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Your Friday Forecast: Sunny, with a 1-in-21-Trillion Chance of Getting Hit by Orbital Debris

The views expressed are those of the author and are not necessarily those of Scientific American.


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Credit: NASA

The orbital realm surrounding Earth is filled with millions of pieces of space junk, some of which occasionally fall back to Earth. Rarely, though, does an entire satellite or spacecraft come back uncontrolled, as NASA expects its Upper Atmosphere Research Satellite (UARS) to do sometime on Friday.

The schoolbus-size UARS [see artist's depiction at left] weighs some 5.7 metric tons, and NASA predicts that 500 kilograms of debris will survive reentry and land somewhere on Earth. Because populated land makes up a small part of Earth’s surface, chances are UARS will land in the ocean or on some vast wildland. Still, orbital debris trackers at the space agency estimate that there is a 1-in-3,200 chance that pieces of UARS will strike someone. That stat has been erroneously reported as the odds that any particular person (say, me) will be hit by UARS debris. In actuality, my odds of being struck down by UARS on Friday are about 1 in 21 trillion, since the risk is spread across almost all of Earth’s 6.7 billion inhabitants.

For a bit of perspective, I am about 14,000 times more likely to be struck dead by lightning on Friday than I am to be struck by UARS. (That estimate is based on rates of fatal U.S. lightning strikes, which show that an average American’s risk of being killed by lightning on any given day is about 1 in 1.6 billion.)

And although the reentry of a UARS-size spacecraft is a relatively rare event, large pieces of debris fall back to Earth without incident on a regular basis. As NASA reports: “Since the beginning of the Space Age in the late-1950s, there have been no confirmed reports of an injury resulting from re-entering space objects. Nor is there a record of significant property damage resulting from a satellite re-entry.”

NASA has set up a Web page to update the public with predictions for the reentry of the satellite. So give it a look, and then go back to planning for an ordinary weekend.

About the Author: John Matson is an associate editor at Scientific American focusing on space, physics and mathematics. Follow on Twitter @jmtsn.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Jennifer Frazer 3:15 pm 09/21/2011

    For the record, I am not worried about getting hit by orbital debris on Friday. I do, however, *love* your headline. :)

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  2. 2. Eliot 10:24 pm 09/21/2011

    Fox news reported that EVERYONE has a 1:3200 chance of being killed by space junk in their life time. The reporter was talking to another reporter and neither one even showed the slightest bit of doubt about what they were saying. Really? 30,000 people die each year globally? I believe only one person, Lottie Williams, has been hit by space jetsam, and was not harmed. Get your news from people who are experts is their field, not some talking head.

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  3. 3. Dr. Strangelove 5:26 am 09/22/2011

    Mr. Matson, I think your math is wrong. It is 1,358 more likely that someone will be struck by that satellite than be struck by lightning. The odds of satellite is 1/3,200 vs. lightning 1/4,347,826 or equivalent to 0.23/1,000,000 mortality rate.

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  4. 4. jmatson 11:42 am 09/22/2011

    There are two problems with your analysis, Strangelove. One is that the 1/3,200 odds are for all people worldwide: there is a 1/3,200 chance that *anyone* will be hit. The lightning figure says that 1 in 4,347,826 people are killed by lightning every year, so those are the odds for *a given individual*. The chance of *anyone* being killed by lightning is far higher: someone in the U.S. is killed by lightning every few days, on average. This is exactly the point of my article–the odds of *anyone* being hit and the odds of *a specific individual* being hit are quite different.

    The second flaw in your analysis is that the lightning figures are for an entire year, whereas I’m talking here about the odds of my being struck by lightning on a specific day (Friday). But either way you choose to look at it, lightning is a far greater danger than this falling satellite.

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  5. 5. Andragogue 4:37 pm 09/22/2011

    Well, I calculated it differently for my Facebook page. I estimated about 5 billion people living in the potential debris path, not the entire population of the Earth. So my odds for any one person in that zone to be about 1 in 15 trillion.

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  6. 6. Andragogue 4:39 pm 09/22/2011

    And, oh! What are the chances that a Velikovskian will get hit?

    ;-)

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  7. 7. mpcahn 9:14 pm 09/22/2011

    Good article, sadly your readership is mostly to smart to fall for outrageous claims about chances of death. Maybe you should get this article syndicated to the fox news website.

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  8. 8. Emanuel Of God 2:56 am 09/23/2011

    A lonely lizard in the Sahara desert who is already within range of a pit viper has decided to attempt a meal of a deadly scorpion who has focused on the vibrations of a sleeping researcher that has traveled 5,000 miles in hope of catching a glimpse of some of the re-entry of this failing satellite when a small chunk of debris strikes the lizard and because of the shape of the space junk it had attained a speed 4 miles above the speed of sound creating a gigantic boom for such a small piece of metal. All but the lizard survive one more day but the researcher has been bitten by a small dengue carrying mosquito and will be dead in a month, the viper eats the scorpion the next day after recovering from trauma it does not understand and my odds of being hit decrease by one. I suppose this is a non linear world at its best and while thinking about that I fail to notice the bus aimed directly at me with its annoying horn blasting away.

    I look forward to the race that learns from statistics and wonder what a scorpion taste like before and after a near miss of something that is muted by the distance between it and the lizard in the sand does nothing to change the taste or deadly affect it is to have on the viper. Nothing but numbers they all are dead before the satellite begins its reentry

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  9. 9. jgrosay 12:01 pm 09/23/2011

    The french comic characters Asterix an Obelix feared nothing but the sky falling into their heads. This is a fear connected to B.C. beliefs, and part of this may be printed in our minds as part of our culture, heritage, and family history. You better keep this fears alive, as destroying it can destroy many linked things, and one never knows which ones. Salut +

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  10. 10. MrTaylorPhysics 12:31 pm 09/25/2011

    NASA’s claim “Nor is there a record of significant property damage resulting from a satellite re-entry.” is a bit misleading. The USSR’s satellite Kosmos 954 re-entered in early 1978 and crashed over the Canadian Arctic. According to a Wikipedia article: “the Canadian government billed the Soviet Union $6,041,174.70 for actual expenses and additional compensation for future unpredicted expenses; the U.S.S.R. eventually paid the sum of three million Canadian dollars.” I regard spreading radioactive debris over my country as property damage!

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