Lunar Prospector
Encyclopedia
The Lunar Prospector mission was the third selected by NASA
for full development and construction as part of the Discovery Program
. At a cost of $62.8 million, the 19-month mission was designed for a low polar orbit
investigation of the Moon
, including mapping of surface composition and possible polar ice
deposits, measurements of magnetic
and gravity fields, and study of lunar outgassing events. The mission ended July 31, 1999, when the orbiter was deliberately crashed into a crater
near the lunar south pole
in an unsuccessful attempt to detect the presence of water.
Data from the mission allowed the construction of a detailed map of the surface composition of the Moon, and helped to improve understanding of the origin, evolution, current state, and resources of the Moon. Several articles on the scientific results were published in the journal Science.
Lunar Prospector was managed out of NASA Ames Research Center
with the prime contractor Lockheed Martin
. The Principal Investigator for the mission was Dr. Alan Binder. His personal account of the mission Against all Odds is highly critical of the bureaucracy of NASA overall, and of its contractors.
The spacecraft was a graphite-epoxy drum, 1.36 m (4.5 ft) in diameter and 1.28 m (4.2 ft) high with three radial 2.5 m (8.2 ft) instrument booms. A 1.1 m (3.6 ft) extension boom at the end of one of the 2.5 m booms held the magnetometer. Total initial mass (fully fueled) was 296 kg (652.6 lb). It was spin-stabilized (nominal spin rate 12 rpm) with its spin axis normal to the ecliptic plane. The spacecraft was controlled by six hydrazine
monopropellant
22-newton thrusters (two aft, two forward, and two tangential). Three fuel tanks mounted inside the drum held 138 kg (304.2 lb) of hydrazine pressurized by helium. The power system consisted of body-mounted solar cells which produced an average of 186 W and a 4.8 A·h rechargeable NiCd battery
. 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 with 64 kilobyte
s 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 queue 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 Dr. Eugene Shoemaker (April 28, 1928 – July 18, 1997), astronomer and co-discoverer of Comet Shoemaker-Levy 9
, to the moon for a space burial
.
Following launch on January 7, 1998 UT (6 January EST) aboard a four-stage Athena II
rocket
, the Lunar Prospector had a 105 hour cruise to the Moon. During the cruise, the three instrument booms were deployed. The MAG and APS collected calibration data, while the GRS, NS, and ER outgassed for one day, after which they also collected calibration data in cis-lunar space. The craft was inserted into an 11.6-hour period capture orbit about the Moon at the end of the cruise phase. After 24 hours Lunar Prospector was inserted into a 3.5-hour period intermediate orbit, followed 24 hours later (on January 13, 1998) by transfer into a 92 km (57.2 mi) x 153 km (95.1 mi) preliminary mapping orbit, and then on 16 January by insertion into the near-circular 100 km (62.1 mi) altitude nominal lunar polar mapping orbit with an inclination of 90 degrees and a period of 118 minutes. Lunar calibration data was collected during the 11.6- and 3.5-hour orbits. Lunar mapping data collection started shortly after the 118 minute orbit was achieved. The data collection was periodically interrupted during the mission as planned for orbital maintenance burns, which took place to recircularize the orbit whenever the periselene or aposelene was more than 20 km (12.4 mi) to 25 km (15.5 mi) from the 100 km nominal orbit; this occurred about once a month. On December 19, 1998, a maneuver lowered the orbit to 40 km (24.9 mi) to perform higher resolution studies. The orbit was altered again on 28 January to a 15 km (9.3 mi) x 45 km (28 mi) orbit, ending the one year primary mission and beginning the extended mission.
The mission ended on July 31, 1999 at 9:52:02 UT (5:52:02 EDT) when Lunar Prospector was deliberately targeted to impact 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 construction as part of NASA's Discovery Program. Total cost for the mission was $62.8 million including development ($34 million), launch vehicle (~$25 million) and operations (~$4 million).
, 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.
