The second of five spacewalks on the servicing mission to the Hubble Space Telescope is in progress, with astronauts Mike Massimino and Michael Good swapping out old parts on the 19-year-old telescope to help extend its lifetime in space.

Massimino (also known as Astro_Mike, the tweeting astronaut) and Good ventured out of space shuttle Atlantis at 8:49 this morning (Eastern Daylight Time) to replace Hubble's three pairs of gyroscopes, which measure the telescope's motion and help keep it pointed in the proper direction.

As with yesterday's spacewalk, the astronauts have hit some minor snags: One of the two-gyro units refused to mount properly, so the spacewalkers retrieved a refurbished spare to use in its place. The delay pushed the spacewalk roughly an hour behind schedule.

The final major task of the spacewalk will be replacing half of Hubble's six batteries (the other half will be swapped out in a later spacewalk), which launched on the telescope in 1990 and have greatly surpassed their design lifetimes of five years. The 125-pound (57-kilogram) batteries power Hubble during the night portion of its orbit, when solar power is not available.

Photo of Massimino (left) and Good (inside telescope, on robotic arm) working on Hubble today: NASA TV