Astronauts have struggled to complete the most critical repair to the Hubble Space Telescope in a second spacewalk.
Mission specialists Mike Good and Mike Massimino put a refurbished pair of gyroscopes into the telescope after a new set refused to go in.
Besides the gyroscopes - to orient it precisely - Hubble got fresh batteries to ensure five more years of life.
Despite the setbacks, scientists said Hubble would function well, pointing to ever distant objects in the cosmos.
Friday's troubled spacewalk was the longest yet, lasting eight hours.
"At times, I felt like I was wrestling a bear," Mike Massimino was quoted as saying by AFP news agency, as he and Mike Good struggled to install the gyroscopes, or "rate sensing units" (RSUs).
Previously, only three of the six gyroscopes worked. But after today's marathon spacewalk, Hubble has four brand new sets and two refurbished ones. Only two are needed to orient the telescope properly.
Troubled mission
The spacewalk was the 20th undertaken in the service of Hubble, but was Mike Good's first.
"Welcome to the wonderful world of working in a vacuum," Mike Massimino told him as he exited the airlock
The first part of the spacewalk was to replace the three RSUs, each of which contains two gyroscopes.
While the first RSU went in as planned, the second one did not seat properly on its plate. The crew opted to place the third RSU in the slot of the second.
The same problem occurred when the RSU meant for the second slot was placed into the third, so the crew opted to install a refurbished unit instead.
But Hubble's deputy senior project scientist, Mal Niedner, said he was not concerned that the astronauts had to resort to refurbished gyroscopes, which lack the latest anticorrosive wiring.
"It's the difference between an A and an A-plus," he was quoted as saying by AP news agency.
The three batteries that were replaced were the original equipment installed on Hubble 19 years ago, intended to have just a five-year lifespan.
On Thursday, the telescope's Wide Field Camera was replaced, giving the telescope an even deeper view into space - and thus into the history of the Universe.
A data processing unit that failed in 2008 was also replaced.
In three further spacewalks to be undertaken in the next three days, two spectrographs will be installed and the remaining three of Hubble's batteries will be replaced
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