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The Doppler Gravity Experiment (DGE) will improve the best current models of the Moon's gravitational field, enabling future lunar missions to use fuel more efficiently. The Moon has a much "bumpier" gravitational field than the earth, with small anomalies due to mass concentrations (or "mascons") on the surface, and a large asymmetry due to the fact that the crust is thicker on the far side of the Moon.  

gravity map image
An example of GEOID Gravity mapping from the Clementine mission

These bumps cause an orbiting spacecraft to speed up or slow down.To hear more information about mascons, please select one of the following:

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The DGE will, in effect, draw a map of the bumps.The DGE, unlike the other experiments aboard Lunar Prospector, requires no extra instrumentation. All of the data is collected simply by communicating with the spacecraft. As the spacecraft orbits the Moon, its speed can always be determined by the Doppler effect, the same effect that causes a police siren to sound higher when the police car is moving toward you and lower when it is moving away from you.

gravity map image
An example of GEOID Gravity mapping from the Clementine mission

  The "siren," in this case, is the spacecraft's radio signal, whose frequency shifts slightly as it moves toward Earth or away from it. By tracking the velocity of the spacecraft, mission scientists can infer the forces acting upon it. For over 99 percent of the duration of the mission (excepting only periods when the engines are being fired) the only force on Lunar Prospector will be gravity.

Thus, by simply circling the Moon and sending signals back to Earth, Lunar Prospector will map the Moon's gravitational field. Lunar Prospector will complete its gravitational map in two months. However, the results of the DGE will be greatly improved if the mission extends beyond the first year. Plans for the extended mission, which will last from the end of the first year to whenever the spacecraft runs out of fuel, call for the spacecraft to descend to an altitude of 30 miles (48 km) and then 6 miles (10 km). At an altitude of 6 miles (10 km), the precision of the gravity data will be improved by a factor of over 100.

Go to Doppler Gravity Results | Go to Instrument Introduction