One of Hubble's tools fails: observatory loses a sharp ultraviolet eye.

Last week, an instrument on the Hubble Space Telescope stopped working, shutting astronomers' only sharp ultraviolet eye on the universe.

On Aug. 3, a malfunction--possibly a short circuit--developed in a 5-volt power supply that drives moving parts in the observatory's Space Telescope Imaging Spectrograph (STIS). That device's only other power supply had stopped working in 2001.

"It's as if STIS suffered a stroke and is now paralyzed," says senior Hubble project scientist David S. Leckrone of NASA's Goddard Space Flight Center in Greenbelt, Md. Any hope of reviving the instrument, he says, would require a repair mission to the telescope.

The other three scientific instruments on Hubble remain healthy. These include a recently revived near-infrared camera and the Advanced Camera for Surveys, which records both ultrasharp visible-light images and low-resolution spectra at visible light and near-ultraviolet wavelengths. The fourth instrument takes images in visible light.

Despite the loss of STIS, Hubble "remains a marvelous observatory and a unique facility for learning about the universe," says John Bahcall of the Institute for Advanced Study in Princeton, N.J.

NASA is now considering whether it could fix STIS as part of a proposed robotic mission to repair and upgrade Hubble some 3 years from now (SN: 7/24/04, p. 56). The agency's decision will depend on the determination of exactly what went wrong with the power supply and whether the robotic system now under study could handle the repair, said NASA associate administrator for science Al Diaz during an Aug. 10 teleconference.

The planned mission would include installation of the Cosmic Origins Spectrograph, which can take ultraviolet spectra of fainter objects than STIS can but doesn't have as high a resolution.

Installed by astronauts during a Hubble-servicing mission in 1997, STIS lasted 2.5 years beyond the 5-year lifetime for which it had been designed. The instrument had contributed about 30 percent of all Hubble observations. Like a prism, STIS separated light into its component colors, and it then recorded the intensity of wavelengths ranging from the near-ultraviolet to the near-infrared. It also took images in both ultraviolet and visible light.

STIS could simultaneously sample some 500 points of light. This enabled the instrument to take a rapid census of supermassive black holes at the ('enters of galaxies by measuring the telltale velocities of a string of stars on either side of the hole. That same capability enabled STIS to study extended objects, such as the disks around young stars. Those disks have the potential to form planets.

In 1999, a team including David Charbonneau, who is now at Harvard University, used STIS to detect the atmosphere of a planet beyond the solar system and reveal the atmosphere's composition (SN: 11/20/99,p. 324). No other instrument has accomplished this feat.

"I think the science case for a Hubble repair is compelling and was so even prior to the demise of STIS," says Charbonneau. "The events of the past week have just made that discussion all the more urgent."