March 27, 2008

Comets Hitting Earth

Stupid Question ™
July 9, 1998
By John Ruch
© 1998

Q: Cataclysmic events are big at the movies right now. When’s the last time a comet hit Earth?

A: Asteroids and comets are the only space objects big enough to have caused the 130-odd giant craters still visible on Earth, and statistics tell us both kinds of objects must have hit us in the past.

But at those impact energies, the object itself is destroyed. Asteroid and comet craters look the same, and it turns out some asteroids are comets. So answering this question requires some educated guessing.

A comet is a “dirty snowball” of ices and minerals left over from the formation of the solar system. Occasionally one gets knocked out of its regular distant orbit and heads for a tight orbit of the Sun, with a giant tail of gas and debris spewing off as it heats up.

Asteroids are large bits of a would-be planet that orbit mostly between Mars and Jupiter. A few asteroids—perhaps 2,000—are in eccentric orbits that bring them dangerously close to Earth.

Many of these near-Earth asteroids are probably extinct or dormant comets. When the Sun has burned off all of a comet’s ice, or has melted the minerals into a protective crust, the comet becomes an asteroid.

A comet’s tail debris remains in orbit, causing annual meteor showers when the Earth passes through them. And most of the meteoric dust that Earth picks up from space—hundreds of tons a day—comes from comet debris. Comet stuff probably impacted Earth this very second.

Widely accepted theory holds that an impact that made a 100-mile-wide crater 65 million years ago off the Yucatán peninsula resulted in the mass extinction of the dinosaurs and many other forms of life. Judging by the impactor’s velocity, it was probably a comet. (Asteroids are generally slower.) Nobody’s as sure about the other craters worldwide.

Then there’s the “Tunguska Event” of June 30, 1908. That’s when something came from the sky and exploded five miles above Siberia, felling 600 square miles of forest and knocking people off their feet 35 miles away. Current theory and evidence suggest it was a 100-yard-wide rocky asteroid split off from a comet.

There’s a history of odd, possibly similar blasts, including a Feb. 1, 1994 explosion over the Pacific. And a fall of large chunks of ice in China in April 1995 was theorized to be cometary, though it’s unlikely ice that small would survive an 18 miles per second plunge through the atmosphere.

Near-Earth asteroids are likely to give us our next “comet” impact, but NASA has yet to convince Congress to fund a program to map and track them. The price tag would be $300 million—a bit less than it cost to make and market “Deep Impact” and “Armageddon.”

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