Astronauts aboard space shuttle Atlantis are preparing to return to Earth after an ambitious and risky mission to re-fit the Hubble telescope.

There are two chances to land on Friday: one at 1500 BST (1000 EDT) and a second at 1639 BST (1139 EDT).

If bad weather scuppers either of those opportunities, the shuttle will try to land on Saturday in Florida or at Edwards Air Force Base, California.

The mission was intended to give a new lease of life to Hubble.

The orbiting observatory is regarded as one of the most important scientific tools ever built.

The fifth and final mission to service Hubble has been hailed as a great success.

Over five spacewalks, astronauts installed new instruments and thermal blankets, repaired two existing instruments, replaced gyroscopes and batteries.

The only disappointment was the failure to restore the high resolution channel (one of three) on Hubble’s main camera – the Advanced Camera for Surveys (ACS).

Cloudy skies and stormy weather could yet pose a problem for Friday’s landing attempts, Nasa has said.

The shuttle has enough supplies to remain in orbit until Monday.

The US space agency has cleared Atlantis for its fiery re-entry into Earth’s atmosphere following in-flight inspections of its heat shield by the crew.

The Hubble telescope was released from the shuttle’s robotic arm on Tuesday.

Atlantis ready to return to Earth