On Dec. 16, a large asteroid passed closer to Earth than any such event since 1969. The 1-kilometer-wide (.62 miles) space rock, named 1998 WT24, about five times farther from Earth than the Moon.

NASA's Steve Ostro used the Deep Space Network antenna at Goldstone, California to make radar images of the asteroid, which are seen in a new animation.

Scientists study asteroids so they can learn what they are made of, how dense they are, and how they rotate.

"We need to know the structure of these Earth-approaching objects in case we ever have to deflect or destroy one," says Donald Yeomans, an asteroid researcher at NASA's Jet Propulsion Laboratory.

About 500 NEOs have been found, and an equal number are thought to still lurk out there undiscovered.

In fact, 1998 WT24 gets awarded an even more fear-inducing designation: Potentially Hazardous Asteroid. PHAs are asteroids known to come within about 0.05 astronomical units (AU) of Earth's orbit (4.6 million miles) and are larger than a few hundred meters. Such objects, if they hit the planet, could destroy a city or cause widespread regional damage.

No PHAs or NEOs that are catalogued currently threaten Earth. Over time, however, their orbits around the Sun can be perturbed by the gravity of Earth and other planets, and in centuries to come, one could be deflected toward Earth.

In addition to professional studies of 1998 WT24, amateurs photographed the rock, which was visible in moderately large backyard telescopes. Another chance to see an asteroid this close will come again in 2004, then not again until 2027.

At least that's the hope.