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. This data effectively maps the distribution of various important elements across the Moon. For example, the Lunar Prospector GRS has 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 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 believed 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.
The Gamma Ray Spectrometer was a small cylinder which was mounted on the end of one of the three 2.5 m (8.2 ft) radial booms extending from the 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 100 km (62.1 mi) and with a surface resolution of 150 km (93.2 mi).
in the polar craters of the Moon, an estimated 3 billion metric tons (260 billion US gallons).
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%. The Moon has a number of permanently shadowed craters near the poles with continuous temperatures of -190 C. 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 neutron spectrometer was a thin 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 150 km (93.2 mi). 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 neutron spectrometer consisted of two canisters each containing helium-3 and an energy counter. Any 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 on the Moon's crust at a given location. Large quantities of hydrogen would likely be due to the presence of water.
activity peaking during the mission, the lunar data is obscured by solar interference. NASA has stated that the information can eventually be recovered by subtracting out the effects of the solar activity. In the meantime, however, the APS has not yielded any useful results.
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 daughter 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 a cube approximately 18 cm (7.1 in) on a side colocated with the neutron spectrometer on the end of one of the three radial 2.5 m (8.2 ft) Lunar Prospector science booms. It contained ten silicon detectors sandwiched between gold and aluminum disks 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 150 km (93.2 mi) and a precision of 10%.
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". 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 has identified three new near-side mascons (mass concentrations).
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 is required for this experiment, only the near-side gravity field could be estimated using this Doppler method. The experiment is 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 200 km (124.3 mi) 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 50 km (31.1 mi) and then to 10 km (6.2 mi), 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.
(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 100 km (62.1 mi) in diameter, has therefore been referred to as "the smallest known magnetosphere, magnetosheath
and bow shock
system in the Solar System". 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 nT could be measured with a spatial accuracy of about 3 km (1.9 mi) 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 100 km (62.1 mi) 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 0.8 m (2.6 ft) boom—a combined 2.6 m (8.5 ft) from the Lunar Prospector in order to isolate it from spacecraft generated magnetic fields. The ER and MAG instruments had a combined mass
of 5 kg (11 lb) and used 4.5 watt
s of power.
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
for full development and construction as part of the Discovery Program
Discovery Program
NASA's Discovery Program is a series of lower-cost, highly-focused American scientific space missions that are exploring the Solar System. It was founded in 1992 to implement then-NASA Administrator Daniel S. Goldin's vision of "faster, better, cheaper" planetary missions...
. At a cost of $62.8 million, the 19-month mission was designed for a low polar orbit
Polar orbit
A polar orbit is an orbit in which a satellite passes above or nearly above both poles of the body being orbited on each revolution. It therefore has an inclination of 90 degrees to the equator...
investigation of the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, including mapping of surface composition and possible polar ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
deposits, measurements of magnetic
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
and gravity fields, and study of lunar outgassing events. The mission ended July 31, 1999, when the orbiter was deliberately crashed into a crater
Shoemaker (lunar crater)
Shoemaker is a lunar crater that is located near the southern pole of the Moon, within half a crater diameter of Shackleton. It lies to the south of the crater Malapert, to the east of Haworth, and just to the west of the similar-sized Faustini. The rim of Shoemaker is circular and worn, with some...
near the lunar south pole
South Pole
The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where the Earth's axis of rotation intersects its surface. It is the southernmost point on the surface of the Earth and lies on the opposite side of the Earth from the North Pole...
in an unsuccessful attempt to detect the presence of water.
Data from the mission allowed the construction of a detailed map of the surface composition of the Moon, and helped to improve understanding of the origin, evolution, current state, and resources of the Moon. Several articles on the scientific results were published in the journal Science.
Lunar Prospector was managed out of NASA Ames Research Center
NASA Ames Research Center
The Ames Research Center , is one of the United States of America's National Aeronautics and Space Administration 10 major field centers.The centre is located in Moffett Field in California's Silicon Valley, near the high-tech companies, entrepreneurial ventures, universities, and other...
with the prime contractor Lockheed Martin
Lockheed Martin
Lockheed Martin is an American global aerospace, defense, security, and advanced technology company with worldwide interests. It was formed by the merger of Lockheed Corporation with Martin Marietta in March 1995. It is headquartered in Bethesda, Maryland, in the Washington Metropolitan Area....
