DISTANT DESTINATIONS

An upcoming NASA selection between very different robotic spacecraft for a $700-million mission scheduled to fly by 2010 is emblematic of the strains that are likely to beset the U.S. agency--and its international partners--for years to come as they struggle to map the new pay-as-you-go U.S. exploration program.

In choosing between a vehicle to return lunar samples from the Aitkin Basin at the Moon's south pole, and an orbiter that would circle Jupiter from pole to pole to see if the gas giant has a solid core, NASA managers must strike a balance between human space spectaculars close to home and the equally spectacular science possible deeper in space.

DUBBED MOONRISE, the Aitkin Basin mission would play right into President Bush's call for a return to the Moon as a nearby proving ground for eventual human missions to Mars. The "Juno" polar orbiter at Jupiter would provide valuable data about both the formation of our own solar system and what astronomers are seeing when they detect gas giants around other stars.

Each was selected for more study from a group of seven proposals vying to be NASA's next "New Frontiers" mission. Both were high on the list of 10-year priorities for planetary exploration set by the National Research Council (NRC) in 2002. NASA says both are getting equal treatment in the evaluation process that should lead to a selection next spring.

Even so, Moonrise would appear to have the edge. There may be water ice at the lunar south pole that humans working there could use for life support and propellant. Juno would be solar-powered at a time when NASA wants to build advanced nuclear power and propulsion systems for exploration, and has already started a major effort to explore the icy moons of Jupiter with a nuclear-powered orbiter.

That doesn't mean selection of Moonrise is a lock. Ongoing work on the nature and origins of the outer planets ensures there will be a significant scientific constituency for more missions there in parallel with the Moon/Mars push.

"Outer planet research will remain a priority item in Solar System exploration," says Orlando Figueroa, who oversees planetary exploration at NASA headquarters as deputy associate administrator for programs in the Science Mission Directorate. "The [President's] vision speaks of the Moon, Mars and beyond. If there is ever going to be hope for a 'beyond,' investigations and research beyond Mars need to be a strong component of our strategic plan."

The U.S. space agency has enlisted the aid of academia and industry in developing new strategic plans for 13 areas, including Solar System exploration, in light of the new space initiative. As the Solar System planners work, they may be guided by some new science from the "beyond" that Figueroa mentioned.

On Jan. 14 Europe's Huygens probe is scheduled to parachute into the murky atmosphere of Saturn's moon Titan. Delivered to the ringed giant's system by NASA's nuclear-powered Cassini spacecraft, Huygens may be able to penetrate the mysteries hidden by the hydrocarbon haze that shrouds the planet-sized moon to answer such basic questions as whether the surface is solid or liquid (see p. 63).