The standby plan, intended to prevent a disaster like last year's destruction of the shuttle Columbia, which killed all seven astronauts on board, would call for the first crew to find safe haven on the International Space Station if the shuttle is incapable of a safe landing.
Then a second shuttle would be launched to the space station to bring the first orbiter's crew back to Earth.
"For the first flight, we're going to have the capability to do this," said Michael Kostelnik, a senior official in Nasa's shuttle and space station programme in Cape Canaveral.
"The second vehicle would be able to launch and go to the International Space Station and pick up the first crew if we had a problem.
"There will probably be more changes on this flight than any previous flight" since the first, in 1981, Kostelnik said.
"Our experience for subsequent flights will be determined by our success and our problems with the first flight"
senior official, Nasa
The plan does not require both shuttles to be on their launch pads at the same time.
If no problem develops with the first flight, led by shuttle commander Eileen Collins, the rescue crew, led by Steven Lindsey, will make the next scheduled flight to the station, but the rescue contingency will remain an option in the future, Kostelnik said.
"Our experience for subsequent flights will be determined by our success and our problems with the first flight," he said.
Nasa had hoped to return to shuttle flight in September or October, but space agency spokesman Allard Beutel said on Thursday the expected launch window has been postponed to 6 March 2005, through 18 April 2005, to give Nasa time to test its external fuel tank and develop a camera boom that will let shuttle crews examine the outside of the craft.
The flight will be the first after a large number of recommended shuttle modifications, from removal of the fuel-tank foam insulation piece that broke off and damaged Columbia's wing, to a new docking manoeuvre that requires some non-powered flight when the shuttle reaches the space station.
The crew all lost their lives when
returning to Earth
Nasa is addressing a problem risk-assessment experts have warned of ever since the Columbia Accident Investigation Board mandated a lengthy list of safety reforms after Columbia blew apart upon re-entry over Texas last year.