The first Gravity Recovery And Interior Laboratory spacecraft (GRAIL-A) entered the Moon‘s orbit at 5pm Eastern on Saturday, by 6pm GRAIL-A was in an orbit 56 miles by 5,197 miles around the moon, which takes about 11.5 hours to complete. The second probe, GRAIL-B, is scheduled to do the same at 5:05pm Eastern time on Sunday. As of 6pm last night, GRAIL-B was 30,018 miles from the moon, closing in at 896 mph. Once in orbit, the twin probes will spend the next few weeks moving closer to the Moon, reducing the orbit time from 11.5 hours to about two hours. By March 2012, when the 82-day science phase of the mission begins, both GRAILs will be in a near-polar, near-circular orbit with an altitude of about 34 miles.
Technology aboard both spacecraft will measure changes in relative velocity, which scientists will use to create a high-resolution map of the Moon‘s gravitational surface.