Did Earth once have two moons? It may take two lunar spacecraft to find out.

NASA's twin GRAIL spacecraft entered lunar orbit in a two-day span ending January 1st. GRAIL is short for Gravity Recovery and Interior Laboratory.

Both craft will now ease down to tighter orbits around the moon. By March, they’ll be formation-flying at low altitude. Radio waves that keep tabs on the distance between them will allow the GRAIL craft to detect variations in the moon's gravitational field as it tugs on one of them slightly more than the other.

By mapping the gravitational field, lunar scientists can tell where mass is hidden below the surface. And that should help them get a better handle on the moon's interior structure.

GRAIL may also reveal the nature of the rugged highlands on the farside of the moon. A recent hypothesis suggested that the elevated farside terrain could be the remains of an ancient, secondary moon of Earth. Such a moonlet would have collided in a giant splat against the main moon billions of years ago. If GRAIL can solve the mystery of the lunar highlands, the mission will itself be a big hit.

—John Matson

[The above text is a transcript of this podcast.]

