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Falling space debris rarely causes harm, US astronomer says

Giant chunks of manmade space junk -- like the dead satellite that the US government is trying to shoot down -- regularly fall to Earth. Yet no one has ever been reported hurt by them.

Chunks of debris weighing two tonnes or more from satellites and rocket parts fall uncontrolled every three weeks or so, according to an analysis by a Harvard University astronomer who tracks satellites and space debris.

And that is just based on the last three years. Go back a decade or so when countries didn't try to control these falling objects. Back then, 2 tonne chunks fell to Earth much more frequently said Jonathan McDowell, who runs Jonathan's Space Report, which tracks the world's space launches and satellites.

It's likely that 50 to 200 "large" pieces of manmade space debris return to Earth every year, according to the Center for Orbital and Reentry Debris Studies. Bill Ailor, the center's director, like those at NASA's Johnson Space Center, said he was asked by the government not to comment specifically on the current satellite re-entry issue.

In the past 40 years, about 5.4 million kilograms of manmade space junk has survived re-entering Earth's atmosphere, according to the orbital debris center.

Yet experts in the field know of only one report of a person being hit by space debris.

Lottie Williams of Tulsa, Oklahoma, was struck on the shoulder in 1997 by a small piece of debris from a discarded piece of a Delta rocket. She was unhurt.

The reason space junk doesn't regularly hit people is simple: About 70 percent of the Earth's surface is water.

And on average there are about 130 people per 2.6km2 of land on Earth, but people do not take up a lot of space.

Far more than 99.9 percent of the land on Earth is not occupied by a person at a given time, according to rough calculations by researcher Alex de Sherbinin of Columbia University.

There is no one place on Earth that is more prone to space junk than others. Where satellites fall depend on their particular orbit.

So the orbital debris center that studies the issue puts the odds of anyone being hurt by any piece of re-entering space junk at one in a trillion, saying you are far more likely to get hit by lightning.
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