On May 7, 1994, the end of a remarkable event occurred. On that date the Clementine spacecraft, a joint Strategic Defense Initiative Organization (SDIO) and NASA initiative, ended its mission to the Moon when a thruster misfired until it had used up all of its fuel, leaving the spacecraft spinning at about 80 RPM with no spin control. Until this point it had been a model of success, as SDIO tested sensors and spacecraft components under extended exposure to the space environment while NASA undertook scientific observations of the Moon. The observations included imaging at various wavelengths, including ultraviolet and infrared, laser ranging altimetry, and charged particle measurements.
On this anniversary of the end of this mission, it seems appropriate to reflect on it and its legacy, especially the revelation that icy asteroids may have deposited large quantities of frozen water in polar impact craters on the Moon. Announced after the completion of the mission, some scientists asserted that Clementine found evidence of the possibility of ice being buried deep in craters on the lunar South Pole where the sun never shines.
It probably arrived there as a result of asteroid impacts millions of years ago; something akin to a snowball tossed into a deep freeze and left undisturbed. Everyone had thought of the Moon as grey, lifeless, and totally devoid of anything useful in the aftermath of the Moon landings, but these newer findings suggested otherwise.
Operated on a shoestring budget, the Clementine spacecraft was launched on January 25, 1994, from the West Coast launch complex at Vandenburg Air Force Base, and achieved lunar insertion on February 21. Lunar mapping took approximately two months, but as the mission was beginning a new phase in May 1994, the spacecraft malfunctioned. In spite of this, Clementine mapped more than 90 percent of the lunar surface.
The late 1996 revelation from scientists that data returned by Clementine suggested that ice existed from an asteroid crash at the Moon’s Shackleton Crater at the South Pole re-energized lunar science. The temperature there never rises above about -170 degrees C, and any ice there could remain frozen for extremely long periods of time.
Excitement over this discovery spurred the team developing Lunar Prospector, a small, spin-stabilized craft that would “prospect” the lunar crust and atmosphere for minerals, water ice, and certain gases; map the Moon’s gravitational and magnetic fields; and learn more about the size and content of the Moon’s core. Launched on January 6, 1998, Lunar Prospector began its short-term mission to globally map the Moon.
Lunar Prospector’s most significant discovery, announced on March 5, 1998, was confirmation that something between 10 to 300 million tons of water-ice was scattered inside the craters of the lunar poles. Not only was ice found—as expected—in the Aitken Basin of the lunar South Pole, but also in the craters of the North. To many scientists’ surprise, Lunar Prospector detected nearly 50 percent more water ice in the North than in the South. From this data, mission scientists also can infer that the ice crystals must be dispersed over a large surface area: 5,000 to 20,000 square kilometers at the South Pole and 10,000 to 50,000 square kilometers at the North Pole. Based on this data, some lunar scientists have estimated that the total quantity of water ice may be close to one cubic kilometer.
To help determine how much ice might be present in July 1999, at the end of its mission, NASA deliberately crashed Lunar Prospector into a crater near the Moon’s South Pole, in the hope that detectable quantities of water would be liberated. Spectroscopic observations from ground-based telescopes did not reveal anything.
There are some scientists who question that the data from Clementine and Lunar Prospector should be interpreted as evidence of lunar ice. Observations from the Arecibo radar observatory in Puerto Rico in 2006, suggest that the purported evidence of lunar ice may actually be a false positive associated with rocks containing hydrogen ejected from young craters. If true, this does not prohibit the possibility that there might be water ice in permanently shadowed craters, but it calls into question the evidence thus far supporting that conclusion.
The Lunar Reconnaissance Orbiter (LRO) launched in 2009 was specifically designed to help resolve this debate. It had an impactor aboard, LCROSS (Lunar Crater Observation and Sensing Satellite), that directly bored into the permanently-shadowed regions near the Moon’s pole to create a crater, throwing tons of debris and potentially water ice and vapor above the lunar surface. As it did so, the LRO flew through taking measurements of hydrated minerals that suggested again that there was water on the Moon.
There remains some controversy about the nature and extent of water on the Moon; but there is agreement that additional scientific research is necessary to determine finally the answer to this mystery.
The discovery of ice on the Moon portends enormous consequences. From ice, humans could create water, oxygen, and hydrogen. The latter could be used to produce rocket fuel and generate electricity. If all of this proves out it makes human research stations on the Moon more feasible than ever before.