GOOD NIGHT, TWO MOONS?

By Jeffrey Kluger

The cow probably had a hard enough time jumping over one moon, never mind two. Yet there is a good chance, according to a new paper appearing in the journal Nature, that the moon we know so well once had a sister.

Astronomers are not sure how our moon was formed, but increasingly they have come to suspect it was born in violence, blasted away from Earth by a collision with a planet-like object at least as big as Mars. To test this theory, a research team from Tokyo University and the University of Colorado, Boulder, developed a computer program that would simulate such a cosmic crack-up and let the scientists watch it play out. As the investigators predicted, the planet--roughly half the size of Earth--was annihilated by the collision, surrounding Earth with a Saturnesque ring of rubble that coalesced into a sphere. "We expected this," says Robin Canup, a Colorado research associate involved in the study. "It was no surprise that a moon formed."

What was a surprise was what else formed. In nine of the 27 trials, the computer impact produced not just one lunar satellite but a pair of them. "If the ring took shape far enough from Earth," says Canup, "the debris separated, creating an inner moon and a second, outer one."

Alas, the two-moon system could not last long--at least on the computer. Held so tightly in the grip of Earth's gravity, the inner moon eventually grew unstable, crashing into the digital planet within 100 years. If not for this, generations of real Earthlings might have learned to dance by the light of the silvery moons.

--By Jeffrey Kluger