WASHINGTON - Government auditors faulted NASA on Tuesday for failing to adequately investigate cheaper, safer alternatives to the space shuttle for delivering parts and supplies to the International Space Station.

With just five years until the three remaining shuttles retire, NASA has 28 flights scheduled to complete work on the orbiting research laboratory. That's a flight rate of more than five shuttle missions per year, which even agency leaders acknowledge could prove to be too many.

Lawmakers in Congress have pressed the agency to examine lower-cost missions to the space station using expendable rockets and cargo containers.

Last year, NASA headquarters officials dismissed the notion as too costly and risky. But those conclusions lacked supporting documentation, according to a 10-month study by the Government Accountability Office, the investigative arm of Congress.

"It's now clear NASA had not done its homework on vetting space shuttle alternatives," said Sen. Sam Brownback, R-Kan., one of several lawmakers who asked the GAO to investigate NASA claims last year.

The GAO findings likely will have little real influence on NASA because agency officials, spurred on by new administrator Michael Griffin, are conducting more detailed analysis of the technology and costs involved in shifting some of the space station missions from the shuttle to other delivery vehicles.

At a May Senate hearing, Griffin acknowledged the shuttles may not be up to making 28 flights in five years. He said shuttle program engineers and managers are considering dropping some of the missions.

A new cargo-only module could be employed to replace the lost shuttle missions, Griffin said. The agency is currently evaluating 26 proposals from companies interested in ferrying cargo to and from the space station. International partners Japan and Europe are developing automated cargo haulers that can launch on rockets. Currently, Russia's Progress automated cargo spacecraft are the only means of resupplying the outpost while the U.S. shuttles are grounded.

NASA officials indicated they agree with the GAO's findings and outlined ongoing efforts to accelerate the design and production of a cargo-only launch vehicle.

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