Space Travel

GRAIL Twin Spacecraft Mission to the Moon

Earthdate: 2012:0101

One of NASA's GRAIL spacecraft maneuvers into lunar orbit. Image credit: NASA/JPL-Caltech

On January 1, 2012, the second of two GRAIL (Gravity Recovery And Interior Laboratory) spacecraft nestled into orbit about our Moon.

The mission objective is one of mapping the Moon's gravity and using that data to increase our understanding of the Moon's interior and thermal history. The mission was launched September 10, 2011.

Twin GRAIL spacecraft above Moon signaling each other and Earth. Image credit: NASA/JPL-Caltech

For this mission, both GRAIL spacecraft will work together to sense changes in gravity from the Moon below as they pass over mountains, craters and maria. And then they will send this information back to Earth.

The Moon stands 384,399 kilometers (238,906 miles) away from Earth. It takes a little over one second for light and radio signals to traverse this distance. But it takes several days for a rocket to cross this gulf between our two worlds.

GRAIL made use of a nearly four-month, low-energy trans-lunar cruise by way of the Sun-Earth Lagrange point L1. This helped to reduce fuel requirements, protect instruments and reduce the velocity of the two spacecraft at lunar arrival. One key concern was to achieve an extremely low 50 kilometer (31 mile) orbit for each spacecraft. The intended separation between the spacecraft (which arrived 24 hours apart) was 175–225 kilometers (109–140 miles). With such extremely tight tolerances in the flight plan, this left almost no room for error correction. As a consequence, the launch window had a width of one second, thus providing only two opportunities for launch each day.

Twin GRAIL spacecraft working together to map Moon's surface. Image credit: NASA/JPL-Caltech

Using radio signals between the spacecraft, NASA will be able to tell the distance between them at any given moment. When one spacecraft flies over lunar features, such as craters, mountains and masses hidden beneath the surface, the range between the two GRAIL spacecraft will change a small amount.

NASA scientists will then convert this information into a high-resolution gravitational map of the Moon. From this data, scientists will be able to understand the processes beneath the surface which affected the structure of the Moon. From this, they will increase our understanding of all the terrestrial worlds in our Solar System.