CAPE CANAVERAL, Fla. — NASA could keep its shuttle fleet operating through 2015 and close a five-year gap in U.S. human space flight, but the cost would top $11 billion, an internal agency study shows.

The risk to U.S. astronaut crews would rise dramatically; plans for lunar exploration would be severely hampered.

But NASA could retain critically skilled workers during the transition between the shuttle program and Project Constellation, the nation's bid to return astronauts to the moon by 2020.

"This option eliminates the gap between shuttle and Constellation operations and allows NASA and its contractors to maintain the critical skills necessary to successfully operate future human space flight programs," NASA officials wrote in a draft of the study, a copy of which was obtained by Florida Today.

Charts show the current shuttle work force — which now numbers 11,900 — would gradually decrease to 7,900 in 2015 rather than drop to zero in 2011.

NASA expects one-third of that current shuttle work force — or about 3,500 people — to lose Kennedy Space Center contractor jobs after shuttle fleet retirement. The other two-thirds are expected to move on to the new moon program or find other work at KSC.

NASA now is operating under a Bush Administration directive to finish the International Space Station and retire the shuttle fleet by September 2010.

The U.S. then would rely on Russia to fly American astronauts to and from the station until Ares 1 rockets and Orion spacecraft are ready to fly in March 2015.

The Russian invasion of Georgia last August prompted President-elect Barack Obama and others to question the reliance on Russia.

NASA Administrator Mike Griffin ordered two studies aimed at minimizing the gap. One examined extending shuttle operations; the other focuses on speeding up the Constellation project.

Several options were examined in the shuttle extension study. Two were considered the most viable.

• Option 1: Add three shuttle flights to the current backlog of nine missions, extending fleet operations through 2012. Cost: $5 billion.

• Option 2: Add up to 13 flights through 2015. Cost: $11.4 billion.

The chance of losing a shuttle crew increases significantly in either case.

The probability of a catastrophic accident during nine remaining flights is 1 in 8, the report said. Adding three flights through 2012 increases the odds to 1 in 6; adding 13 flights through 2015 increases the probability to 1 in 4.

The risk of losing a crew on any given flight is 1 in 77, the report said.

A two-year extension would be a boon for International Space Station operations. Three supply runs and crew rotations would be added. Only "minor negative impacts" to Constellation program milestones would be expected.

A five-year extension would push back the moon program. Engine test, rocket manufacturing and launch facilities would not be freed up in time to restart lunar exploration by 2020. The anticipated delay: Up to three years.

Decision deadline: May. NASA would have to pursue either option by then to reverse shuttle program shutdown plans.