Journey Into Space: Gravity, Orbits, and Collisions
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Meteor Attack!
Mona Chiang 

Oct 19, 1998—Is the sky falling? It might seem that way when a meteor storm pelts Earth next month.

The scene: New York City's bustling streets. Suddenly, fiery trails ignite the sky as space rocks as big as basketballs and Volkswagen Beetles smash through Grand Central Station. They topple the landmark Chrysler Building, and one boulder blasts straight through three skyscrapers before gouging a crater into a crowded street. Is it the start of an alien takeover?

No, it's a startling meteorite shower-and the opening scene of this past summer's blockbuster movie Armageddon. Real meteor showers are streaks of light produced by meteoroids, or tiny space fragments, as they burn up in Earth's atmosphere. Could film fiction ever become reality?

Good question. On November 17, the most intense meteor shower in more than 30 years is due to strike Earth. "Such massive meteor storms are brief and extremely rare," says astronomer Peter Jenniskens at NASA's Ames Research Center in Moffett Field, California.

NOVEMBER SHOWERS
The Leonid meteor shower (so called because it seems to radiate from the constellation of Leo) occurs each November, usually from the 14th to the l9th. The showers typically occur when Earth crosses comet Tempel-Tuttle's orbit around the sun. The shower's cause: The trail of dust particles that both precede and follow the comet. This past February, Tempel-Tuttle swept as close as it gets to the sun during the comet's 33-year orbit, shedding a thicker than-normal trail of comet dust.

The biggest storms always occur after perihelion (per-uh-HEEILyuhn) passage, or the comet's closest approach to the sun, explains David Lynch, research scientist with The Aerospace Corporation in El Segundo, California.

SITTING DUCKS
Armageddon is not yet at hand. During the Leonid meteor shower, meteoroids ranging in size from a grain of sand to small pebbles disintegrate as they shoot through Earth's atmosphere. But there's no danger they'll strike the planet's surface.

The 500-plus satellites orbiting Earth, however, are sitting ducks for space dust that can speed up to 60,960 meters (200,000 ft) per second-much faster than .22-caliber bullets. "Every satellite up there could get hit," Lynch says. "But with really teeny particles." It's highly unlikely a space rock will rip a hole through a satellite. Still, cometary grains could gouge solar panels with pits, sandblast the reflective coating off mirrors, and short out satellites' electrical circuits.

The mere threat of crippled TV and phone satellites has engineers scrambling to protect precious space hardware. The Hubble Space Telescope, for example, will be angled away from any possible debris to protect its lenses, mirrors, and other optics.

Where will the spectacular fireworks show be most visible on Earth? People in Asia will have ringside seats. During the storm's peak, which usually lasts from 90 minutes to two hours, observers may witness as many as 10,000 streaking meteors. The rest of us will have to settle for a tamer shower of about 50 meteors per hour before and after the storm's peak.

SPACE INVADERS
Tiny meteoroids lighting up the sky are relatively harmless compared to incoming comets or asteroids, larger space rocks that orbit the sun. Like meteoroids, these space travelers are remnants of leftover debris from the formation of the solar system about 5 billion years ago. But unlike their benign cousins, comets and asteroids have the capability to annihilate life on Earth.

Scientists say an asteroid or comet one kilometer (0.6 mi) or more in diameter could prove large enough to cause global destruction. "Anything smaller might be a problem, but a local problem," says Donald Yeomans, director of NASA's Near-Earth Object Program Office in Pasadena, California. In other words, a smaller object slamming into Earth could wipe out a city, but spare the entire human race. With this in mind, scientists are tracking near-Earth objects (NEOs) -asteroids or comets approaching within 48 million km (30 million mi) of Earth-that could potentially hit our planet. As of August, scientists have discovered about 535 NEOs, 245 of them with diameters larger than one kilometer. Yeomans estimates about 2,000 to 2,500 NEOs have the potential to destroy all life on Earth.

In fact, one such asteroid is scheduled to approach our planet around Thanksgiving. On November 25, asteroid 1996 FG3 will pass within 5.7 million km (3.5 million mi) of Earth-almost 15 times the distance from here to the moon. "It's not dangerous," assures Yeomans. "But it's a fairly close approach."

Because there are more asteroids than comets in Earth's neighborhood, we're far more likely to get struck by an asteroid. Fortunately asteroids are also easier to track. Most of them lie between the orbits of Mars and Jupiter and follow a fairly circular path around the sun. "There's a very good chance we would discover an Earth-threatening asteroid decades prior to its predicted impact," Yeomans says.

The same doesn't apply to comets, however. Most comets stray from way beyond the farthest planets, follow highly elliptical orbits, and usually don't begin to outgas (form their telltale tails of vaporized ice and dust, due to the sun's heat) until they reach well within Jupiter's orbit, about three times the distance between Earth and the sun. Because of their stealth, new comets are often discovered only a few months before they cross Earth's orbit. And it's difficult to predict if a comet will collide with Earth, because the gases it releases can change the comet's direction. "New comets are sort of a wild card," says Yeomans. Fortunately, they're also relatively rare.

DOOMSDAY?
With such threatening neighbors, could doomsday be at hand? Are movies like Armageddon and Deep Impact foretelling the end of Earth as we know it? Not really, experts say. According to David Rabinowitz of Jet Propulsion Laboratory's NearEarth Asteroid Tracking (NEAT) team, collisions with a space rock huge enough to cause global catastrophe may occur once every 100,000 years on average.

But NASA isn't resting easy with such statistics. The space agency plans to track down 90 percent of the large (more than one kilometer) NEOs in the next 10 years. And thanks in part to last summer's Earth-slamming blockbusters, NEOtracking programs are grabbing more attention than ever before. The U.S. government has doubled funding for these programs to $3 million this year.

Just think what a sequel to Armageddon could do.
   
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