Now, Lunar Prospector, launched 7 January, 1998, continues with a NASA mission of the low-cost "Discovery" Program, like last summer's highly successful Mars Pathfinder lander and rover. The missions of the Discovery series combine both technology-development and scientific objectives, all within a strict low-cost discipline. As a result, the Agency assumes a level of risk that would have been considered unacceptable in the great flagship missions of the past. As the Moon has by now been well-photographed by Lunar Orbiter, Apollo, and Clementine, Lunar Prospector has no imaging instruments at all, but is devoted to detailed mapping of the physical and chemical environment. The instruments include a Magnetometer, Electron Reflectometer, Gamma-Ray Spectrometer, Neutron Spectrometer, Alpha Particle Spectrometer, and a Doppler Gravity Experiment.
The simple spinning spacecraft, launched on a new low-cost solid-fueled Lockheed "Athena" rocket, was placed into a low polar, Sun-synchronous orbit, so that (except for eclipses of the Sun by the Earth) it is always in sunlight. At Full and New Moon it is also constantly in view of the Earth, but at First and Last Quarter it is over the lunar far-side and out of touch for varying periods ranging up to nearly an hour. In order to obtain the best possible spatial resolution, the 118 min orbit is at a altitude of just 100 km, which may be lowered even further, to 50 km and then even to 10 km, when the 1-year initial survey is complete. Because of perturbations by the Earth and Sun, and also the "mascons" (irregularities in the lunar gravitational field, evidently caused by clumps of high-density material), low lunar orbits are not stable for long periods and require constant correction, using small thrusters on board. Once their propellant is exhausted, Prospector will eventually crash into the Moon. Nevertheless, the entire mission could last as long as three years.
The Magnetometer and Electron Reflectometer will yield sensitive, high spatial-resolution maps of the Moon's magnetic field, with sensitivity down to 0.01 nT and 3 km resolution. Similarly, the Doppler Gravity Experiment will determine the near-side field with 200 km spatial resolution from S-band tracking of the initial orbit, ranging down to a few km in the 3-year extended mission. The Alpha Particle Spectrometer detects possible lunar outgassing events, specifically those involving radon (which decays by alpha particle emission) release, and thus searches for signs of low-level tectonic activity on the Moon. Obviously such outgassing would be of great interest in understanding the interior chemistry and structure of the Moon, and would open the door to much further exploration and investigation.
Finally, the Gamma-Ray Spectrometer and Neutron Spectrometer could lead to the discovery of lunar ice, which would likely be of critical importance in any future development of lunar resources. Such ice has in fact been reported by Clementine, based on radar echos from a deep crater near the lunar South Pole which is in perpetual shadow; yet the result is unconfirmed and remains controversial. Nevertheless, if shielded from direct sunlight, the temperature is calculated to remain low enough that any ice trapped there would not evaporate into the vacuum, not even over the 4.5 Gy age of the Moon.
The high-energy cosmic rays (mostly protons) which constantly bombard the Moon will penetrate many meters into the surface, producing complex showers of secondary radiations. These in turn strike atomic nuclei in the lunar regolith, producing myriad short-lived radionuclei, which mostly decay with the emission of characteristic gamma-ray lines, as well as stimulating the emission of numerous high-energy secondary neutrons. The gamma rays, being highly penetrating, allow us to map the elemental and isotopic composition of the Moon from orbit, to a depth of a good fraction of a meter at least. (Thus are the plodding prospector and his burro displaced, at least for airless bodies!) A start at this sort of exploration was made by the Apollo Gamma-Ray Spectrometer, but, because of the low orbital inclination dictated by the requirements of the landings, only a limited region was ever examined, excluding the poles.
The neutrons not only themselves stimulate the emission of the gamma-rays, but in a hydrogen-rich environment such as ice, they are efficiently moderated and slowed to low energies. Also, some fraction of them will be captured by hydrogen, forming deuterium, with the emission of a famous and unmistakable gamma-ray line at 2.2 MeV. Even buried under several centimeters of lunar regolith (as is almost inevitable, given the eons of meteoritic bombardment to which the Moon has been subjected), the combination of slow neutrons and the 2.2 MeV gamma-ray line will be an unambiguous indicator of ice if it exists anywhere near the surface.
It is still too early to gauge the practical consequences of such a discovery in detail, but they could well be very great. It is known from the Apollo missions that oxygen, in the form of metallic oxides, is quite abundant on the Moon. Almost every other economically important element can also be found there in useful quantities. The obvious exception, needed both to support human biology and for production of rocket fuel, is hydrogen. At this moment, hydrogen appears to be the missing key to the economic exploration and development of the inner Solar System, especially the Moon. At a cost of $83 M (including launch), and considering the cost ($10,000 per kgm, conservatively) of transport to the Moon, the discovery of, say, 0.1 cubic kilometer of ice could make Lunar Prospector one of history's better investments.