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

On a marathon spacewalk Sunday, two astronauts overcame repeated obstacles to make the second of two historic repairs to the Hubble Space Telescope.

One bolt that had to be extracted proved so stubborn that astronaut Michael Massimino resorted to brute force to rip it out of the telescope.

His efforts paid off. Hubble’s $166 million chemical analyzer, dead for nearly five years, came back to life after Massimino and a colleague rewired its electronics. The device makes a fingerprint of cosmic objects by separating light. It is good for finding black holes and examining the atmosphere of planets outside our solar system.

When Mission Control announced that the chemical analyzer had passed the “aliveness test” administered by engineers on Earth, the astronauts in space cheered.

“That sounds great,” Massimino said. “Thanks so much.”

Saturday, another team of astronauts revived a Hubble camera that broke two years ago.

Never before have astronauts tried to repair Hubble’s scientific instruments. Because those instruments weren’t designed for maintenance in space, working on them poses major challenges to astronauts wearing stiff, thick space gloves.

Sunday’s outing was the fourth of five spacewalks planned for the crew of space shuttle Atlantis, which is paying the last service call to the storied telescope. The seven Atlantis astronauts want to rejuvenate the Hubble to ensure it will last at least five more years.

The astronauts ran into a trio of unwelcome surprises during Sunday’s outing, which ran so long that they never got to their second scheduled chore, installation of insulation on the Hubble.

First, Massimino couldn’t undo a bolt holding a handrail in place — a major problem, because the handrail blocked access to the failed Space Telescope Imaging Spectrograph. Massimino yanked the handrail free after getting a go-ahead from Mission Control. Engineers estimated the task would take 60 pounds of force.

Then Massimino had trouble installing a device designed to grab 100-plus tiny screws he had to take out. The device was supposed to sit on the door to the spectrograph and keep the screws from floating into the Hubble’s workings.

Finally Massimino’s power screwdriver went dead, leading the exasperated astronaut to blurt out, “Oh, for Pete’s sake!” He had to break off working and travel to a distant toolbox to get a spare screwdriver.

The refrigerator-sized spectrograph has made major contributions to astronomy since it was added to Hubble in 1997. It doesn’t take photos but instead analyzes the composition of stars and other objects in the universe.

It detected black holes at the center of many galaxies and helped scientists do a definitive study of a star in the last stages of life. It was the first to analyze the atmosphere of a planet orbiting a star other than the sun.

Nearly every spacewalk made by the Hubble crew has run into unexpected difficulties. Thursday, spacewalker Andrew Feustel had to muscle another stuck bolt out of place. If he’d failed, a new camera would’ve had to return to Earth rather than being installed on the Hubble.

Friday, Massimino and spacewalking partner Michael Good had so much trouble replacing Hubble’s gyroscopes that they fell 90 minutes behind schedule.

The mission’s final spacewalk is scheduled for Monday. Feustel and partner John Grunsfeld will have to try to finish up the work that Massimino and Good didn’t have time for.

Hubble analyzer fixed, but not without headaches