WASHINGTON -- The senior U.S. senators from Florida and Texas are pushing back against NASA's plan to retire the U.S. space shuttle fleet by the end of the decade regardless of whether a replacement vehicle is ready to enter service by then.

Sens. Kay Bailey Hutchison (R-Texas), chairwoman of the Senate Commerce science and space subcommittee, and Bill Nelson (Fla.), her Democratic counterpart, introduced a bill June 21 that would require NASA to keep the space shuttle orbiter flying until a new crew transport vehicle has flown.

NASA Administrator Mike Griffin is adamant about retiring the shuttle in 2010, but has said he intends to accelerate development of the proposed Crew Exploration Vehicle (CEV) in order to minimize any gap in the United States' ability to put humans in space. NASA previously had planned to field the CEV in 2014. Although Griffin has said he wants the CEV to be ready before then, he has not made its availability a precondition for retiring the shuttle.

The bill sponsored by Hutchison and Nelson would change that.

Specifically, the bill says, "In order to ensure continuous human access to space, the Administrator may not retire the Space Shuttle orbiter until a replacement human-rated spacecraft system has demonstrated that it can take humans into Earth orbit and return them safely."

The space shuttle language is included in a broader measure, S. 1281, which authorizes appropriations for NASA for 2006-2010.

While the bill endorses NASA's new exploration goals, which include returning astronauts to the Moon by 2020 in preparation for eventual trips to Mars and beyond, it parts with NASA on both shuttle retirement and on plans to eliminate international space station-based research that does not directly support the space agency's exploration plans.

The Senate Commerce Committee is scheduled to vote on the legislation June 23. Lawmakers in the House of Representatives, meanwhile, intend to introduce their own NASA authorization bill June 27. The House version of the bill, according to sources familiar with it, would not require NASA to keep flying the shuttle until the CEV is ready. The House and Senate must sort out any differences between their respective versions of a bill before it can become law.

Congress last sent a NASA authorization bill to the White House for the president's signature in 2000.