Twin space probes will enter the moon’s orbit on New Year’s weekend in an effort to measure its gravitational field and determine what lies beneath the surface, NASA said recently.

The washing machine sized twin Gravity Recovery and Interior Laboratory (GRAIL) probes were launched from Cape Canaveral Air Force Station on Sept. 10, 2011.

GRAIL-A is set to arrive in lunar orbit on Saturday December 31 at 4:21 p.m. EST, NASA said Friday. GRAIL-B will enter orbit 24 hours later at 5:05 p.m. EST on Sunday January 1 2012.

The GRAIL probes are set to give NASA a better understanding of how Earth and other rocky planets in the solar system formed.

The twin probes took off from Florida’s coast in September and will arrive at the moons orbit during the New Year’s Eve weekend but data collection won’t begin until May.