Despite ongoing work to launch its next shuttle flight by July, NASA's greatest challenge lies further ahead in the coming shift to a new spaceship by 2014, the U.S. space agency's chief said Tuesday.

NASA administrator Michael Griffin said that proper handling of the agency's workforce while it is refocused - and reduced - for the Crew Exploration Vehicle is vital for nation's spaceflight future as the shuttle fleet approaches a 2010 retirement.

"To be clear, NASA will not need as many engineers and technicians on the shop floor to operate and maintain the CEV and Crew Launch Vehicle (CLV) as we do today with the space shuttle," Griffin told the Senate's science and space subcommittee, adding that both new vehicles are designed to be simpler and cheaper than NASA orbiters. "Change is hard, but if we don't act now to bring it about, we will not develop the space program that we want to have."

Some shuttle workers will be transferred to other CEV support programs where their skills can be applied, Griffin added.

Griffin also maintained that he is optimistic NASA will meet its July window to launch the STS-121 shuttle mission to the International Space Station (ISS), marking the agency's second test flight following the 2003 Columbia disaster. Shuttle program managers are expected to pick a launch date from the mission's July 1-19 window on Thursday, he added.

NASA hopes to launch as many as three shuttle missions this year - beginning with STS-121 and followed by two ISS construction flights - pending the resolution of ongoing external tank wind tunnel tests to verify modifications to the tank's foam insulation.

Griffin said the shuttle is key to completing the ISS by 2010, but the space station's value as a science platform will be muted without a reliable orbital transit system beyond the shuttle's retirement.

"We can only realize the potential of the space station if we have a robust space transportation system to ferry crew, experiments and equipment to and from the station," Griffin said.

Senators made it clear that minimizing any gap in U.S. human spaceflight capability during the gulf spanning the 2010 shuttle retirement and 2014 deadline for the CEV is paramount.

"If there is a capability for Americans to go into space between 2010 and 2014, I will feel much more secure," Sen. Kay Bailey Hutchinson (R-Texas), chair of the space and science subcommittee, told Griffin.

But the NASA chief stressed that technological and funding challenges made a seamless transition almost impossible, given the space agency's tasks of completing the ISS before the shuttle fleet retires, its exploration push to fly manned CEV flights by 2014 and the agency's goal of returning astronauts to the Moon by 2020.

"We are technology limited to the 2011 to 2012 timeframe, we're funding limited for later dates than that," Griffin said, adding that he hopes to secure commercial cargo - and later crew - access to orbit and the ISS during that 2010 to 2014 gap.