The Sky is Falling!
Saturday’s Science Section of the New York Times had a colorful piece by William Broad on the problem of space debris complicated by the reckless destruction of one of its weather satellites by the Chinese. The gist is that a terrible problem in space navigation is getting catastrophically worse. There is no doubt the Chinese ASAT test was irresponsible, even negligent. But there is, as always, a deeper agenda within the ink-saturated pages of the NYT. By linking a looming crisis in space transportation to a space weapon, the impetus for an arms ban in space is implicitly carried. If one detonation in space creates a dire situation, dozens or more must definitely make space travel forever impossible. Any weapons in space take us closer to disaster. The argument, based on the very faulty logic and images of the NYT essay, plays right into the Chinese strategy. Clearly a great deal of damage can be inflicted on a spacecraft (or EVA astronaut) by a piece of debris traveling at relative velocities in excess of 40,000 kph. A single fleck of paint sloughed from a separating boost vehicle is thought to have pitted the windshield of the Space Shuttle as badly as a piece of gravel bouncing off the windshield of a car traveling along a highway. But such collisions are still extremely rare.
Let’s look at some of the basic flaws in Broad’s article. It starts with the very dramatic and colorful renderings of earth orbit. Note the 10,000-plus bits of space junk larger than a softball are portrayed as significant blobs. In fact, were these depicted to scale (relative to an earth roughly nine inches in diameter), most would be invisible to the most powerful microscope. Indeed, the space junk that looms so ominously in the NYT graphics would be so insignificant they would exist only in theory—as no direct evidence of them could be detected. Each of the red, yellow, and blue dots is shown larger than the metropolitan area of the city of Houston, TX. In truth, if all of the space junk, known and hypothesized, were jumbled together, it could be piled into a single vacant black in the aforementioned city, with room left over for municipal parking. It’s a big sky out there, folks. Collisions are bad, but just how likely is a catastrophic crash?
Broad claims the debris cloud will expand continuously, apparently increasing its mass and destructiveness over time. No doubt the ability to detect the debris will expand, as sensors are cross tabulated and estimates refined (going from 800 to 1000 bits of detectable debris in this case), which is a quite different thing than spontaneous generation of new debris. The apparent growth in the model is from the increasing dispersion of debris over time. The implosion of the satellite created by the hyper-velocity ASAT sent the debris out in a three dimensional cloud (not unlike a large firework display). This means that as the debris gets farther from the center of the implosion some of it will burn up in the atmosphere and some will move out of earth orbit altogether. Some bits will track along a relatively stable orbital path and remain a nuisance/threat for thousands (hardly millions, as claimed) of years. The key factor here is that as the debris cloud expands the debris effectively disperses (distance between bits of detritus increases) making it less of a navigation hazard than if the same cloud were to stay roughly contiguous in a usable orbital slot. The catastrophic cascade effect referenced is a particularly scary and dramatic argument. In this case, a large piece of debris collides with another large piece (on the order of a spent rocket body or decrepit satellite) smashing the latter into thousands of other bits which seek out more large bodies to crash into, and so forth and so on, until space junk is present on the order of 300-1000 times its current density. The likelihood of such a chain of events occurring today is so remote that the author of the idea now agrees that it was a fanciful and essentially ridiculous idea. Only if one projects a geometric progression of orbital debris growth along the curve evidenced in the past fifty years is the possibility even remotely possible. Before the Chinese ASAT, it was something on the order of a fifty percent chance (of one occurrence) within the next thousand years. Now it is probably something like a fifty percent chance of one occurrence in 970 years.
Of course, all this presumes an unchanging future. No assumptions about the possibility of cleaning up space junk (and there are numerous interesting proposals out there on how this might be accomplished), learning from previous mistakes (the earliest space launches were the dirtiest), or limiting blast damage from kinetic engagements can be considered. As for the last comment, the Old Soviet Empire tested up to twenty of its massive, SL-11 launched co-orbital ASATs in the 1970s and ‘80s, and achieved about a fifty percent success rate. It was a typically ham-fisted approach. Essentially they maneuvered a massive shotgun shell into proximity of the target satellite (about 5 km or so) and then detonated, allowing the momentum of the ASAT to dictate the vector of the blast pattern. Anyway, after the first attempt riddled the orbit with detritus, the Soviets altered the direction of the intercept so that the velocity of the blast would be toward the earth’s atmosphere (down its gravity well). In this way, not only was the ASAT debris quickly and efficiently disposed of in the upper atmosphere (quite attractively, I imagine, if one happened to be watching in the night sky) but the entire target, too—shreds and all—was pushed into the atmosphere to burn up rapidly and cleanly. It is impossible to argue that these tests actually made orbits safer (by taking out the dead hulks of old satellites), but the latter ones did not add any significant debris to orbit. The Chinese could use a similar approach to future tests by simply altering their engagement trajectory. Kinetic kill is not the only space weapon envisioned, and in fact the least desirable. A laser or other directed energy weapon based in space could disable satellites without creating any debris. Smaller bits of debris could become target practice for lower levels of generated energy, giving them a push into decaying orbits for complete burn disposal. The important point here is that no one knows what space war will look like in the coming centuries. But it likely will come. The Chinese have placed an exclamation point on the preceding sentence.
This debris problem is vital to the anti-weaponization platform. If they can argue a single test might ruin spaceflight forever, then oh-my-gosh Chicken Little, a war in space would inevitably eliminate the ability to operate there FOREVER. That the world will be forced to abandon space travel because of increasing debris due to some mindless warrior ethos expanding into the heavens is preposterous.