NASA greeted the New Year in style: the two GRAIL spacecraft arrived to the Moon on New Year's Eve and Day, and after a 40-minute braking maneuver, each entered their preliminary orbits. The probes are currently on a wide elliptical orbit and further burns will bring them down to 50 km above the surface over the next two months. Once there, they will map the gravitational field of the Moon which will help to determine it's internal structure.

The long journey to the Moon. GRAIL travelled on a low-energy transfer orbit, venturing out to the L₁ Sun-Earth libration point. It might be counterintuitive, but this route actually requires less fuel than the direct one.

Although the scientific program is just about to start, the coast phase already raised the hopes for an extended mission. The probes were not designed to survive the long darkness of the June 4^th lunar eclipse (it's a small mission after all), so originally no extension was planned. Data from the solar panels and batteries show however that they could manage the eclipse. In case of an extension, the probes would be raised first to 300 km altitude for better chances during the event an then lowered back to about 25 km over the surface. That's pretty close – the highest point on the Moon is almost 11 km, higher than Mt. Everest! The probes will also swap places: the Sun will shine from the other side of the orbit, so they both have to rotate 180 degrees. The swap is needed to keep the correct geometry of the instruments.

An extended mission would allow a second mapping with even better resolution. At the nominal altitude, the probes will only be able to measure the detailed gravitational profiles of large, complex craters but the lower altitude will provide such profiles for simple bowl-shaped craters too. Measuring the structure of the crust beneath the simplest craters will help to better understand the data from the complex craters with central peaks and multiple, terraced walls. The extended mission would last through most of December 2012.

For more details about GRAIL, check out our earlier pieces about the mission.

László Molnár

Image sources and credits:

1.) 2.) NASA

3.) Lockheed Martin