Giant chunks of manmade space junk — like the dead satellite that the U.S. 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 tons 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's 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, two-ton 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 12 million pounds 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, Okla., 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 is water.

And on average there are about 130 people per square mile of land on Earth, but people don't 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.

Using Columbia University's population density maps, McDowell calculated that at the highest possible risk, there's a 1-in-10,000 chance that the dead satellite could hit a person. However, it's probably closer to one in a million, McDowell said.

That doesn't take into account toxic fumes from the ton of frozen and dangerous hydrazine rocket fuel, which is the reason Pentagon officials said they needed to shoot down the dead satellite. The Centers for Disease Control and Prevention put out a notice to local public health officials warning of the toxicity of the fuel.

McDowell is skeptical, however, given the odds.

"My gut reaction is that this is just completely bogus," McDowell said of the decision to shoot down the satellite based on a public health threat. He doesn't completely discount the danger of the rocket fuel, however.

This is the type of risk that shouldn't be reduced to mere numbers, said David Ropeik, a Boston risk communications expert who has consulted with the Bush White House and Department of Homeland Security.

"It's the nature of the risk, not the number," said Ropeik, co-author of the book "Risk." "A good question can be asked whether it is the public's worry that is driving this or whether the government is concerned about the harm that it can cause, even though the chances of that are low."