Countdown to the final mission for the US space shuttle Discovery has begun, with Nasa confident that fuel tank cracks are fixed and the shuttle is ready for its 39th and last launch tomorrow.

The aging shuttle is “in good shape,” Nasa test director Jeff Spaulding said, and the weather forecast was 80 per cent favourable for pleasant conditions at launch time, (21.50 GMT) tomorrow.

Nasa experts have repeatedly voiced confidence in repairs that shored up the brackets on the external fuel tank, after cracks emerged upon fuelling just ahead of the planned November launch, postponing the mission until now.

Another mishap befell the mission in January, when astronaut Tim Kopra was injured in a bicycle accident. He has been replaced by another veteran astronaut, Steve Bowen.

Discovery, which first flew in 1984, will be the first shuttle to enter retirement when it concludes this mission. The other two remaining in the fleet, Atlantis and Endeavour, are slated for their final flights later this year.

The shuttle has been a “workhorse, ambassador, scientist and equal opportunity emissary,” Nasa said on its website, noting that Discovery has flown more missions than any other spacecraft and made 5,600 trips around the earth.

Discovery was the first shuttle to be piloted by a woman in 1995 and carried the first female shuttle commander into space in 1999 – both times the same pioneer, Eileen Collins.

Other historic firsts to come aboard Discovery were the first African American spacewalker, Bernard Harris in 1995, and the first sitting member of Congress to fly in space, Senator J ake Garn in 1985. In addition to the 180 people Discovery will have carried into space by the end of this mission, the shuttle sent the Hubble Space Telescope into orbit and launched the Ulysses robotic probe on its journey toward the Sun.

Discovery was also the first “to rendezvous with the Russian Mir Space Station, and it delivered the Japanese Kibo laboratory to the International Space Station”, Nasa noted.

This mission is due to last 11 days and the six-member crew will deliver and install a new module to the International Space Station.

The Permanent Multipurpose Module will provide room for extra storage and space for experiments.

Discovery will also bring Robonaut 2, “the first dexterous humanoid robot in space”, said Nasa.

Astronauts will first test how it works in microgravity before figuring out how upgrades could graduate the robot to a full-fledged space assistant.

The three remaining US shuttles are due to become museum pieces once the final shuttle mission takes place, leaving the Russian space program’s Soyuz capsule as the sole method to bring astronauts to and from the ISS.

Endeavour is set for take-off on April 19 and Atlantis is scheduled for June 28, though funding for Atlantis remains in question.

There were initially five space shuttles in the fleet – Challenger exploded shortly after liftoff in 1986 and Columbia disintegrated on its way back to earth in 2003.

The sixth shuttle, Enterprise, did test flights in the atmosphere but was never flown into space. It is already on display at a museum outside Washington.

Earlier this year, the US company SpaceX succeeded in sending its Dragon space capsule into orbit and back, but it will likely be several years before a private US spaceflight can carry crew and cargo to the ISS.