NASA astronauts head out on last Hubble spacewalk

Gearing up for their fifth and final spacewalk today, the crew of the space shuttle Atlantis hoped to install the last set of batteries and a sensor that should help the Hubble Space Telescope point at its celestial targets.

During the eighth day of the 11-day mission, astronauts John Grunsfeld and Andrew Feustel set out this morning for their third spacewalk of the mission.

The astronauts are largely focused on installing a battery pack in the telescope. Another battery module, which weighs 460 pounds, was successfully installed last week. The batteries power Hubble during the nighttime sections of its orbit.

The astronauts also will be trying to replace one of the three fine guidance sensors on the telescope. The optical sensor, comprising mirrors, lenses, prisms, beam splitters and photomultiplier tubes, gives the orbiter information so it can steadily aim its telescope at its targets. The fine guidance sensors also can take precise measurements of the position and motion of stars.

If the astronauts have time, they also hope to install a stainless steel blanket on the outside of the telescope. The blanket, which should offer thermal protection for equipment bays, was supposed to be installed during a spacewalk yesterday but a stuck bolt used up a lot of the astronauts' time and they were unable get to the blanket installation.

Actually, the stuck bolt nearly derailed the repair of a key instrument - the Space Telescope Imaging Spectrograph, which is NASA's major black hole hunter. It acts as a sort of outer space prism, separating light from the universe into its component colors, giving scientists a "wavelength fingerprint" of any object. That data can offer clues to an object's temperature, chemical composition and motion.

The spectrograph is designed to pick up the light and color signature of black holes. It also helps astronomers map the motion of gas affected by a black hole's gravitational pull.

Yesterday's trouble arose when Mission Specialist Mike Massimino, who has been Twittering about his experience in space, was unable to remove a bolt from the spectrograph's handrail. Eventually, he worked around the issue by simply yanking the handrail until the head of the bolt snapped off. After that, the two astronauts were able to finish the repair job.

On Tuesday, the seven-astronaut crew will redeploy the telescope. They're scheduled to return to Earth on Friday.

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