Astronauts on the space shuttle Atlantis have begun the first of five spacewalks servicing the Hubble Space Telescope.
During the six hours and 30 minutes they spend outside the shuttle, Nasa astronauts John Grunsfeld and Drew Feustel will remove Hubble's Wide Field Planetary Camera 2 and replace it with the new Wide Field Camera 3, which will allow Hubble to capture a wide range of images in ultraviolet to near-infrared wavelengths.
The astronauts will also replace a failed science data processing computer that delayed Atlantis's launch last October. The Science Instrument Command and Data Handling Unit sends commands to Hubble's science instruments and formats science data for transmission to the ground.
Finally, Grunsfeld and Feustel will install a mechanism that will allow a spacecraft to capture Hubble and take it out of orbit at the end of its life.
Hubble and Atlantis are flying 350 miles (563 kilometres) above the Earth. The orbit is littered with space junk, some of which led to a minor scare for Nasa last night.
The US Air Force noticed that a 10cm piece of space debris was on course to come close to the shuttle. Left over from the destruction of a satellite by China in 2007 during a weapon test, the debris was predicted to come within 2.7 kilometres of Atlantis.
Nasa decided that it didn't need to move Atlantis out of the way and the debris missed the shuttle.
However, the International Space Station is watching a different piece of debris from its lower altitude that has a slight chance of coming close on Friday.
Last night, mission control told the astronauts that Atlantis' heat shield was in such good shape following Monday's launch that no further inspection would be needed next week.
As for Hubble, after seven years in space Nasa said the telescope looked in surprisingly good shape.
Flight controllers "gasped" when the telescope first came into view. "It's an unbelievably beautiful sight," said Grunsfeld. "Amazingly, the exterior of Hubble, an old man of 19 years in space, still looks in fantastic shape."
No one will visit Hubble after the Atlantis astronauts leave next week, so Nasa has crammed as much as it can into the five spacewalks and poured more than $1 billion into the mission.