Live in Orbit: Mike and Mike Prep for Spacewalk

Atlantis astronauts Mike Massimino and Mike Good are breathing pure oxygen before beginning the mission's second spacewalk, which is expected to start around 8:45 a.m., about about a half-hour later than scheduled.

The 40-minute procedure purges nitrogen from the spacewalkers' bloodstreams to minimize their chance of experiencing decompression sickness. Already, the crew cabin's air pressure has been dropped to 10.2 psi, reducing the amount of time needed to breathe the pure oxygen.

Once that's complete, depressurization of the shuttle's airlock will take about 15 minutes.

Then "Mike and Mike," both 46, will step out into the vacuum of space and begin working on the Hubble Space Telescope. It's the third spacewalk for Massimino, a veteran of the STS-109 trip to Hubble in 2002, and the first for Good, a.k.a "Bueno."

The spacewalk's first task is actually the mission's top priority, though it doesn't involve the sexier installation or repair of science instruments.

Massimino and Good will replace three units that hold Hubble's six gyroscopes, without which the observatory wouldn't be able to point its instruments and stay steady enough to collect data.

Massimino, dubbed EV-3, will be wearing a suit with broken red stripes on the legs. His helmet camera will show No. 19 in the bottom right of the NASA TV picture. Good, EV-4, will have dashed, barber pole-style red stripes on his suit. His helmet camera will show No. 20.

You can follow the whole spacewalk live here on The Flame Trench. Just click on the NASA TV image on this page to launch a live viewer.

Check out this updated NASA TV schedule for a timeline of upcoming mission events.

And this Flight Day 5 Execute Package contains more detailed timelines and notes sent to the crew by the Mission Control Center in Houston.

IMAGE NOTE: Click to enlarge the tightly cropped image showing shuttle Atlantis in silhouette during solar transit on Tuesday, May 12, taken from Florida. The image was made before Atlantis and the crew of STS-125 had grappled the Hubble Space Telescope. Photo Credit: NASA/Thierry Legault