HOUSTON, Texas (AFP) - - The Atlantis shuttle crew began final steps Tuesday to release a revamped Hubble Space Telescope back into its orbit after a grueling repair mission aimed at extending "humanity's quest for knowledge."

US astronauts who hauled the 19-year-old orbiting observatory into the ship's payload bay last week and spent the past five days giving it a new lease on life were preparing for a fond farewell when they release it from the shuttle's robotic arm at 1253 GMT.

This is US space agency NASA's last mission to the aging but beloved Hubble, whose spectacular images have helped broaden humankind's understanding of the universe as it peers ever deeper into the cosmos.

As part of the operation, mission specialist Megan McArthur reached out and grabbed onto Hubble using the robotic arm and prepared to lift the delicate stargazer out of the payload bay.

"We just maneuvered to the release position," McArthur told Mission Control here exactly one hour prior to release.

Teams on the ground will then command open the telescope's aperture door, a large shutter that protects Hubble's primary and secondary mirrors.

Commander Scott Altman and Pilot Greg Johnson will carefully guide Atlantis away, before subtly firing its thrusters to establish a safe distance between the ship and the telescope.

During the last planned human interaction with the iconic observatory, spacewalking astronauts finished their work on Hubble on Monday with a sentimental final outing that lasted about seven hours and left nothing on their mission's wishlist unfulfilled.

The shuttle crew equipped Hubble with upgrades that will extend its life by at least five years.

NASA's shuttles, facing retirement by the end of next year, launched the space telescope in 1990. Shuttle crews returned in 1993, 1997, 1999 and 2002 to upgrade the telescope with new science instruments and replace failed parts.

"Hubble has returned to flagship status. It now has a full arsenal of instruments and tools for astronomers to make new discoveries," said Jon Morse, NASA's chief astrophysicist.

"Our work is just beginning. We have thousands of astronomers around the world waiting to get their data. They are chomping at the bit."

The refurbished telescope will undergo three to four months of re-commissioning, a period during which each of the four cameras and spectrographs either installed or repaired by the Atlantis astronauts will be checked and re-calibrated before scientists resume their observations.

"This is a really tremendous adventure we've been on, a very challenging mission," said John Grunsfeld, the astronomer-turned-astronaut who led three of the shuttle mission's five spacewalks.

"Hubble is not just a satellite, it's about humanity's quest for knowledge."

The Atlantis astronauts plan to return to Earth on Friday, landing at NASA's Kennedy Space Center in Florida.

After Atlantis gingerly pulls away from Hubble, the astronauts will examine the shuttle's heat shield for signs of damage from impacts with space debris and tiny meteoroids.

Hubble's 11-day mission carries a higher risk than NASA's usual mission to the International Space Station because of an accumulation of man-made space debris at the telescope's higher altitude.

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