As the moments tick away before tonight's scheduled launch of the space shuttle Endeavour to the International Space Station (ISS), another countdown is underway: Only a handful of launches remain before the shuttle program's scheduled retirement in 2010. When President-elect Barack Obama takes office two months from now, he and his aides will need to decide quickly whether or not to hold to that date, a determination that will have major implications for the future of U.S. space exploration.

The U.S. Government Accountability Office (GAO), Congress's investigative arm, has identified the shuttle's future as one of the most urgent issues facing the incoming Obama administration. "NASA has already begun the process of shutting down production and transitioning people, equipment and resources to new endeavors," GAO director of acquisition and sourcing management Cristina Chaplain says. She adds that the longer the decision is delayed, the more difficult it will be to keep operating the shuttle safely and cost-effectively. "Moreover, putting off a decision may hamper the transition itself and keep NASA from pursuing new space transportation development," Chaplain wrote in an e-mail.

At present, the scheduled replacement for the low Earth orbit–only shuttle is the Constellation program, which would allow astronauts to return to the moon and target even more distant destinations such as Mars. But even assuming a 2010 shuttle phaseout, Constellation won't be ready for manned flights until at least 2015.

The shuttle's retirement, and the introduction of what became the Constellation program, are key points of Pres. George W. Bush's 2004 Vision for Space Exploration. Another central tenet of that plan is a target for returning astronauts to the moon by 2020, a goal that some deem problematic.

Bush's plan "added what seems to many people to be an arbitrary and sort of artificial deadline of landing people on the moon," says Jim Bell, an astronomy professor at Cornell University and recently named president of the Planetary Society, a nonprofit that advocates for space exploration. "We're not saying that NASA shouldn't go to the moon, we're not saying the moon's not worthy of exploration," Bell says. "What we're saying is that those kinds of things could be the right thing to do, but not in this artificially constructed timescale."

The Planetary Society this week unveiled its road map for the future of the space program, timed to provide the incoming administration with outside scientific guidance. The document advocates for increasing international partnerships, setting Mars as the ultimate target for human exploration, and allowing the shuttle to enter obsolescence as planned in 2010.

"We are absolutely in favor of its scheduled retirement," Bell says, praising NASA Administrator Michael Griffin for supporting the plan and noting that the Columbia Accident Investigation Board recommended the shuttle be replaced as quickly as possible. "The longer that NASA hangs on to it, the more of—unfortunately—a liability it becomes."

Some see a lapse in NASA's ability to command manned space missions as unacceptable. Sen. Kay Bailey Hutchison (R) of Texas, where space exploration is a major industry, wrote an op-ed for the Houston Chronicle last year warning that NASA would need to rely on Russian spacecraft for transport to and from the ISS during the five-year gap between 2010 and 2015.

"As the world's leader in space technology, it is simply unacceptable that we will be in this position of technological dependency," she wrote. "Our national security depends on our ability to explore space without relying on nations who may not always have our best interests at heart." (These sentiments were echoed in an e-mail from Hutchison's press secretary today.)

Where NASA will head during Obama's presidency is an open question. (His aides did not immediately respond to requests for comment.) As reported by the Washington Post last November, during the Democratic primary, Obama announced that his $18-billion-a-year package for early education and K-12 would be funded by "delaying the NASA Constellation Program for five years." Later drafts of the education plan did not include the Constellation language.*

But a new position paper from the Center for American Progress (CAP) may offer a glimpse of the future. The CAP is a think tank headed by John Podesta, former chief of staff to Pres. Bill Clinton and one of the top aides on Obama's transition team. According to the Orlando Sentinel, the CAP paper states: "The decision to phase out the shuttle by 2010 should be reconsidered; it should be flown until a suitable replacement becomes available."

Bell fears that extending the shuttle's timeline will only push back other priorities, especially given the current financial climate. A 2010 retirement would free up the funding to pursue Constellation, he says, and would keep NASA on track. "The American space program," he says, "needs to move on."

*A more recent campaign document devoted solely to space exploration, in contrast, proposes expediting Constellation and extending the shuttle's service by at least one flight to narrow the five-year gap.