Grail Twin Probe to Study Earth’s Moon Gravity Field

Two spacecrafts are set to enter orbit around Earth's moon over the New Year's weekend, in the latest lunar mission to measure the uneven gravity field and determine what lies beneath the moon' core.

The near-identical Grail spacecraft, short for Gravity Recovery and Interior Laboratory, which skyrocketed from the Florida coast in September, have been independently traveling to their destination and will arrive 24 hours apart.

On New Year's Eve, one of the Grail probes will fire its engine to slow down so that it could be captured into orbit. This move will be repeated by the other the following day.

The chances of the probes overshooting are slim since their trajectories have been precise, engineers said. Getting struck by a cosmic ray may prevent the completion of the engine burn and they won't get boosted into the right orbit.

After it enters orbit, the spacecraft will spend the next two months flying in formation and chasing one another around the moon until they are about 35 miles above the surface with an average separation of 124 miles. However, data collection won't begin until March, astronomers said.

"Both spacecraft have performed essentially flawlessly since launch, but one can never take anything for granted in this business," said mission chief scientist Maria Zuber of the Massachusetts Institute of Technology.

During the probe's orbit, changes in the lunar gravity field will cause them to speed up or slow down, changing the distance between them. Radio signals transmitted by the spacecraft will measure the slight distance gaps, allowing researchers to map the underlying gravity field.

These information can help scientists deduce what's beneath the lunar surface and explain why the far side of the moon is more rugged than the side that faces Earth.

While many new information about the moon is expected from the probes, the possibility of sending astronauts back may not happen soon as the Constellation program was canceled last year by the government.

Officially known as Grail-A and Grail-B, the name of the probes were taken from a contest hosted by NASA several months ago to submit new names. The probes will be christened with the winning names after the second orbit insertion.