Space Telescope Imaging Spectrograph (STIS), the most versatile spectrograph ever to fly on Hubble, ceased operations in August 2004 due to the failure of its power supply. In order to restore STIS to operational status, astronauts will perform a never-before-attempted on-orbit replacement of an electronics board inside STIS's main electronics box. On Earth this operation is relatively simple, but in space many challenges confront the astronauts as they work to replace the failed board including working to remove 111 tiny, non-captive screws with astronaut gloves.
The Cosmic Origins Spectrograph (COS) that will be added during Servicing Mission 4, and STIS are highly complementary and are very complimentary to each other providing scientists with a full set of spectroscopic tools for astrophysical research. The STIS instrument's accomplishments include determining the atmospheric composition of an exoplanet as well as spectra and images at ultraviolet and visible wavelengths of the Universe from our solar system out to cosmological distances.
Wide-Field Planetary Camera 2 (WFPC2) was installed in Dec 1993 and used to obtain high resolution images of astronomical objects. This camera was removed in the last servicing mission so it is no longer in service.