NASA’s lunar probes will test theory of why one side of the moon is lopsided

Another theory suggests that the backside hump is a tidal bulge. Planets and moons sport such bulges when they get tugged at — and the Earth tugs on the moon a lot. The problem: Tidal bulges tend to be symmetrical, so there should be a bulge on both sides of the moon.

Asphaug’s theory requires a very specific sequence of events some 4.5 billion years ago, when the infant Earth was a molten ball.

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Long before life appeared, rocky debris ricocheted around the early solar system. Something the size of Mars plowed into the Earth, sending huge globs of molten material hurtling into space. The largest glob coalesced into the moon. This catastrophic-impact theory of moon formation is widely accepted by scientists.

To that, Asphaug and Jutzi threw in a twist: What if a second, smaller glob of Earth-stuff also got blasted free? If it launched at a particular angle, the glob would have coalesced into a second body and drifted behind the moon in roughly the same orbit.

After a few million years, the pull of the sun would have drawn the smaller moon closer to the bigger moon. Eventually, the two bodies collided — in slow motion. A fast collision would have excavated a giant crater. But a slow collision — just the type predicted by the computer simulations — would have pancaked the small moon onto the surface, leaving evidence for GRAIL to spot.

It’s a quirk of happenstance that GRAIL will be able to test the theory at all. Zuber proposed the $400 million mission five years ago, long before Asphaug and Jutzi published their idea. Zuber wanted to probe other, more general questions: Does the moon have a solid core? How long did the moon take to cool after it formed? And did the moon once have a magnetic field?

“You might think we already know all there is to know about the moon,” said Zuber. ”Of course, that’s not the case.”

The twin GRAIL probes arrived in a high lunar orbit this weekend, but they won’t begin collecting data until March.

By then, thrusters will have dropped the pair to just 35 miles above the surface. Flying in formation — one ahead of the other — the probes will map minute fluctuations of the moon’s gravity over its entire surface. This new gravity map will be 100 to 1,000 times as accurate as current maps. From it, scientists will infer the internal structure of the moon “from crust to core,” Zuber said.

Asphaug said there’s an even better way to test the long-shot idea, though it’s one that GRAIL can’t carry out: Study rocks from the far side of the moon. The Apollo astronauts collected hundreds of pounds of moon rocks — but all of them came from the Earth-facing side.