Like a distant relative that earthlings were just getting to know when their visits ended, the moon has continued to beckon.
In a week's time, a tiny spacecraft known as Lunar Prospector is scheduled to answer the call - the first dedicated lunar-exploration mission in 25 years.
At a time when high-profile planetary missions are beaming back detailed images of Jupiter, Mars, and asteroids, a return to the moon might seem ho-hum. Yet, some researchers say, the moon not only has a fascinating story of its own to tell, but it is also mankind's most likely proving ground for building bases on other planetary bodies.
"If, as I do believe, we'll have man back on the moon in the not-too-distant future and build a base, we have to learn to live off the land," says Alan Binder, director of the Lunar Research Institute in Gilroy, Calif., and the mission's chief architect. "We need to learn where we can get the resources" to sustain a lunar outpost.
The United States is not alone. Japan is scheduled to launch a mission in early 1999 that includes probes to burrow beneath the lunar surface. The Europeans and Russians also are said to be planning unmanned moon missions.
Making new maps
Scheduled for launch on Jan. 5 from the Kennedy Space Center at Cape Canaveral, Fla., the squat, cylindrical Lunar Prospector will spend 18 months mapping the moon's composition, magnetic fields, and gravity.
Manned Apollo orbiters, which swept across the sky along the lunar equator while a pair of astronauts collected samples and set up experiments on the surface below, mapped only about 20 percent of the moon, notes Michael Drake, director of the Lunar and Planetary Laboratory at the University of Arizona at Tucson.
"We've really only just scratched the surface," he says.
The agenda for what has become a $63 million Discovery mission was set shortly after the Apollo program ended, according to Dr. Binder. At that time, NASA set up a panel of scientists to weigh Apollo's discoveries against the range of researchers' questions. The gaps Apollo left behind became the objectives for Lunar Prospector to help meet.
The panel placed the highest priority on mapping the crust's composition. To generate those maps, the 660-pound orbiter carries three different kinds of spectrometers.
The gamma-ray spectrometer is designed to detect the radioactive decay of elements such as uranium, potassium, and thorium, as well as the unique signatures of gamma rays given off when cosmic rays interact with aluminum, iron, and silica.
The alpha-particle spectrometer, similar to those flown during Apollo missions, focuses on gases such as carbon monoxide, carbon dioxide, and nitrogen, which periodically vent from beneath the lunar surface. Such gases, Binder notes, could be tapped for life-support systems.
The neutron spectrometer has perhaps the most intriguing mission of all - to help determine if water exists on the moon. If water is present, it could be used to prepare everything from oatmeal to rocket fuel at lunar outposts.
A year ago, a Pentagon spacecraft named Clementine beamed back radar data that suggested water-ice deposits might exist deep inside the dark recesses of polar craters. Such deposits could result from comets colliding with the moon.
But researchers using the Arecibo radio telescope in Puerto Rico interpreted radar returns similar to Clementine's as simply rough surfaces.
Lunar Prospector's neutron spectrometer will attack the problem by measuring the abundance of hydrogen, one of water's two chemical elements. If it detects hydrogen concentrations at the poles in excess of concentrations deposited at the surface by the solar wind, "we'll assume the hydrogen is locked up in water," Binder says.
How it formed
Detailed information on the moon's composition, as well as information about its structure, also will shed light on the moon's origin, according to Dr. Drake.
"The popular theory for the moon's formation is that when the earth was 60 to 70 percent assembled, it was struck by an object at least as massive as Mars," he explains. The core of that "protoplanet" merged with Earth's, while debris formed a ring around Earth.
Recent modeling by a team at the University of Colorado at Boulder suggests that the moon could have formed within a year of the impact and would have accounted for about half of the debris. The rest fell back to Earth.
Using Lunar Prospector's data on the moon's composition and core and comparing them with Earth's, researchers should be able to estimate the bulk composition of the reckless protoplanet. This will help clarify giant-impact theories, Drake says.