Mission specialists and Mike Good are in the midst of the second-to-last . The spacewalk, the second of the mission for both Massiminio and Good, is focused on , or STIS.

The basically 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.

The spectrograph, which was installed on Hubble during a 1997 shuttle service mission, suffered a power supply failure in 2004 and has since been in safe mode, which means it's off but protected, NASA said.

Massimino and Good are scheduled to replace a low-voltage power supply board that contains the failed power converter.