The final launch of the space shuttle Discovery was delayed on Tuesday after a problem with an electrical system was discovered during pre-launch engine tests.
The launch is now aimed for Thursday if NASA engineers can fix the problem.
Having lasted three times as long as most vehicles used by Earth-bound individuals, Discovery will launch as the US space agency NASA retires the ageing shuttles and begins to transition routine flights to commercial providers.
The launch was already delayed for two days because of repairs of helium and nitrogen leaks in one of the shuttle's engine pods and subsequent inspections.
The mission will deliver the last major US contribution to the International Space Station (ISS) - an extra room - along with supplies, including a human-like robot, known as Robonaut 2 (R2), the first-such robot ever sent to space.
The oldest vehicle in the operating space shuttle fleet, Discovery entered construction in 1979 and blasted off into space for the first time in 1984.
After Discovery's planned 11-day mission, the workhorse of the fleet will have spent nearly a year in orbit, made more flights than any other shuttle and carried more crew members.
In February, the shuttle Endeavour is slated to make the absolutely last shuttle programme flight to the ISS, although NASA is pushing for funding for another flight for Atlantis in summer 2011.