Rediscovering the Moon: 20 Years Since Lunar Prospector

Twenty years ago, NASA’s Lunar Prospector spacecraft began an 18-month mission of discovery, revealing the Moon to potentially harbor vast water-ice reserves at its poles and offering tantalizing clues of a small, iron-rich core and the most diminutive magnetic field then known to exist in the Solar System. “A voyage to rediscover the Moon” was Public Affairs Officer (PAO) George Diller’s remark at 9:28 p.m. EST on 6 January 1998, as a four-stage Athena 2 booster smoothly delivered Lunar Prospector into the night from Space Launch Complex (SLC)-46 at Cape Canaveral Air Force Station, Fla. Diller’s words echoed the fact that a full quarter-century had passed since humans last flew to the Moon and underscored a reality that lunar exploration had gone almost unnoticed. The astonishing discoveries of Lunar Prospector caused a stark re-evaluation of scientific thinking about our nearest celestial neighbor. Indeed, its findings continue to resonate today, as efforts to settle the Moon with humans gather pace.

Lunar exploration with machines and humans, which began in the late 1950s, saw its genesis as an offshoot of Cold War geopolitics between the United States and the Soviet Union. The Russians sent the first automated probe to the Moon and photographed the never-before-seen farside, as well as achieving the first successful soft-landing on the lunar surface. Meanwhile, America despatched a fleet of Lunar Orbiters and Rangers to extensively map the Moon, and seven Surveyors to touch down on the surface, precursors of the triumphant first footsteps by humans during Project Apollo. In 1990, Japan became the third nation to launch a homegrown lunar spacecraft, after which NASA’s Clementine orbiter saw the United States return to the Moon for the first time in over two decades.

Lunar Prospector rose from the drawing board and into planning, definition and production through NASA’s low-cost “Discovery Program”, which officially came into being in the early 1990s. Its goal was to develop limited-scope deep-space missions for less than $150 million, in stark contrast to the agency’s previous flagship endeavors. The first Discovery mission was the Near Earth Asteroid Rendezvous (NEAR) Shoemaker, launched in February 1996, and the second was Mars Pathfinder, which rose from Earth in December 1996 and delivered the Sojourner rover onto the surface of the Red Planet in July 1997. Lunar Prospector’s cost came in at $62.8 million, with the intention that it would conduct low-polar-orbiting exploration of the Moon, mapping its surface constituents, its potential water-ice deposits, its magnetic and gravitational fields and global “outgassing” events. Before Lunar Prospector, over 75 percent of the Moon was not mapped in detail and fundamental questions about its formation and evolution remained unanswered.

In late 1994, NASA solicited proposals for the third round of Discovery missions and in February 1995 Lunar Prospector was judged “mature enough” to enter full development and construction. Early plans called for it to be launched in June 1997 and in March of that year the construction of the spacecraft was completed by prime contractor Lockheed Martin in Sunnyvale, Calif. Physically, it was drum-shaped, some 4.6 feet (1.4 meters) in diameter and 4 feet (1.3 meters) tall, bristling with three equidistantly-spaced instrument booms, each measuring 8 feet (2.5 meters) in length. An extension on one of these booms positioned a magnetometer at a suitable distance to ensure no interference from the spacecraft’s intrinsic magnetic field. Spin-stabilised, Lunar Prospector weighed 650 pounds (295 kg) and was equipped with six hydrazine thrusters for maneuvering and body-mounted solar cells for electrical power. In order to reduce costs, the spacecraft had no on-board computer and instead was controlled from Earth, with scientific data downloaded continuously.

It carried six primary instruments, including the combined electron reflectometer and magnetometer to examine lunar magnetic fields. A gamma-ray spectrometer was tasked with performing the first “global” measurements of gamma-ray spectra from the surface, thereby deriving clues about the chemical composition. Together with an adjoining experiment, the neutron spectrometer, Lunar Prospector would search for evidence of hydrogen concentrations and perhaps water-ice in the shadowed polar regions. Rounding out the spacecraft’s scientific toolkit were an alpha particle spectrometer to examine the Moon’s tenuous “atmosphere” and tectonic activity and a Doppler gravity experiment to determine internal mass distribution within the surface.

By the time Lunar Prospector emerged from Lockheed Martin’s manufacturing facility and entered a lengthy phase of functional and environmental testing, its launch had slipped to late September 1997. This was again rescheduled for late November, due to the need for additional testing of the four-stage Athena 2 launch vehicle. Built by Lockheed, the Athena’s first flight in August 1995 suffered a control system failure and the booster was remotely destroyed by the Range Safety Officer. Its second mission in August 1997 successfully delivered NASA’s Lewis spacecraft into orbit, although the payload later failed, due to a design flaw in its attitude-control system. In late October 1997, Lunar Prospector’s launch was rescheduled again for no sooner than 5 January 1998, in order to allow adequate time to test, review and prepare the Athena 2 vehicle.

