Washington - NASA gave the green light Monday to the May 31 launch of Discovery shuttle, which will carry the largest payload ever to the space station and mark a half-way point to mothballing the shuttle programme.

The mission will be the tenth shuttle flight since NASA returned to space in July 2005 after the 2003 Columbia disaster, marking "something of a milestone," said William Gerstenmaier, director of NASA's human exploration of space.

After the upcoming mission, only ten more shuttle flights are scheduled until the ageing spacecraft are mothballed for good in 2010, he noted.

Delivery and installation of the long-awaited centre piece of Japan's Kibo laboratory, named for the Japanese word for hope, is the main goal of the mission, NASA officials told reporters in a webcast news conference from the launch station at Cape Canaveral, Florida.

NASA also said it has confidence in the ongoing Russian investigation into the near catastrophe last month, when a returning Soyuz spacecraft carrying three astronauts hit the atmosphere at the wrong angle and crashed to Earth 400 kilometres off target in the steppes of Kazakhstan.

The perilous entry scorched the hatch and melted down the craft transmitting device, cutting off communication. A Soyuz craft is normally parked at the orbiting International Space Station as an emergency escape vehicle for the three resident astronauts.

Gerstenmaier said the Russians were carrying out a "very thorough" investigation but the officials still had "a lot of work ahead of them."

"We know Soyuz is acceptable for emergency return. We haven't seen anything that says it's not acceptable," Gerstenmaier said. (dpa)