SCIENCE probably played no great role in George Bush鈥檚 decision. Yet the week before his announcement, lunar science mailing lists and message boards were buzzing with excitement. From a scientific point of view, many lunar scientists clearly felt this could hardly be a better time for an announcement that the US is returning to the moon. Thirty lean years have given them lots of time to do their homework. Today, their understanding of the moon extends far beyond the discoveries of the Apollo years, thanks to a productive series of unmanned missions: Galileo in 1990, Clementine in 1994 and Lunar Prospector in 1998. And that has produced a whole new set of open questions in lunar and planetary science that researchers are keen to tackle through manned expeditions.
鈥淭here have been three major paradigm shifts since 1990,鈥� says Carl茅 Pieters, a geologist at Brown University in Providence, Rhode Island. First was the recognition of the magnitude and character of the South Pole-Aitken basin, a depression on the far side of the moon that stretches from the south pole nearly to the equator. It is the largest impact crater in the solar system and the impact that caused it may even have pierced the moon鈥檚 mantle.
The basin is heavily eroded by later impacts and could be the oldest geological feature on the moon or Earth. Being able to date it, and craters within it, would test the important theory that the early Earth and the moon experienced a cataclysmic bombardment by meteorites 4 billion years ago, which might even have killed off Earth鈥檚 first attempts at life. Such features do not survive on Earth because of resurfacing by plate tectonics.
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Pieters鈥檚 second shift stems from the discovery of hydrogen 鈥� which might mean water 鈥� at the moon鈥檚 south pole by NASA鈥檚 Clementine orbiter in 1994, a finding confirmed four years later by Lunar Prospector. This hydrogen appears to lie in permanently shadowed craters, where the temperature is as low as 鈭�200 掳C. At these temperatures, water deposited by the comets that must have landed there could have remained, without subliming away into space. 鈥淭his discovery has set people reeling,鈥� says Pieters, because it opens up the possibility of mining the ice for a water supply or for hydrogen to turn into rocket fuel.
Somewhat lost in the excitement over ice as a resource is its significance for planetary science. It would provide a unique record of cometary composition over billions of years, and perhaps show how much of Earth鈥檚 water could have come from extraterrestrial sources.
The third paradigm shift, courtesy of Lunar Prospector, was the discovery that the moon is not nearly as homogeneous as the Apollo missions鈥� observations suggested. All the missions went to an equatorial region on the near side which we now know is contaminated with material from the nearby Imbrium basin. This material is unusually high in a combination of potassium, thorium and other rare-earth elements, and was colourfully named KREEP (K for potassium, REE for rare-earth elements and P for phosphorus). One theory holds that this was the last part of the moon鈥檚 crust to crystallise, and that elements that do not go readily into crystalline forms accumulated there.
Whatever the origin of KREEP, the Apollo rock samples may well not be typical of the moon. Researchers are itching to find out what the possibly geologically different crust on the moon鈥檚 far side is like. In particular, they would like to understand how the moon cooled, by looking at any differences between the far side鈥檚 solidified lava lakes and the extensive lava seas of the near side.
For all these reasons, a thorough exploration of the South Pole-Aitken basin ranks at the very top of lunar scientists鈥� agenda. A survey of planetary scientists by the US National Research Council in 2002 put it as the third most important target in the solar system after Mars and Pluto, even for unmanned missions. In this context, visiting the moon without investigating South Pole-Aitken, as unfortunately happened with the Apollo missions, seems like visiting the Grand Canyon National Park but skipping the canyon. In fact, two groups 鈥� one headed by Michael Duke of the Colorado School of Mines, in Golden, which includes Pieters as a member, and the other headed by Jeffrey Taylor of the University of Hawaii in Honolulu 鈥� have submitted proposals to NASA for a robotic mission to the basin. On a budget of $700 million or less, it would fly in 2008 or 2009, and bring back the first lunar rock samples since those collected by the Soviet Union鈥檚 Luna 24 in 1976. But now the teams are scrambling to think how such a mission would fit into a manned space programme. 鈥淭here are other missions we could do with the same equipment,鈥� says Duke, such as preparing the way for manned flights in the same way that the Ranger and then the Surveyor missions in the early 1960s prepared the ground for Apollo. For example, robotic explorers could test the availability of ice at the south pole and measure the climatic conditions there, which so far have only been estimated from models.
A moon expedition also holds out great opportunities for some scientists who have no interest in the science of the moon itself. Paul van Susante of the Colorado School of Mines has been working on designs for an ultra-large telescope, 30 metres in diameter, that would be built into a permanently shadowed crater near the south pole. It would be an astronomer鈥檚 dream. Not only is the sky there perpetually dark, the telescope would also enjoy the benefits of a planet with negligible seismic activity and no atmosphere to blur its vision. Because of the telescope鈥檚 size and its uninterrupted viewing time, 鈥渋t would have the ability to do in 17 days all the work that the James Webb Telescope [planned successor to the Hubble Space Telescope] will do in its 10-year lifetime,鈥� van Susante says. In particular, it would vastly speed up astronomers鈥� efforts to find Earth-like planets orbiting other stars.
Not everyone is convinced, however. 鈥淭he moon is a wonderful place for astronomy. But are there places where we can do it better?鈥� asks Daniel Lester of the McDonald Observatory at the University of Texas, who has worked on design proposals for several ground-based and space-based telescopes. Our experience with Hubble and other orbiting telescopes has taught us how to keep them cold and steady, producing useful images. But some of the technology required to build a moon-based telescope does not even exist yet.
Looming large over any moon base will be its role as a practice facility for missions to Mars 鈥� possibly even a launch pad for the journey itself (see Destination Mars). A long-term simulation of a Mars mission would involve six months on the space station, a year on the moon, and six more months on the space station during which medical researchers would be able to study the long-term effects on astronauts of low gravity, isolation and harsh space conditions. Yet in case of an emergency, the astronauts would be only a four-day space flight from home.
Over the years since the moon dust settled on Eugene Cernan鈥檚 last footsteps, our neighbour in space has had to fight for attention at NASA, overshadowed by the seemingly much more interesting bodies of the outer solar system. 鈥淚t has been relegated to third-class status: Mars first, the rest of the solar system second, and the moon third,鈥� says Paul Spudis of the Applied Physics Laboratory at Johns Hopkins University in Baltimore, who is on Taylor鈥檚 moon exploration team. But with a new manned programme on the cards, that is likely to change, and lunar scientists are ecstatic about it. 鈥淚t鈥檚 just such an obvious platform for doing anything else in space,鈥� says Pieters. 鈥淚 can鈥檛 imagine the moon not being a critical part of our space endeavour.鈥�
