After Hours

Geek Trivia: The cold hard truth

Why are meteorites an unlikely cause of earthbound explosions or fires?

Fifty-one years ago, Ann Hodges of Sylacauga, AL became the recipient of that rarest and rightfully unwelcome of all astronomic gifts—a meteorite-induced injury. On Nov. 30, 1954, an eight-pound H4 chondrite meteorite crashed through the roof of her home and ricocheted into her hip, leaving her with a nasty bruise—but earning her a place in meteorite lore as the first confirmed modern case of a meteorite directly harming a human being.

Of course, this was hardly the first case of documented meteorite landings. In fact, 43 years prior to Hodges' historical injury, a more famous group of rocks made landfall.

On June 28, 1911, a series of meteorites hit the earth near Nakhla, Egypt, bearing with them geologic material that originated on Mars. These so-called Nakhlites have since become centerpieces in the ongoing scientific debate over the possible past or future existence of life on Mars.

But scientific conundrums are only part of the fame and mystery surrounding the Nakhla meteorites—at least as far as space rock aficionados are concerned. Many sources claim that the Nakhla meteorite storm, which included a series of atmospheric detonations and earth impacts by more than 40 extraterrestrial stones, is also an example of that incredibly rare event: a meteorite fatality!

Unconfirmed reports claim that a meteorite killed a dog outside Nakhla. However, given that the event occurred nearly a century ago, it's unlikely that we'll ever authoritatively corroborate these claims.

What makes the Nakhla fatality reports so difficult to believe—and Ann Hodges' unfortunate luck all the more extraordinary—is the fact that it's highly unlikely for a meteorite to strike anyone or anything, let alone kill something or someone. Thousands of meteorites bombard the earth every day, but the vast majority are micrometeorites—grains of rock the size of sand or dust that drift to earth as a result of upper-atmosphere explosions of meteors.

Those meteorites large enough to cause injury during impact are highly unlikely to hit a target as small—and as mobile—as a human being (or a dog), especially considering that almost all humans reside on dry land, which covers only about a third of the Earth's surface. Even in Hodges' case, the meteorite crashed into her house and bounced into her while she was napping (read: an unmoving target).

While there are numerous reports of meteorite-induced death or injury, many of these erroneously cite scorching-hot, high-velocity meteorites as the cause of an otherwise unexplained earthbound explosion or fire—which is extremely unlikely.


Why are so-called scorching-hot, high-velocity meteorites an unlikely cause of earthbound fires and explosions, despite numerous unconfirmed reports to the contrary?

As opposed to what you may have seen in the movies, meteorites are not terribly warm when they hit the ground, and some are even frost-coated. Untrained observers assume that since meteors in the upper atmosphere are visible due to combustion, earth-impacting meteorites must be hot as well. The reality is quite the opposite.

First of all, friction causes the combustion visible from meteors, but it's friction from air rubbing against air—not air rubbing against the meteorite. Meteorites strike the atmosphere at speeds of roughly 30,000 miles per hour. At that rate, the air can't move out the meteorite's way fast enough, creating compression in front of it.

This compression heats the air to the point of combustion, creating the familiar image of a shooting star. But the combustion never actually touches the meteorite. The compression happens so fast that it creates a pocket of relatively cool, slow air between the meteorite and the compressed air, a phenomenon called standoff shock.

The transferred heat literally peels layers off the space rock as part of a process called ablation. With the hot layers peeled away, the core of the meteorite remains cool, thanks to exposure to the vacuum of space. High-altitude air may chill it further once it slows down enough to shed its combustive fireball.

By the time meteorites hit earth, they're moving at perhaps only 200 mph, and they can be as cold as hail or snow. That's why, contrary to popular belief, Ann Hodges was in virtually no danger of being burned by her meteorite fragment.

When meteorites have caused documented damage, such as the Hodges incident, it's been because of their speed and mass, not their temperature. Thus, we have another Hollywood astro-myth exploded in the name of Geek Trivia.

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The Quibble of the Week

If you uncover a questionable fact or debatable aspect of this week's Geek Trivia, just post it in the discussion area of the article. Every week, yours truly will choose the best post from the assembled masses and discuss it in the next edition of Geek Trivia.

This week's quibble comes from the November 9 edition of Geek Trivia, "Before the age of the Dyna-Soar." TechRepublic member Rosenthj pointed out yet another corollary aircraft I didn't have the space to mention.

"Actually, the Germans experimented with a vertically launched, manned version of the V-1 concept—though rocket-powered with the same engine used by the Me-163—called the Natter (Viper) Ba-349B-1. So this was the first vertically launched, manned, rocket-powered weapon. See this link for a fascinating history."

Extra points for the great link, dear reader. To the rest of you: Keep those quibbles coming!

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The Trivia Geek, also known as Jay Garmon, is a former advertising copywriter and Web developer who's duped TechRepublic into underwriting his affinity for movies, sci-fi, comic books, technology, and all things geekish or subcultural.

About Jay Garmon

Jay Garmon has a vast and terrifying knowledge of all things obscure, obtuse, and irrelevant. One day, he hopes to write science fiction, but for now he'll settle for something stranger — amusing and abusing IT pros. Read his full profile. You can a...

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