Atlantis astronauts Mike Massimino and Mike Good have begun the fourth of five spacewalks during the final mission to repair the Hubble Space Telescope. The two 46-year-olds, paring up for the second time, switched on their spacesuit battery power at 9:45 a.m. EDT, after completing more than 90 steps needed to get started.
They'll soon float from the shuttle's airlock into the cargo bay to get to work on the telescope 350 miles above central Australia. Today's spacewalk is the fourth in as many days and the 22nd to service Hubble, which was deployed in 1990 and first repaired in 1993.
You can watch it live by clicking on the NASA TV image above to launch a viewer.
The spacewalkers have a daunting task: Repair the Space Telescope Imaging Spectrograph, or STIS, which stopped functioning in 2004.
Like Saturday's work on the Advanced Camera for Surveys, it was not designed to be repaired in orbit and the operation has been likened to brain surgery.
To install a new low voltage power supply, Massimino must remove a cover plate fastened to the telescope by 111 small screws that could damage the telescope if they float away.
That's more than three times as many screws as mission specialist John Grunsfeld had to handle with ACS on Saturday, but they're easier to reach.
The crew learned this morning that the ACS repair had been only partially successful, despite their outstanding work Saturday.
Tests overnight confirmed the camera's wide-field channel is working well, but a high-resolution channel still had power problems. The two channels were knocked out by power failures in 2006 and 2007.
For today's spacewalk, Massimino will wear a spacesuit identified by broken horizontal red stripes, and his helmet camera will show No. 19 in the bottom right corner of the TV screen. This is his fourth spacewalk, including two on STS-109 in 2002. The first three totaled 22 hours, 42 minutes.
Good's spacesuit has barber pole red stripes, and his camera shows No. 20. He'll primarily ride the shuttle's robotic arm. This is his second spacewalk, after participating in the eighth-longest spacewalk ever with Massimino on Friday - nearly eight hours.
The spectrograph surgery is slated to take up about four-and-a-half hours of the planned six-hour, 30-minute spacewalk. The final task would be to install a protective thermal cover on one of the telescope's bays (Bay 8).
Here's some material to help follow along with the 11-day STS-125 mission:
This NASA TV schedule provides a timeline of events coming up today and throughout the remainder of the mission.
This Flight Day 7 Execute Package offers a more detailed timeline for today and notes sent to the crew by mission controllers in Houston.
Here's an official mission summary and press kit.
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