected or can be sent, as a matter of routine, to fly in formation with the high-value target. A decoy doesn't have to be a throwaway subsatellite, it could be an inflatable spar a few tens of meters long with a pseudo-target at the end to attract the on-rushing kinetic kill vehicle away from the real spacecraft. Such a decoy could be deployed in a matter of minutes, and even re-stowed afterwards for future re-use.

Even the simple suspicion that a target may have such a capability would discourage a potential attacker. And the realization that a target might also be able to detect and characterize even a failed attack would be an additional deterrent. There would be no way for the attacking country to get away with attempted mayhem.

These engineering angles to China's ground-launched kinetic kill system suggest to me that the hardware's intended target isn't up in space at all. It is more probably a calculated move on the board—in this case, not for chess but for Go. If we focus too closely on the specific man and its neighborhood, we may miss the strategy for the game as a whole and wind up losing.

Ultimately, the central issue of this ASAT demonstration isn't about engineering. But good engineering assessment may enable us to determine what the issue is not, and that's a clue toward figuring out what it really could be. Then, and only then, can we develop a fruitful strategy.

James Oberg, today's guest blogger, is a lifelong "space nut" who worked 22 years at NASA Mission Control in Houston. He is the NBC News space analyst and a long-time contributor to IEEE Spectrum. In 1999, he published Space Power Theory for the U.S. Space Command. You can learn more about his work at his Web site.