The asteroid passing by Earth later today is going to come pretty close to Earth -- about 200,000 miles distant. But it will come even closer to the moon, missing the Earth's only natural satellite by 150,000 miles.

What would happen if a large asteroid did strike the moon? Could it knock it clear out of its orbit?

Various objects hit the moon quite frequently, creating new craters on its surface. The gif image at right shows a meteoroid hitting moon on May 2, 2006, an impact which created a crater about 14 meters wide and three deep. The small white flash on the right-hand side is the impact occurring, but at a speed seven times slower than reality. At regular speed, the flash lasted four-tenths of a second. The rock that collided with the moon was only about 10 inches across. If such a rock were to head toward Earth, it would burn up in the Earth's atmosphere and never reach the ground.

It would take another nearly moon-sized object to actually move the moon, Clark Chapman, a scientist at the Southwest Research Institute told Popsci. If that were to happen, the moon wouldn't survive the impact. Former Popsci writer Sandeep Ravindran likens it to "whacking an egg with a golf club," which could send chunks of the moon hurtling toward Earth, and potentially jeopardizing life on Earth as a result of disruption to the lunar cycle. Fortunately, no known nearby asteroids come anywhere close to that size.

As for little 2005 YU55: If it had run a collision course with the moon it would have caused a good-sized crater -- about four kilometers across, according to Don Yeomans, director of NASA's Near-Earth Object Program. That's not nothing, but it isn't much of anything, either.