International cooperation in human spaceflight may be facing its toughest test since the cold war.

The immediate concern: Will US astronauts be able to ride Russian rockets between 2010, when the last shuttle is retired, and 2015, when the National Aeronautics and Space Administration expects to launch a replacement?

Russian spacecraft are how NASA plans to send its astronauts to the International Space Station. But with tensions rising over Russia’s invasion of Georgia and a US-Polish missile deal, some lawmakers and space-policy analysts worry that the US Congress – or Russia itself – could scuttle the plan. If tensions fail to ease over the longer term, the space station could shift from an orbiting laboratory to a geopolitical bargaining chip.

The possibility that international tensions could limit US access to the space station for at least five years “is a real concern,” says Ray Williamson, an analyst with the Secure World Foundation, a space-policy think tank in Superior, Colo. But the notion that the US needs an alternative right away is a bit premature, he adds.

Proposals range from extending the shuttle program beyond 2010 to cutting a deal with China, which is ramping up its own human spaceflight effort. Each option faces big budgetary or political challenges.

Already, Bush administration officials reportedly have suggested that the full range of US-Russian ties need to be reviewed in light of Moscow’s actions in the former Soviet Republic of Georgia. Those tensions appeared to have risen another notch Wednesday when US Secretary of State Condoleezza Rice and Polish Foreign Minister Radek Sikorski inked a deal under which the US would install antiballistic missiles in northeastern Poland.

For the US, the challenge lies in the way the Bush administration crafted its 2004 vision for space exploration. It called for an end to the shuttle program in 2010 and the launch of a replacement, the Ares I and its Orion capsule, by 2015. NASA is working on the Ares I system, along with other major components of its Constellation program, with that deadline firmly in mind. Constellation aims to return humans to the moon by 2020. But that schedule leaves at least a five-year gap with no homegrown way to send astronauts to the space station.

The US endured a nearly six-year gap in human spaceflight between the Apollo and shuttle programs. But this time around, the US has a destination in orbit that it has paid big money to build and maintain.

NASA Administrator Michael Griffin has called the gap and the need for Russia’s help “unseemly.” To try to narrow the gap, the agency initially set an internal target for launching Ares 1 with its Orion crew capsule in September 2013. But with more-refined cost estimates in hand, and based on an assumption of no new money likely from Congress to support a 2013 deadline, the agency has moved that internal date to September 2014.

“The space community has been trying to yell about this for years, but people didn’t pay a lot of attention,” says George Whitesides, executive director of the National Space Society, a space-exploration advocacy group in Washington. “The biggest lesson we should have learned” from the shift from Apollo to the shuttle “is the need to think through the transition between vehicles without gaps.”

The first signals about the future may well come from Congress, which is weighing whether to grant NASA a waiver this year from the Iran, North Korea, and Syria Non-Proliferation Act. It did so in 2005 so that NASA could buy astronaut launch services from Russia through 2011. NASA officials have said they need to place their order soon given the lead time the Russians say they to build the new Soyuz capsules NASA would need.

Short of holding its nose and granting the waiver, Congress appears to have few options.

The US could try to play a China card, notes Vincent Sabathier, a senior fellow at the Center for Strategic and International Studies in Washington and a former official with the French space agency CNES. In an online commentary last week, he noted that China doesn’t represent an immediate solution. But Chinese space officials have expressed an interest in participating in the space-station program, he notes. And tighter links between the US and Chinese space programs could represent important confidence-building measures affecting other areas of US-Chinese ties.

But such a move would require a profound change in US policy that would be a tough sell, especially at this point in the US political cycle, according to Joan Johnson-Freese, a specialist in international space policy who heads the department of national-security studies at the US Naval War College in Newport, R.I.

Thus, if Congress turns its back on a waiver for NASA , “that leaves us in a situation of keeping the shuttle on line,” she says. “Then we’re eating our seed corn once again.” The reason: Unless a new president and Congress give NASA enough money to extend the shuttle program – which already is winding down and likely would require a costly recertification of the remaining shuttles – cash to keep the shuttles running probably would come from money NASA plans to spend on the Constellation program.

At the end of the day, “Congress will act pragmatically” and grant the waiver, predicts Peggy Finarelli, a senior fellow at the Center for Aerospace Policy Research at George Mason University in Fairfax., Va.

While US-Russian relations appear to be growing more rocky, she doesn’t anticipate the space station becoming a bargaining chip for either country. Hearkening back to the Carter administration’s response to the Soviet invasion of Afghanistan and its cancellation of science and technology exchanges in protest, she says that the White House could make that move then because “there was nothing there where you had multibillion-dollar programs where you had invested a lot of your future.”

Moreover, over the past 10 to 15 years, the two countries’ space programs and even their space-launch industries have become tightly intertwined. “These relations are not so easy to damage,” says Mr. Whitesides. That, he says, gives some cause for hope that the space station will remain a symbol of international cooperation in space, despite sometimes rocky relations between partners on Earth.