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And I'm David Greene, in Washington.

Later this week, a retired NASA satellite that's the size of a school bus will finally fall back towards Earth. Most of it will burn up in the atmosphere. But about two dozen pieces are expected to hit our planet somewhere. The biggest piece will weigh about 300 pounds. NPR's Nell Greenfieldboyce reports on your risk of being hit by this falling space junk.

NELL GREENFIELDBOYCE: The Upper Atmosphere Research Satellite has been orbiting the Earth for 20 years. Now it's about to come back down. NASA emphasizes that in the history of the space age, there have been no confirmed reports of falling space junk hurting anyone. But that doesn't mean no one has ever been hit.

Lottie Williams lives in Tulsa, Oklahoma. Back in 1997, she was walking through a park around 4:00 a.m. when she saw what looked like a shooting star.

LOTTIE WILLIAMS: It was just a big ball of fire, shooting across the sky at just a fast speed.

GREENFIELDBOYCE: A little while later, she felt a tap on her shoulder. When she turned around, there was no one there, but something fell to the ground. It was a small piece of burned mesh. An analysis later showed it's most likely part of a returning Delta II rocket - that fireball in the sky.

(SOUNDBITE OF LAUGHTER)

WILLIAMS: I mean, that was one of the weirdest things that ever happened to me.

GREENFIELDBOYCE: And I think you're the only one like in the world that that's ever happened to.

WILLIAMS: Yes. Mm-hmm. That's true.

GREENFIELDBOYCE: I told her about the huge NASA satellite expected to fall to Earth on Friday, plus or minus a day.

WILLIAMS: It's not going to come across Oklahoma again, is it?

GREENFIELDBOYCE: They don't know where it's going to be.

WILLIAMS: Oh, they don't know?

GREENFIELDBOYCE: No.

WILLIAMS: Goodness. Mm.

GREENFIELDBOYCE: To find out why it's so hard to know when and where, I called Brian Weeden. He's a space junk expert with the Secure World Foundation. He used to do re-entry analysis in the Air Force. He says there can be error in observing orbiting objects with radar and telescopes.

The atmosphere's density constantly changes. You have to factor in storms, winds, the object's size and shape.

BRIAN WEEDEN: And all of that makes it a very complex issue and one that's very, very hard to predict accurately.

GREENFIELDBOYCE: He compares it to dropping a coin in a fish tank and trying to guess where it will land. Still, Weeden is not worried.

WEEDEN: The Earth is 70 percent water, and within the portions of it that are only land, there's a lot of land.

GREENFIELDBOYCE: NASA has calculated the odds that someone might get hit this time around. Mark Matney is an orbital debris scientist at Johnson Space Center in Houston. He says the risk is one in 3,200.

MARK MATNEY: That 1 in 3,200 is the probability that someone, somewhere on the Earth will be hit by a piece of debris of sufficient size to cause injury.

GREENFIELDBOYCE: But Matney says your own personal risk of being hit by a piece of this satellite is far, far lower.

MATNEY: Something like one in trillions for any one person.

GREENFIELDBOYCE: He says these days, NASA's satellites and space vehicles have to meet standards that lower the risk even more. Sometimes they're designed to be steered down into the ocean. But this satellite was launched before that policy took effect.

Matney says that space junk falls back to Earth all the time. Usually, no one notices. Large objects the size of this satellite return about once a year. This one is getting advance publicity because it's the biggest NASA satellite to make an uncontrolled re-entry in about three decades.

MATNEY: If you're lucky enough to be near the re-entry at nighttime, you should see a spectacular show.

GREENFIELDBOYCE: He says NASA will be announcing the latest predictions on this re-entry as they become available.

Nell Greenfieldboyce, NPR News.

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