The U.S. space agency has again delayed the launch of the final mission of the space shuttle
NASA postponed the launch from Wednesday until at least Thursday after discovering electrical problems.
Earlier delays were caused by leaks of nitrogen and helium and because a critical payload hardware was not ready in time.
When the 11-day mission begins, the shuttle will carry six American astronauts to the International Space Station, as well as deliver a final module for the U.S. portion of the orbiting laboratory.
The shuttle will also carry the first humanoid robot into space, Robonaut 2 - or R2.
With human-like arms, a powerful-looking torso and a gold-helmet head, R2 looks something like a comic book superhero.
Once aboard the space station, the robot will become a permanent resident, where, initially, it will do little more than hold tools. NASA envisions R2 will eventually assist astronauts during spacewalks.
On its return to Earth, will be retired along with shuttles and after a final mission by in February. has completed 38 missions in 26 years. It is the oldest of the three shuttles.
After NASA retires its shuttle fleet, partners in the space station will use Russian and private rockets to send crew and supplies to the orbiting laboratory.