Watch for falling rock

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As a kid I was always disappointed when we drove past “Watch for Falling Rock” signs in the mountains and no rocks actually fell. (I had a similar reaction to deerless “Deer Crossing” signs.)

Obviously we were just driving in the wrong places, because on September 15 a very impressive fall of rock occurred in Peru, when a meteorite slashed out of the sky and smashed into the ground at a town called Carancas, near the Bolivian border.

People up to 20 kilometres away heard the impact, which shattered windows in the local health centre a kilometer away. The resulting crater, more than 13 metres wide and several metres deep, filled with water almost immediately—water that witnesses reported was bubbling. A column of smoke or steam rose for several minutes, and many witnesses said they smelled a sulfurous stench.

All of this would have been enough to get the impact a lot of attention in the media even without what happened next: reports started surfacing that “scores” (one source said 200) villagers had taken ill, suffering headaches and vomiting, in the wake of the impact.

More people than just me immediately thought of Michael Crichton’s The Andromeda Strain, in which a deadly pathogen is brought to Earth from space.

The reality, fortunately, seems to have been less mysterious—and far less deadly—than that, New Scientist magazine reports. Scientists from the Peruvian Institute for Geology, Mining and Metallurgy visited the site to investigate, and released their report last week.

There were a few unusual things about the meteorite. The recovered fragments were stony, which is odd because stony meteorites normally break apart in the air: the ones that make it all the way to the ground are typically nickel-iron.

Don Yeomans of NASA’s Jet Propulsion Laboratory in Pasedena told New Scientist that the object, which may only have been basketball-sized, was probably a fragment from something much larger, maybe three metres in diameter, that broke up at an altitude of 50 kilometres. “One would expect a three-metre…meteorite to hit the Earth’s atmosphere a few times each year and sometimes a fragment makes it to the ground.”

The reports of boiling water in the crater and steam rising for half an hour are also odd. Even though we think of meteorites as being blazing hot, they’re actually very, very cold, having been chilled in interplanetary space for eons. A meteorite’s outer layers burn off as it streaks through the air, but its interior remains icy, and so the surface does not remain hot after impact—certainly not hot enough to boil water.

The kinetic energy of the impact may have heated the ground, or the air trapped and heated at the front of the meteorite as it descended may have provided the necessary heat.

As for the mysterious illnesses: well, the scientists found that only about 30 villagers, not 200, had any complaints. Twenty children from the local school are also being examined. No cause of their illness has yet been determined.

There has been speculation that poisonous compounds in the soil or water, such as cyanide, might have been released, but Lionel Jackson of the Geological Survey of Canada has a simpler explanation: he thinks those made ill were simply stunned by “a big explosion in a very quiet area of the world.”

So there’s no Andromeda Strain, for which we can be thankful. However, the Peruvian meteorite does remind us that there are big chunks of rock hurtling around in space with our planet’s number on them. Never mind the titans like the 10-kilometre-wide monster that slammed into the Yucatan peninsula 65 million years ago and probably doomed the dinosaurs, or the 50-metre-wide meteorite that blasted out the four-kilometre-wide Arizona Meteor Crater 50,000 years ago, or the object that exploded over Tunguska, Siberia, with the force of a hydrogen bomb not quite a century ago, flattening trees over hundreds of square miles.

As the Peruvian meteorite crater shows, even a rock the size of a basketball landing in your backyard could really ruin your day.

When you come right down to it, the whole planet should be under a “Watch for Falling Rocks” sign.

Permanent link to this article: https://edwardwillett.com/2007/10/watch-for-falling-rock/

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