Space shuttle Discovery's final mission is off until February, three months late because of fuel tank cracks that are stumping engineers.
Nasa's top spaceflight managers said they need more time to understand the cracking, which cropped up following a failed launch attempt in early November.
Discovery remains on the launch pad, holding a load of equipment for the International Space Station.
The launch team plans to conduct a fuelling test by month's end - rigging the external tank with gauges and sensors, then loading it up - in hopes of cracking the elusive crack problem.
"Analysis can only get you so far," said shuttle programme manager John Shannon. "It's time to go test."
Bill Gerstenmaier, head of Nasa's space operations, said lift-off tentatively is set for around February 3, the opening of the next practical launch window. That will push the final mission of shuttle Endeavour back a full month, into April.
A series of unmanned cargo ships from other countries are due to fly to the orbiting lab in the next few months, complicating matters. Also on tap is the arrival of a new station crew in mid-December, via a Russian Soyuz spacecraft.
Six astronauts are assigned to the Discovery mission, as well as the first humanoid robot bound for space, Robonaut, which will stay packed aboard the shuttle.
Mr Gerstenmaier said the postponements should not affect Nasa's effort to secure funding for an extra shuttle mission next summer. Nor should they hinder space station activities.
Nasa is retiring its shuttle fleet next year under the direction of the White House, after 30 years of flight.