As you may already have heard, the Space Telescope Imaging Spectrograph experienced a malfunction on Wednesday, May 16. The evidence available so far has shown that a fuse in the primary power feed to the STIS blew, most likely the result of a short circuit somewhere between the fuse and the power convertors within the STIS. While we are still in the early stages of understanding what happened, we are hoping to fully restore STIS operations within the month of July. In the meantime, we believe we have an adequate pool of WFPC2 programs to maintain a good observing efficiency with HST.
As always with anomalies of this sort, the HST Project at GSFC is leading an intensive analysis effort, with the participation of NASA and LMSC engineering and parts experts, STScI and GSFC engineers and scientists, and Ball Aerospace engineers. The goal of this effort is to use all available information to try to pin down the exact cause of the problem, so that appropriate remedial steps can be taken.
Fortunately, STIS has a fully independent side of electronics (hereafter referred to as side 2) that can be used to operate the instrument. There is a very high likelihood that switching to side 2 will bypass the short circuit and allow continued STIS operations. Reviews are underway to define the areas within flight software, commanding, data bases, testing and verification, and calibration of mechanisms that require attention in order to switch to operations using the side 2 chain.
A primary uncertainty at this time is whether the STIS CCD will have active temperature control in the future (the Side 2 thermistor used for temperature ontrol of the CCD failed before launch and was not replaced). The suspect fuse that feeds power to the STIS Side 1 electronics can be replaced easily during servicing, but if a short occurred, then its use may not be possible or advisable. The possibility of cross-strapping the CCD temperature control using only parts of the Side 1 electronics within STIS will be investigated. In the absence of active CCD temperature control an on-orbit calibration program will be conducted to quantify any effect on science operations. The latter are expected to be quite minimal for most scientific applications. We will quantify the effect of environmental temperature on the CCD sensitivity as a function of wavelength, dark current, and bias level and structure. We will also have to re-determine the slit positions, since different encoders are used on side 2. Hence we will enable imaging and slitless spectroscopy observations first, and wait with enabling slitted spectroscopy until the slit positions have been determined and put into the appropriate database.
As a rough estimate, it currently seems likely that STIS will not be restored to normal science productivity until mid- to late July 2001. We will update this estimate as the details of the required work become clearer. In the mean time, observing calendars have been quickly reworked with WFPC2 observations replacing scheduled STIS science. With the inclusions of Snapshot targets it is expected that observing efficiency can be maintained at a high level. The HST observing community has been notified of this interruption in STIS observing, and STScI will attempt to respond to individuals' needs and maintain high science productivity from HST. In some cases this might mean switching STIS imaging to WFPC2 if this would save the science for a program already underway.