1998's $63 million 354-pound Lunar Prospector spacecraft's 18-month lunar orbit, was first of NASA's Discovery series of faster, better, cheaper space explorations, first to explore the moon's surface on a purely scientific mission since Apollo 17 in 1972. The mission was envisioned 11 years ago by space colonization advocate Alan Binder, its primary purpose to survey lunar resources: the elements of its rocks, its magnetic and gravity fields and the possibility of polar water ice. Prospector completed over 6,800 orbits, dipping to within 6.2 miles as its 5 instruments analyzed the moon's chemistry, gravity and magnetic fields. Prospector did everything expected - mapped the moon's surface and resources and discovered the lunar core. Reading the moon's poles, an instrument detected hydrogen's chemical signature, suggesting water's presence.
Water ice would be key
Lunar ice can provide drinking water for lunar bases near the poles, so water would not have to be brought from Earth. The water could also irrigate crops in underground lunar farms or be chemically broken down into its basic elements: oxygen for breathing and hydrogen for rocket fuel or electrical generators. The lunar base could become a launch point for interplanetary probes no longer having to lift the weight of fuel from Earth's much stronger gravity. Prospector detected large quantities of hydrogen at the moon's poles, indicating presence of water ice. Prospector discovered hydrogen when it detected neutrons emanating from the moon's surface. Their interaction with lunar soil indicated hydrogen. Most lunar hydrogen came from solar winds. Shadowed depths of the moon's polar craters are never exposed to solar winds. Hydrogen there would be from water-bearing meteors and comets. Water ice may have existed for billions of years in these shadow reservoirs of temperatures below -280 degrees F. With frozen water in the crater the collision would vaporize the ice and create a vapor of water molecules. Sunlight would quickly break the water down into hydrogen and hydroxyl, a chemical formed when ultraviolet radiation frees a hydrogen atom from water. If the vapor cloud is dense enough, sunlight will cause the hydroxyl molecules to be visible to powerful telescopes using ultraviolet radiation detectors.
Tons of buried ice?
6.6 billion tons of water ice may be buried in the top foot and a half of lunar soil at shadowed bottoms of craters in the polar regions. Prospector mapped hydrogen distribution on the moon's surface, inferring presence of water. What's the form of the hydrogen? Water ice, or implanted by solar winds? If it's water ice, it's brought by comets and meteors. Its primary mission complete, Lunar Prospector was maneuvered into a 16-mile elliptical orbit at times 6.2 miles above the moon's surface. For 6 months it took a high-resolution look at the moon to confirm previous findings. Prospector's current orbit was too low to provide a sufficiently steep shot necessary to plunge into the crater. Likelihood of hitting the crater is 99%.
The experiment has only a 10% chance of success because water ice could be anywhere in the crater rather than evenly distributed. Not finding anything doesn't mean anything. Too many things can go wrong. We might not hit enough water to make a signal. It's marginal. If we don't succeed, if we don't hit water - and there's no guarantee we'll hit it - another mission will have to fly. Crashing Prospector in a controlled way can do one more bit for science, prove it's water and save $100 million. Crashing Prospector hopes to squeeze more data from the spacecraft and determine once and for all if water ice exists in lunar soil. Estimates show as much as 200 million metric tons of water mixed in the top 18 inches of lunar soil near the poles. Only a fraction - 100 pounds (12 1/2 gallons) lasting 13 hours and taking 3 months to analyze - may be vaporized by the spacecraft's crash, enough to prove presence of water. Low on fuel, Prospector would crash anyway.
Doubts at Stanford
Not all researchers accept the premise the crash produced proof of water. A Stanford electrical engineering professor and geology professor theorize that lunar ice, if it ever existed, reacts chemically with dust to form a substance similar to water-containing minerals in a concrete paste. They contended the crash could produce a water vapor plume even with water not previously present. Binder's not impressed with their theory because the shadowed bottom of the crater is so cold it would slow such chemical reactions. Not that they're wrong, but their analysis won't hold water. It will take several months to analyze data. Prospector's impact produced sufficient heat to vaporize water ice but not enough to break down mineral crystals. If we see water vapor, it's because it hit water ice. Icy comets may have delivered water to the moon, their impact locking water ice at bottoms of craters where the sun never shines. Prospector found indirect evidence for water ice.
TRICKY MANEUVER
Guiding the spacecraft to a precise impact in the crater was a tricky maneuver but everything went on schedule. The plan was for the spacecraft to fire rockets to slow down and start a speedy dive toward a crater. After coming in at a very low angle, it was to clear a half-mile-high rim by 1/2 mile, then crash into the 2 1/2-mile-deep crater, releasing energy equal to that of crashing a 2-ton car at over 1,100 MPH, generating enough heat to melt lunar ice and create a cloud of water vapor detected by Earth and space telescopes.
24 hours before the dive, Ames radioed orders to fire the spacecraft's thrusters, putting it into an elliptical orbit ranging from 6.2 to 142 miles altitude, giving controllers the angle they need to crash into the 2 1/2-mile-deep crater without hitting the rim, with barely enough fuel, just before the
spacecraft slipped behind the moon and lost contact with Earth. It was a time-delayed command telling the spacecraft precisely when to fire its thrusters for the final blast into the crater. They ignited the final fuel reservoir and sent Lunar Prospector on a death march. 1 hour later it hit a targeted crater The 3,780-mph impact of the spacecraft expected to. with the force of a 2-ton truck traveling 1,100 miles an hour. The ejected plume, up to hundreds of kilometers above the surface, was watched by observatories around the world. Hubble, the Submillimeter Wave Astronomy Satellite in Earth orbit, earthbound McDonald Observatory in west Texas and Hawaii's Keck Telescope observed the crash.
Since Prospector's radio signals were not picked up again, the spacecraft must have hit the moon. The precise impact point, however, wasn't clear. Everything went normally, leaving every reason to believe it got to the impact site. Had the automatic rocket firing behind the moon not gone as planned, the spacecraft would have reappeared, still in orbit and sending radio signals. No dust cloud or other visible indication of impact was detected by telescopes focused on the lunar south pole. Prospector was to complete its highly successful voyage of discovery by smashing near the lunar south pole, into a shadowed crater believed to contain frozen water.
The spacecraft could explode on the moon with enough force to spiral a plume of dust and vapor into its sky. Earth and space telescopes searched the plume for water, hoping for a visible plume or dust cloud from the impact. No immediate sightings were reported from observatory telescopes focused on the moon. The spacecraft skimmed at a near-horizontal angle over the rim of a 26-mile crater near the moon's south pole, slamming it into the permanently shadowed base of the far wall. MacDonald telescopes continue taking data, recording measurements in ultraviolet light, analyzed for the chemical signature of water. Powerful Earth telescopes disappointingly detected no dust cloud. Lack of dust, however, does not mean a search for water on the moon failed. Telescopes using UV filters will continue looking for clues that water vapor was splashed into the lunar sky by the impact.
The crash site is the final resting place for USGS astronomer Eugene Shoemaker, one of the world's leading lunar impact authorities. After a car crash killed him a lipstick-size metal container filled with his ashes was glued to Prospector's brace before launch.