NASA's GRAIL mission will map the Moon's gravity

After its planned launch this morning was canceled due to upper wind levels, a Delta II rocket carrying NASA's Gravity Recovery and Interior Laboratory (GRAIL) has been rescheduled to tomorrow morning. The GRAIL mission will incorporate two unmanned spacecraft - GRAIL-A and GRAIL-B - which will fly in formation over the Moon's surface, measuring variations in its gravity. Using this data, scientists hope to learn more about the Moon's thermal history, and how other rocky planets within the inner solar system developed.

Each of the spacecraft are about the size of a washing machine, and have a combined weight of just 1,600 pounds (726 kg). They will be riding side-by-side aboard the rocket, and will be deployed separately once it reaches orbit. GRAILs A and B should take three and a half months to reach the Moon, with A scheduled to arrive this New Year's Eve, and B following on New Year's Day of next year.

After spending five weeks establishing their lunar orbit, the two spacecraft will be maneuvered into formation, and the three month science phase of the mission will begin. During that time, the Moon will rotate three times beneath the orbiters, each 27-day rotation constituting one mapping cycle.

Using a precision formation-flying technique, the twin GRAIL spacecraft will map the moon's gravitational field, as depicted in this artist's rendering

Each time the lead spacecraft encounters a gravitational mass, it will speed up and move forward. When the second spacecraft encounters that same mass, it will also speed up and catch back up with its counterpart. By comparing the differences in the distances between the two units, for each of the three mapping cycles, variations in the Moon's gravitational fields can be determined.

Middle school students will also have the chance to obtain photos of requested lunar targets, via GRAIL's onboard MoonKAM system.

All images courtesy NASA