Offering a wistful farewell, astronauts aboard the shuttle Atlantis released the Hubble Space Telescope back into its orbit Tuesday, concluding the US space agency's final mission to repair and upgrade the revolutionary observatory.

The shuttle crew released the beloved telescope at 1257 GMT, sending Hubble "back on its own for the final time," said Kyle Herring, mission commentator.

The separation marked the end of NASA's human missions 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.

Mission specialist Megan McArthur, who maneuvered the Hubble using the Atlantis's robotic arm and lifted the delicate stargazer out of the payload bay, released the telescope as the two spacecraft sailed at an altitude of 560 kilometers (350 miles) over Africa.

The astronauts and Mission Control remained largely quiet during the delicate detachment, letting the magnitude of the moment speak for itself before offering poignant remarks on the 19-year-old telescope that has been painstakingly repaired.

"Hubble has been released, it's safely back on its journey of exploration," Atlantis Commander Scott Altman informed Mission Control.

"Looking back at this mission, it has been an incredible journey for us as well. I think it has demonstrated the triumph that humans can have when they overcome challenges presented to them," said Altman.

"Not everything went as we planned, but we planned a way to work around everything. We've been able to do some incredible things together. And that's the thing I think about Hubble -- we've done it together and now Hubble can continue on its own, exploring the cosmos."

The astronauts faced a series of obstacles over the five grueling daily spacewalks required to conduct the overhaul.

There were stripped bolts, and a power ratchet that went dead at a crucial moment. One of the shuttle's four spacewalkers had to work through the pain of a poorly-fitting spacesuit boot. And when high technology failed in an attempt to remove a stuck railing, the astronauts resorted to more primitive means to pry it loose: brute force.

But their efforts left Hubble equipped for at least another five years of operations -- and the promise of new discovery.

"It's wonderful to see Hubble, the most famous scientific instrument of all time, newly upgraded and ready for action thanks to you," Mission Control's Dan Burbank told the Atlantis crew.

Altman and Pilot Greg Johnson carefully guided Atlantis away following the release, and subtly fired its thrusters to establish a safe distance between the shuttle and the telescope.

Spacewalking astronauts finished their work Monday with a sentimental final outing that lasted about seven hours and left nothing on their mission's wishlist unfulfilled.

They fitted it with two new science instruments and repaired two others that had been sidelined for years with electrical problems; they installed new gyroscopes and batteries to fortify the precision pointing systems; and they applied patches to the outside of the telescope where solar radiation had damaged the soft exterior.

"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 after an 11-day mission, landing at NASA's Kennedy Space Center in Florida.