NASA's upcoming service call on the Hubble Space Telescope won't happen till late September or early October. Until recently it had been scheduled to begin in late August. Rumors about the slip have been circulating for weeks. They were confirmed yesterday by Space Shuttle program manager John Shannon, as reported in the Houston Chronicle.As with most other launch delays over the history of the Hubble program, the source of the slip isn't the telescope it's the shuttle. NASA needs more time to prepare two giant external fuel tanks for the mission, which will be flown on the shuttle Atlantis. Normally only one tank is needed. But if anything goes wrong during ascent, the astronauts won't be able to seek safe haven at the International Space Station, which is in a different orbit than Hubble. So NASA will have a second shuttle on the launch pad, poised to come to Atlantis's rescue if needed.
You probably remember that in 2004 NASA administrator Sean O'Keefe cancelled the upcoming servicing mission. He made the controversial move because of safety concerns after the February 2003 Columbia disaster. When Michael Griffin took over as NASA chief two years later, he reexamined the issue and concluded that servicing Hubble is actually no more risky than visiting the space station as long as that second-shuttle-on-the-pad rescue option is available.
Why does Hubble need servicing again? After all, it's been repaired and upgraded four times already. Well, space is a harsh and unforgiving environment, with extreme temperature swings, an unending rain of meteoroids and high-energy particles, and other hazards. Hubble has been up there for 18 years now, and some of its critical parts including the batteries that power its instruments through orbital night are original equipment. So some amount of refurbishment is needed just to keep the telescope operating.
More important, though, is keeping Hubble at the forefront of scientific discovery. The way to do that is to replace old instruments with new state-of-the-art ones. So not only will Atlantis's astronauts deliver new batteries, gyroscopes, insulation, and other mechanical and electrical components, but they'll also install the Wide Field Camera 3 and the Cosmic Origins Spectrograph. These two new detectors will enhance the telescope's "discovery power" (the product of its areal coverage and sensitivity) by a factor of 100 or so, ensuring that Hubble will continue to reveal new cosmic wonders until and even after the larger James Webb Space Telescope goes into orbit around 2013.
You can follow preparations for the Hubble servicing mission at the STS-125 website (in NASA parlance, it's Space Transportation System mission number 125).