The gravitational field of the Moon has been determined by the tracking of radio signals emitted by orbiting spacecraft. The principle used depends on the Doppler effect, whereby the line-of-sight spacecraft acceleration can be measured by small shifts in frequency of the radio signal, and the measurement of the distance from the spacecraft to a station on Earth. Since the gravitational field of the Moon affects the orbit of a spacecraft, it is possible to use these tracking data to invert for gravitational anomalies. However, because of the Moon's synchronous rotation it is not possible to track spacecraft much over the limbs of the Moon, and the far-side gravity field is thus only poorly characterized. The gravitational acceleration of the moon is 1.63 m/s2, about 16.7% that of earth's. Because weight is a measurement in which gravitational acceleration is an important factor, things on the moon will weigh only 16.7% of which they weigh on the earth.
The major characteristic of the Moon's gravitational field is the presence of mascons, which are large positive gravitational anomalies associated with some of the giant impact basins. These anomalies greatly influence the orbit of spacecraft about the Moon, and an accurate gravitational model is necessary in the planning of both manned and unmanned missions. They were initially discovered by the analysis of Lunar Orbiter tracking data, since navigation tests prior to the Apollo program experienced positioning errors much larger than mission specifications.
The origin of mascons are in part due to the presence of dense mare basaltic lava flows that fill some of the impact basins. However, lava flows by themselves can not explain the entirety of the gravitational variations, and uplift of the crust-mantle interface is required as well. Based on Lunar Prospector gravitational models, it has been suggested that some mascons exist that do not show evidence for mare basaltic volcanism. The huge expanse of mare basaltic volcanism associated with Oceanus Procellarum does not possess a positive gravitational anomaly.
- ↑ http://www.easycalculation.com/physics/classical-physics/learn-gravitational-acceleration.php
- ↑ Paul Muller and William Sjogren (1968). "Masons: lunar mass concentrations". Science 161: 680–684. doi:10.1126/science.161.3842.680.
- ↑ A. Konopliv, S. Asmar, E. Carranza, W. Sjogren, and D. Yuan (2001). "Recent gravity models as a result of the Lunar Prospector mission". Icarus 50: 1–18. doi:10.1006/icar.2000.6573.