For the first time since 2001, both of NASA’s launch pads are occupied. One of them will be the base of Atlantis' lift off on October 10,on its Hubble repair mission, while at the second launch pad is parked Endeavour, the shuttle which will be on stand by in case a rescue mission will be required.

The mission of the STS-125 crew aboard Atlantis is to return to the Hubble Space Telescope for one last time before the shuttle retires in 2010. The 11-day visit will include 5 spacewalks for repairs and upgrades to the telescope meant to ensure it’s functioning for another five years or more.

Commander Scott Altman, Pilot Gregory C. Johnson, and Mission Specialists Andrew Feustel, Michael Good, John Grunsfeld, Mike Massimino and Megan McArthur form the fifth shuttle crew to fly to the Hubble telescope.

NASA had intended to mothball the Hubble before the new telescope was in place, a decision that was met with protests among astronomers who have been able to look into space 2.2 billion light years and more because they don't have to peer through Earth's atmosphere.

Missions to the space station are easier because ISS crew is on hand to help inspect the shuttle. The ISS also offers up to three months refuge for visiting crew in case of an emergency. The Hubble, which orbits 580 kilometers above Earth, offers neither. That means the shuttle would have to survive on its own for up to 25 days, with Endeavour on stand-by at a separate launch pad for a rescue mission.

If Atlantis won’t be able to ensure the return home of the astronauts, Endeavour will be ready to launch, and will be manned by the flight deck crew of STS-123.

A year ago, the Hubble telescope's most far-seeing camera shut down due to a possible power failure and other problems, prompting NASA engineers to put the entire telescope on temporary standby. The Advanced Camera for Surveys (ACS) was installed in 2002 in a special shuttle mission to replace the old space camera - in orbit since 1990 - and was hailed as the gateway to some of humankind's most spectacular views of the universe.

The STS-125 mission aims to install a cosmic origins spectrograph and to replace a wide field camera in operation since 1993 with a Wide Field Camera 3. This latest camera will be the first on the Hubble that can cover everything from the ultraviolet to the infrared spectrum.

The aging telescope is also in need of new batteries, new gyroscopes and a new thermal blanket to insulate it from the severe temperature swings as it orbits Earth every 45 minutes. Also scheduled for fixing is the Advanced Camera for Surveys.

The Hubble mission started back in 1990, when the shuttle Discovery launched and released the telescope into the orbit. Since then, it has circled around Earth over 97,000 times, and has provided numerous answers in ways that would have been impossible from Earth observations.

Theoretically, the James Webb observatory will replace Hubble in 2013 the earliest. The Hubble Space Telescope (HST) was first conceived in 1946 by astronomer Lyman Spitzer, constructed since 1979 and launched in 1990.

STS-125 is facing also another great risk: to be compromised by orbital debris. Earlier this month the shuttle program manager John Shannon revealed that in case of STS-125 the chances for a catastrophic collision with debris are 1 in 185, compared with the usual 1 in 300.

Although space missions are known to have their share of danger, from launch to landing, the orbital debris increase the risk like never before, making the mission even more complicated.