A new flight plan: back to the moon.

After going in circles around our own planet for more than 30 years, astronauts may finally have gotten permission to leave home. In January, President Bush unveiled an ambitious plan for a manned mission to Mars, using the moon as a testing area and stepping-stone. But for many planetary scientists, the moon is a desirable destination in and of itself. Although the Apollo missions brought back nearly 400 kilograms of rocks, scientists still know precious little about the moon's topography, gravitational field, and overall composition.

Both manned and robotic missions could greatly expand that limited view, says James Head of Brown University in Providence, R.I. Filling in those knowledge gaps could not only make the lunar face more familiar but also provide new insight about the early history of the inner solar system, including the era when life formed on Earth, he adds.

The highest-resolution global topographic maps of the moon now available were acquired in 1994 from an instrument aboard the Department of Defense's Clementine mission. However, those maps depict the surface with an average resolution not nearly precise enough for discerning the depth of craters and elevations of mountains, let alone choosing landing sites for people.

NASA'S modest Lunar Prospector mission in 1997 didn't carry a camera, though its detectors provided a coarse map of the moon's elemental composition.

"There never was a full-fledged remote-sensing mission to the moon, so we don't have a full sense of the character of the moon's resources," says Carle Pieters of Brown University. "Our knowledge of the moon in fact is nowhere near as good as it is for Mars."

MOONSTRUCK The first step to becoming better acquainted with the moon is to send an orbiting observatory that can map large swaths with a resolution as fine as a half-meter, says Head, who worked on the Apollo missions. The six landing sites for the Apollo missions were scrutinized that closely, but these sites, all on the moon's near side, represent only a tiny portion of the lunar surface.

For a detailed look at the composition of and resources on the moon's surface, including specific minerals and possibly water, and a mapping mission would also require a spectrometer capable of analyzing moonlight over a wide range of visible and near-infrared wavelengths. That spectrometer would be similar to one now orbiting the Red Planet on the European Space Agency's Mars Express.

Such an orbiting mission would provide not only data of scientific interest but also the basics for sending people back to the lunar surface, which under the Bush plan would happen by 2020. A fleet of orbiting robots could home in on regions where astronauts might safely land. The orbiters would also identify locations where a robotic lander could extract hydrogen and oxygen from rocks and soils to create rocket fuel.

Scientists anticipate that a robotic filling station on the moon would ultimately mean that missions from Earth wouldn't have to carry the fuel needed for the return journey. At first, robots would make only a small amount of fuel to demonstrate the feasibility of the plan, notes James B. Garvin, lead scientist for moon and Mars exploration at NASA headquarters in Washington, D.C.

From the vantage point of researchers, one of the most compelling reasons to explore the moon is to read the history of the solar system that the moon's geology chronicles, Head says. That's because the moon hasn't undergone major upheavals in the more than 4 billion years since its birth.

In that period, by contrast, Earth has had numerous facelifts. Ongoing volcanic eruptions, the shifting of the continents due to the movement of tectonic plates, and other large-scale makeovers have erased Earth's original surface.

Theorists argue that the moon formed when a Mars-size asteroid walloped the young Earth, sending into terrestrial orbit an amalgam, some of which coalesced into the moon. The moon therefore serves as "a geologic record of the formative years of our own planet, the childhood which is no longer accessible to us," says Head.

To explore that early era, there may be no better place than the South Pole--Aitken basin, a 2,500-kilometer-wide impact site on the moon's far side, says Michael B. Duke of the Colorado School of Mines in Golden. Stretching from the moon's south pole to a crater 15[degrees] from the equator, the basin was excavated when a mountain-size body struck the area no later than 3.9 billion years ago. That's just around the time that life arose on Earth, perhaps triggered by a related series of impacts.

Planetary scientists have proposed that terrestrial life gained a foothold when comets or other debris bombarding Earth delivered a supply of organic material. No one expects to find life in the South Pole--Aitken basin. But mineral deposits there may reveal the composition of the material that entered the inner solar system and perhaps gave a foothold to life.

"The compositional information and information on the depth and mechanics of excavation of the basin are fundamental problems of planetary origin and history, which likely can be investigated nowhere else in the solar system," Duke reported last March at the annual Lunar and Planetary Science Conference in Houston.

While the basin might be good for planetary research, any location on the moon's far side would also be suitable for establishing an observatory that peers out into the universe, says Pieters. That's because the far side faces away from the radio noise and other electromagnetic disturbances emanating from our planet.

Even as the idea of returning to the moon kindles scientists' hopes for a new era of lunar science, President Bush's vision may relegate basic science to a back seat. "This is not the moon for the moon's sake but for [human space] exploration," notes Garvin.

For example, even though many scientists might opt to send astronauts to the South Pole--Aitken basin, other less scientifically intriguing regions might make more practical sense for producing fuel and setting up a base camp where astronauts could practice maneuvers future explorers might eventually do on Mars.

Such sites might include "the peaks of eternal light," which are mountains at the moon's south pole just outside the Aitken basin. They are bathed in sunlight for 85 percent of the lunar day and therefore offer a location particularly rich in solar energy, which could be used to power equipment.

A REALISTIC PLAN? The President's proposal calls for a $1 billion increase in NASA'S budget over the next 5 years, with an additional $11 billion coming from canceling the space shuttle program and ending outlays for the space station by 2010. But those moneys are just a start on realizing the entire vision of traveling all the way to Mars.

Many space scientists recall an earlier plan, announced in 1989 by President George H.W. Bush from the steps of the National Air and Space Museum in Washington, D.C. The venture was soon abandoned after NASA put a $400 billion price lag on it.