. The Principal Investigator for the mission was Dr. Alan Binder. His personal account of the mission Against all Odds is highly critical of the bureaucracy of NASA overall, and of its contractors.
Spacecraft and subsystems
Hydrazine
Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...
monopropellant
Monopropellant
Monopropellants are propellants composed of chemicals or mixtures of chemicals which can be stored in a single container with some degree of safety. While stable under defined storage conditions, they react very rapidly under certain other conditions to produce a large volume of energetic gases...
22-newton thrusters (two aft, two forward, and two tangential). Three fuel tanks mounted inside the drum held 138 kg (304.2 lb) of hydrazine pressurized by helium. The power system consisted of body-mounted solar cells which produced an average of 186 W and a 4.8 A·h rechargeable NiCd battery
Nickel-cadmium battery
The nickel–cadmium battery ' is a type of rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes....
. 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
Harris Corporation
Harris Corporation is a Florida-based international communications equipment company that produces wireless equipment, electronic systems, and both terrestrial and spaceborne antennas for use in the government, defense, and commercial sectors. It is also the largest private-sector employer in...
80C86 with 64 kilobyte
Kilobyte
The kilobyte is a multiple of the unit byte for digital information. Although the prefix kilo- means 1000, the term kilobyte and symbol KB have historically been used to refer to either 1024 bytes or 1000 bytes, dependent upon context, in the fields of computer science and information...
s of EEPROM
EEPROM
EEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration...
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 queue buffer
Circular buffer
A circular buffer, cyclic buffer or ring buffer is a data structure that uses a single, fixed-size buffer as if it were connected end-to-end.This structure lends itself easily to buffering data streams.-Uses:...
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 Dr. Eugene Shoemaker (April 28, 1928 – July 18, 1997), astronomer and co-discoverer of Comet Shoemaker-Levy 9
Comet Shoemaker-Levy 9
Comet Shoemaker–Levy 9 was a comet that broke apart and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of solar system objects. This generated a large amount of coverage in the popular media, and the comet was closely observed by...
, to the moon for a space burial
Space burial
Space burial is a burial procedure in which a small sample of the cremated ashes of the deceased are placed in a capsule the size of a tube of lipstick and are launched into space using a rocket...
.
Mission profile
Athena II
The Athena II is an American small expendable launch system which was used for three launches between 1998 and 1999, and which is scheduled to return to service in 2012. It is a member of the Athena family of rockets, along with the smaller Athena I...
rocket
Rocket
A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...
, the Lunar Prospector had a 105 hour cruise to the Moon. During the cruise, the three instrument booms were deployed. The MAG and APS collected calibration data, while the GRS, NS, and ER outgassed for one day, after which they also collected calibration data in cis-lunar space. The craft was inserted into an 11.6-hour period capture orbit about the Moon at the end of the cruise phase. After 24 hours Lunar Prospector was inserted into a 3.5-hour period intermediate orbit, followed 24 hours later (on January 13, 1998) by transfer into a 92 km (57.2 mi) x 153 km (95.1 mi) preliminary mapping orbit, and then on 16 January by insertion into the near-circular 100 km (62.1 mi) altitude nominal lunar polar mapping orbit with an inclination of 90 degrees and a period of 118 minutes. Lunar calibration data was collected during the 11.6- and 3.5-hour orbits. Lunar mapping data collection started shortly after the 118 minute orbit was achieved. The data collection was periodically interrupted during the mission as planned for orbital maintenance burns, which took place to recircularize the orbit whenever the periselene or aposelene was more than 20 km (12.4 mi) to 25 km (15.5 mi) from the 100 km nominal orbit; this occurred about once a month. On December 19, 1998, a maneuver lowered the orbit to 40 km (24.9 mi) to perform higher resolution studies. The orbit was altered again on 28 January to a 15 km (9.3 mi) x 45 km (28 mi) orbit, ending the one year primary mission and beginning the extended mission.
