Even as NASA prepares to resume space shuttle flights next May, there is growing debate about whether the orbiter fleet should be retired earlier than called for in the agency's current plans.

Officially NASA says it plans to complete the international space station as close to 2010 as possible and then stop flying the shuttle. Finishing the space station, NASA officials have said, will require 28 shuttle flights spread over six years. Assuming NASA gets off three flights in 2005, the space agency would have to maintain an average flight rate of five missions per year to retire the shuttle fleet on schedule. While some agency officials believe more than 28 flights might be needed to finish the station and get it on a stable footing, others are assessing whether the program could get by with fewer flights.

NASA acknowledges its shuttle schedule could easily slip by a year or more as problems inevitably crop up. NASA announced Oct. 29 that it was now targeting May 12 to June 3 for return to flight, a two month delay from the previous planning window. Few expect NASA to be ready to retire the shuttle any earlier than 2012 given the vagaries of space flight and the shuttle fleet's historical flight record.

The chief executive of shuttle solid rocket booster manufacturer ATK told analysts in an Oct. 28 conference call that he expects the shuttle to be flying well beyond the 2010 retirement date President George W. Bush set for the fleet in January when the president outlined a new exploration-driven agenda for the U.S. space agency.

"I haven't spoken with anybody who believes the space shuttle program will wind down in 2010," Dan Murphy, president and chief executive officer of the Minneapolis, Minn.-based company, told analysts. "NASA is adhering to the program as it is. All outside experts ... believe the existing program has to fly through 2014."

Such talk worries space exploration advocates who sense an erosion of resolve among some NASA officials and industry executives to retire the shuttle fleet and get on with exploration.

With space shuttle costs expected to reach $5 billion annually in the years ahead, according to government and industry sources, a two-year postponement of the space shuttle's retirement would translate into a $10 billion hit to new exploration-driven efforts.

Louis Friedman, director of the Pasadena, Calif.-based Planetary Society, said the only way NASA can afford to move out beyond Earth orbit in the current budget climate is by making a firmer commitment to retiring the shuttle by 2010 and getting on with the development of the Crew Exploration Vehicle (CEV).

"NASA has been wonderful about embracing the vision for space exploration. The PR face of NASA has been wonderful on this subject," Friedman said. "At the same time, they have not embraced it programmatically. The moon program hasn't really started, there has been talk about delaying the CEV a few years, and the 28 [shuttle] flights still stick out."

Friedman said NASA could dramatically reduce the number of shuttle missions it needs to fly if it moves aggressively to offload space station resupply missions and even some assembly tasks to unmanned rockets.

The Planetary Society published a report in July that advocates limiting the space shuttle to launching the remaining U.S. elements -- a configuration known as U.S. Core Complete -- and relying on other launchers to get NASA's European and Japanese partners' modules into place. Such an approach, the report says, could permit NASA to retire the shuttle after flying as few as six to eight missions. The report, "Extending Human Presence into the Solar System," was written by a team of former NASA officials led by retired astronaut Owen Garriott and Michael Griffin, a senior manager at Johns Hopkins University's Applied Physics Laboratory who led NASA's short lived exploration initiative in the early 1990s.

NASA space flight officials say such a dramatically truncated space shuttle manifest is unrealistic.

William Readdy, NASA's associate administrator for space operations, told a Senate panel in May that switching to expendable rockets at this stage in the game would cost more and take longer than finishing the station with the shuttle.

"The [international space station] was designed to be carried into space and assembled using the space shuttle. The components have already been built, tested and most of them have been integrated and are awaiting launch at Kennedy Space Center," Readdy told the Senate Commerce science, technology and space subcommittee. "Switching to expendable launch vehicles at this point would result in what we estimate to be a minimum of four or five years delay in resuming [international space station]  assembly, require significant investment in new capabilities and require redesign and retesting of space station elements."

Sen. Sam Brownback (R-Kan.), the subcommittee's chairman, said during the hearing that he was not convinced that NASA had conducted a thorough assessment of alternatives to using the shuttle. Brownback and his counterpart in the House of Representatives, Rep. Dana Rohrabacher (R-Calif.), subsequently asked the U.S. Government Accountability Office (GAO) to look into the matter.

Alan Li, the GAO official heading up the review, said his team is interviewing NASA personnel at Johnson Space Center in Houston and at headquarters here to learn more about the depth of analysis behind Readdy's hearing pronouncements. Li told Space News the GAO would be ready to report its findings to Congress in the spring. Other U.S. government officials said NASA and other administration personnel are assessing whether as many as five flights could be cut from the manifest, but are not said to be contemplating anything as aggressive as the 2008 retirement backed by the Planetary Society.

NASA spokesman Allard Beutel said space shuttle officials settled on the 28-flight manifest only after careful consultation with international space station program officials in Houston and abroad. He said the 28 flights are what are seen as necessary to achieve the space station configuration agreed to when the space station partners met in July.

Michael Kostelnik, NASA's deputy associate administrator for the space shuttle and space station programs, told reporters Oct. 29 that he and NASA Comptroller Steve Isakowitz -- the agency's top budget official -- began an effort about a month ago to see how many of the planned 28 flights are absolutely essential. Kostelnik said the planning exercise was driven by budget concerns.

Another space shuttle report in the works is expected to add further to the retirement debate when it is published early next year. The Space Shuttle Children's Fund awarded a two-year grant to George Washington University here in March to study astronaut safety. Portions of the draft report leaked to the Internet site NASA Watch, which highlighted the authors' call for an independent assessment to analyze the feasibility of using "more robotic missions and expendable launch vehicles in lieu of the Space Shuttle" to reduce the number of shuttle missions NASA has to fly. The report also references interviews with unnamed astronauts and other government officials -- described as holding a minority view point -- that feel that the space shuttle ought to either never fly again or be restricted to a very small number of flights.

Joseph Pelton, the lead author on the paper and director of the Advanced Communications Research Institute at George Washington, said the report is still very much a work in progress and that he regrets that excerpts had been made public. He said a draft has been circulated for review by NASA officials but declined to provide Space News with a copy. Pelton said his team does not have a firm position on whether the shuttle could or should be retired earlier than 2010.

"We certainly have not taken a stance yet. That was just reporting that came from interviews," Pelton said of the "scuttle shuttle" view point expressed in the report. "There was a strong minority that felt that way, and a majority that felt the other way. We still have a long ways to go until we make our final recommendation."