The Meteorites That Fell To Earth

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The asteroid Vesta has been in the news lately, thanks to new images from NASA’s Dawn spacecraft that nicely showcases Vesta’s unusual shape. Per Phil Plait at Discover Magazine, as the second largest asteroid, “its gravity should be strong enough to crush it into a sphere. But it’s not a ball; it’s lumpy and stretched out and… has an enormous circular depression at its south pole which flattens that entire hemisphere of the little world.”

The Dawn mission should also provide more information on the various minerals that make up Vesta, although here there is a little less mystery, thanks to meteorites — pieces of Vesta that broke off and fell to Earth — that have been recovered and analyzed using spectroscopy to determine their composition. Those meteorites have the same spectral signature as Vesta.

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Just for clarification, “meteoroid” refers to a chunk of debris (usually a piece that has broken off from a comet or asteroid) that burns up on its way through Earth’s atmosphere, giving off a visible flash of light in the night sky. That flash of light is a “meteor” and there can be more than one, i.e., during meteor showers. “Meteorite” is whatever part of the original meteroid is left that lands on Earth.

The first person to suggest they were actually rocks from space was German physicist Ernst Chladni in 1794, who was ridiculed by his colleagues at the time for the suggestion, although he was vindicated nearly 10 years later after a spectacular display of thousands of meteorites fell into L’aigle, France in April 26, 1803.

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Meteorites come in all sizes, but very large ones are rare. One of the oldest (originally weighing around 250 pounds) fell into a wheat field in a small commune in northeastern France called Ensisheim on November 7, 1492. Naturally the inhabitants assumed the object was magical — this was pre-Chladni — and started chipping off pieces as souvenirs, perhaps thinking it would bring them good fortune.  But the remainder of the meteorite is intact and on display in the Regency Palace.

Another famous meteorite is the so-called Willamette Meteorite (see image, right), the largest such object found in North America and the sixth largest in the world. The thing weighs 15.5 tons and is primarily made of iron, with a bit of nickel thrown in for good measure.

Meteorites aren’t always too careful about where they land, with 57 recorded strikes on human structures in the 20th century. There are claims that in 1911, a meteorite struck and killed a dog in Egypt, but the account is hotly disputed. More recent accounts claim strikes on a cow, a horse, and a parked car, among other unfortunate targets.

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The first recorded instance of a meteorite striking a person was in Alabama on November 30, 1954, when a large meteorite crashed through the roof of one Ann Hodges — who was just napping on the sofa — bounced off her radio and struck her on the hip and hand. Hard. She was badly bruised, and to add insult to injury, she was renting the house. Her landlady sued for possession of the offending rock, in hopes of auctioning it off to pay for the damage to the house.

Fortunately, this seems to be a rare occurrence, although in 2009, Wired reported that a German teenage boy was hit on the hand by a meteorite while walking home from school. He described seeing a large ball of light, then felt a pain in his hand, followed by “an enormous bang like a crash of thunder,” telling the Daily Telegraph that “my ears were ringing for hours afterwards.” The meteorite was large enough to leave a foot-wide crater in the ground.

So it’s kind of nasty when meteorites attack. But that doesn’t mean they don’t have their own special kind of beauty. That’s what one of the Physics Buzz bloggers discovered recently while visiting the National Museum of Natural history in Washington, DC, likening the interior of an iron meteorite on display to an abstract painting:

Originally, these meteorites were made of only one very hot alloy, called taenite. As the taenite cooled very slowly over time – as little as 100 degrees Celsius per million years in some meteorites – another alloy called kamacite formed and grew alongside it. The two alloys formed bands next to one another, producing the beautiful pattern on display at the museum. The criss-crossing bands of alloys are known as Widmanstätten, or sometimes Thomson, patterns. Count Alois von Beckh Widmanstätten of Austria discovered the pattern when he heated a slice of a meteorite in 1808. Because the two alloys oxidized at different rates when heated, bands of different colors appeared on the slice.

That kind of Widmanstatten pattern doesn’t just happen when you slice through an iron meteorite, though. These days, the rocks are ground, polished, and cleaned, then dipped into nitric acid, at which point the telltale bands should appear.

Vesta’s meteorites, alas, are unlikely to have this pretty pattern because they’re not in the iron-nickel category. In fact, they have a very unique composition: they are made almost entirely of the mineral pyroxene, common in lava flows, suggesting that Vesta might once have been much hotter, such that heavier minerals sank to its core and lighter ones drifted to the surface.

See? You don’t have to be a nickel-iron meteorite to be special.