Hubble history

1977 - Project begins 1985 - Hubble built 1990, 24 April - Hubble launched 1990, 18 May - First light 1993, December - Flawed mirror corrected 1997, February - Second servicing mission 1999, December - Emergency service to repair gyroscopes 2002, March - Repairs not done in 1999 2010 - End of Hubble mission

The new wing arrays are smaller - each just eight metres long - but will deliver 20% more power than the old pair, damaged by eight years of wear and tear.

The HST is 12 years into a 20-year mission and has already established itself as one of history's finest scientific instruments, having proven the existence of super-massive black holes, witnessed the formative stages of solar systems and given scientists their best estimate yet for the age of the Universe.

But the current servicing mission, being undertaken by a seven-member crew on the Columbia shuttle, should substantially improve Hubble's abilities.

Nasa confident

The new Advanced Camera System (ACS) to be fitted later this week will be able to see finer detail and will have a much wider field of view than the Wide Field Camera that it replaces.

The trickiest upgrade, however, is likely to come on Wednesday, when Nasa has to "switch off" Hubble so astronauts can fit a new power control unit.

The spacewalkers will have to work fast to prevent the extreme cold of space doing damage to sensitive components on the observatory.

There is also no guarantee the power will come back on once the unit is installed - although Nasa says it is confident it will happen.

Columbia launched from the Kennedy Space Center on Friday and is set to land there on 12 March after releasing Hubble back into orbit.