SM4 STIS & ACS Repair Contingency Plans

As part of the preparations for all HST Servicing Missions, contingency plans are developed in anticipation of any event that might lead to a shortened servicing schedule with fewer EVAs. The contingency plans take many factors into account, with astronaut safety being of prime importance. An important element in formulating the plans is a community-based assessment of the relative scientific priority of the science instruments that are scheduled for attention during the servicing mission.

At the April 2007 Space Telescope Users Committee meeting, NASA Headquarters formally asked the STUC to provide input on the relative scientific priorities that should be assigned to the tasks of repairing STIS and ACS during Servicing Mission 4. This judgement will serve as one component in determining the overall relative priorities of the repair activities. The STUC's assessment was to be made under the assumption that both COS and WFC3 are installed and fully operational, and that vital space craft maintenance and safety related activities have been completed successfully.

The STUC agreed to provide a recommendation by May 15th. They consulted with the HST User Community on this issue, soliciting responses to the following question:
Based on scientific considerations, which instrument, ACS or STIS, should have the higher priority for repair?
The STUC asked that interested parties submit brief comments describing the scientific rationale for their preference. The closing date for those comments, which were sent directly to the STUC via a dedicated e-mail address set up at OCIW, was 14 May 2007. A total of ~50 responses were received.

The STUC made a formal response to the HST Project and the STScI Director on May 15 2007. The main points are summarised as follows:

1. There is strong scientific support in the community for repairs to both instruments.
2. There was a modest majority in favour of STIS among the members of the community who replied to the STUC request for input.
3. The STUC themselves had a slight majority in favour of ACS, but recognised that other considerations (technical, scheduling, cost) may play a greater role in deciding the priorities in the final EVA schedule.

In support of the STUC's activity, STScI is providing this web page, which collects information on the technical capabilities of the four instruments: ACS, COS, STIS and WFC3.

Reference Documents

Overview