The latest issue of NASA's Orbital Debris Quarterly News (I kid you not) is hot off the presses, and it includes a fascinating article on how increased solar activity can help to sweep Earth's orbit clean of space junk.
I know what you're picturing: you're picturing the sun just tossing out giant explosions that blast past Earth, vaporizing everything in their way. The reality is slightly significantly less violent, much (to the relief of our astronauts on the ISS), but it's still effective enough to make a noticeable dent in the volume of space junk we've got whizzing around up there.
One of the effects of increased solar radiation is that Earth's atmosphere heats up a little bit, and that causes it (the thermosphere, specifically) to swell outwards into space. Keep in mind that there isn't some point where Earth's atmosphere just stops and space begins: rather, you just get fewer and fewer molecules of atmosphere the farther up you go. So, when the thermosphere swells up, bits of space junk start smashing headlong into more and more molecules of atmosphere, and every time they do, they slow down a little tiny bit. The cumulative effect is that eventually, the junk slows down enough that it can't maintain a stable orbit, and it heads back to Earth, usually burning up along the way. Non-junk satellites (and the ISS) have the same problem with atmospheric drag, but they've also got thrusters (or until recently, space shuttles) to give them the occasional boost to a higher orbit.
As a sample case, NASA has been tracking the debris from China's little satellite-destroying performance back in 2007, and they've found that of the 3,218 cataloged pieces of debris that they've been keeping an eye on, six percent has been destroyed. But half of that six percent has deorbited within just the last year, correlating nicely with increased solar activity. Further increases up through the end of next year should help out with the space junk problem even more, but as the solar cycle calms down after 2013, we'll have to go back to trying out crazy ideas instead. LINK
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