Geoffrey Forden, an MIT physicist and expert on the Chinese space program, was recently interviewed by John Johnson Jr. of the LA Times on the topic of the recent proliferation of space junk from China's missile test last year ("China Added to Space Debris" April 16, 2008). The news item caught my eye with the new-to-me and very impressive sounding word "supercriticality." Apparently that is what happens when space junk collides in a cascade of impacts. It is apparently very, very bad. Space is becoming cluttered to the extent that clutter will beget clutter, and in time it won't be possible to have something orbiting earth without it being damaged in a collision. This is what happens when space gets a "supercritical debris density" - which has already happened in some areas of space, according to NASA (see the "Cocktail Party Physics" link below for the specific reference).

Shooting down satellites isn't the sole purview of China-- the US also recently took a defunct spy satellite down to test its machinery.

... Forden called the Bush administration decision to shoot down its satellite "bad policy" because it could encourage other nations to build their own antisatellite weapons.

The difference is that the US target was in low enough orbit that the debris will burn up on reentry relatively soon. The debris from the Chinese missile test was in a much higher orbit, so the millions of pieces of debris that explosion created will remain up there clunking into the working satellites for hundreds of years to come.

Forden said the threat of supercriticality was a warning that it was time to treat space not as a vast junkyard, but as a natural resource that must be protected the same way we were learning to protect resources on Earth.

Turns out we need better policy in space, too!

• The Union of Concerned Scientists on space junk (Fall 2007) - apparently it is time to reassess the Outer Space Treaty
• Science blog "Cocktail Party Physics" on space junk (April 16, 2008)
  including some of the ideas for cleaning up space such as "laser brooms" and "terminator tethers"