A 2000 law restricting U.S. purchases of Russian technology has put America's future on the international space station in doubt, raising the prospect that the United States would be unable to keep astronauts on the station beyond April 2006.

The law makes it illegal for NASA to pay Russia to fly astronauts to the orbiting laboratory, and Russian space officials insist they cannot afford to carry the Americans for free after they fulfill an agreement to provide 11 free trips.

The dilemma could cripple President Bush's initiative to send humans back to the moon and eventually to Mars, because the United States could not use the station to research the effects of prolonged weightlessness on the human body.

Congress has questioned NASA about how it plans to escape the impasse, but NASA has offered no solutions. Deputy Administrator Frederick D. Gregory told reporters in a recent telephone news conference that a high-level meeting last month of the 16 space station partner nations did not address the matter.

NASA officials said they are studying whether a preexisting bilateral agreement may allow the agency to obtain Russian equipment without running afoul of the law, but they acknowledged that this would raise difficult legal and national security issues.

The restrictions imposed under the Iran Nonproliferation Act of 2000 forbid the United States from buying anything from Russia for the space station unless the U.S. president certifies that Russia is not exporting nuclear, chemical or biological warfare technology or know-how to Iran.

Conceived in a Republican Congress as a means of stiffening President Bill Clinton's response to Russian support for Iran's nuclear ambitions, the act passed unanimously and continues to receive bipartisan support.

Until recently, however, it was largely ignored. Russia, under a 1996 bilateral "balance agreement," must provide 11 Soyuz spacecraft to ferry U.S. astronauts and Russian cosmonauts between Earth and the space station at no cost. Soyuz cannot be reused. The 11th free Soyuz is scheduled for liftoff in October 2005 and with a planned return to Earth in April 2006. After that, Russia intends to limit passengers to those who pay.

"This is a situation where America's very valid concerns about Iran are ending up having a dramatic impact on the space station program," said Rep. Dana Rohrabacher (R-Calif.), chairman of the subcommittee on space and aeronautics. He is a space station advocate and the author of the law's space station restrictions. "Of all the problems we have faced, this is one of the most serious," he said.

Administration officials said Bush has no immediate plans to certify Russia's good behavior -- nor will he suspend the act by invoking its exception for "imminent loss of life or grievous injury" to the astronauts.

Congress and the administration agree that the law does not allow an end around, such as buying Russian technology through a third party or a new barter agreement. And a Democratic proposal to amend the law has won only three supporters in 17 months.

The quandary means that the United States, which has invested $33.5 billion in the station since 1985, will be unable to use it unless Russia changes its policy.

Officials from the Russian Aviation and Space Agency warn that their country can no longer afford to fly astronauts for free, and after next year will accept only paying customers -- scientists from other nations, and, if it can find them, more space tourists such as the two men who reportedly paid $20 million apiece to fly to the station in 2001 and 2002.

The United States will be able to fly astronauts to the station for short stays once the grounded space shuttle returns to service, which is planned for next year. But shuttle passengers cannot stay longer because only Soyuz can dock permanently and stand by as a "lifeboat" in case the crew needs to evacuate. Without a permanent presence, U.S. scientists and astronauts will be unable to study the effects of prolonged weightlessness, identified as the station's only research goal under the president's moon-Mars initiative.

Congressional misgivings about Russia and Iran arose in the early 1990s after Russia agreed to build a nuclear power plant near the Iranian city of Bushehr. "Its whole purpose is to make nuclear weapons," Rohrabacher said in a telephone interview. "Iran is a major producer of oil and gas. It doesn't need a nuclear plant."

After 1995, concern intensified because of "all kinds of [other] proliferation activities," recalled Rep. Howard L. Berman (D-Calif.), a senior member of the International Relations Committee and a co-sponsor of the restrictions.

The law, which arose from suspicion that Clinton's supportive relationship with then-Russian President Boris Yeltsin was causing him to overlook the Iranian connection, requires the president to impose trade sanctions on any foreign entity that helps Iran acquire nuclear, chemical or biological weapons technology.

Lawmakers added the space station clause as a stiffener. "We wanted to put [everyone] on notice that we should not have high-level cooperation, even in space, if the Russians were using their technological skills to help Iran build a nuclear weapon," Rohrabacher said. "It was a very good idea."

The White House fought it, arguing "that we were using the diplomatic tools available . . . and that passing this law would undercut our ability to influence the Russian government," said Gary Samore, former director for nonproliferation and export controls on Clinton's National Security Council staff. But fearing a veto override, in the end "we held our noses and signed it," Samore said in a telephone interview from the London offices of the International Institute for Strategic Studies, where he directs research in nonproliferation.

As the clock ticks down on the balance agreement, the Bush administration has begun to consider alternatives. "Obviously, we'll have to look at different strategies," one senior administration official said. "But there's still a fair bit of water to go over the dam before we get to that point."

The official, who declined to be identified by name because of Bush administration policy, did not rule out eventual certification of Russian compliance, but said the administration has no immediate plans to take that step. The official also saw no way to use the "imminent" danger exemption as long as a Soyuz craft lifeboat is at the station.

Also off the table is the possibility of buying Soyuz spacecraft through intermediaries or negotiating a new barter agreement. Deputy Assistant Secretary of State Steven Pifer told Congress last year that such tactics "would likely be viewed by many as an evasion of the law."

NASA is pursuing the possibility that additional Soyuz might be available under the existing agreement, which authorizes the United States and Russia to trade goods and services "for the life of the station," but it is far from clear whether this wording would admit Soyuz purchases beyond the original 11.

"How this is done under the Iran Nonproliferation Act is a legitimate question for the future," said NASA spokesman Glenn Mahone. "There's no good answer for that, but we have some time to decide."

Congress could amend the law, but a modest effort to allow "extraordinary" purchases from Russia after the Columbia tragedy in February 2003 has picked up only three signatures.

"The concern is that if you try to bypass the law, you're breaking it, and it's there for a reason," said Rep. Nick Lampson (D-Tex.), author of the amendment and ranking minority member of the space subcommittee.

But in the years since passage of the nonproliferation act, the congressional climate toward Iran has "grown worse, not better," Berman said, especially after Iran was forced to acknowledge it had set up a clandestine nuclear program separate from Bushehr. Iran said the program is for peaceful purposes.

"The Iranians have made it clear they are moving forward on the bomb," Rohrabacher said. "Even though I have more focus than most people on making the space station a success, I am not going to do anything that would signal a weakening of our resolve."

The Bush administration, at least for now, has decided to appeal to Russia's goodwill and its desire to reaffirm its position as a leader and "reliable partner among space-faring nations," the senior administration official said.

There is little evidence that these blandishments are having an effect. Russian Aviation and Space Agency spokesman Konstantin Kreidenko said in an interview that while there have been "some working discussions" about the United States purchasing Soyuz, "Americans didn't ask for free flights," an impossibility after 2005.

"From then on," Kreidenko said, "we won't be able to send people for free."

Correspondent Peter Baker in Moscow contributed to this report.

Soyuz prepared to dock with the international space station in April. After 2006, the astronauts may not stay aboard the space station because of a law prohibiting the United States to pay Russia to fly astronauts to the station.