Florida ? Atlantis blasted off Monday for the Hubble Space Telescope on the most delicate and dangerous repair job ever in orbit — a mission so risky that for the first time a second shuttle stands ready to rescue the seven astronauts if something goes wrong.

It will be the last visit to the aging observatory, and the work will include five spacewalks in an especially high orbit strewn with space junk. The astronauts will try to fix equipment that was never designed to be tinkered with in space, and they will not have the option of using the international space station as a safe harbor in a crisis.

The improvements will extend the life of one of the space program’s proudest achievements — a technological marvel that has yielded breathtaking pictures of distant galaxies and some of the celestial violence that shaped the universe.

The crew will replace Hubble’s batteries and gyroscopes, install two cameras and take a crack at fixing two broken science instruments.

Mission Control wasted no time informing the astronauts that an early look at the launch video had uncovered nothing of concern. Some debris was spotted coming off the fuel tank, but nothing appeared to strike Atlantis. The astronauts inspected the thermal shielding on their crew cabin Monday evening; a full-scale survey of the shuttle was set for today.