Space / Space Flight

Lowest Risk Moon Appeal: Close, geology contains clues to Earth’s early evolution Risks: No air, harsh temperatures Mars Appeal: Water, atmosphere, could have supported life in the past advertisement | article continues below Risks: Thin air, lengthy transit

When President Bush announced the goal of manned flights to the moon and Mars, NASA immediately started shifting resources to prepare for its new mandate. The news also emboldened space enthusiasts to dust off their dreams and begin reconsidering the tantalizing range of potential destinations for human exploration—focusing, of course, on the nearest and least hostile locales.

The obvious first stop is the moon, but only because it is just a three-day trip away. Other than that, the moon is not exactly inviting. It lacks an atmosphere, and pulling oxygen out of the silicon dioxide soil would present a tough engineering challenge. Water on the moon, if it exists at all, is probably confined to frozen deposits in a few shadowed craters near its poles. And temperatures rise and fall by a brutal 450 degrees Fahrenheit over the course of a lunar day.

Moderate Risk Asteroids Appeal:  Asteroid geology records early solar system development Risks: No air, no water Europa Appeal: May have saltwater ocean that could support life  Risks: Long voyage, radiation

Gary Martin, NASA’s future technology architect, believes it is important to return to the moon anyway, because the lunar surface is the perfect place to test the technologies and procedures needed for subsequent visits to Mars and beyond. “It will let us try out space suits, figure out how best to do geology on a powdery surface, establish human-robotic interfaces, stuff like that,” he says.

Mars is the friendliest spot beyond Earth, hands down. Even so, the surface is a barren landscape, where human life would hang by a high-tech thread. The atmosphere is just 1/130 as dense as Earth’s, affording little protection from solar radiation, and it consists mostly of unbreathable carbon dioxide. Visiting astronauts would have to create their own oxygen, perhaps using solar-generated electricity to extract it from the highly oxidized Martian sands.

Danger! High Risk Venus Appeal:  Earth's near twin Risks: Sulfurous clouds, crushing atmosphere, broiling heat Jupiter Appeal: The biggest planet, fascinating atmospheric chemistry  Risks: No surface to land on

Our number three target might be one of the large satellites of Jupiter. Europa in particular beckons because it seems to have a salty ocean beneath its miles-thick crust of cracked ice. A manned mission could explore whether life has taken hold in this unique environment. The surface abounds with water, which could be split into oxygen and hydrogen to provide air and fuel for the trip home.

A trip to Europa would probably take several years using current technology, however. Equally daunting, the satellite orbits deep within Jupiter’s awesome magnetic field, which traps and holds an enormous supply of lethally charged particles. Humans would have to be shielded as they drilled down to Europa’s mysterious ocean. “Maybe people could control robotic machines from Jupiter’s outer moon Callisto, where the radiation is much less powerful, especially on its back side,” Martin suggests.

After that short wish list, the options grow more problematic. Saturn’s moon Titan may have intriguing hydrocarbon lakes underneath its opaque nitrogen atmosphere. But Titan is frigid (about –290°F) and twice as distant as Europa. Humans could potentially visit Mercury, especially if they aim for the planet’s poles, but conditions there are even harsher than on the moon. Far easier would be the many rocky asteroids between Mars and Jupiter. These small bodies are relatively nearby, require little fuel for landing and takeoff, and probably contain remarkable geologic histories.

The four Jovian planets—Saturn, Uranus, Neptune, and Jupiter—are no go’s. They lack solid surfaces on which to land, plus their fearsome gravity and thick clouds of ammonium compounds would make departure a task akin to that of an insect struggling free from a piece of flypaper. Venus is the closest planet to Earth but also arguably the deadliest. Its steady 850°F temperatures and its crushing surface pressures would overwhelm any conceivable space suit design.

For now, robots handle all the dangerous assignments. The Soviet Venera landers managed to photograph Venus’s torrid surface; the NEAR spacecraft survived touchdown on asteroid Eros; early in 2005 the Huygens probe will parachute onto Saturn’s hydrocarbon-covered moon, Titan.

Some astronomers question whether the time is right to divert scarce funding for human space adventures. Astrophysicist Fred Espenak of the Goddard Space Flight Center speaks for many of his colleagues: “With the current state of technology, planetary science can be done much more inexpensively with robot missions than with manned ones.”

Yet few can deny the romantic appeal of human feet treading the rusty deserts of Mars. Sooner or later, that romance will most likely become reality.

Photographs: top to bottom, Courtesy of U.S. Geological Survey/NASA/JPL (2); Courtesy of Johns Hopkins University/Applied Physics Laboratory/NASA/JPL; Courtesy of NASA/JPL; NASA/SPL/Photo researchers