Tuesday saw a strangely historic event occur—the first accidental collision in space of two orbiting satellites: a 1,200-pound communications satellite in full-working order, and a 1-ton aging Russian satellite that's been presumed dead for five to ten years.
The collision occurred 500 miles over Siberia, and it looks like NASA is pinning the blame on the Russian satellite, Cosmos 2251, which was "out of control" according to NASA space debris expert Mark Matney. The functioning Iridium satellite was launched in 1993 and had a role in a constellation that brings satellite-comms to remote parts of the world. Due to the resilient nature of the constellation, the service is still running, and a "spare" unit will mop up the new hole in the network soon.
We're used to the idea of space being "clean" and "empty" and of orbiting satellites spinning silently and precisely through the shining void, so the idea of a collision between two satellites, let alone two such large ones, is at first a hugely surprising thing.
But there's an inevitability about this accident, which comes down to the physics of gravity and hard statistics. Satellites need to share orbital space due to orbital mechanics and the particular purpose of each satellite. The number of launches rises every year since Sputnik, and available space is getting cluttered with working and defunct satellites.
More worrying is the increasing amout of space garbage. The U.S. Space Surveillance Network tracks as much debris as it can find—currently it's tracking 18,000+ pieces over 4-inches in size. Things like discarded rocket stages, debris from launch vehicles, lost tools fumbled by astronauts, debris caused when a satellite is hit by other debris and so on. There are millions of smaller pieces, such as flecks of paint or tiny metal fragments, and they all pose a risk to satellites, the ISS, and launch vehicles like the Shuttle.
And now there's a new debris cloud at 500 miles with over 600 pieces of a largish size, and countless thousands of smaller pieces. NASA's confident the ISS, which orbits some 270 miles lower down, and the upcoming Shuttle mission are safe—but it's monitoring the debris to ensure none of it gained enough delta-vee to change its orbit to a mission-threatening one.
There have been proposals for decades to do something about space debris—which poses a particular risk to astronauts performing spacewalks, since space suits are resilient enough for the harshness of space, but not strong enough to resist a metal fragment impacting at orbital velocity.
NASA is researching a "laser broom" for the ISS, which will detect and push incoming debris out of the ISS's path. And there's a proposal to fit low-earth-orbiting satellites with a "terminator tether" which will lower a cable into the ionosphere when the satellite's lifespan is over—the resulting drag on the cable would pull the satellite into the atmosphere where it would burn up. There's also lots of work to develop protective materials for spacecraft and space suits, including nanotechnology "shield" material.
Currently no-one's exactly sure how to clean-up space junk, including the Inter-Agency Space Debris Coordination Committee which is a collaboration between international space agencies. There are several proposals to deploy huge super-material "nets" that would filter debris from selected orbits, though that's obviously hugely technologically challenging. But as more and more debris is left in orbit, at some point someone will need to innovate and inject the billions of dollars it'll need to mop it all up