In this photo provided by NASA, astronaut Andrew Feustel, left, STS-125 mission specialist, navigates near the Hubble Space Telescope on the end of the remote manipulator system arm on May 16, 2009. (AP)
Two US astronauts embarked on a fourth spacewalk on Sunday to revive a long inactive instrument aboard the 19-year-old Hubble telescope once used to study super massive black holes.
Mike Massimino and Mike Good emerged from the airlock of the shuttle Atlantis to begin work on the Space Telescope Imaging Spectrograph at 1345 GMT (1915 hrs IST), 30 minutes later than scheduled to allow Good to adjust the fit of the right boot on his spacesuit.
NASA said Saturday's efforts to revive a second Hubble instrument, the Advanced Camera for Surveys, had been a partial success, after two of three internal imagers were recovered.
Hubble will remain anchored in the payload bay of Atlantis until Tuesday, when the seven shuttle astronauts expect to finish their ambitious refurbishment of the telescope requiring five daily spacewalks.
The work under way aboard Atlantis is intended to extend Hubble observations for at least another five years.
"At this point, we are feeling really good," said Preston Burch, NASA's Hubble program manager. "Hubble has reached a new high in terms of capabilities. We have also made huge strides in terms of restoring the health of the observatory."
During today's spacewalk, Massimino and Good will attempt to replace an internal circuit board on spectrograph.