Just passing by.

A giant space rock is headed our way. Is Earth doomed, or is the threat of impact just a false alarm? On September 29, a city-size asteroid (space rock that revolves around the sun and is smaller than a planet) named Toutatis (too-TAH-tiss) will pass within 1 million miles of Earth. That's only four times farther than the moon--a close shave by space standards.

But according to astronomers, Earthlings have nothing to fear from Toutatis, the largest known asteroid to ever come so near. NASA scientists use telescopes to track "Near-Earth Asteroids (NEAs)," or asteroids that cross Earth's orbit (path around the sun). "We know their trajectories [paths] well into the future," says Steven Ostro, a NASA astronomer.

Although Earth is safe from Toutatis for now, the future forecast isn't as bright. The tug of gravity (force that pulls two objects together) from the sun and planets gradually changes Toutatis's orbit. That means a few hundred thousand years from now, the 4.6 kilometer (3 mile)-long by 2.4 km (1.5 mi)-wide rock may hit Earth.

The impact wouldn't wipe out humans, but it could kill millions, says David Morrison, an astronomer at NASA. He adds, "I wouldn't want to be on Earth when it hits."

THREAT OF NEAR-EARTH ASTERIODS

  AN ASTEROID      ENTERS EARTH'S       POTENTIALLY
THE SIZE OF ...    ATMOSPHERE ...       CAUSING ...

   Dust and          Continually      Shooting stars
    pebbles

 A blue whale         Every few       Powerful shock-
                      centuries       wave traveling
                                         100 miles

 A three-mile-        Every 10            Global
   wide rock        million years       devastation

SOURCE: NASA, ERIC ASPHAUG, WALL STREET JOURNAL, AND DAVID MORRISON

Answer

About 439 new NEAs were detected between 2003 and 2004. (Answers will vary slightly.)