Shoemaker (lunar crater)
Shoemaker is a lunar crater that is located near the southern pole of the Moon, within half a crater diameter of Shackleton. It lies to the south of the crater Malapert, to the east of Haworth, and just to the west of the similar-sized Faustini. The rim of Shoemaker is circular and worn, with some...
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 construction as part of NASA's Discovery Program. Total cost for the mission was $62.8 million including development ($34 million), launch vehicle (~$25 million) and operations (~$4 million).
Scientific experiments
The spacecraft carried six instruments: a Gamma Ray SpectrometerGamma Ray Spectrometer
Gamma Ray Spectrometer is a gamma-ray spectrometer on the 2001 Mars Odyssey spacecraft, a space probe orbiting the planet Mars since 2001. Part of the United State's NASA's Mars Surveyor Program, it returns geological data about Mars' surface such as identifying elements and the location of water...
, a Neutron Spectrometer, a Magnetometer
Magnetometer
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature...
, 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)
The Lunar Prospector GRSGamma Ray Spectrometer
Gamma Ray Spectrometer is a gamma-ray spectrometer on the 2001 Mars Odyssey spacecraft, a space probe orbiting the planet Mars since 2001. Part of the United State's NASA's Mars Surveyor Program, it returns geological data about Mars' surface such as identifying elements and the location of water...
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. This data effectively maps the distribution of various important elements across the Moon. For example, the Lunar Prospector GRS has 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 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
KREEP
KREEP, an acronym built from the letters K , REE and P , is a geochemical component of some lunar impact melt breccia and basalt rocks...
(potassium, rare-earth element, and phosphorus containing material, which is believed 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.
The Gamma Ray Spectrometer was a small cylinder which was mounted on the end of one of the three 2.5 m (8.2 ft) radial booms extending from the 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 100 km (62.1 mi) and with a surface resolution of 150 km (93.2 mi).
Neutron Spectrometer (NS)
Based on Lunar Prospector Neutron Spectrometer (NS) data, mission scientists have determined that there is indeed water iceIce
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
in the polar craters of the Moon, an estimated 3 billion metric tons (260 billion US gallons).
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%. The Moon has a number of permanently shadowed craters near the poles with continuous temperatures of -190 C. 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 neutron spectrometer was a thin 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 150 km (93.2 mi). 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 neutron spectrometer consisted of two canisters each containing helium-3 and an energy counter. Any 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
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
-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 on the Moon's crust at a given location. Large quantities of hydrogen would likely be due to the presence of water.
The Alpha Particle Spectrometer (APS)
The Alpha Particle Spectrometer (APS) was damaged during launch, ruining one of the five detecting faces. Additionally, due to sunspotSunspot
Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature....
activity peaking during the mission, the lunar data is obscured by solar interference. NASA has stated that the information can eventually be recovered by subtracting out the effects of the solar activity. In the meantime, however, the APS has not yielded any useful results.
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
Radon
Radon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as the decay product of uranium or thorium. Its most stable isotope, 222Rn, has a half-life of 3.8 days...
gas and its daughter product, polonium
Polonium
Polonium is a chemical element with the symbol Po and atomic number 84, discovered in 1898 by Marie Skłodowska-Curie and Pierre Curie. A rare and highly radioactive element, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores. Polonium has been studied for...
. These putative outgassing
Outgassing
Outgassing is the release of a gas that was dissolved, trapped, frozen or absorbed in some material. As an example, research has shown how the concentration of carbon dioxide in the Earth's atmosphere has sometimes been linked to ocean 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 a cube approximately 18 cm (7.1 in) on a side colocated with the neutron spectrometer on the end of one of the three radial 2.5 m (8.2 ft) Lunar Prospector science booms. It contained ten silicon detectors sandwiched between gold and aluminum disks 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 150 km (93.2 mi) and a precision of 10%.
