Scientists who study Earth's moon have two big regrets about the six Apollo missions that landed a dozen astronauts on the lunar surface between 1969 and 1972. The biggest regret, of course, is that the missions ended so abruptly, with so much of the moon still unexplored. But researchers also lament that the great triumph of Apollo led to a popular misconception: because astronauts have visited the moon, there is no compelling reason to go back.

In the 1990s, however, two probes that orbited the moon--Clementine and Lunar Prospector--raised new questions about Earth's airless satellite. One stunning discovery was strong evidence of water ice in the perpetually shadowed areas near the moon's poles. Because scientists believe that comets deposited water and organic compounds on both Earth and its moon, well-preserved ice at the lunar poles could yield clues to the origins of life. Just as important, though, was the detection of an immense basin stretching 2,500 kilometers across the moon's far side. Carved out by an asteroid or comet collision, the South Pole--Aitken Basin is a 13-kilometer-deep gouge into the lunar crust that may expose the moon's mantle. It is the largest impact crater in the entire solar system.

Thanks to rock samples collected by Apollo astronauts, lunar geologists know that impact basins on the moon's near side were created about 3.9 billion years ago. South Pole--Aitken is believed to be the moon's oldest basin, so determining its age is crucial. If it turns out to be not much older than the near-side basins, it would bolster the "lunar cataclysm" theory, which posits that Earth and its moon endured a relatively brief but intense bombardment about half a billion years after the creation of the solar system. Planetary scientists are at a loss to explain how such a deluge could have occurred.

These discoveries have put the moon back on the exploration agenda, but some scientists are unenthusiastic about the lunar missions that have been scheduled so far. The European Space Agency expects to launch a lunar orbiter called SMART-1 in March, but the craft's primary goal is to test an ion engine similar to the one already tested in NASA's Deep Space 1 mission. Lunar-A, a Japanese probe to be launched this summer, is designed to implant seismometers on the moon by hurling missile-shaped penetrators into the surface, but technical difficulties have limited the craft to only two penetrators, so the risk of failure is high. The Japanese space agency is planning a more ambitious mission named SELENE for 2005, but this lunar orbiter will not be able to answer the fundamental questions posed by the Clementine and Lunar Prospector findings. Says Alan Binder, the principal investigator for Lunar Prospector: "We need to get to the surface, dig it up and see what's there."

An upcoming series of NASA missions, called New Frontiers, will most likely include an unmanned lunar lander that could scoop up about one kilogram of rock fragments from the South Pole--Aitken Basin and then rocket the samples back to Earth for detailed analysis. Michael B. Duke, a Colorado School of Mines geologist who proposed a similar mission in 2000, says the selection of the landing site is critical. Ideally, the site would have impact melt rocks revealing both the age of the basin and the composition of the lunar mantle and would also be close enough to the South Pole so that researchers could test for the presence of ice. To minimize risk, the best solution would be to send landers to more than one site, but that might break the mission's budget, which will probably not exceed $650 million.

The notion of sending astronauts back to the moon seems even more far-fetched given NASA's money troubles. But the agency offered a glimmer of hope last October when Gary L. Martin, NASA's first "space architect," sketched out a possible next step for human exploration: positioning a small space station at the L1 Earth-moon Lagrangian point, where the gravitational pulls of Earth and its moon cancel each other out. Located only 65,000 kilometers from the moon--one sixth the distance between the moon and Earth--this point would provide easy access to the lunar surface (and to Mars as well). But with NASA still struggling to assemble the International Space Station in low-Earth orbit, nobody is expecting to see a replay of Apollo anytime soon.

   
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