Editing Lunar Prospector (section)

== Instruments ==
The spacecraft carried six instruments: a [[Gamma Ray Spectrometer]], a [[Neutron spectrometer]], a [[Magnetometer]], an [[Electron Reflectometer]], an [[Alpha Particle Spectrometer]], and a Doppler Gravity Experiment. The instruments were omnidirectional and required no sequencing. The normal observation sequence was to record and downlink data continuously.

=== Gamma Ray Spectrometer (GRS) ===
[[File:Lunar prospector instrument lp grs fs lg.gif|thumb|''Lunar Prospector'' [[Gamma Ray Spectrometer]] (GRS) ]]
[[File:Lunar Thorium concentrations.jpg|thumb|[[Thorium]] concentrations on the Moon, as mapped by ''Lunar Prospector'']]
The ''Lunar Prospector'' [[Gamma Ray Spectrometer]] (GRS) produced the first global measurements of [[gamma-ray]] spectra from the lunar surface, from which are derived the first "direct" measurements of the chemical composition for the entire lunar surface. 

The GRS was a small cylinder which was mounted on the end of one of the three {{convert|2.5|m|abbr=on}} 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 {{convert|100|km|abbr=on}} and with a surface resolution of {{convert|150|km|abbr=on}}.<ref>{{cite journal | journal = Science | date = 1998 | volume = 281 | issue = 5382 | pages = 1484–1489 | doi = 10.1126/science.281.5382.1484 | title = Global Elemental Maps of the Moon: The ''Lunar Prospector'' Gamma-Ray Spectrometer | author = D. J. Lawrence |author2=W. C. Feldman |author3=B. L. Barraclough |author4=A. B. Binder |author5=R. C. Elphic |author6=S. Maurice |author7=D. R. Thomsen | pmid = 9727970 | bibcode=1998Sci...281.1484L| doi-access =  }}</ref>

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.<ref>{{cite web|title=Iron Distribution - ''Lunar Prospector''|url=http://lunar.arc.nasa.gov/results/images/ironmap.jpg|publisher=[[NASA]]|access-date=July 14, 2008|url-status=dead|archive-url=https://web.archive.org/web/20080626081909/http://lunar.arc.nasa.gov/results/images/ironmap.jpg|archive-date=June 26, 2008}}</ref>

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 radioactive decay of elements contained in the Moon's crust; and
# elements in the crust bombarded by cosmic rays and solar wind particles.
The most important elements detectable by the GRS were uranium (U), thorium (Th), and potassium (K), radioactive elements which generate gamma rays spontaneously, and iron (Fe), titanium (Ti), oxygen (O), silicon (Si), aluminum (Al), magnesium (Mg), and calcium (Ca), elements which emit gamma rays when hit by cosmic rays or solar wind particles. The uranium, thorium, and potassium in particular were used to map the location of [[KREEP]] (potassium, rare-earth element, and phosphorus containing material, which is thought  to have developed late in the formation of the crust and upper mantle, and is therefore important to understanding lunar evolution). The GRS was also capable of detecting fast (epithermal) neutrons, which complemented the neutron spectrometer in the search for water on the Moon.

=== Neutron Spectrometer (NS) ===
[[File:Lunar prospector instrument lp ns fs lg.gif|thumb|''Lunar Prospector'' Neutron Spectrometer (NS)]]
[[File:78783main epistaS.jpg|thumb|''Lunar Prospector'' Neutron Spectrometer data showing excess hydrogen at the Moon's south pole. Magenta and dark blue show highest hydrogen concentrations.]]
Based on the ''Lunar Prospector'' Neutron Spectrometer (NS) data, mission scientists have determined that there is evidence for [[Lunar water|lunar water ice]] in the polar craters of the Moon,<ref>{{cite web|title=Neutron Spectrometer Results |url=http://lunar.arc.nasa.gov/results/neures.htm |publisher=NASA |access-date=July 14, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20080531054648/http://lunar.arc.nasa.gov/results/neures.htm |archive-date=May 31, 2008 }}</ref> an estimated 3 billion [[tonne]]s (800 billion US gallons).

