Lunar Prospector (1998-1999)

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Overview

The Lunar Prospector was designed for a low polar orbit investigation of the moon, including mapping of surface composition and possible deposits of polar ice, measurements of magnetic and gravity fields, and study of lunar outgassing events.

Data from the 19 month mission allowed construction of a detailed map of the surface composition of the moon, and improved our understanding of the origin, evolution, current state, and resources of the moon.

The spacecraft carried 6 experiments: A Gamma Ray Spectrometer, Neutron Spectrometer, Magnetometer, Electron Reflectometer, 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.

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).

Mission

Following launch on January 7, 1998 UT 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 January 13, 1998) by transfer into a 92x153 km preliminary mapping orbit, and then on January 16 by insertion into the near-circular 100 km altitude nominal lunar polar mapping orbit with an inclination of 90° 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 re-circularize 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 December 19, 1998, a maneuver lowered the orbit to 40 km to perform higher resolution studies.

The orbit was altered again on January 28, 1999, to a 15x45 km orbit, ending the 1 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 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.

Craft

The spacecraft was a graphite-epoxy drum, 1.37 m in diameter and 1.28 m 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 held the magnetometer. It was spin-stabilized (nominal spin rate 12 rpm) with its spin axis normal to the ecliptic plane. The spacecraft was controlled by 6 hydrazine monopropellant 22-N thrusters, two aft, two forward, and two tangential. Three fuel tanks mounted inside the drum held 138 kg 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-hr 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. There was no on-board computer, all control was from the ground, commanding a single on-board command and data handling unit. Data were downlinked directly and also stored on a solid-state recorder and downlinked after 53 minutes, to ensure all data collected during communications blackout periods were received.

Experiment Summary

Gamma Ray Spectrometer (GRS)

• This 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 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 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 NS in the search for water on the moon.
• The GRS was 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 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 took 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 achieved global coverage from an altitude of approximately 100 km and with a surface resolution of 150 km.

Neutron Spectrometer (NS)

• This was designed to detect minute amounts of water ice which may exist on the moon. It could 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° 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 solar wind implanted hydrogen.
• The NS was a thin cylinder colocated with the APS at the end of one of the three radial Lunar Prospector science booms. The instrument had a surface resolution of 150 km. For the polar ice studies, the NS examined the poles to 80° latitude with a sensitivity of at least 10 ppm of hydrogen. For the implanted hydrogen studies, the NS examined the entire globe with a sensitivity of 50 ppm.
• The NS consisted of two canisters each containing helium-3 and an energy counter. Any neutrons colliding with the helium atoms gave an energy signature which was detected and counted. One of the canisters was wrapped in cadmium and one in tin. The cadmium screened out thermal (low energy or slow-moving) neutrons while the tin did 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 indicated the number of thermal neutrons detected, which in turn indicated 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.

Alpha Particle Spectrometer (APS)

• This 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 will help in a determination of the style and rate of lunar tectonics.
• The APS was a cube approximately 18 cm on a side colocated with the NS on the end of one of the three radial 2.5 m 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 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 made 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)

• This was designed to learn about the surface and internal mass distribution of the moon. This was accomplished by measuring the doppler shift in the S-band tracking signal as it reaches Earth, which can be converted to spacecraft accelerations. The accelerations can be processed to provide estimates of the lunar gravity field, from which the location and size of mass anomalies affecting the spacecraft orbit can be modeled. Estimates of the surface and internal mass distribution give information on the crust, lithosphere, and internal structure of the moon.
• This experiment provided the first lunar gravity data from a low polar orbit. Because line-of-sight tracking was required for this experiment, only the near-side gravity field 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 nor power requirements. The experiment gave 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. The extended mission, in which the spacecraft descended to an orbit with an altitude of 50 km and then to 10 km, improved on this resolution 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 could be used for uplink and downlink, or a medium gain helix antenna could be used (downlink only).
• 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/sec) for the omnidirectional antenna, and -0.0172 Hz (-1.12 mm/sec) for the medium gain antenna. LOS data was sampled at 5 sec to account for the approximately 5 second spin rate of the spacecraft, leaving a residual of less than 0.1 mm/sec.

Magnetometer (MAG)

• This was 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 was located on the end of one of the three radial Lunar Prospector booms, on a 0.8 meter pole extending from the ER. It was 2.6 m from the Lunar Prospector in order to isolate it from spacecraft generated magnetic fields. It was a triaxial fluxgate magnetometer similar in design to the instrument used on Mars Global Surveyor. The MAG measured the magnetic field amplitude and direction at spacecraft altitude with a spatial resolution of about 100 km when ambient plasma disturbances were minimal.

Electron Reflectometer (ER)

• This was designed to collect information on the lunar remnant paleomagnetic fields. The ER measured the energy spectrum and direction of electrons. This information was used to determine the location and strength of magnetic fields. The moon has no global magnetic field, but does have weak localized magnetic fields at its surface. This experiment helped to 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 and the electronics package were located at the end of one of the three radial science booms on Lunar Prospector. The ER worked by measuring 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 measured field strengths as small as 0.01 nT with a spatial accuracy of about 3 km at the lunar surface.

Craft Data