The Sky Is Falling! But No Big Deal

Dick writes about Earth and planetary science for Science magazine.

NASA announced today that a satellite it sent into orbit in 1991 is coming back down by the end of this month, all 6 tons of it. No one can say in advance exactly where the remains of the Upper Atmosphere Research Satellite (UARS) will hit, but history and intensive calculations show that the risk to humans is extremely low. Really.

Q: What is the danger, really?

A: NASA calculates that the chances of a piece of UARS injuring a human are about 1 in 3200. The chances that an American will die in an auto accident in a year are roughly 1 in 10,000. Sound high? Then think of it in terms of your chances of being hit by UARS's cosmic debris: about 1 in 22 trillion. And besides, in the 54-year history of the space age, no one has ever been hit by falling debris, despite a steady rain of satellites and rocket bodies. Last year, one object a day fell back, almost all of them incinerating themselves before hitting Earth. A moderate-sized chunk fell each week on average. Typically, one object the mass of UARS comes down each year.

Q: The history is reassuring, but how can anyone calculate such odds?

A: It's a matter of physics and knowing the reentering object. NASA scientists and engineers considered what pieces UARS would break into as it hits the atmosphere, the pieces' shapes, and their composition. All that goes into a computer program that calculates whether each piece gets hot enough to burn up and, if not, how far it can travel before hitting. Then it's a question of what there is to hit. You can eliminate anything on the 70% of the planet that's water, everything poleward of 57° latitude over which UARS does not orbit, and for all practical purposes the empty areas of the world, such as the Amazon, most of Australia, the Sahara, the Tibetan Plateau, much of Siberia, and even a lot of the western interior of the United States. Turns out people are a pretty tough target for UARS's debris, which will hit a total area of just 22 square meters.

Q: When will we know where it's going to hit?

A: When it hits. The U.S. Air Force will be tracking UARS by radar on its way in, but even 2 hours before reentry, there will still be so much error in its prediction that the 800-kilometer-long debris-strewn field could be anywhere along a 10,000-kilometer-long track.

Q: So I won't likely get hit, but will there at least be something to see?

A: Oh, yes, if you are anywhere near its reentry track. Should be quite a sight, they say, with 26 surviving pieces blazing in, the biggest weighing 150 kilograms. A \$750 million fireworks display.

Q: What about other big things still in orbit?

A: NASA is committed to bringing those down in controlled reentries. The international space station, weighing in at over 400 tons, would be driven down into a predictable, empty spot like the mid-Pacific Ocean.

Correction: The cost of the "fireworks display" (the cost of the UARS mission) in the next-to-last answer was misstated. It is \$750 million.

Posted in Space