Washington - For nearly 20 years the Hubble Space Telescope has kept its orbiting eye trained on the universe, and with the launch of space shuttle Atlantis to repair the ageing instrument scientists hope it will continue to provide important discoveries.

The fifth and last mission to repair Hubble is to launch at 2:01 pm (1801 GMT) Monday from the Kennedy Space Centre in Cape Canaveral, Florida. The Atlantis crew will undertake five spacewalks, adding two new instruments, repairing two others and replacing other hardware in frequently delicate operations.

Scientists say the upgrades, which US space agency NASA hopes will extend Hubble's life span until at least 2014, will continue to provide clues about the origin and nature of the universe.

Since its launch in 1990, Hubble has helped scientists to place the age of the universe at 13.7 billion years, learn that black holes are at the centre of most galaxies, monitor planetary formation and discover that the universe is expanding at an ever-faster pace.

With the addition of new instruments and repairs of others, scientists still hope to learn about the gases between galaxies and take detailed pictures of distant stars.

The risky mission to repair Hubble has been plagued by delays, with NASA indefinitely postponing it because of problems with the orbiting telescope's mechanisms. A launch slated for October 14, 2008, was cancelled.

NASA reluctantly scheduled the service mission under pressure from space enthusiasts who were alarmed at the prospect that Hubble would shut down for years until the planned 2011 launch of a successor, the James Webb Space Telescope.