President Bush's plan to send astronauts to the Moon and Mars doesn't change much in the near term. The immediate goal is still to replace the space shuttle, a task NASA has been struggling with for years. In the long term, it gives the space agency the clear goal that most observers agree it badly needs. The technology for human and robotic exploration of the Moon and Mars poses serious challenges. But the biggest problem will be sustaining a viable program over the long years it will take for astronauts to reach Mars.

Since President Nixon cancelled the Apollo Program in 1972, NASA's human spaceflight program has focused on near-term goals-first building the shuttle, then a permanently occupied space station. Both ran far over budget and behind schedule. The station is still incomplete, and as a long-term goal it leaves NASA literally going around in circles.

Last year, the Columbia Accident Investigation Board agreed, saying the lack of long-range goals for human spaceflight was hurting the space program. A National Academy of Sciences workshop came to the same conclusion in a report released January 14, the day of Bush's announcement. "You need a long-term goal for human spaceflight," said Lennard Fisk, chair of the academy's space studies board. Participants agreed the goal should be staged exploration with a long time horizon, not a crash project like Apollo. "Let's systematically go forth into the solar system," Fisk said. "If you take that approach, with a lot of interim steps, it's not a budget-busting thing." Bush's proposal "appears to be very consistent with what we were asking for," Fisk said in a telephone interview.

The plan that Bush outlined Wednesday at NASA headquarters would return the space shuttle to flight with the goal of completing assembly of the International Space Station by 2010. Once that job is done, the shuttle will be retired. That scenario requires spending about $4 billion a year on shuttle operations, plus the extra cost of recovering from the Columbia disaster. NASA's current shuttle launch manifest includes two return-to-flight test missions, six missions to complete the core of the station, and 14 subsequent missions to deliver crews and equipment to the station. At four missions a year, that will take NASA into 2010.

Bush did not mention a main concern of astronomers-a service mission to the Hubble Space Telescope that had been planned for next year before the Columbia accident. But on Friday afternoon, January 16 [after this story was initially posted], NASA Administrator Sean O'Keefe devastated astronomers by cancelling it, leaving Hubble likely to cease operations by 2007. O'Keefe blamed the need to devise special safety procedures for the mission, which cannot reach the International Space Station. 

The plan calls for the United States to narrow its space-station research to focus on the effects of space flight on humans. That reflects concern on the effects of long-duration weightlessness, which O'Keefe said in a separate press conference need to be mitigated. Bush set no deadlines for ending U.S. support for the station, but O'Keefe showed a chart that zeroed station spending in 2017.

To replace the shuttle, Bush wants to build a new Crew Exploration Vehicle (CEV), which unlike the shuttle could go beyond low Earth orbit. His goal is  to begin tests of an unmanned version of this craft by 2008, with the first manned mission "no later than 2014." This maiden voyage would probably be a modest one: delivering astronauts to the space station. That's roughly the timetable that NASA had set for the Orbital Space Plane, last year's planned successor to the shuttle.

O'Keefe said the new spacecraft would be an evolvable design that could be fine-tuned to the needs of individual missions. That's a big step beyond the one-size-fits-all approach of the shuttle and earlier proposed replacements. Flying from the ground to low-Earth orbit, from Earth orbit to the moon, and returning to the ground all have different requirements, says Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics. "It makes sense to have a set of vehicles built around the same core."

Versions of the CEV would return to the moon. Robots would arrive first, by 2008, to explore the surface and begin preparing for human exploration. Astronauts would follow, with extended missions starting between 2015 and 2020. Bush mentioned the possibility of mining lunar resources and using them to fuel spacecraft that could be launched from the Moon's low gravity. That capability is probably far off. Recent observations of perpetually shadowed craters near the lunar pole failed to find the water ice suggested by earlier observations, and whole new technologies would have to be developed that today are the realm of science fiction. Nonetheless, the moon is a logical step. "It teaches us operating on another world in a gravity well," says McDowell, lessons vital to exploring Mars "that we can and should learn on our friendly little airless world three days from home." Robots would head to Mars first, following the tire tracks of the Pathfinder and Spirit landers. Bush did not set a target date for a manned Mars mission. O'Keefe said the timetable for Martian exploration will depend on the program's progress in coming years.