Gravity Recovery and Interior Laboratory (GRAIL) mission

Article By:

Asmar, Sami W. Jet Propulsion Laboratory, National Aeronautics and Space Administation, Pasadena, California.

Zuber, Maria T. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts.

Last reviewed:2014

DOI:https://doi.org/10.1036/1097-8542.YB150523

Measuring the gravitational fields of the planets is a very powerful tool for remotely investigating the structure of their interiors. Planetary gravity is typically measured via radio tracking of spacecraft near their targets, either orbiting or performing a fly-by, and sensing the pull from the mass of the planet. When a spacecraft transmits a radio signal to Earth, the Doppler effect causes a shift in the frequency of the signal received at Earth that is proportional to the motion of the spacecraft. Perturbation to the trajectory by the gravitational forces causes the detectable motion. This “line-of-sight tracking” method requires that the spacecraft be in view of the Earth, so this method cannot be employed at the Moon. The Moon is synchronously locked in its orbit around Earth and shows us only its near side, with no possible Doppler tracking from the far side. Having two spacecraft, instead of one, solves this challenge. They fly in formation and track each other as first demonstrated in Earth orbit by the Gravity Recovery and Climate Experiment (GRACE) mission using twin orbiters. NASA's Jet Propulsion Laboratory provided a version of the GRACE payload to fly on the Gravity Recovery and Interior Laboratory (GRAIL) lunar mission. The gravity instrument carried on each GRAIL spacecraft conducts dual one-way ranging measurements, which are used to develop the lunar gravity field map. This required precisely measuring how much and how fast the separation distance between the two GRAIL spacecraft changes. The measurable distance is about the size of a blood cell, and the velocity is clocked to about a millionth of a meter per second.

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