NASA will have a tough time pursuing research projects at the International Space Station because of the scheduled retirement of the shuttle next year and the station's closure in 2015, government auditors reported Tuesday.

"NASA faces several significant challenges that may impede efforts to maximize research utilization of the ISS," according to the 39-page Government Accountability Office report.

Challenges include the pending retirement of the shuttle in 2010 and an anticipated delay after that before subsequent space vehicles are developed to carry people. Other problems include the high cost of launches, a lack of dedicated funding for space station research and the uncertain future for the space station beyond 2015.

The report was prepared for Sen. Bill Nelson, D-Fla., and Rep. Bart Gordon, D-Tenn., chairman of the House Science and Technology Committee. Nelson said the report explains why the space station deserves more attention to maximize its potential and why a new vehicle is needed to get there after the shuttle retires.

"It would make no sense to abandon the International Space Station in 2015, only five years after completion," said Nelson, who pushed for links between space station work and experiments at Kennedy Space Center. "We also need a vehicle to get there after the shuttle is retired, which is one reason why I've urged the president to adequately fund NASA."

Rep. Bill Posey, R-Fla., said the report explains why it's too soon to retire the shuttle.

"This report makes the case for adding shuttle flights ... because absent these additional flights, the utilization of the ISS will fall far short of its full potential," Posey said.

The report underscores the importance of shortening the gap between the shuttle and the successor vehicle, Rep. Suzanne Kosmas, D-Fla., said.

"The GAO's report is another reminder that we must keep fighting to minimize the spaceflight gap in order to protect jobs and to maximize the return on the $100 billion invested in the International Space Station," Kosmas said.

Under Bush administration policy, NASA budgeted to stop participating in the International Space Station in 2015, providing a five-year window for research after the station is completed next year. Congress has directed NASA to take steps to use the station until at least 2020, although no decisions on extensions have been made yet.

Research began even as the space station was assembled. By February 2009, U.S. research at the station and on shuttle flights to and from the station resulted in 160 publications. Subjects ranged from protein crystallization to plant growth to human research.

But a 2004 report shifted the focus of research to human spaceflight, and away from biology and physical sciences. In 2002, there were 26 experiments on biology, 11 on biotechnology, 13 on fundamental physics and 23 on materials science. By 2008, there were 13 on biology and five on materials science, according to the GAO report.

"Hardware needed for research projects was also canceled or delayed by NASA or commercial developers, either because of the change in research priorities or other constraints, such as the pause in shuttle flights after the loss of Space Shuttle Columbia," the report said.

Costs are expected to rise with the use of Russian rockets after the shuttle retires. NASA estimated it costs $44,000 per kilogram to launch experiments; the Agriculture Department reported paying $250,000 for a compartment the size of a shoe box.

"NASA officials have stated that it is significantly more expensive to conduct research on board the ISS than on Earth, and the agency now views lack of funding for research as the major challenge to full research utilization of the ISS," the report said.