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 {{convert|150|km|abbr=on}}. 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|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 {{convert|-190|C|F}}. 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 Alpha Particle Spectrometer (APS) ===
[[File:Lunar prospector instrument lp aps fs lg.gif|thumb|''Lunar Prospector'' Alpha Particle Spectrometer (APS) ]]
The Alpha Particle Spectrometer (APS) was a cube approximately {{convert|18|cm|abbr=on}} colocated with the neutron spectrometer on the end of one of the three radial {{convert|2.5|m|abbr=on}} ''Lunar Prospector'' science booms. It contained ten silicon detectors sandwiched between gold and [[aluminum disk]]s arranged on five of six sides of the cube. Alpha particles, produced by the decay of radon and polonium, leave tracks of charge on the silicon wafers when they impact the silicon. A high voltage is applied to the silicon, and the current is amplified by being funneled along the tracks to the aluminum disk and is recorded for identification. The APS was designed to make a global examination of gas release events and polonium distribution with a surface resolution of {{convert|150|km|abbr=on}} and a precision of 10%.

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.

=== 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>

=== Electron Reflectometer and Magnetometer (MAG/ER) ===
[[File:Lunar prospector instrument lp er fs lg.gif|thumb|''Lunar Prospector'' Electron Reflectometer (ER)]]
[[File:Lunar prospector instrument lp mag fs lg.gif|thumb|''Lunar Prospector'' Magnetometer (MAG)]]
[[File:Moon ER magnetic field.jpg|thumb|Total magnetic field strength at the surface of the Moon, derived from the electron reflectometer experiment.]]
The Magnetometer and Electron Reflectometer (collectively, MAG/ER) detected anomalous surface magnetic fields on the Moon, which are in stark contrast to a global [[magnetosphere]] (which the Moon lacks). the Moon's overall magnetic field is too weak to deflect the [[solar wind]], but MAG/ER discovered a small surface anomaly that can do so. This anomaly, about {{convert|100|km|abbr=on}} in diameter, has therefore been referred to as "the smallest known magnetosphere, [[magnetosheath]] and [[bow shock]] system in the Solar System."<ref>{{cite web |title=Magnetometer / Electron Reflectometer results |url=http://lunar.arc.nasa.gov/results/magelres.htm |publisher=NASA |access-date=July 14, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20100527121330/http://lunar.arc.nasa.gov/results/magelres.htm |archive-date=May 27, 2010 }}</ref> Due to this and other magnetic features of the Moon's surface, hydrogen deposited by [[solar wind]] is non-uniformly distributed, being denser at the periphery of the magnetic features. Since hydrogen density is a desirable characteristic for hypothetical lunar bases, this information may be useful in choosing optimal sites for possible long-term Moon missions.

The electron reflectometer (ER) and [[magnetometer]] (MAG) were designed to collect information on the lunar [[magnetic field]]s. 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 [[mineral]]s 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 [[electron]]s. The instrument measured the pitch [[angle]]s of [[solar wind]] electrons reflected from the Moon by lunar magnetic fields. Stronger local magnetic fields can reflect electrons with larger pitch angles.  [[Field strength]]s as small as 0.01 [[nanotesla|nT]] could be measured with a spatial accuracy of about {{convert|3|km|abbr=on}} 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 {{convert|100|km|abbr=on}} when ambient [[Plasma (physics)|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 {{convert|0.8|m|abbr=on}} boom&mdash;a combined {{convert|2.6|m|abbr=on}} from ''Lunar Prospector'' in order to isolate it from spacecraft generated magnetic fields. The ER and MAG instruments had a combined mass of {{convert|5|kg|abbr=on}} and used 4.5 [[watt]]s of power.