A major asteroid passed by the Earth this week, and another is heading this way.

An asteroid visible in the night sky

Asteroids are similar to comets and meteors. But they are larger, myriad chunks of rock up to several hundred kilometers in diameter, generally orbiting the sun between Mars and Jupiter.  There are those that approach Earth, as seen in countless home movies; and others that hit it, often ending up in museums.  Then there are those with really big impacts, seen in films.

We experienced this week what astrophysicist Ben Oppenheimer and others are calling, in cosmic terms, a close encounter. A large asteroid known as 2004-XP14 came within about 435,000 kilometers of Earth, about the distance between the Earth and the moon.

Ben Oppenheimer

The Earth has been hit before.  Astrophysicist Ben Oppenheimer says, if 2004-XP14 had hit Earth, the damage would have been felt and seen. "This rock would probably cause a crater about 10 to 20 miles in diameter. In 1908 there was a fireball that blew up over Siberia, and in fact, this caused all kinds of controversy for a long time as to what it really was. In the end it seems it was a fireball, probably a bit smaller than this one, but it wiped out about 700 square kilometers of forest -- burned trees, leveled trees -- in all directions." 

Some geologists studying impact craters have suggested asteroids may be linked to the extinction of the dinosaurs 65 million years ago.  Another major asteroid heading toward Earth is called Apophis, after the Egyptian god of chaos. 

The passing of Apophis will be a closer, says Oppenheimer. "In 2029 Apophis will pass about ten times closer to the Earth in the sky than the 2004 XP."

Estimates differ, but researchers at the U.S. space agency, NASA, say it will safely pass about one million kilometers from Earth. Nevertheless Oppenheimer says, "This will be a spectacular event, we've got to look for it."

Scientists estimate 2004 XP14 will have about 10 more passes by Earth this century. None is expected to pose a threat to the planet.