The U.S. space agency now says it has postponed the launch of the space shuttle until Friday.
NASA had planned to launch Wednesday on an 11-day mission. But the launch was delayed due to a problem with a fuel cell used to provide electricity to the shuttle.
was originally set to depart last month, but bad weather caused a series of postponements.
Space agency engineers hope to launch this week so that long-delayed work on the International Space Station can resume. Construction stopped after the shuttle disaster in February 2003 forced a long moratorium on shuttle flights.
Failure to launch the shuttle this week could delay the mission until October.
Astronauts on the mission are expected to attach a 17-ton addition to the space station. It will include a new set of giant solar energy panels that will reach 80 meters in length when fully unfurled.
The launch schedule date is dictated by the need to avoid conflict with a planned September 14 Russian launch with a new Russian-American space station crew.