NASA’s latest setback on the road to completing the international space station comes as the U.S. space agency prepares to roll out its plan for replacing the space shuttle fleet with the new vehicles that will be needed to carry astronauts and equipment to the Moon.

The Space Shuttle Discovery’s return to flight July 26 was marred by a close call with a falling chunk of insulating foam that broke free from its external tank about two minutes after liftoff. NASA officials previously had said they made great strides in reducing tank debris in the two and a half years since a chunk of foam brought down the Columbia, agency officials said. After the July 26 launch incident, they admitted they were wrong and said they clearly have more work to do and will not fly the shuttle again until the foam problem is solved.

While NASA Administrator Mike Griffin said the space agency would fix the external tank’s remaining foam shedding problems “in short order” and launch again — perhaps this year — he also said the incident is a reminder of why the White House has called for retiring the shuttle by the end of 2010 and replacing the three-orbiter fleet with an inherently safer space transportation system.

Griffin told reporters July 29 that NASA has settled on launch vehicle designs that make use of the space shuttle’s major components, including the main engines, solid rocket boosters and external tank. Griffin said NASA’s plans, the results of an intensive Exploration System Architecture Study Griffin ordered in late April to plot a return to the Moon by 2020, would be publicly unveiled in “a few weeks.”

Sources with detailed knowledge of the study results said NASA will need to spend $5 billion to develop the crew launcher, a price tag that includes the solid rocket booster-based vehicle itself, a new upper stage and all necessary launch infrastructure.

The Crew Exploration Vehicle, which NASA hopes to field around 2011, is expected to cost another $5 billion to develop and would be designed both to service the space station and to carry astronauts to lunar orbit. A heavy-lift launcher capable of delivering 125 metric tons of cargo to low Earth orbit would be finished after the smaller crew launch vehicle, according to NASA’s plan, and would also cost in the neighborhood of $5 billion to develop.

The Crew Exploration Vehicle, according to NASA’s plan, will be a capsule capable of accommodating three people and a limited amount of cargo for space station missions, a crew of four for a lunar mission and up to six people to dock with an awaiting Mars-bound vehicle.

Unlike the shuttle orbiter, which is mounted to the side of the external tank and in the path of foam and other debris coming off the tank, the Crew Exploration Vehicle would launch at the top of the stack — out of the way of any debris the booster and upper stage might shed . NASA would adopt the same type of in-line design for the heavy-lift launcher as well, putting the cargo canister up top.

“As long as we put the crew and valuable cargo up above the tank we don’t care what they shed,” Griffin said July 29, explaining to reporters why he remains confident in the shuttle-derived approach even after Discovery’s close call.

Prior to Discovery’s return to flight, NASA also was closing in on its options for completing the international space station. Sources familiar with the planning effort said agency officials have been looking at two main scenarios.

One scenario involves conducting 17 shuttle flights before retiring the shuttle in September 2010, and includes launching Europe’s and Japan’s space station modules, but not outfitting them with additional science equipment. The other scenario involves conducting 11 shuttle flights and postponing launching the international partners’ modules until NASA completes its proposed heavy-lift launcher.

Knowledgeable sources said the 17-flight scenario will become more tentative with each month the shuttle is down and more or less untenable if the shuttle is not flying again come spring.

While Griffin said NASA has not given up on flying in September, experts outside the agency said it is unlikely the shuttle will be cleared for flight in time for that launch window.

“September seems awfully close,” said James Hallock, a member of the Columbia Accident Investigation Board. “I think they’ve got a lot of things to do.”

NASA would have one last shot for the year in November, but that window is only a couple days long. NASA first attempted to launch Discovery July 13, but that launch campaign was stood down nearly two weeks to give the agency time to troubleshoot a not yet fully resolved fuel sensor glitch.

One former spaceflight official said NASA could be facing a longer delay than it is ready to admit.

Griffin said he is not ready to concede defeat and has appointed a “tiger team” to tackle the foam problem and find a fix.

Griffin also said that until NASA knows more, he will not rule out getting the international partners’ space station modules launched before the shuttle is retired. He also said NASA’s position has not changed on sending a shuttle crew to service the Hubble Space Telescope, which needs new batteries and gyroscopes to keep operating beyond 2008.

“We want to service Hubble if we can,” he said. “We will not know if we can until we have done the two test flights that comprise our return-to-flight sequence.”

NASA was widely praised for swiftly deciding to stand down the shuttle fleet until it understands and fixes the foam problem. White House spokesman Scott McClellan said President George W. Bush is confident in Griffin and his team at NASA and appreciates the agency’s commitment to safety. House Science Committee Chairman Sherwood Boehlert (R-N.Y.) and space and aeronautics subcommittee chairman Rep. Ken Calvert (R-Calif.) issued a joint statement saying “NASA is handling this situation exactly as it should.” Former NASA Administrator Sean O’Keefe told Space News that NASA’s recent actions are proof positive that a safety-oriented mindset has taken hold since the Columbia accident. “If there was still any doubt that the culture has changed, this should end it,” O’Keefe said.

A number of space policy observers looking beyond the immediate engineering challenge ahead to the bigger picture, saw the turn of events as a significant setback for NASA.

“We have a series of commitments that require flying the shuttle, that’s why this is such a nasty problem with no clear escape route,” said John Logsdon, a Columbia Accident Investigation Board member who directs the Space Policy Institute at George Washington University here. “I don’t envy the people who have to figure their way out of this corner.”

Brian Berger is editor in chief of and the SpaceNews magazine. He joined in 1998, spending his first decade with the publication covering NASA. His reporting on the 2003 Space Shuttle Columbia accident was...