But who’s worrying about those big rocks in the sky?

But who’s worrying about those big rocks in the sky?

Image by urikyo33 from Pixabay

Originally published 6 January 1997

Patient: Doc, I’m telling you, my fear has become com­pul­sive. I can’t sleep. Can’t eat. Can’t work. I go around look­ing up at the sky. Yes­ter­day I was almost run down in the street while gaz­ing upward.

Doc­tor: Lie down on the couch here, and we’ll talk about it. Make your­self com­fort­able. Now tell me, when did you first start wor­ry­ing about aster­oids falling from the sky?

Patient: About a year ago. I watched a TV pro­gram about the extinc­tion of the dinosaurs. A big chunk of rock smashed into the Earth about 64 mil­lion years ago. Blast­ed so much dust into the atmos­phere that the Earth was wrapped in cold and dark for years. Wiped out two-thirds of all plants and ani­mals, includ­ing the dinosaurs. It’s all I think about.

Doc­tor: Hmmm. Six­ty-four mil­lion years ago. That was a long time ago, was­n’t it?

Patient: Yeah, Doc, I know, I know. But it’s not like it has­n’t hap­pened more recent­ly. Mete­or Crater in Ari­zona is only a few tens of thou­sands of years old. That blast was 2,000 times stronger than the bombs that wiped out Hiroshi­ma and Nagasa­ki. Can you imag­ine if one of those things hit Ari­zona today?

Doc­tor: Tens of thou­sands of years? That’s still a long time.

Patient: The Earth is pock­marked with craters, some younger than the one in Ari­zona. In 1908, an object 60 meters wide (198 feet) dis­in­te­grat­ed in the atmos­phere over the Tun­gus­ka region of Siberia. Dev­as­tat­ed hun­dreds of square miles of for­est. If it had hit above Boston…

Doc­tor: Siberia?

Patient: Astronomers esti­mate that a 50-meter object should hit the Earth about every 50 years or so. We’re overdue!

Doc­tor: Sure­ly that’s a sta­tis­ti­cal aver­age. It could be a thou­sand years before the next impact.

Patient: Or 10 min­utes! Large aster­oids zip past the Earth all the time. In 1989 a 100-meter (328 feet) rock missed us by only 400,000 miles. In ear­ly 1991 a large object came with­in 100,000 miles, and there were oth­er near miss­es in 1993 and 1994. The mete­ors and comets we detect are prob­a­bly only a small frac­tion of the ones that zip by.

Doc­tor: Aren’t you for­get­ting that three-quar­ters of the Earth­’s sur­face is water? Does­n’t that con­sid­er­ably reduce the chance of a mete­or’s hit­ting land?

Patient: Water! That’s worse. If a 500-meter (1,640 feet) object smashed into the Atlantic, the tidal wave would wash against the Berk­shires! We would­n’t have a chance.

Doc­tor: Have you con­sid­ered that your fear might be just a tiny bit irrational?

Patient: Would you be dis­mis­sive if I were here because of a fear of flying?

Doc­tor: Fear of fly­ing is com­mon. Planes go down fre­quent­ly. Crash­es get big play in the media because of the large num­ber of vic­tims. But, you know, the prob­a­bil­i­ty of being killed in an air­plane crash is 100 times less than in an auto­mo­bile acci­dent. It’s the hype and calami­tous cir­cum­stances sur­round­ing air crash­es that cause fear of flying.

Patient: Doc, I’ll grant you that impacts like the one that wiped out the dinosaurs are infre­quent, but the num­bers of peo­ple who would be killed make a 747 crash look like small pota­toes. The con­se­quences of such an impact could elim­i­nate the entire human pop­u­la­tion. A small­er, Tun­gus­ka-like event might only hap­pen once a cen­tu­ry, but is like­ly to kill tens of thousands.

Doc­tor: Would­n’t you say that…

Patient: The astronomers Clark Chap­man and David Mor­ri­son have worked out the sta­tis­tics — divid­ed the num­bers of prob­a­ble deaths by the his­toric fre­quen­cy of impacts. The prob­a­bil­i­ty of being killed by an aster­oid or comet is about the same as for an air­plane crash. Less than for an auto­mo­bile acci­dent or homi­cide, but much greater than a fire­works acci­dent or bot­u­lism poisoning.

Doc­tor: Then you will at least agree that wor­ry­ing about aster­oids is as irra­tional as fear of flying?

Patient: Doc, we’re not talk­ing about ratio­nal­i­ty here. If I could deal with this ratio­nal­ly, I would­n’t be on your couch. We have a gov­ern­ment agency, the FAA, that wor­ries about whether air­plane trav­el is safe. We have laws to keep me safe from fire­works and bot­u­lism, both of which are far less like­ly to kill me than an aster­oid or comet. But who’s wor­ry­ing about rocks from the sky?

Doc­tor: Have you considered…

Patient: Chap­man and Mor­ri­son esti­mate that a Tun­gus­ka-like event will hap­pen at least once each cen­tu­ry, and with cur­rent pop­u­la­tion den­si­ties kill 10,000 peo­ple. A 200-meter object will on aver­age hit every 10,000 years and cause 100,000 fatal­i­ties, or five times that many if it hits in the ocean. A one-kilo­me­ter object will col­lide with Earth every 200,000 years and kill 20 mil­lion — or pos­si­bly 20 bil­lion. It’s been a long time since…Doc, Doc…

Doc­tor: Uhh, yes?

Patient: Why do you keep glanc­ing upwards?


In 2013, an esti­mat­ed 20-meter object explod­ed over the Chelyabin­sk region of Rus­sia, pro­duc­ing a shock­wave that shat­tered win­dows and caused injuries over an area 100-kilo­me­ters wide. The mete­or went com­plete­ly unde­tect­ed pri­or to impact with the Earth­’s atmos­phere. ‑Ed.

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