Lunar Prospector

Organization:	NASA
Major Contractors:	Discovery Program
Mission Type:	Planetary Science
Satellite of:	Moon
Launch:	January 7, 1998 at 02:28:44 UTC
Launch Vehicle:	Athena 2
Mission Highlight:	Entered lunar orbit January 11, 1998, 10:28 UTC
Mission Duration:	570 days - Lunar impact on July 31, 1999 at 9:52:02 UTC near lunar south pole.
Mass:	158 kg
NSSDC ID:	1998-001A
Webpage:	NASA NSSDC Master Catalog
Orbital elements
Semimajor Axis:	6,478.2 km
Eccentricity:	.00046
Inclination:	90.55°
Orbital Period:	117.9 minutes
Apogee:	101.2 km
Perigee:	99.45 km
Orbits:	~7060
Instruments
Gamma Ray Spectrometer (GRS) :	Maps of elemental abundances on the lunar surface
Lunar Prospector Neutron Spectrometer (NS) :	Detect minute amounts of water ice
Alpha Particle Spectrometer (APS) :	Detect radon outgassing events on the surface of the Moon
Doppler Gravity Experiment (DGE):	Learn about the surface and internal mass distribution of the Moon
Magnetometer (MAG):	Map weak lunar magnetic fields
Electron Reflectometer (ER):	Collect information on the lunar remnant paleomagnetic fields

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 will allow construction of a detailed map of the surface composition of the Moon, and will improve understanding of the origin, evolution, current state, and resources of the Moon.

1 Scientific experiments

2 External links

Contents

1 Spacecraft and Subsystems 2 Mission Profile

Spacecraft and Subsystems

The spacecraft is a graphite-epoxy drum, 1.37 meters in diameter and 1.28 meters high with three radial 2.5 m instrument booms. A 1.1 m extension boom at the end of one of the 2.5 m booms holds the magnetometer. Total initial mass (fully fueled) was 296 kg. It is spin-stabilized (nominal spin rate 12 rpm) with its spin axis normal to the ecliptic plane. The spacecraft is controlled by 6 hydrazine monopropellant 22-Newton thrusters, two aft, two forward, and two tangential. Three fuel tanks mounted inside the drum hold 138 kg of hydrazine pressurized by helium. The power system consists of body mounted solar cells which produce an average of 186 W and a 4.8 amp-hr rechargeable NiCd battery. Communications are through two S-band transponders, a slotted, phased-array medium gain antenna for downlink, and an omnidirectional low-gain antenna for downlink and uplink. There is no on-board computer, all control is from the ground, commanding a single on-board command and data handling unit. Data are downlinked directly and also stored on a solid-state recorder and downlinked after 53 minutes, to ensure all data collected during communications blackout periods are received.

The probe also carried a small part 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.

Mission Profile

The spacecraft was launched January 7, 1998 by a three-stage Athena 2 rocket. The mission ended July 31, 1999 when the Lunar Prospector was deliberately targeted to impact in a permanently shadowed 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 plume was observed.

Following launch on 7 January 1998 UT (6 January EST) aboard a three-stage Athena 2 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 13 January 1998) by tranfer into a 92 x 153 km preliminary mapping orbit, and then on 16 January by insertion into the near-circular 100 km 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 to 25 km from the 100 km nominal orbit, about once a month. On 19 December 1998, a maneuver lowered the orbit to 40 km to perform higher resolution studies. The orbit was altered again on 28 January to a 15 x 45 km orbit, ending the 1 year primary mission and beginning the extended mission. The mission ended on 31 July 1999 at 9:52:02 UT (5:52:02 EDT) when Lunar Prospector was deliberately targeted to impact in a permanently shadowed area of a 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 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 6 experiments: 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)

The fundamental purpose of the Gamma Ray Spectrometer (GRS) experiment will be to provide global maps of elemental abundances on the lunar surface. The GRS is designed to record the spectrum of gamma rays emitted by: 1) the radioactive decay of elements contained in the Moon's crust; and 2) elements in the crust bombarded by cosmic rays and solar wind particles. The most important elements detectable by the GRS are 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 will be 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 is also capable of detecting fast (epithermal) neutrons, which will complement the neutron spectrometer in the search for water on the Moon. The Gamma Ray Spectrometer is a small cylinder which will be mounted on the end of one of the three 2.5 m radial booms extending from the Lunar Prospector. It consists of a bismuth germanate crystal surrounded by a shield of borated plastic. Gamma rays striking the bismuth atoms produce a flash of light with an intensity proportional to the energy of the gamma ray which is 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 will be required to generate statistically significant results. At nine passes per month, it will 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 is used in the detection of fast neutrons. The GRS will achieve global coverage from an altitude of approximately 100 km and with a surface resolution of 150 km.

