The space shuttle's pending upcoming mission to continue maintenance and repair work on the Hubble telescope, which had been set for August 28, will be pushed back four to five weeks, NASA said Thursday.

"Right now Hubble's mission is scheduled for August 28; we really cannot make that date with the external tank processing," shuttle program manager John Shannon explained at a press conference Thursday.

"The changes we have made add about four to five weeks of processing time on those two tanks," he said.

The shuttle's mission to Hubble -- orbiting at 600 kilometers -- is highly unusual because it requires a second shuttle to be ready to launch on standby.

To do that two external tanks have to be prepared at the same time, which makes production a bigger challenge and takes longer.

In case of a major problem with the Hubble mission the shuttle and its crew of seven cannot head to dock at the International Space Station (ISS). So a second shuttle needs to be at the ready, to be launched in case of emergency.

"I would expect that sometime this month we will come forward and not just have the Hubble mission date ... but really some pretty good numbers for the next six to ten flights," Shannon said.

Hubble revolutionized astronomy when it was launched in 1990 as the first orbiting space telescope, but some of its devices have since failed, reducing its capabilities.

NASA plans to install new batteries, a wide-angle camera and other technology to sharpen Hubble's gaze into the universe and extend its working life by up to 10 years, mission officials have said.

The next shuttle launch carrying the main module of the Japanese lab Kibo is scheduled for May 31. It will be the 10th mission since shuttle flights resumed after the February 2003 Columbia disaster.

Following the upcoming mission, there will be another 10 or so shuttle flights before the current fleet is retired in 2010.