Doppler Gravity Experiment (DGE)
The Doppler Gravity Experiment (DGE) was the first polar, low-altitude mapping of the lunar gravity field. The Clementine missionClementine mission
Clementine was a joint space project between the Ballistic Missile Defense Organization and NASA...
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". 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 has identified three new near-side mascons (mass concentrations).
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 is required for this experiment, only the near-side gravity field could be estimated using this Doppler method. The experiment is 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 200 km (124.3 mi) and precision of 5 mGal (0.05 mm/s²) in the form of spherical harmonic
Spherical Harmonic
Spherical Harmonic is a science fiction novel from the Saga of the Skolian Empire by Catherine Asaro. It tells the story of Dyhianna Selei , the Ruby Pharaoh of the Skolian Imperialate, as she strives to reform her government and reunite her family in the aftermath of a devastating interstellar...
coefficients to degree and order 60. In the extended mission, in which the spacecraft descended to an orbit with an altitude of 50 km (31.1 mi) and then to 10 km (6.2 mi), 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.
Electron Reflectometer and Magnetometer (MAG/ER)
The Magnetometer and Electron Reflectometer (collectively, MAG/ER) detected anomalous surface magnetic fields on the Moon, which are in stark contrast to a global magnetosphereMagnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
(which the Moon lacks). The Moon's overall magnetic field is too weak to deflect the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
, but MAG/ER discovered a small surface anomaly that can do so. This anomaly, about 100 km (62.1 mi) in diameter, has therefore been referred to as "the smallest known magnetosphere, magnetosheath
Magnetosheath
The magnetosheath is the region of space between the magnetopause and the bow shock of a planet's magnetosphere. The regularly organized magnetic field generated by the planet becomes weak and irregular in the magnetosheath due to interaction with the incoming solar wind, and is incapable of fully...
and bow shock
Bow shock
A bow shock is the area between a magnetosphere and an ambient medium. For stars, this is typically the boundary between their stellar wind and the interstellar medium....
system in the Solar System". Due to this and other magnetic features of the Moon's surface, hydrogen deposited by solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
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
Magnetometer
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature...
(MAG) were designed to collect information on the lunar magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
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
METEOR
METEOR is a metric for the evaluation of machine translation output. The metric is based on the harmonic mean of unigram precision and recall, with recall weighted higher than precision...
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
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
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
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
.
The ER determined the location and strength of magnetic fields from the energy spectrum and direction of electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s. The instrument measured the pitch angle
Angle
In geometry, an angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle.Angles are usually presumed to be in a Euclidean plane with the circle taken for standard with regard to direction. In fact, an angle is frequently viewed as a measure of an circular arc...
s of solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
electrons reflected from the Moon by lunar magnetic fields. Stronger local magnetic fields can reflect electrons with larger pitch angles. Field strength
Field strength
In physics, the field strength of a field is the magnitude of its vector value.In theoretical physics, field strength is another name for the curvature form...
s as small as 0.01 nT could be measured with a spatial accuracy of about 3 km (1.9 mi) at the lunar surface. The MAG was a triaxial fluxgate magnetometer similar in design to the instrument used on Mars Global Surveyor
Mars Global Surveyor
The Mars Global Surveyor was a US spacecraft developed by NASA's Jet Propulsion Laboratory and launched November 1996. It began the United States's return to Mars after a 10-year absence. It completed its primary mission in January 2001 and was in its third extended mission phase when, on 2...
. It could measure the magnetic field amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
and direction at spacecraft altitude with a spatial resolution of about 100 km (62.1 mi) when ambient plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
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 0.8 m (2.6 ft) boom—a combined 2.6 m (8.5 ft) from the Lunar Prospector in order to isolate it from spacecraft generated magnetic fields. The ER and MAG instruments had a combined mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
of 5 kg (11 lb) and used 4.5 watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
s of power.