February 21, 2008
Greater Chance of Being Hit by Lightning than Space Debris
About 12 million pounds of manmade space junk, such as the dead satellite the U.S. government shot down last night, re-entered the Earth's atmosphere over the past 40 years, according to the Center for Orbital and Reentry Debris Studies.
However, no one has ever been reported hurt by it. The reason is simple, and mainly a matter of probability; 70 percent of the Earth's surface is water. And greater than 99.9 percent of the land is not occupied by a person at a given time, according to AP reports of rough calculations by researcher Alex de Sherbinin of Columbia University.
On average, there are about 130 people per square mile of land on Earth, and people simply don't take up a lot of space.
Indeed, only one person has ever reportedly been hit by space debris. Lottie Williams of Tulsa, Oklahoma, was hit on the shoulder by a small piece of debris from a discarded piece of Delta rocket in 1997. According to an AP report, she was not hurt in the incident.
Analysis by a Harvard University astronomer who tracks satellites and other space debris showed chunks of fragments weighing up to several tons from satellites and rocket parts have routinely fallen uncontrolled once every three weeks or so within the last few years. Prior to that, when countries did not try to control these falling objects, two-ton chunks routinely fell to Earth, according to Jonathan McDowell, who runs Jonathan's Space Report, which tracks space launches and satellites around the world.
Bill Ailor, director of the Center for Orbital and Reentry Debris Studies, told AP its likely that 50 to 200 "large" pieces of manmade space debris return to Earth every year, adding that he was asked by the U.S. government not to comment specifically on the current satellite re-entry issue.
But despite the large amount of debris that returns to Earth, the orbital debris center estimates the odds of anyone being hurt by any piece of spacejunk is one in a trillion, far less likely than the chances of being struck by lightning.
McDowell used population density maps of Columbia University to calculate the highest possible risk, and found only a 1-in-10,000 chance that a dead satellite could actually strike a person. However, he told AP the real risk is probably closer to one in a million.
However, that number does not take into consideration risks from exposure to fumes from the satellite's toxic hydrazine rocket fuel, which is the reason Pentagon officials said they had to shoot down the dead satellite.
The U.S. Centers for Disease Control and Prevention (CDC) issued warnings this week to local public health officials about the fuel's toxicity.
But considering the small odds of satellite debris actually hitting any people though, McDowell is skeptical for the stated reasons the satellite was shot down.
"My gut reaction is that this is just completely bogus," McDowell told AP, of the decision to shoot down the satellite based on a public health threat. However, he didn't completely discount the danger of the rocket fuel.
David Ropeik, a Boston risk communications expert and co-author of the book "Risk", told AP that this is the type of risk that shouldn't be reduced to mere numbers. Ropeik has consulted with both the Bush White House and the Department of Homeland Security.
"It's the nature of the risk, not the number," he said, "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."
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