NASA officials are searching for ways to keep a skilled U.S. aerospace workforce on hand during the anticipated four-year operating gap between the space shuttle and the agency’s future astronaut transport.

Current plans call for retiring the space shuttle fleet by about 2010 as part of the space exploration vision set by U.S. President George W. Bush last year. That vision calls for a renewed push in human exploration of the Moon and eventually Mars.

But to do that, NASA will need a new astronaut -carrying spacecraft — dubbed the Crew Exploration Vehicle — that is still in the early planning stages and not expected to make its first manned flight before 2014. Meanwhile, NASA and its international space station partners hope to continue operating that orbital facility through 2016.

It’s a recipe for trouble, experts warned at the third Integrated Space Operations Summit (ISOS) held here during the last week of March.

“That to me is the period where we could lose it all,” said John Douglass, president and chief executive officer of the Aerospace Industries Association, a Washington-based trade group. “To have a program that ends without a replacement starting up … it’s somewhat incomprehensible.”

Earlier in March , the U.S. Government Accountability Office released a report criticizing NASA for not doing enough to keep its skilled workforce beyond the retirement of the space shuttle. Such skills are vital for the development of a new crew-carrying vehicle or a shuttle-derived heavy-lift launcher .

“Some folks who were faced with the announcement of the vision assumed that this [shuttle] program was over today and have already left for other pursuits,” said David Throckmorton, deputy director of NASA’s Stennis Space Center in Mississippi, who led a summit panel studying the shuttle program’s mission execution.

Much of the ISOS summit revolved around easing the looming transition period spanning the shuttle’s retirement and the beginning of manned missions aboard the Crew Exploration Vehicle . NASA officials repeatedly stressed the need to find a way to retain the skills of its current workforce while recruiting engineers to pursue future human space exploration missions.

Lesa Roe , deputy director of NASA’s Langley Research Center, Hampton, Va., said shuttle-specific programs employ about 20,000 people both inside and outside the space agency, with various efforts spread across 640 facilities throughout the country.

“Thirty-four percent of facility usage in the agency is used by the shuttle program,” Roe said. “What we need to do is not wait. We need to go ahead and maintain those skills and look at it as a return on an investment, which we will need in a couple of years.”

There are some doubts that NASA can meet its current target of completing construction of the international space station by 2010, a feat shuttle officials say will take an average of five shuttle flights per year — an estimated 28 missions overall — to accomplish.

“Five flights a year might be a bit optimistic,” Dennis Granato, director of missile defense programs at Northrop Grumman , said during a discussion on industry capabilities for shuttle transition.

Granato said NASA followed the 1986 Challenger accident, in which the shuttle exploded during launch with seven astronauts aboard, with a robust flight program that peaked at seven missions in a single year. But that momentum ebbed, dropping back to less than five missions a year leading up to the launch of Columbia.

Currently, NASA plans to launch up to three shuttle missions this year, with flight windows opening in mid-May, mid-July and early December, using the Discovery and Atlantis orbiters. The shuttle Endeavour has been undergoing a major overhaul since 2003.