Every once in a while there a news station hears about some satellite collision or some rocket upper stage failure and, briefly, we are reminded about the problem of space junk. Over the last 53 years of humans launching objects into space we have gotten into the habit of leaving some extra pieces floating around. NORAD tracks tens of thousands individual bits of space debris through various optical and radio tracking techniques. There is estimated to be as many as 600,000 pieces larger than a centimeter.
Just what is all the junk? Wrappers from all that astronaut ice cream? Most is actually pretty benign: flecks of paint, or ablative material from rocket engines. There is some larger stuff in there too: loose bolts, empty fuel tanks, dead satellites, and even a bag of tools.
There are a couple of interesting implications for all this material floating around. Not too long ago I was talking to a friend of mine, Liz Argall, about a large alien ship approaching Earth. We were wondering how much it would interfere with our local ecosystem of communication and science satellites. And what about space debris? Luckily space is huge. Despite there being so much junk, there is also a huge volume that it occupies. And so the chances that a ship near Earth would get hit by anything is pretty low. Also assuming you have a ship large and complex enough to survive interstellar travel a few loose screws wont hurt it.
In fact it’s unlikely that anyone here would notice these collisions. There are reports of people seeing flashes on the Moon caused by meteorites slamming into its rocky surface. For a while no one believed the observations until someone finally caught an event on camera. Now we actually look for them. But these flashes are associated with heavy, solid objects hitting the Moon. A solid meteorite about a few inches wide can hit the moon with one Gigajoule of energy, the equivalent of about a quarter ton of TNT. But even then it’s pretty hard to see unless you are looking for it. Most space debris collisions are going to be nearly undetectable from the ground.
So if alien space ships won’t cause to much commotion, what will? There is a theory call the Kessler Syndrome. Proposed by a NASA scientist in the 70’s it proposes that there is a critical density of junk and satellites at which point random collisions create more debris, which collides with more objects, causing yet more junk until all of low Earth orbit is unusable and all our communication satellites get destroyed. There is, of course, some debate about how likely of a scenario this is. But it is possible. As it stands the International Space Station has be moved a couple of times a year in order to avoid a “close call”. Being cautious is good. Still, actual collisions with any working satellite are rare.
Hopefully we’ll get to cleaning up some of the junk somehow. Proposed methods include a laser broom (heating one side of an object with a laser beam so that the hot side throws off material and slows down until if falls back to earth) or sweeper satellites built to collect the larger pieces and plunge them back to Earth (and safely burn up on re-entry). Luckily there is a natural cleaning mechanism—the Earths atmosphere. Below about 300km there is enough drag from occasional air molecules that light objects fall out of orbit in a few years.