U.S. astronauts have completed the fourth in a series of five spacewalks to repair the Hubble Space Telescope.
Astronauts Mike Good and Mike Massimino on Sunday replaced the power supply unit for the Space Telescope Imaging Spectrograph, a powerful instrument that allows NASA to identify black holes and to monitor galaxies.
The two spacewalkers had trouble unbolting a handrail that had to be removed in order to start their repairs. While they were not able to loosen one of the handrail's bolts, they were able to bend it out of the way and proceed with their work.
Their spacewalk lasted more than eight hours. It was the sixth-longest spacewalk in the history of the U.S. space program.
The problems of that handrail bolt and a power tool with a weak battery caused the astronauts to abandon one of the tasks they had scheduled for Sunday's spacewalk, the replacement of some worn insulation on the telescope. Mission planners are considering adding that chore to Monday's fifth and final spacewalk.
The five spacewalks are intended to upgrade the 19-year-old telescope and to extend its life for at least five more years.
The mission is more dangerous than others because the telescope is sharing an orbit with debris left behind by satellite collisions and rocket launches.
The astronauts are further challenged because they can only work with the supplies they are able to carry with them on the shuttle. In missions to the International Space Station, astronauts have enough supplies on the station to last up to three months.
Some information for this report was provided by AFP, AP and Reuters.