Subject: [DPS Members] DPS Mailing #07-10: More on HST Servicing

May 1st 2007

Here's a quick update from Kenneth Carpenter, HST Operations Project
Scientist, providing additional details about the HST servicing mission
options.  Please continue to send your input to Phil Nicholson, at


   Update on SM4 Priorities & Plans for Repair of Science Instruments

                 HST Project/Goddard Space Flight Center

The HST Project/GSFC would like to clarify some of the statements
made recently regarding plans for repair of HST Science Instruments
during SM4. The Space Telescope User Committee (STUC) has been asked
to provide input on the relative scientific priorities of repairing the
Space Telescope Imaging Spectrograph (STIS) and the Advanced Camera
for Surveys (ACS). We wish to emphasize that the Project is working
to find a way to repair both instruments during SM4. However, we do
not know for certain, and may not know until the mission is in
progress, whether there will be time for only one repair or both. The
request we have made to the STUC is for guidance on the relative
priority of the repairs *in the event that only one instrument can be
repaired* (whether due to lack of total Extra-Vehicular Activity
(EVA) time or due to a mission-shortening contingency, or some other
unanticipated reason).

A number of ACS repair options are currently under investigation,
including both the Wide Field Channel (WFC) and High Resolution
Channel (HRC). Although we cannot ascertain a priori whether the
condition of the ACS electronics will allow for HRC recovery, it is a
goal of the repair effort that the HRC also be recovered.  It should
also be kept in mind that the ACS Solar Blind Channel (UV camera) is
operating now and is expected to continue to operate after SM4,
independent of any repair efforts.

The community should be aware that science-at-stake is only one of
several factors that will enter into the final decisions regarding
which instrument may or may not be repaired and in which order those
repairs might happen during the mission.  These other factors
include, for example, technical feasibility, likelihood of success,
and ease of the required EVA tasks. In fact, NASA HQ has not formally
approved either repair - the ACS repair, in particular, is a
relatively recent concept and requires further development before SM4
priorities and plans can be finalized.

We do welcome and encourage comments from the community via the STUC,
but ask the community to keep the above in mind when preparing input to
the User's Committee.

Further details on this process and the capabilities of the
ACS and STIS are available at