US astronauts on the shuttle Atlantis said Wednesday they were amazed at how successful their grueling mission to revamp the aging Hubble telescope had been, given the number of tough obstacles they faced.

Speaking to reporters earlier, the smiling seven crew members spoke with pride of their mission to give the 19-year-old stargazer a new lease on life to peer back further into the origins of the universe.

"I never imagined actually the flight would go as interestingly as it has," said mission commander Scott Altman.

"It's amazing looking back at how hard things looked a couple of times, more difficult than I ever expected. But then to overcome and wind up with everything done in the way it was -- we were very successful."

US President Barack Obama took notice of the successful mission also, making plans for a late afternoon private call from the White House to orbit to congratulate the crew.

Over five daily spacewalks, the crew repaired and restored the Hubble in NASA's final visit to the observatory, giving it at least five more years -- if not 10 -- to continue to explore the mysteries of the cosmos.

Altman's crew faced multiple obstacles as they overhauled the telescope, with most of the spacewalks running into overtime.

They had to overcome stuck bolts, a power tool that lost battery power at a crucial point as well as spacesuit problems that included an ill-fitting boot and a damaged glove.

John Grunsfeld, who led three of the spacewalks with astronaut Drew Feustel, said he could not help but think of the space observatory as a living entity.

The two men could not resist giving the telescope a friendly farewell nudge when they finished the last outing, but the "bump" knocked a protective cover off an observatory antenna, which they then had to scramble to replace.

"I was a little retrospective, thinking about the amazing journey Hubble is about to undergo," said Grunsfeld, an astronomer turned astronaut who has logged three missions to Hubble over a decade, the most of any astronaut.

The successful outcome was a testament to the value of humans rather than robots in space, the astronauts said.

"Many of the things we did were not possible robotically," said Grunsfeld. "That is the real utility of humans. We are able to deal with situations that are unexpected and we do that all day long."

None of the astronauts seemed more vexed than Mike Massimino, who led two of the long spacewalks, including Sunday's outing in which he had to rip away a handrail with a stuck bolt.

The stuck fastener threatened to prevent him from reviving a 10 year-old black hole hunting instrument that was shut down by an electrical problem in 2004.

The crew's only woman, Megan McArthur, the shuttle's robot arm operator, was impressed by the telescope's majesty as she released it back into space from the Atlantis's cargo bay Tuesday.

"What I felt most was just awe and the human ingenuity in creating such an amazing thing," said McArthur.

Another of the spacewalkers Mike Good recalled how before the trip he would stand in the driveway of his Houston home and scan the night skies hoping for a glimpse of Hubble.

"When I finally got to meet it face to face, it was awesome," said Good. "It was a great experience."

The shuttle crew is due to return to Earth on Friday, touching down at NASA's Kennedy Space Center.

On Wednesday, mission control told the astronauts to turn off lights and other equipment to conserve electricity in case stormy weather forces a postponement in their return.

With the shuttle fleet facing retirement at the end of next year, the US space agency has no plans for future visits to the space telescope, a cooperative project with the European Space Agency.

"I truly believe this is a very important moment in human history, and I think it's an important moment for science," David Leckrone, NASA's chief Hubble astronomer, told a news briefing.

"Just using what Hubble has already done as a starting point, it's unimaginable that we won't dramatically go further than that."

International space efforts meanwhile marked another milestone Wednesday as it gave the US, Russian and Japanese crew aboard the International Space Station permission to drink water recycled from urine and humidity recovered from the breathing air.

The recycled water will make it easier for the orbital base to begin housing six rather than three full time astronauts.