The science objectives of the GRAIL mission are to produce a high-resolution map of the Moon's gravitational field and use it to deduce the Moon's interior structure and composition, to increase understanding of the Moon's thermal history, and ultimately to improve knowledge of how Earth and its rocky neighbors in the inner Solar System developed into the diverse worlds we see today.
The engineering objectives are to enable the science mission to be carried out. This requires two important sets of activities:
- placing two spacecraft into the same, carefully chosen lunar orbit at the right time and the right distance apart, an extremely challenging set of maneuvers never before attempted in deep space
- calibrating and removing all the "noise" that is expected to contaminate the delicate measurements
The orbit into which the two spacecraft must be placed is nearly circular and nearly over the Moon's poles. The altitude and distance separating the two spacecraft will need to vary. When flying low and close together, the spacecraft will be more sensitive to localized gravity effects, such as an individual mountain or crater. When farthest apart and at their highest altitudes, they will be more sensitive to global gravitational effects, which reveal information about the Moon's core and lower mantle. At positions in between, the spacecraft will be sensitive to regions between the crust and lower mantle.
The part of the mission that occurs at the Moon is timed to fit between two lunar eclipses which will occur on Dec. 10, 2011, and June 4, 2012, so the solar-powered spacecraft will not have to go without sunlight for the time it would take to fly through Earth's shadow. Science measurements must begin on March 8, 2012, to take full advantage of the 82 days during which the orientation of the Sun relative to the two spacecraft will allow their solar panels to face the Sun while the antennas through which the spacecraft will measure their separation point to each other.
The two GRAIL spacecraft will document variations in the Moon's gravity field by measuring subtle movements toward and away from each other, caused by those gravitational differences. In order to isolate the movements due to gravity, GRAIL scientists and engineers have been working together to determine how much spacecraft movement should be attributed to extraneous factors including pressure from sunlight -- both direct and reflected off the Moon -- from heat radiated by the Moon and by the spacecraft, by minute amounts of gas escaping from the spacecraft, and by expansion and contraction of various parts of the spacecraft in response to heating and cooling.