Rick Searfoss was a rookie astronaut when that proposal came out. Jazzed by the plan, Searfoss' class incorporated pictures of the moon and Mars into the design of the insignia they wore on their space flight suits. "We had this starry-eyed view ... of how all this stuff was going to happen," Searfoss remembers.

In retrospect, he says, the elder Bush's proposal "called for too much. It called for the moon, Mars, and it called for the space station." By cutting the space station and the shuttle, he says, the current plan is a "much more measured approach."

"This is the poor man's [route] to human space flight," says Garvin.

Perhaps too poor. That's what Norm Augustine, the former chief executive officer of Lockheed-Martin in Sunnyvale, Calif., told President Bush's newly appointed space exploration advisory commission last month. In 1991, Augustine headed a similar commission. He estimates that NASA's entire budget--currently set at $15 billion a year--over the next decade might not be enough to fund the newly proposed plan.

If the ambitious proposed mission joins current plans without NASA receiving a greater financial boost, prospects for space science studies that have long been in the works become bleak, some scientists point out. For example, the President's recently proposed budget postpones a joint Department of Energy-NASA spacecraft to study dark energy--the mysterious entity that is accelerating cosmic expansion--and also a space-based experiment to search for gravitational radiation (SN: 11/30/02,p. 339).

And then there's the decision to eliminate any further shuttle missions to repair or upgrade the Hubble Space Telescope. Without maintenance, that orbiting telescope could stop functioning as early as 2006 instead of lasting into the next decade.

NASA administrator Sean O'Keefe says that his decision was based solely on safety issues. However, Rodger Thompson of the

University of Arizonain Tucson and some other astronomers contend that the real reason for cutting Hubble short is to save money for the moon-Mars initiative.

"I'd love to go to Mars," says Thompson, but an early demise of Hubble "is a pretty high price to pay today for a program that may never get funded."

MARS TEST BED The President's plan has people living and working on the moon for extended periods "If we can develop a closed life support system on the moon, then we'll be able to do the same thing at least as easily on Mars," says Duke.

As a dress rehearsal for investigating the Red Planet, lunar explorers would build a base, assemble a filling station, and learn how to manipulate fragile grains of material for scientific analysis. The advantage, of course is that the moon is just 3 days away, while traveling to Mars could be a nearly yearlong sojourn.

However, the demonstration site may provide obstacles that Mars visitors would never encounter. The moon lacks the shield provided by even a thin planetary atmosphere, such as Mars, so lunar explorers will have to withstand the relentless bombardment of harmful solar radiation and pelting rain of micrometeoroids. Despite its windstorms, Mars might turn out to be a more hospitable place because it has more water and other essential materials, says Duke.

If it's to build upon astronauts' lunar experience, a human mission to Mars must follow sooner rather than later, says Garvin. Bush's plan doesn't specify when the journey to the Red Planet might occur.

But NASA shouldn't delay another 20 years beyond 2020, the target for returning to the moon, says Garvin. If the agency waits too long, he cautions, "then all the technology lessons learned from the moon will be lost."

RELATED ARTICLE: NASA's nuclear legacy: using fission to journey to the red planet.

For many space enthusiasts, sending people to Mars has long been the ultimate dream. President George W. Bush, in his new space-exploration plan, doesn't specify a date for a human journey to the Red Planet. But some space scientists hold that much of the technology is ready to roll.

Shortly after Neil Armstrong's historic moon walk in 1969, NASA rocket scientist Wernher Von Braun told a House of Representatives science committee that the space agency would be ready to fly people to Mars by 1981. Braun based his prediction on research NASA had been conducting since the early 1960s on using nuclear reactors to propel spacecraft into deep space.

In 1961, NASA and the Atomic Energy Commission founded the Nuclear Engines for Rocket Vehicle Applications (NERVA) program. The NERVA design for rockets was relatively simple. A nozzle would be attached to a reactor in which the fission of uranium-235 releases tremendous amounts of heat. Liquid hydrogen would flow around the reactor, absorbing heat and vaporizing. That would provide a propulsive force as the hydrogen rushes out the nozzle.

By the time the program ended, after 12 years, it had tested 20 nuclear reactors at Jackass Flats in Nevada for potential use in powering rockets.

One of the reactors tested under the NERVA program generated 55,000 pounds of thrust for 62 minutes, more than enough to carry a rocket out of Earth's orbit and fly it to the Red Planet, says Start Borowski of NASA's Glenn Research Center in Cleveland. The hydrogen turbo pumps, valves, nozzles, and other equipment required for a nuclear-propelled rocket are already in operation on chemically propelled rockets, he notes.

"The NERVA program during the 1960s demonstrated capabilities that exceed what we need today to get to Mars," Borowski says.

NASA planners are likely to reconsider nuclear-propulsion designs. These systems would not only shorten flight times to Mars, but they would also require much less mass than do standard rocket engines using liquid-or solid-chemical fuels, Borowski says.

Under NASA's Prometheus program, nuclear propulsion has been proposed for robotic missions to the icy moons of Jupiter a decade from now. Nuclear powered rockets are also being considered for voyages to near-Earth asteroids in the more distant future.

NERVA was disbanded in January 1973, a month after the lab Apollo mission. By that time, Borowski notes, the Cold War's race to the moon had been won, the public had tired of watching Apollo landings, and the space agency was turning its attention to what appeared to be more exciting opportunities--the space shuttle and space station. Ironically, these two programs now seem about to be terminated.--R.C.