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Lunar Prospector was a spacecraft that orbited the Moon for 19 months in 1998-99. From a low polar orbit, it mapped surface composition including lunar hydrogen deposits, measured magnetic field and gravity field fields, and studied lunar outgassing events. The mission ended July 31, 1999, when the orbiter was deliberately crashed into a crater near the lunar south pole.
Data from the mission provided detailed mapping of the surface composition of the Moon, and helped to improve understanding of the origin, evolution, current state, and resources of the Moon. The mission identified the presence of hydrogen, implying deposits of lunar water. Several articles on the scientific results were published in the journal Science.
Lunar Prospector was the third mission selected by NASA for full development and construction as part of the Discovery Program. It was managed by NASA Ames Research Center with the prime contractor being Lockheed Martin; it cost $62.8 million. The Principal Investigator for the mission was Alan Binder. His personal account of the mission, Lunar Prospector: Against all Odds, is highly critical of the bureaucracy of NASA overall, and of its contractors.
Communications were through two S band transponders, a slotted, phased-array medium-gain antenna for downlink, and an omnidirectional low-gain antenna for downlink and uplink. The on-board computer was a Harris 80C86 (based on Intel's 8086) with 64 of EEPROM and 64 kilobytes of static RAM. All control was from the ground, the computer echoing each command to the ground for verification there. Once the command was ground-verified, an "execute" command from the ground told the computer to proceed with execution of the command. The computer built telemetry data as a combination of immediate data and also read from a circular buffer which allowed the computer to repeat data it had read 53 minutes earlier. This simple solid-state recorder ensured that all data collected during communications blackout periods would be received, providing the blackout was not longer than 53 minutes.
The probe also carried a small amount of the remains of Eugene Shoemaker (April 28, 1928 – July 18, 1997), astrogeologist and co-discoverer of Comet Shoemaker-Levy 9, to the Moon for a space burial.
In 2013 an unidentified piece of space junk was discovered in an unstable orbit around the Earth, and assigned the provisional number WT1190F. After it crashed into the Indian Ocean, the object was identified as probably the translunar injector of Lunar Prospector.
The mission ended on July 31, 1999 at 9:52:02 UT (5:52:02 EDT) when Lunar Prospector was steered into a deliberate collision in a permanently shadowed area of the Shoemaker crater near the lunar south pole. It was hoped that the impact would liberate water vapor from the suspected ice deposits in the crater and that the plume would be detectable from Earth; however, no such plume was observed.
The Lunar Prospector mission was the third mission selected by NASA for full development and launch as part of NASA's Discovery Program. Total cost for the mission was $63 million including development ($34 million), launch vehicle (~$25 million) and operations (~$4 million).
The GRS was a small cylinder which was mounted on the end of one of the three radial booms extending from Lunar Prospector. It consisted of a bismuth germanate crystal surrounded by a shield of borated plastic. Gamma rays striking the bismuth atoms produced a flash of light with an intensity proportional to the energy of the gamma ray which was recorded by detectors. The energy of the gamma ray is associated with the element responsible for its emission. Due to a low signal-to-noise ratio, multiple passes were required to generate statistically significant results. At nine passes per month, it was expected to take about three months to confidently estimate abundances of thorium, potassium, and uranium, and 12 months for the other elements. The precision varies according to element measured. For U, Th, and K, the precision is 7% to 15%, for Fe 45%, for Ti 20%, and for the overall distribution of KREEP 15% to 30%. The borated plastic shield was used in the detection of fast neutrons. The GRS was designed to achieve global coverage from an altitude of approximately and with a surface resolution of .
The instrument mapped the distribution of various important elements across the Moon. For example, the Lunar Prospector GRS identified several regions with high iron concentrations.
