Hyperactive sun clears space junk ? for now

Low Earth orbit just got a free spring-clean, thanks to the sun. It turns out that increased solar activity in recent years has removed some of the satellite debris that clogs this region, making it temporarily safer for other satellites and astronauts.

The sun will hit an 11-year peak in its activity ? the solar maximum ? in 2013. As this approaches, small increases in solar radiation warm the outer layer of Earth's atmosphere, called the thermosphere, forcing it to expand into space. This places atmospheric molecules in the path of low orbiting debris, which brake their orbital velocity and cause them to re-enter the atmosphere sooner than expected, where they usually burn up.

In the latest edition of NASA's Orbital Debris Quarterly News, the agency's chief scientist Nicholas Johnson notes that the billowing thermosphere has accelerated the rate of burn up of debris from Fengyun-1C, a satellite China destroyed in an anti-satellite missile test in 2007, and from the 2009 collision between Russia's Kosmos 2251 and the US Iridium 33. This is a "welcome, albeit brief, respite from an otherwise growing orbital debris population", he says.

It's a short-lived respite since, in the long term, climate change will warm the planet while cooling the thermosphere ? and so keep junk aloft longer ? says space debris researcher Hugh Lewis at the University of Southampton, UK.

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