AFP - US astronauts aboard the shuttle Atlantis released the Hubble Space Telescope on Tuesday, after the US space agency's final mission to repair and upgrade the orbiting observatory.
  
Astronauts released the telescope at 1257 GMT, sending Hubble "back on its own for the final time," said Kyle Herring, mission commentator.
  
The delicate release marked the end of 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.
  
The 19-year-old observatory spent six days in the shuttle's cargo bay undergoing an overhaul conducted over five daily spacewalks.
  
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.
  
"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."
  
During Tuesday's release, astronauts Mike Massimino and Mike Good were standing by in the shuttle's airlock ready to respond with another spacewalk should there be problems setting Hubble free. However, the operation unfolded without incident.
  
"It's wonderful to see Hubble, the most famous scientific instrument of all time, newly upgraded and ready for action thanks to you," shuttle communicator Dan Burbank told the Atlantis crew from Mission Control.
  
Altman and Pilot Greg Johnson carefully guided Atlantis away following the release, and prepared to subtly fire the shuttle's 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 dramatically increase the telescope's power and 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.
  
With Atlantis separated 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.