How did Earth get its moon?

August 15, 2001 Posted: 7:30 PM EDT (2330 GMT)

http://www.cnn.com/2001/TECH/space/08/15/moon.impact/?related

CNN) -- We've admired it for centuries. We've sent probes to circle it. We've sent men to walk on it.

But we still don't know exactly how the Earth got its moon.

Now a group of scientists think they have a better handle on a popular theory about the creation of the moon.

New computer models appear to support the so-called "giant impact theory" first proposed in the mid-1970's. According to that theory, the moon formed after the Earth was rammed by a huge object, possibly something as big as Mars.

"It is known that giant collisions are a common aspect of planet formation," said Erik Asphaug of the University of California-Santa Cruz.

But earlier models couldn't reconcile an impact, or impacts, with the current composition and mass of the Earth and moon -- and with the way they are hanging in today's solar system.

Astronaut Buzz Aldrin walked on the moon in July of 1969

Using the new model, researchers at the Southwest Research Institute and the University of California at Santa Cruz, created high-resolution simulations that show that an oblique impact by an object with 10 percent of the mass of Earth could have ejected sufficient iron-free material into Earth's orbit to eventually coalesce into the moon, while also leaving the Earth with its present mass and correct initial rotation rate.

The simulation also implies that the moon formed near the very end of Earth's formation, some 4.5 billion years ago.

The moon is believed to have played an important role in making the Earth habitable because of the stabilizing effect it had on the tilt of Earth's rotation.

The findings appear in Thursday's issue of Nature.

--CNN producer Janis Winogradsky contributed to this report.