The fundamental purpose of the GRS experiment was to provide global maps of elemental abundances on the lunar surface. The GRS was designed to record the spectrum of gamma rays emitted by:
The neutron spectrometer was a narrow cylinder colocated with the Alpha Particle Spectrometer at the end of one of the three radial Lunar Prospector science booms. The instrument had a surface resolution of . The neutron spectrometer consisted of two canisters each containing helium-3 and an energy counter. Any thermal neutrons colliding with the helium atoms give an energy signature which can be detected and counted. One of the canisters was wrapped in cadmium, and one in tin. The cadmium screens out thermal (low energy or slow-moving) neutrons, while the tin does not. Thermal neutrons are cosmic ray-generated neutrons which have lost much of their energy in collisions with hydrogen atoms. Differences in the counts between the two canisters indicate the number of thermal neutrons detected, which in turn indicates the amount of hydrogen in the Moon's crust at a given location. Large quantities of hydrogen would likely be due to the presence of water.
The NS was designed to detect minute amounts of water ice which were believed to exist on the Moon. It was capable of detecting water ice at a level of less than 0.01%. For the polar ice studies, the NS was slated to examine the poles to 80 degrees latitude, with a sensitivity of at least 10 ppm by volume of hydrogen. For the implanted hydrogen studies, the NS was intended to examine the entire globe with a sensitivity of 50 ppmv. The Moon has a number of permanently shadowed craters near the poles with continuous temperatures of . These craters may act as cold-traps of water from incoming comets and meteoroids. Any water from these bodies which found its way into these craters could become permanently frozen. The NS was also used to measure the abundance of hydrogen implanted by solar wind.
The APS was designed to detect radon outgassing events on the surface of the Moon. The APS recorded alpha particle signatures of radioactive decay of radon gas and its byproduct product, polonium. These putative outgassing events, in which radon, nitrogen, and carbon dioxide are vented, are hypothesized to be the source of the tenuous lunar atmosphere, and may be the result of the low-level volcanic/tectonic activity on the Moon. Information on the existence, timing, and sources of these events may help in a determination of the style and rate of lunar tectonics.
The APS was damaged during launch, ruining one of the five detecting faces. Additionally, due to sunspot activity peaking during the mission, the lunar data was obscured by solar interference. The information was eventually recovered by subtracting out the effects of the solar activity.
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 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 and precision of 5 mGal (0.05 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 and then to , this resolution was expected to improve by a factor of 100 or more.
The downlink telemetry signal was transmitted at 2273 MHz, over a ±1 MHz bandwidth as a right-hand circularly polarized signal at a nominal power of 5 W and peak power of 7 W. Command uplinks were sent at 2093.0542 MHz over a ±1 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 Hz (27.3 mm/s) for the omnidirectional antenna, and −0.0172 Hz (−1.12 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 mm/s.
The detailed data collected has shown that for low lunar orbit the only stable or "frozen orbits" are at inclinations near 27º, 50º, 76º, and 86º.
The electron reflectometer (ER) and magnetometer (MAG) were designed to collect information on the lunar . the Moon has no global magnetic field, but it does have weak localized magnetic fields at its surface. These may be paleomagnetic remnants of a former global magnetic field, or may be due to meteor impacts or other local phenomena. This experiment was to help map these fields and provide information on their origins, allow possible examination of distribution of on the lunar surface, aid in a determination of the size and composition of the lunar core, and provide information on the lunar induced magnetic dipole.
The ER determined the location and strength of magnetic fields from the energy spectrum and direction of . The instrument measured the pitch of solar wind electrons reflected from the Moon by lunar magnetic fields. Stronger local magnetic fields can reflect electrons with larger pitch angles. as small as 0.01 nanotesla could be measured with a spatial accuracy of about at the lunar surface. The MAG was a triaxial fluxgate magnetometer similar in design to the instrument used on Mars Global Surveyor. It could measure the magnetic field amplitude and direction at spacecraft altitude with a spatial resolution of about when ambient plasma disturbances are minimal.
The ER and the electronics package were located at the end of one of the three radial science booms on Lunar Prospector. The MAG was in turn extended further on a boom—a combined from Lunar Prospector in order to isolate it from spacecraft generated magnetic fields. The ER and MAG instruments had a combined mass of and used 4.5 of power.
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