By Paul Rincon
Science reporter, BBC News, Austin
Nasa has announced details of a challenging mission to "rescue" the Hubble Space Telescope.
HUBBLE SPACE TELESCOPE
Named after the great US astronomer Edwin Hubble
Launched in 1990 into a 600km-high circular orbit
Equipped with a 2.4m primary mirror and five instruments
Length: 15.9m; diameter: 4.2m; Mass:
Without an upgrade, the multi-billion dollar orbiting observatory is likely to fail in 2010 or 2011.
The planned fix will provide a massive boost to Hubble's capabilities, giving it greater sensitivity and a larger field of view.
The mission, by space shuttle Atlantis, will make Hubble 90 times more powerful than its original version.
It could also extend the telescope's lifetime by more than a decade.
Nasa officials and astronomers outlined their plans at a meeting of the American Astronomical Society in Austin, Texas.
"Hubble will be able to work productively out into the middle, if not the end of the next decade instead of its current projected demise due to wear and tear," Dr Alan Stern, associate administrator for science at Nasa, told the BBC News website.
"In addition, we're giving it new capabilities: 10 times or 20 times the sensitivity in terms of spectroscopy for probing the ancient Universe, a much larger field of view and much greater wavelength capabilities than it has ever had before.
"It will be a totally new machine."
Sandra Faber, a professor of astronomy at the University of California, Santa Cruz, said that the upgrade mission would make Hubble 90 times more powerful than its original incarnation.
Whereas Hubble's original incarnation could only see 10 galaxies at a time, this one would be able to see 900.
The upgrade mission, the fifth and final flight to fix Hubble, is scheduled to launch in August 2008, although that date is now in doubt.
The Hubble repair mission will have to wait for two higher priority shuttle launches before it can go ahead.
The first of these, which will loft Columbus, the European Space Agency's main scientific contribution to the International Space Station, was due to fly in January, but will now have to be delayed until February at the earliest.
Wear and tear
Hubble remains operational, but gyroscope failures have given it limited steering.
The Advanced Camera for Surveys (ACS) was the most-used instrument on the telescope until its failure last January, after five years of operations.
The Space Telescope Imaging Spectrograph (STIS), a sophisticated instrument which separated the light from different celestial objects into its components, failed in 2004.
Remarkable pictures taken by the orbiting telescope
Astronauts will repair these existing instruments and install two new ones, which will be carried into orbit on the space shuttle Atlantis. The new instruments are the Wide Field Camera 3 (WFC3) and the Cosmic Origins Spectrograph (COS).
The WFC3 will be Hubble's first "panchromatic" camera with a wide field of view and able to take amazingly sharp images over a wide range of colours.
COS will probe the "cosmic web", the large-scale structure of the Universe, which is determined by the gravity of dark matter and can be traced by galaxies and intergalactic gas.
Atlantis will rendezvous with Hubble, grab the telescope with its robotic arm and pull it on to a work platform to give astronauts easy access to its interior.
John Grunsfeld is one of the astronauts who will revamp Hubble over the course of five spacewalks, each about six-and-a-half to seven hours long.
"They will be filled with content, there's no time to take a breather and look around," he told BBC News.
John Grunsfeld is a veteran of four space shuttle missions
"It's going to be a marathon at a sprint pace for 11 days on orbit."
Launched in 1990, the Hubble Space Telescope is now regarded as one of the most important instruments in the history of astronomy. It has made a remarkable contribution to our understanding of the origin and evolution of the Universe.
Hubble has obtained the deepest views of the cosmos, finding high-interest objects for other observatories to investigate in detail. Its studies of the Universe's expansion also dramatically refined the best estimates for the age of the cosmos.
Its pictures have also produced hard evidence for the existence of black holes and confirmed theories of planetary formation.
Following the Columbia disaster in 2003, which killed seven astronauts, another mission to service Hubble was considered too hazardous.
This was because astronauts would not be able to use the International Space Station as a safe haven if the shuttle sustained damage on launch.
Nasa has now accepted the risk of the mission, but will have another shuttle ready to launch immediately to bring the crew home if the servicing mission is endangered.
You can listen to the Hubble servicing astronauts talk about their mission on BBC Radio 4's Frontiers website. The astronomers who will use the upgraded Hubble will discuss their hopes on the BBC World Service Discovery programme on Wednesday 9 January at 1230 GMT. If you miss it, you can catch up with the broadcast at the Discovery website
SERVICING THE HUBBLE SPACE TELESCOPE
Shuttle Atlantis will grab Hubble with a robotic arm and pull it on to a work platform to allow astronauts easy access to its interior
Hubble has six gyroscopes that are critical to its control and pointing systems. These have started to fail and all will have to be replaced
Six new batteries will rejuvenate the electrical system; astronauts will attach new thermal blankets to insulate sensitive components
The telescope has two instrument bays; the COS and WFC3 will be slid into racks made vacant by the removal of older instruments
An attempt will also be made to repair the Space Telescope Imaging Spectrograph (STIS) which stopped working in 2004