Meteor attack!

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 bum 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 19th. The showers typically occur when Earth crosses comet Tempel-Tuttle's orbit around the sun (see miniposter, p. 12). 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-HEEL-yuhn) 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 (see chart). 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 Near-Earth Asteroid Tracking (NEAT) team, collisions with a space rock huge enough to cause global catastrophe may occur once every 100,000 years on average (see timeline, p. 9).

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, NEO-tracking 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.

RELATED ARTICLE: TIMELINE

Earth's Greatest Hits

Ever since Earth formed more than 4 billion years ago, it's been pelted with rocks from outer space. Here are some of Earth's greatest hits:

212 million years ago

A comet that shattered into small pieces may have created live large impact craters on Earth, including one of the largest known (on land, the 100-km- (62-mi-) wide Manicouagan crater in Quebec, Canada. The impact may have caused the extinction of about 80 percent of species on Earth.

65 million years ago

An asteroid about: 10 to 20 km (6 to 12 mi) in diameter slammed into Mexico's Yucatan Peninsula and wiped out: half on Earth's species, including dinosaurs.

35 million years ago

An underwater crater 89-kin (55-mi) wide formed when a 1.6-km- (1-mi-) wide asteroid smashed off the coast of Virginia.

49,000 years ago

The famous Meteor Crater in Arizona may be all that's left, of a 46-m (150-ft) meteorite that slammed into Earth.

90 years ago

An asteroid exploded above The Tunguska Valley in Siberia, Flattening more than 2,000 sq. Km (800 sq. mi) of forest and Killing hundreds of reindeer.

RELATED ARTICLE: What's What In Space

                   Meteoroids                 Asteroids

SIZE          From grains of sand to      50 meters (164 ft) across
              about 50 meters (164 ft)    or bigger; Ceres, the
              across                      largest, is about 940 km
                                          (584 mi) in diameter

COMPOSITION   Rocky or metallic           Rocky or metallic (iron
                                          and nickel)

ORIGIN        All over the solar system   Asteroid belt, between
                                          Mars and Jupiter

POPULATION    Zillions (unknown)          Estimated at several
                                          million

COMMENTS      Friction from Earth's       Asteroids are materials
              atmosphere makes a          left over from the
              meteoroid glow and form a   formation of the solar
              streak of light called a    system and never
              meteor. A meteoroid that    combined to forma planet.
              reaches Earth is called a
              meteorite.

              Comets

SIZE          About I km (0.6 mi) in
              diameter on average;
              usually less than 10 km
              (6 mi) in diameter for the
              nucleus

COMPOSITION   Mixture of frozen gases
              and grains of dust

ORIGIN        Kuiper belt, just beyond
              the orbits of Neptune and
              Pluto; Oort cloud, past the
              farthest planets, about one
              light-year (9.5 trillion km)
              from the sun

POPULATION    Unknown

COMMENTS      When a comet approaches
              the sun, it develops a
              cloud of dust and vaporized
              gas, called the coma,
              around the nucleus, and
              an enormous tail of dust
              and gas.

RELATED ARTICLE: BLAST IT! (MAYBE NOT)

As asteroid is on a collision course with Earth. All life as we know it is in jeopardy. Hollywood's ready answere: Send a crew of astronauts to plant a nuclear warhead deep inside the asteroid and blow it up. At least that's the gimmick in last summer's meteoric flicks Deep Impact and Armageddon.

But is it really our best shot? Hardly-"Blowing up an asteroid is the worst thing you can do," says Donald Yeomans of NASA's Near-Earth Object Program Office in California. "Then you've got a shotgun blast instead of a bullet." In other words, nuking one huge chunk of rock might blow it into smithereens, but it could also unleash a torrent of shattering debris headed straight toward us on Earth.

A safer bet may be to gently nudge the asteroid "a few millimeters a second," says Yeomans. His scenario: First, NASA would learn to identify any Earth-threatening object a decade or two prior to impact. Then, NASA might send an unmanned spacecraft to set off a nuclear blast in space, right in front of the asteroid, but not directly on its surface. The blast would vaporize the front side of the asteroid, but would nudge its orbit out of harm's way. "The resulting rocket-like thrust on the asteroid slows it down enough so that in 20 years or so, it will miss the Earth," Yeomans explains.

But what if we had, say, only six months warning? The possibility may be unlikely, but so far scientists have tracked only 10 percent of potentially hazardous near-Earth objects. "I doubt there's a ready solution or any ready hardware," say Robert McMillan, an astrophysicist with the Lowell Observatory NEO Survey program in Flagstaff, Arizona. In other words, duck!

RELATED ARTICLE: hands on science

CREATE A CRATER

When a large meteorite slams into Earth, it often leaves behind a crater. Do all meteorites leave the same imprint? Try this.

You need:

newspapers * shoebox * 4 cups flour * 4 cups salt * index card * cinnamon or paprika * different sized marbles * meterstick * ruler

To do:

1. Spread newspapers on the floor in front of a wall. Then set the shoebox on the papers against the wall.

2. Tape a meterstick on the wall next to the shoebox.

3. Mix the flour and salt in the shoebox. Sweep the index card over the mixture to even out the top. Sprinkle cinnamon or paprika over the mixture to help craters show up better.

4. Hold a marble at the top of the meterstick. Drop it in the shoebox. Carefully pick up the marble.

5. Use a ruler to measure the crater's diameter and depth. Record measurements on a piece of paper.

6. Select a bigger marble and repeat steps 4 and 5. Do the craters look the same? Are they the same size?

7. Pick a smaller marble than the first one. Repeat steps 4 and 5. How is this crater different from the others?

Conclusions:

Which marble made the deepest crater? Which one made the largest crater?

Don't stop now!

Most meteorites hit Earth at an angle. Does the angle at which a meteorite hits affect the size and shape of a crater? Find out.

RELATED ARTICLE: THE SHOWER COMETH

On November 17, the Leonid meteor shower will display the most spectacular sky show in more than 30 years. The shower's source, comet Tempel-Tuttle, had a close brush with the sun earlier this year, causing the comet to shed a thicker-than-normal dust trail. Here's a view of the comet's elliptical orbit in our solar system.

RELATED ARTICLE: Where do comets come from?

Astronomers believe a large population of comets reside beyond the farthest planets in our solar system, about 5 to 9.5 trillion km from the sun. This cometary "residence" is called the Oort cloud after Dutch astronomer Jan Oort, who first suggested its existence. Comets originating from the Oort cloud take about 100,000 to 1 million years to complete one orbit around the sun. A few comets, however, exist within the solar system, just outside the orbit of Neptune. This region, called the Kuiper belt (not shown) is home to comets with shorter orbital periods, like Comet Tempel-Tuttle, whose orbit takes it near the sun every 33 years.