Satellite Collisions and the Kessler Syndrome

Yesterday, for the first time ever, two large satellites collided in earth orbit. Though the actual damage caused by the accident is small – both were destroyed utterly, but one was non-functional and the other apparently has eight in-orbit backups – the event could mark an unfortunate turning point in space exploration.

Space, even just the space directly above earth, is enormous relative to the number and size of things that happen to be in it. But that number is only growing, and since stable orbits require velocities that are dozens of times the speed of bullets, collisions need not involve large objects in order to cause massive damage. Worse, as Donald Kessler argued in a 1991 article on the increasing dangers of space debris, we need not add any more satellites to low earth orbit for the number of objects to increase: rocket pieces, discarded accidentally or intentionally during launch, often stay in orbit indefinitely; and debris caused by the breakup of satellites during collisions has the potential to spread out and endanger other satellites.

At a certain critical mass, the safety of every object leaving earth is jeopardized, as chain reactions of collisions continually add debris, causing more collisions. This potential condition is perhaps predictably known as the Kessler Syndrome, and while we're not in danger of that state anytime soon, yesterday's major collision is probably just the first of many. And it's added a significant amount of debris in and of itself. Before, there were only 17-18,000 object capable of causing catastrophic damage to an artificial satellite. Now, after a single collision, SpaceflightNow quotes Michael Carey, the Air Force Brigadier General in charge of the surveillance program: "As of about 12 hours ago, I think the head count was up (to around) 600 pieces." The number will certainly rise before they're done, and they won't have a clear picture for days at least.

And with all of this in mind, the AP article on the recent collision manages to paint a scary picture even for our relatively infrequent manned flights:

Litter in orbit has increased in recent years, in part because of the deliberate breakups of old satellites. It's gotten so bad that orbital debris is now the biggest threat to a space shuttle in flight, surpassing the dangers of liftoff and return to Earth...

"The collisions are going to be becoming more and more important in the coming decades."