The space shuttle Atlantis and a crew of seven astronauts are officially set for a planned May 11 launch to give the Hubble Space Telescope one last upgrade, NASA announced Thursday.

After more than half a year of delays, top shuttle mission managers found that Atlantis and its crew are ready to overhaul the 19-year-old Hubble for the final time. Liftoff is set for 2:01 p.m. EDT (1801 GMT) on launch day.

NASA announced the official launch date after a day-long meeting at the agency's Kennedy Space Center launch site in Florida to review the shuttle's readiness for flight.

In a NASA first, the space agency announced the shuttle launch decision via the micro-blogging Web site Twitter.

"The poll is unanimous. We are GO for a May 11 launch for space shuttle Atlantis' STS-125 mission to service Hubble," NASA spokesperson John Yembrick wrote in the Twitter announcement.

NASA will discuss the upcoming launch during a press conference on NASA TV at 5:00 p.m. EDT (2100 GMT).

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Last flight to Hubble

Commanded by veteran spaceflyer Scott Altman, Atlantis and its crew are slated to launch on an 11-day mission to Hubble.

Five back-to-back spacewalks are scheduled to add new instruments, replace broken gyroscopes and old batteries, as well as attach a docking mechanism for a future robotic vehicle.

The astronauts will also attempt unprecedented repairs on equipment never designed to be fixed in space.

If all goes well, the mission will extend Hubble's mission life through at least 2014, mission managers said.

Initially slated to launch in October 2008, the Hubble servicing mission has been delayed for months after a data handling unit aboard the space telescope failed unexpectedly last year. The added chore of fixing that broken part was added to the flight.

Mission managers initially targeted a May 12 launch for Atlantis, but decided last week to target an earlier liftoff in order to get at least three chances to lift off before standing down due to launch range traffic.

NASA launched the Hubble Space Telescope in April 1990 and has sent astronauts to repair or upgrade the observatory four times. With the space shuttle fleet set to retire in 2010, Atlantis' mission will be the fifth and last service call, NASA has said.

Rescue shuttle on standby

NASA is also prepared if the mission goes substantially awry. While Atlantis is in space, a second space shuttle - the Endeavour orbiter - will be on standby to launch a rescue mission in case of an emergency.

Unlike recent shuttle missions to the International Space Station, where astronauts can seek refuge if their orbiter suffers critical damage, Atlantis astronauts have no such safe haven.

They cannot reach the station from Hubble because the space telescope flies in higher orbit than the orbiting lab and in a different orbital inclination, or tilt relative to Earth's equator.

The mission also has a higher risk of damage from space debris, about a 1-in-221 chance of a critical strike.

Missions to the space station have about a 1-in-300 chance of being struck, NASA officials have said.

The space agency's benchmark for space debris risk is a 1-in-200 chance of a serious hit, they added.

Since Atlantis cannot reach the space station from Hubble's position, Endeavour is primed to launch within a week of a declared emergency with a sparse crew of four astronauts.

According to the plan, Endeavour would rendezvous with Atlantis, where the stricken shuttle's astronauts would perform three spacewalks to abandon ship and return home.

Atlantis would then be disposed of during re-entry over the Pacific Ocean, NASA officials have said.

NASA believes the likelihood of actually needing the rescue mission to be extremely remote.

Atlantis is currently scheduled to land on Friday, May 22 at the end of the Hubble mission.

Once the shuttle returns, NASA would begin preparing Endeavour for a planned June mission to continue construction of the International Space Station.

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