''Lunar Prospector'' was a spacecraft that orbited the
Moon
The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
for 19 months in 1998-99. From a low
polar orbit
A polar orbit is one in which a satellite passes above or nearly above both poles of the body being orbited (usually a planet such as the Earth, but possibly another body such as the Moon or Sun) on each revolution. It has an inclination of abo ...
, it mapped surface composition including lunar hydrogen deposits, measured
magnetic
Magnetism is the class of physical attributes that occur through a magnetic field, which allows objects to attract or repel each other. Because both electric currents and magnetic moments of elementary particles give rise to a magnetic field, m ...
and
gravity
In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...
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
The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is the point in the Southern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True South Pole to distinguish ...
.
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
ice on the Moon. Several articles on the scientific results were published in the journal ''Science''.
''Lunar Prospector'' was the third mission selected by
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
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
The Lockheed Martin Corporation is an American Arms industry, defense and aerospace manufacturer with worldwide interests. It was formed by the merger of Lockheed Corporation with Martin Marietta on March 15, 1995. It is headquartered in North ...
; 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.
Spacecraft and subsystems

The spacecraft was a graphite-epoxy drum, in diameter and high with three radial instrument booms. A extension boom at the end of one of the 2.5 m booms held the magnetometer. Total initial mass (fully fueled) was . 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
Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly hazardous unless handled in solution as, for example, hydraz ...
monopropellant
Monopropellants are propellants consisting of chemicals that release energy through exothermic chemical decomposition. The molecular bond energy of the monopropellant is released usually through use of a catalyst. This can be contrasted with biprop ...
22-newton thrusters (two aft, two forward, and two tangential). Three fuel tanks mounted inside the drum held 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 (based on
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, and Delaware General Corporation Law, incorporated in Delaware. Intel designs, manufactures, and sells computer compo ...
's
8086
The 8086 (also called iAPX 86) is a 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979, is a slightly modified chip with an external 8-bit data bus (allo ...
) with 64
kilobyte
The kilobyte is a multiple of the unit byte for Computer data storage, digital information.
The International System of Units (SI) defines the prefix ''kilo-, kilo'' as a multiplication factor of 1000 (103); therefore, one kilobyte is 1000&nbs ...
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
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.
Mission profile

Following launch on January 7, 1998 UT (January 6 EST) aboard a four-stage
Athena II rocket
A rocket (from , and so named for its shape) is a vehicle that uses jet propulsion to accelerate without using any surrounding air. A rocket engine produces thrust by reaction to exhaust expelled at high speed. Rocket engines work entirely ...
, ''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
cislunar 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 preliminary mapping orbit, and then on January 16 by insertion into the near-circular 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 to from the 100 km nominal orbit; this occurred about once per month. On December 19, 1998, a maneuver lowered the orbit to to perform higher resolution studies. The orbit was altered again on January 28 to a 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 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).
Instruments
The spacecraft carried six instruments: a
Gamma Ray Spectrometer, a
Neutron spectrometer, a
Magnetometer
A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, ...
, 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''
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 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 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)

Based on the ''Lunar Prospector'' Neutron Spectrometer (NS) data, mission scientists have determined that there is evidence for
lunar water ice in the polar craters of the Moon, an estimated 3 billion
tonne
The tonne ( or ; symbol: t) is a unit of mass equal to 1,000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton in the United States to distinguish it from the non-metric units of the s ...
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 . 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
Cadmium is a chemical element; it has chemical symbol, symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12 element, group 12, zinc and mercury (element), mercury. Like z ...
, 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 solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
.
The Alpha Particle Spectrometer (APS)

The Alpha Particle Spectrometer (APS) was a cube approximately colocated with the neutron spectrometer on the end of one of the three radial ''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 and a precision of 10%.
The APS was designed to detect
radon
Radon is a chemical element; it has symbol Rn and atomic number 86. It is a radioactive noble gas and is colorless and odorless. Of the three naturally occurring radon isotopes, only Rn has a sufficiently long half-life (3.825 days) for it to b ...
outgassing events on the surface of the Moon. The APS recorded
alpha particle
Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay but may also be produce ...
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
Sunspots are temporary spots on the Sun's surface that are darker than the surrounding area. They are one of the most recognizable Solar phenomena and despite the fact that they are mostly visible in the solar photosphere they usually aff ...
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)

The Doppler Gravity Experiment (DGE) was the first polar, low-altitude mapping of the lunar gravity field. The
''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". 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 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
The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The ''Doppler effect'' is named after the physicist Christian Doppler, who described t ...
in the
S-band
The S band is a designation by the Institute of Electrical and Electronics Engineers (IEEE) for a part of the microwave band of the electromagnetic spectrum covering frequencies from 2 to 4 gigahertz (GHz). Thus it crosses the convention ...
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.
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
magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...
(which the Moon lacks). the Moon's overall magnetic field is too weak to deflect the
solar wind
The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
, but MAG/ER discovered a small surface anomaly that can do so. This anomaly, about 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
The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
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
A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, ...
(MAG) were designed to collect information on the lunar
magnetic field
A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
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
A meteor, known colloquially as a shooting star, is a glowing streak of a small body (usually meteoroid) going through Earth's atmosphere, after being heated to incandescence by collisions with air molecules in the upper atmosphere,
creating a ...
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
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...
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
In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways:
* An electric dipole moment, electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple ...
.
The ER determined the location and strength of magnetic fields from the
energy spectrum and direction of
electron
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s. The instrument measured the pitch
angle
In Euclidean geometry, an angle can refer to a number of concepts relating to the intersection of two straight Line (geometry), lines at a Point (geometry), point. Formally, an angle is a figure lying in a Euclidean plane, plane formed by two R ...
s of
solar wind
The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
electrons reflected from the Moon by lunar magnetic fields. Stronger local magnetic fields can reflect electrons with larger pitch angles.
Field strength
In physics, field strength refers to a value in a vector-valued field (e.g., in volts per meter, V/m, for an electric field ''E'').
For example, an electromagnetic field has both electric field strength and magnetic field strength.
Field str ...
s as small as 0.01
nT 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
''Mars Global Surveyor'' (MGS) was an American Robotic spacecraft, robotic space probe developed by NASA's Jet Propulsion Laboratory. It launched November 1996 and collected data from 1997 to 2006. MGS was a global mapping mission that examined ...
. It could measure the magnetic field
amplitude
The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...
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
watt
The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
s of power.
See also
*
Lunar resources
*
Luna-Glob
Luna-Glob (, meaning ''Lunar sphere'') is a Moon exploration programme by Roscosmos meant to progress toward the creation of a fully robotic lunar base. When completed, the programme is intended to continue with crewed lunar missions, starting ...
, a current Russian lander program
*
Prospector (spacecraft)
*
Resource Prospector (rover)
*
List of artificial objects on the Moon
This is a partial list of artificial materials left on the Moon, many during the missions of the Apollo program. The table below does not include lesser Apollo mission artificial objects, such as a hammer and other tools, Laser Ranging Retroflect ...
References
External links
*
*
*
*
*
*
*
''Lunar Prospector'' Mission Profileb
NASA's Solar System Exploration*
{{Orbital launches in 1998
1999 on the Moon
Discovery Program
Missions to the Moon
NASA space probes
Spacecraft launched in 1998
Spacecraft that impacted the Moon
Spacecraft that orbited the Moon