Neutron Spectrometer (NS)

The Lunar Prospector Neutron Spectrometer (NS) is designed to detect minute amounts of water ice which may exist on the Moon. It can detect 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 degrees 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 will also be used to measure the abundance of solar wind implanted hydrogen. The neutron spectrometer is a thin cylinder colocated with the Alpha Particle Spectrometer at the end of one of the three radial Lunar Prospector science booms. The instrument has a surface resolution of 150 km. For the polar ice studies, the NS will examine the poles to 80 degrees latitude with a sensitivity of at least 10 ppm of hydrogen. For the implanted hydrogen studies, the NS will examine the entire globe with a sensitivity of 50 ppm. The neutron spectrometer consists of two cannisters each containing helium-3 and an energy counter. Any neutrons colliding with the helium atoms will give an energy signature which will be detected and counted. One of the cannisters is 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 cannisters 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 be due to the presence of water.

The Alpha Particle Spectrometer (APS)

The Alpha Particle Spectrometer (APS) is designed to detect radon outgassing events on the surface of the Moon. The APS will record 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 will help in a determination of the style and rate of lunar tectonics. The APS is a cube approximately 18 cm on a side colocated with the neutron spectrometer on the end of one of the three radial 2.5 m Lunar Prospector science booms. It contains 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 funnelled along the tracks to the aluminum disk and is recorded for identification. The APS will make a global examination of gas release events and polonium distribution with a surface resolution of 150 km and a precision of 10%.

Doppler Gravity Experiment (DGE)

The purpose of the Lunar Prospector Doppler Gravity Experiment (DGE) is 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 modelled. Estimates of the surface and internal mass distribution give information on the crust, lithosphere, and internal structure of the Moon. This experiment will provide 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 can be estimated using this Doppler method. The experiment is a by product of the spacecraft S-band tracking, and so has no listed weight or power requirements. The experiment will give the near-side gravity field with a surface resolution of 200 km and precision of 5 mgal in the form of spherical harmonic coefficients to degree and order 60. A possible extended mission, in which the spacecraft would descend to an orbit with an altitude of 50 km and then to 10 km, would improve on this resolution by a factor of 100 or more.

The downlink telemetry signal is 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 are sent at 2093.0542 MHz over a +/- 1 MHz bandwidth. The transponder is 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). The spacecraft is spin-stabilized, the spin results 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 is 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.

Magnetometer (MAG)

The Lunar Prospector Magnetometer (MAG) will be used primarily to map weak lunar magnetic fields. There is no global lunar magnetic field, but regional crustal magnetic fields do exist. These may be paleomagnetic remnants of a former global magnetic field, or may be due to meteor impacts or other local phenomena. It is hoped that a map of the location and strengths of these fields will provide information on their origin. The experiment also may allow estimates of the size and composition of the lunar core and provide information on the lunar induced magnetic dipole. The magnetometer is located on the end of one of the three radial Lunar Prospector booms, on a 0.8 meter pole extending from the electron reflectometer. It is 2.6 meters from the Lunar Prospector in order to isolate it from spacecraft generated magnetic fields. It is a triaxial fluxgate magnetometer similar in design to the instrument used on Mars Global Surveyor. The magnetometer and electron reflectometer instruments have a combined mass of 5 kg and use 4.5 watts of power. The MAG will measure the magnetic field amplitude and direction at spacecraft altitude with a spatial resolution of about 100 km when ambient plasma disturbances are minimal.

Electron Reflectometer

The Electron Reflectometer (ER) was designed to collect information on the lunar remnant paleomagnetic fields. The Moon has no global magnetic field, but it does have weak localized magnetic fields at its surface. This experiment was to help map these fields and provide information on their origins, allow possible examination of distribution of minerals on the lunar surface, and aid in a determination of the size and composition of the lunar core.

The ER determined the location and strength of magnetic fields from the energy spectrum and direction of electrons. The instrument measured the pitch angles of solar wind electrons reflected from the Moon by lunar magnetic fields. Stronger local magnetic fields can reflect electrons with larger pitch angles.

The ER and the electronics package were located at the end of one of the three radial science booms on Lunar Prospector. (The magnetometer was in turn extended further on a 0.8 m boom which extended from the ER.) The electron reflectometer and magnetometer instruments had a combined mass of 5 kg and used 4.5 watts of power. Field strengths as small as 0.01 nT could be measured with a spatial accuracy of about 3 km at the lunar surface.

External links

• NASA/Ames Research Center: Lunar Prospector

This article or image contains materials that originally came from a NASA website. All NASA information is in the public domain, with the exception of the usage-restricted NASA logo. See the NASA copyright policy for more information.