In the meantime, Lunar Prospector arrived in Florida for processing in early December 1997 and was loaded aboard the four-stage booster. Standing 93 feet (28.2 meters) tall, the Athena 2 boasted three solid-fueled lower stages and a hydrazine-fed fourth stage and could inject up to 4,500 pounds (2,000 kg) into Low-Earth Orbit (LEO) and up to 1,300 pounds (600 kg) to Geostationary Transfer Orbit (GTO). During its short lifetime, the Athena family saw seven launches—between August 1995 and September 2001—and an overall success rate a little over 70 percent. On the night of 6 January 1998, it smoothly delivered Lunar Prospector on a 105-hour voyage to the Moon. During the translunar coast, the spacecraft’s instrument booms were deployed and it completed a 32-minute engine firing at 7:17 a.m. EST on 11 January to settle into an initial “capture” orbit.

Circling the Moon once every 11.6 hours, Lunar Prospector’s orbit was modified several times over the next few days. Firstly, it was moved into an “intermediate” orbit, with a period of 3.5 hours, during which time lunar calibration data was collected, before it was established in a preliminary mapping orbit on 13 January, which carried it as low as 57 miles (92 km) and as high as 95 miles (153 km) above the surface. Three days later, the spacecraft moved into a near-circular lunar-polar-mapping orbit at an altitude of 62 miles (100 km), inclined 90 degrees and circling the Moon every 118 minutes. Over the course of the next year, Lunar Prospector would periodically execute station-keeping burns to recircularise this orbit.

The mission revealed our nearest celestial neighbor in a new light. Early in March 1998, NASA announced the first tentative finding of water-ice, hidden beneath the lunar regolith in permanently-shadowed craters near the north and south poles. Graphs of data ratios from the NS instrument revealed significant dips, which Principal Investigator Dr. Alan Binder explained was “the kind of data signature one would expect to find if water is present”. It was conservatively suggested that 300 million metric tons of ice might exist at the lunar poles, although later estimates suggested that up to six billion metric tons could be buried under 18 in (45 cm) of soil, in more concentrated deposits than originally thought. Water signatures were 15 percent stronger at the north pole, as opposed to the south.

Dr. Binder cautioned that the results were indicative of “significant hydrogen enrichment” and “a telltale signature of water-ice” and noted that improved lunar models conclusively demonstrated that large amounts of hydrogen exist at the Moon’s poles. “The data do not tell us definitively the form of the water-ice,” he explained. “However, if the main source is cometary impacts, as most scientists believe, our explanation is that we have areas at both poles with layers of near-pure water ice.”

Lunar Prospector data also indicated that the Moon might possess a small, iron-rich core, between 140-280 miles (220-450 km) in radius, towards the smaller end of theories at the time. This preliminary conclusion lent some support to hypotheses that much of the Moon was torn away from Earth when a Mars-sized planetesimal impacted in the distant past. The data indicated that the core contained just four percent of the Moon’s total mass, a stark contrast to our own core, which represents about 30 percent of Earth’s mass. The spacecraft also developed the first global maps of lunar elemental composition, delineating large variations of thorium, potassium and iron, and revealed magnetized rocks and one of the smallest magnetospheres in the known Solar System. “The Moon was previously interpreted as just an unmagnetized body without a major effect on what is going on in the solar wind,” explained Dr. Mario Acuna, a member of the team located at NASA’s Goddard Space Flight Center, Greenbelt, Md. “We are discovering that there is nothing simple about the Moon as an obstacle to this continuous flow of electrically charged gas from the Sun.” As well as developing a gravity map of the entire surface, Lunar Prospector found seven previously unknown “mass concentrations” (or “mascons”) on the Moon’s nearside, which are known to create gravitational anomalies.

The spacecraft completed its baseline year-long mission in January 1999 and was extended by six months. By this stage, its orbit had been lowered to 25 miles (40 km) to acquire higher-resolution data and eventually to a path which carried it as low as 9.3 miles (15 km) above the surface. Originally intended to crash into the Moon, a novel concept was developed to deliberately target the spacecraft into one of the permanently shadowed polar craters, in an attempt to detect the plume of water-ice. At 4:52 a.m. EDT on 31 July 1999, it impacted Shoemaker Crater, close to the south pole, at an estimated velocity of 3,800 mph (6,100 km/h). It proved a poignant moment, for some of the ashes of U.S. planetary geologist Eugene Shoemaker—the crater’s namesake—were aboard Lunar Prospector.

Shoemaker Crater, which measures over 30 miles (50 km) in diameter, possesses an outer rim which is high enough to place it in permanent shadow, yet was low enough to allow Lunar Prospector to complete an approach and impact trajectory. It was hoped that the impact would liberate water vapor from suspected ice deposits in the crater and that the resultant plume would be observable from Earth. Scientists cautioned that the chance of success was no greater than 10 percent. According to NASA “no observable signature of water” was detected. Possible explanations included the spacecraft missing its intended target, hitting a rock or dry soil or perhaps water molecules were firmly bound in rocks as hydrated minerals, rather than existing as free ice crystals. However, the end of Lunar Prospector unveiled the Moon in a totally different light and the identification of hydrogen, mascons and the potential for water carries great potential for the establishment of lunar bases and the return of human bootprints to lunar soil.

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