The Space Telescope Imaging Spectrograph (STIS) stopped science operations on 2004-Aug-03, due to the failure of a power supply within the "Side-2" electronics, which had powered the instrument since 2001-May-16, when a short circuit blew a fuse thus knocking out the Side-1 electronics. Currently, STIS is in "safe" mode: the instrument and its on-board computer are switched off but the heaters are on to ensure a healthy and stable thermal environment.
The failed unit delivered power to all the mechanisms within STIS, including the aperture wheel, the Mode Select Mechanism (which selects gratings and mirrors for a given observing mode) and the CCD shutter. At the time of the failure, the STIS was in "idle" mode, in which the light path is completely blocked (a precaution to prevent over-illumination of the MAMA detectors). In the absence of a working power supply, the mechanisms cannot be moved from their current positions, and the instrument is inoperable.
Immediately following the failure, the HST Project office at Goddard Space Flight Center (GSFC) assembled a Failure Review Board (FRB) including engineers from GSFC, the Space Telescope Science Institute (STScI), Ball Aerospace (where STIS was built), and Interpoint (the builders of the failed power supply). The board conducted a thorough review of all available telemetry data to identify the most likely cause(s) of the failure, to evaluate possible work-arounds, to recommend further testing to characterize the problem and/or establish whether or not STIS can be returned to use, and to consider any risks of identical failures occurring in other HST instruments.
The FRB presented its findings and recommendations on 2004-Sep-16. Although it made no definitive conclusion as to the root cause, it localized the failure to be within the Interpoint power converter. A similar anomaly had been identified in 2002-November during the pre-launch testing of the Gravity Probe B, which used the same type of power converter. Unfortunately, the board did not identify any possible work-arounds to restore STIS operations. Nevertheless, the board proposed four tests developed by engineers and scientists at GSFC and STScI:
| Test 1: | Verify the health and viability of the STIS detectors and lamps. This would be useful to know if an attempt to repair STIS were to be made in the future. |
| Test 2: | Attempt to reactivate the Side-1 electronics. During the last servicing mission, astronauts replaced the blown side-1 fuse. The short circuit might have resolved itself (even though the probability for that is very low). |
| Test 3: | Verify that the failure in the mechanism power supply still exists. |
| Test 4: | Test whether the STIS Control Electronics Board (CEB) can be turned on separately while STIS is in safe mode. This test would ensure that certain STIS components can be used as additional load during battery capacity tests, which is important to track the health of the batteries aboard HST. |
Each test also included a thorough description of all risks and benefits associated with its execution.
Senior management at the HST Project Office carefully considered all risks and benefits associated with the execution of the proposed tests. At the time this article is published, Test #1 mentioned above is still under consideration as additional tests are conducted on Interpoint converters, while Tests #2 and #3 have not been approved for execution due to the significant risks involved (if a short circuit would occur, it could entail significant ramifications to not only the health of STIS components, but also that of other HST instruments). For now, the only test approved for execution is the CEB Load test (Test #4), which was successfully executed on 2004-December. In the mean time, STIS will be kept in "safe" mode until the proposed Robotic Servicing Mission.
We are sorry to bring you this sad news about STIS, which brought many unique observing capabilities to Hubble, including very high resolution echelle spectroscopy in the ultraviolet and spatially resolved spectroscopy in the ultraviolet and optical. These and other capabilities will not be restored when Wide Field Camera 3 (WFC3) and the Cosmic Origins Spectrograph (COS) are installed in HST. A steady and high percentage of approved HST observing programs have used STIS throughout its lifetime. On the positive side, project engineers at GSFC continue to look carefully at possible ways to service STIS during the proposed robotic servicing mission.