The Space Telescope Imaging Spectrograph (STIS) was designed and built by a team of scientists and engineers from the GSFC and Ball Aerospace. STIS is primarily a spectrograph, allowing spatially resolved spectroscopy over a wide range of wavelengths. The instrument makes use of three separate detectors to provide multiple operational modes over four different spectral bands. The operational modes include a camera mode (used for target acquisition and deep imaging), long-slit low and medium spectral resolution modes in first order, medium and high-resolution echelle modes, and a very low-resolution prism mode. STIS' scientific capability was demonstrated by a variety of investigations, including the first detection of the atmosphere of an extrasolar planet.

STIS was designed to operate for five years. It has either met or exceeded all its scientific requirements. Initially, STIS was not slated for replacement or upgrade as part of any future servicing mission. Among the major scientific achievements made by scientists using STIS were:

• Conducted an efficient census of galaxies to catalog super-massive black holes. The fraction of galaxies that prove to contain a central massive black hole has proven to be surprisingly large;
• Made the first-ever measurements of the chemical composition of the atmosphere of an extrasolar planet;
• Saw the magnetic "footprints" of the Jovian satellites in Jupiter aurora, and made clear images of Saturn's aurora;
• Studied the dynamics of circumstellar disks, the region around young stars where planets may form;
• Found the first evidence of the high-speed collision of gas in the recent supernova remnant SN1987A;
• Independent confirmation of the age of the universe by finding the coolest and hence oldest white dwarf stars that exist in our galaxy.

On May 16, 2001, STIS Side 1 Low Voltage Power Supply (LVPS) failed after approximately 42,000 hours of operation. Engineers determined this failure was most likely due to a shorted tantalum capacitor. Due to built-in electrical redundancy of all HST instruments, the instrument was switched to draw its power from the Side 2 LVPS and continued to function for approximately 27,000 hours of further operation.

The Side 2 electrical system failed on August 2, 2004. NASA convened an Anomaly Review Board to investigate the cause of the STIS problem and an investigation to determine if the instrument is recoverable. The team conclusively pin-pointed the failure to a converter on LVPS-2 in Main Electric Box 1 (MEB 1). This failure left the instrument fully shuttered and it has remained in Safe mode with the instrument and its on-board computer switched off since the failure. The instrument heaters were left on in order to ensure a stable thermal environment. The team also concluded that by replacing the failed board with a replacement board (LVPS-2R), STIS could be brought back to full functionality.

From shortly after the Side 2 electronic failure, NASA scientists and engineers have been working on ways to repair STIS. Their perseverance has resulted in a fully mature and well tested repair procedure, including many new astronaut tools to perform tasks never perceived when STIS was being built.

"Our goal for this mission is to leave Hubble at the apex of its scientific capabilities," said David Leckrone, Hubble senior project scientist at NASA's Goddard Space Flight Center, Greenbelt, Md. "Our two new instruments, plus the hoped-for repairs of STIS and ACS, will give astronomers a full 'tool box' with which to attack some really profound problems, ranging from the nature of dark matter and dark energy, to the chemical composition of the atmospheres of planets around other stars."

Scientists are confident that some of Hubble's greatest discoveries are yet to come.