Editing Lunar Prospector (section)

=== Doppler Gravity Experiment (DGE) ===
[[File:LPgravityfield.png|thumb|A visualization of the lunar gravity field based on spherical harmonic coefficients determined from ''Lunar Prospector'' data. The left side of the image shows the far side of the Moon where the increased uncertainty in the gravity field can be seen.]]
The Doppler Gravity Experiment (DGE) was the first polar, low-altitude mapping of the lunar gravity field. The [[Clementine mission|''Clementine'' mission]] had previously produced a relatively low-resolution map, but the ''Prospector'' DGE obtained data approximately five times as detailed: the "first truly operational gravity map of the Moon".<ref>{{cite web|title=Doppler Gravity Experiment Results|url=http://lunar.arc.nasa.gov/results/dopres.htm|publisher=NASA|access-date=July 14, 2008|url-status=dead|archive-url=https://web.archive.org/web/20080712080232/http://lunar.arc.nasa.gov/results/dopres.htm|archive-date=July 12, 2008}}</ref> The practical benefits of this are more stable long-term orbits and better fuel efficiency. Additionally, the DGE data is hoped to help researchers learn more about lunar origins and the nature of the lunar core. The DGE identified three new near-side [[mass concentration (astronomy)|mass concentration]] regions.

The purpose of the ''Lunar Prospector'' DGE was to learn about the surface and internal mass distribution of the Moon. This is accomplished by measuring the [[Doppler shift]] in the [[S-band]] tracking signal as it reaches Earth, which can be converted to spacecraft accelerations. The accelerations can be processed to provide estimates of the lunar gravity field, from which the location and size of mass anomalies affecting the spacecraft orbit can be modeled. Estimates of the surface and internal mass distribution give information on the crust, lithosphere, and [[Geology of the Moon|internal structure of the Moon]].

This experiment provided the first lunar gravity data from a low polar orbit. Because line-of-sight tracking was required for this experiment, only the near-side gravity field could be estimated using this Doppler method. The experiment was a byproduct of the spacecraft [[S band]] tracking, and so has no listed weight or power requirements. The experiment was designed to give the near-side gravity field with a surface resolution of {{convert|200|km|abbr=on}} and precision of 5 [[Galileo (unit)|mGal]] (0.05&nbsp;mm/s²) in the form of [[spherical harmonic]] coefficients to degree and order 60. In the extended mission, in which the spacecraft descended to an orbit with an altitude of {{convert|50|km|abbr=on}} and then to {{convert|10|km|abbr=on}}, this resolution was expected to improve by a factor of 100 or more.

The downlink telemetry signal was transmitted at 2273&nbsp;MHz, over a ±1&nbsp;MHz bandwidth as a right-hand circularly [[Polarization (waves)|polarized]] signal at a nominal power of 5 W and peak power of 7 W. Command uplinks were sent at 2093.0542&nbsp;MHz over a ±1&nbsp;MHz bandwidth. The transponder was a standard Loral/Conic S-Band transponder. An omnidirectional antenna can be used for uplink and downlink, or a medium gain helix antenna can be used (downlink only). Since the spacecraft was spin-stabilized, the spin resulted in a bias in the Doppler signal due to the spacecraft antenna pattern spinning with respect to the Earth station of 0.417&nbsp;Hz (27.3&nbsp;mm/s) for the omnidirectional antenna, and −0.0172&nbsp;Hz (−1.12&nbsp;mm/s) for the medium gain antenna. LOS data was sampled at 5 seconds to account for the approximately 5 second spin rate of the spacecraft, leaving a residual of less than 0.1&nbsp;mm/s.

The detailed data collected has shown that for low lunar orbit the only stable or "[[Frozen_orbit|frozen orbits]]" are at inclinations near 27º, 50º, 76º, and 86º.<ref name=nasa20061106>{{cite web |title=Bizarre Lunar Orbits |url=https://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit/ |last=Bell |first=Trudy E. |date=November 6, 2006 |editor-last=Phillips |editor-first=Tony |work=Science@NASA |publisher=[[NASA]] |accessdate=2017-09-08 |archive-date=2021-12-04  |archive-url=https://web.archive.org/web/20211204040014/https://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit/ |url-status=dead }}</ref>