An artist’s rendering of the Lunar Prospector spacecraft. (Credit score: NASA/JPL)
Early on July 31, 1999, human eyes mounted with rapt eagerness on the Moon’s south pole as a spacecraft plunged to its demise in a panoramic bid to uncover whispers of water on a parched world. Though Lunar Prospector was rapidly snuffed out in a shadowy crater, it ignited a seek for the constructing blocks of life on our celestial neighbor for years to return.
Its 18-month voyage to Earth’s solely satellite tv for pc uncovered a spot with the potential to host colossal reservoirs of water ice at its poles, hinted at an iron-rich core, and pinpointed a measly magnetic area. “A voyage to rediscover the Moon,” was how NASA commentator George Diller described it as Lunar Prospector launched into the darkness above Cape Canaveral, Florida, atop an Athena II rocket at 9:28 P.M. EST on Jan. 6, 1998.
Diller was proper. Over 20 years because the final man left his bootprints in lunar mud, the Moon’s uniqueness was amply acknowledged, however its geological lifelessness was very a lot taken as a right. But because the Apollo astronauts perpetually altered the Moon’s picture, Lunar Prospector would achieve this once more.
Mission blueprints
Lunar Prospector rose from the drafting board of NASA’s low-cost Discovery Program, a marketing campaign to discover area with missions priced at lower than $150 million — a fraction of multi-billion-dollar flagship ventures just like the Viking Mars landers, the Galileo Jupiter orbiter, and Voyager’s half-century-and-counting grand tour of the outer photo voltaic system. The ultimate price ticket for Lunar Prospector got here in at a snug $62.8 million.
Coming into growth in 1995, the undertaking focused a 1997 launch for a polar-orbiting lunar mission to map the Moon’s floor composition, assess potential water-ice deposits, and characterize its magnetic and gravitational fields. Constructed by Lockheed Martin, the drum-shaped probe bristled with three equidistantly spaced instrument booms, every measuring 8 ft (2.5 meters) in size.
Six devices — an electron reflectometer, magnetometer, gamma-ray spectrometer, neutron spectrometer, alpha-particle spectrometer, and Doppler gravity experiment — would globally scour a world whose 14.6 million sq. miles (38 million sq. kilometers) equate to a complete land space barely lower than Asia.
Earlier than Lunar Prospector, lower than 25 % of the Moon had been mapped intimately. And key questions on its origin and evolution have been a clean slate.
However hopes of a mid-1997 launch evaporated as a consequence of developmental points. Additional technical maladies with the Athena II rocket pushed the launch into January 1998.
After takeoff, Lunar Prospector took 105 hours — about 4 and a half days — to journey the 240,000 miles (370,000 km) between Earth and the Moon. It slipped into an preliminary “seize” orbit on Jan. 11. It then step by step shaved its orbital interval from 11.6 hours to only underneath 2 (118 minutes, to be actual) earlier than getting into right into a polar “mapping” orbit that carried it simply 62 miles (100 km) above the rugged lunar terrain.
A yr of discoveries — and surprises — had begun.
Figuring out H2O
By March 1998, Lunar Prospector had already tentatively recognized water ice in completely shadowed craters close to the north and south poles. It did so by revealing substantial hydrogen, assumed to be locked inside water. The water signature was some 15 % stronger on the north pole than the south. Hydrogen concentrations on the poles have been additionally thrice larger than the lunar common.
Early calculations estimated that about 300 million metric tons of water ice was locked in rock on the poles, however have been later modified in September of that yr down to six billion metric tons, buried in extremely concentrated deposits beneath 18 inches (45 centimeters) of lunar regolith.
The flooring of completely shadowed craters by no means face the Solar and stay darkish in some instances for greater than 2 billion years. They have a tendency to lie at excessive latitudes the place the Moon’s axis is nearly perpendicular to the course of daylight.
One such crater, named Shoemaker, is 51 miles (32 km) large and resides east of the lunar south pole at 88.1° south latitude. Its antiquity is manifested in a closely worn rim and dozens of tiny craters that scar its inside partitions. Eternally darkish, circumstances at its ground by no means creep larger than -343 levels Fahrenheit (-173 levels Celsius).
Shoemaker at its deepest is 1.5 miles (2.5 km), sufficient to pile three skyscrapers the peak of Dubai’s Burj Khalifa on high of each other. At these depths, so-called chilly traps kind on its ink-dark ground, the place water molecules from comets or different impactors accrue and stay indefinitely.
With substantial hydrogen and water ice at each poles, the proof was persuasive that the Moon may help human colonies. “The info don’t inform us definitively the type of the water ice,” mentioned Lunar Prospector Principal Investigator Alan Binder. “Nevertheless, if the principle supply is cometary impacts … our rationalization is that we now have areas at each poles with layers of near-pure water ice.”
A deep dive
Lunar Prospector accomplished its baseline year-long mission in January 1999 and was then prolonged by m months. Its orbit was lowered to 25 miles (40 km) above the floor to collect higher-resolution information, and it was later lowered once more to 9.3 miles (15 km).
As its mission drew to an in depth, a novel plan was crafted during which NASA would crash the probe right into a completely shadowed crater to check hypotheses that water ice certainly lay underneath the floor. When the probe crashed, researchers reasoned, buried water ice could be blasted into area and visual within the ensuing plume. Shoemaker proved a really perfect candidate: Its rim was tall sufficient to be completely shadowed however low sufficient for Lunar Prospector to chart an strategy and influence trajectory, hitting the crater’s ground with near-bull’s-eye precision.
The mission’s finish and vacation spot carried added poignancy, for aboard Lunar Prospector have been the ashes of geologist Eugene Shoemaker, the crater’s namesake.
Though the chances of punching a gap deep sufficient into Shoemaker’s ground to launch a plume wealthy with water ice have been gauged at no higher than 10 %, the daring dive nonetheless captivated an earthly viewers and seized worldwide headlines.
At 4:52 A.M. EDT on July 31, 1999, Lunar Prospector hit the crater at 3,800 mph (6,100 km/h), garnering the briefest of flashes that was seen from Earth. Sadly, no water was detected, probably as a result of the probe missed its supposed goal level, impacted a rock, or just hit a patch of dry soil.
Regardless of the dearth of outcomes, it didn’t deter scientists from making an attempt once more. In October 2009, the Lunar CRater Commentary and Sensing Satellite tv for pc (LCROSS) hit Cabeus, a south pole crater, this time efficiently revealing water-ice grains within the ejected materials. Later, information from NASA’s Moon Mineralogy Mapper (M3) aboard India’s Chandrayaan-1 probe confirmed water-ice patches are current in lots of completely shadowed craters. And in late 2024, NASA’s PRIME mission will drill for ice at Shackleton Crater, which lies close to Shoemaker on the lunar south pole.
This renewed curiosity in our closest celestial companion owes a lot to Lunar Prospector. Its brief however eventful life teased us with the lure of water on a world beforehand written off as lifeless. And the Moon’s grip on humanity continues to resonate a quarter-century later as we put together for our subsequent grand journey: returning to the lunar floor to remain.
