CAPE CANAVERAL, Fla. — Astronauts strapped into space shuttle Atlantis today for one last flight to the Hubble Space Telescope, an extraordinarily ambitious mission NASA hopes will lift the celebrated observatory to new scientific heights.

The six men and one woman who will attempt the complicated, riskier-than-usual job shouted, waved and raised their fists as they headed out to the pad, eager to get going after waiting seven months to fly. Their flight was delayed last fall, two weeks before the scheduled launch, after the orbiting telescope broke down.

"Let's go!" commander Scott Altman said. "Yeah!"

Near perfect weather was forecast for the afternoon liftoff. NASA also was keeping an eye on the weather at the emergency landing strip in Spain, where there was a slight chance of rain.

The astronauts climbed aboard Atlantis late this morning. A mile away on the other pad stood shuttle Endeavour, primed for a rescue in case Atlantis is battered by launch debris or space junk.

Many of the expected 30,000 onlookers began gathering at Kennedy Space Center for the launch — scheduled for just after 2 p.m. EDT — including space center workers and guests. The Hubble scientists and managers were euphoric to finally be so close to liftoff.

The 19-year-old Hubble, last visited by astronauts seven years ago, is way overdue for a tuneup.

On this fifth and final repair mission, two spacewalking teams will replace Hubble's batteries and gyroscopes, install two new cameras and take a crack at fixing two broken science instruments, something never before attempted. Those instruments, loaded with bolts and fasteners, were not designed to be tinkered with in space.