After 26 years of flights and more than 142 million miles traveled, the NASA space shuttle Discovery is hitting a string of tough luck.

Today's scheduled launch was scrubbed because of a gaseous hydrogen leak near the spacecraft's external tank. This is the fourth time in the past week that Discovery's launch was delayed due to various leaks and electrical issues. Shuttle launch director Mike Leinbach characterised this latest leak as "significant."

NASA now is aiming for a Monday November 8, launch. If that one is postponed as well, the next launch opportunity will be November 30, according to the space agency.

With NASA retiring its entire shuttle fleet, this is Discovery's last planned flight. The shuttle craft, the third added to NASA's fleet and now the oldest still in operation, made its maiden voyage in 1984. Notably, Discovery was the "return to flight" shuttle after both the Challenger and Columbia accidents.

Discovery has had a productive run, travelling 142,917,535 miles in space, making 5,628 earth orbits and carrying a total of 246 astronauts aloft in its 38 flights. When the shuttle does lift off again, it will be making history.

A humanoid robot, dubbed Robonaut or R2, is packed away in Discovery's cargo bay ready to be transported to its permanent residence on the International Space Station.

The 300-pound Robonaut is made up of helmeted head, a torso, two arms and two hands. The robot's hands are designed to mimic human hands so it can use tools already aboard the station for use by the astronauts there.

At this point, Robonaut is only a torso secured to a platform but NASA engineers plan to add either a single leg or two legs, depending on what will be more useful to the robot working inside the space station, as well as outside aiding in spacewalks.

John Olson, director of NASA's Exploration Systems Integration Office, said in an earlier statement that Robonaut is a step in the "quickening pace between human and robotic exploration of space."