POST-OPERATIONS EXOSAT ACTIVITIES
We have been giving some thought to EXOSAT activities in the period' following the end of the EXOSAT orbital operations phase. For the first time ESA will have in its possession a usable and complete data base derived from the operation of one of its satellites. All other ESA missions that have come to the end of their days have been of the principal investigator (P.I.) type, ie. the instrumentation was supplied by national P.I's and the data derived were the property of the P.I.
A paper was prepared for the delegate body, the Science Programme Committee (SPC), setting out the Agency's plans for EXOSAT in the post operational phase. The paper was given unanimous support by the advisory scientific groups ie. the Astrophysics Working Group and the Space Science Advisory Committee. The SPC at its meeting on February 7th, 1986 unanimously approved the plans described in ESA/SPC(86)3.
ESA continues to operate IUE, and like EXOSAT, IUE has built up over the years a
valuable, much used data base. It is anticipated that, if and when ME orbital
operations are terminated a similar scheme will be implemented.
EUROPEAN SPACE AGENCY
EXOSAT's orbital operations will terminate in the latter half, of 1986 due to orbit decay and/or loss of attitude control gas. IUE's orbital operations, from an ESA viewpoint, will terminate either following catastrophic satellite failure, or should funding for further mission extension not be approved.
Both missions represent a very significant investment of the Scientific Programme and both have generated and are continuing to generate invaluable data-bases, which will be used by researchers long after the termination of mission operations.
The advisory bodies, the Astrophysics Working Group and the Space Science Advisory Committee, and the SPC have emphasized, particularly in discussion of the long range plans for a 'European Space Information System', the need, not only to preserve eventually such data in an archive but to keep them readily accessible and useable in the shorter, post-operational term.
2. The needs of post-operational support and timescale
After the transmission from EXOSAT of its last bit, (the situation for IUE being
broadly similar) there will be a backlog of production of final observation
tapes and auto-analysis production to be cleared. There will also be
considerable tidying-up of documentation, including the need for an analysis of
the operations themselves for feed-on to future missions, e.g. ISO and the X-Ray
Cornerstone, etc. While the data remain proprietary to the observer for one year
after despatch, it will be necessary to continue support to the 'last' observers
as is done for current observers. On a longer timescale it can be anticipated
that updating of instrument calibration, software modifications
etc. will be required.
Data from early in the mission are now entering the public domain and eventually, of course, all data will be publicly available and support will be required to enabledata-base access. Again the need for tile interactive facility be reinforced.
It will be essential to provide the infrastructure, equipment, and scientific personnel for the proper support of the scientific communty wishing to exploit the EXOSAT data. This infrastructure must include the scientists of tile Observatory Team familiar with EXOSAT's instrumentation and the data analysis systems. For the support to the community to be of the right calibre these scientists will pursue their research activities (based on their own observations and archive retrieval) which, during tile orbital operational phase have had to take second place to the support role.
It will be essential for the data in the data-base to be brought to a common standard, e.g. with respect to current instrument calibration, format etc., bad or insignificant data sets will have to be purged and the data-base properly structured for long-term archive retrieval as needed, for example, by a 'data centre', see ESA/SPC(85)17. This can best be done by scientists actively working with the data.
Defining the time of arrival of the last bit as To the best estimate for the clearing of backlogs, finalising documentation etc. is of tile order of six months to a year, i.e. TO+1 . During this interval the requirements on those of the Observatory Team dealing solely with operations will reduce to zero.
It is very difficult to predict the rate of decay of tile interest in EXOSAT data, but the next major international missions, fully accessible by the world community, AXAF and the ESA X- Ray Cornerstone, will not fly until the mid-1990's. In the meantime specialist missions, (ROSAT, ASTRO-C and SAX) will fly. Observations with these satellites will, without any doubt, lead to the question - what did EXOSAT see? - if only because of the ubiquitous property of most X-ray sources - time variability.
The response to a questionnaire to the IUE 'community' indicated that intensive use of the HE data should be foreseen for at least 5 years after termination of orbital operations and this has been used as a guide for EXOSAT.
For present purposes for EXOSAT it is planned that, in tile period up to TO+4
years, the scientific manpower level of the Observatory Team would gradually
reduce and that support is provided to the 'level-of-effort' available. The
Executive limits the scope of this activity to 4 years at the present time, it
being assumed that after this period the
'European Space Information System' or something similar will
be in place. In this four year period it is the intention to use the ESIS
Network, to be established, to provide catalogue and eventually data access to
The Executive will monitor the use of the EXOSAT data-base and report back to the SPC as TO+4 years approaches.
3. Staffing and budget
The Observatory Teams performing the Scientific Operations of EXOSAT and IUE comprise scientific supernumeraries, contract staff and with support from some regular staff members. Currently for full operations the EXOSAT Team comprises some 24 persons. This number would reduce to about the 50% level at TO+1 year and to 25% level at TO+4 years. Associated with staff costs would be the running expenditure for missions etc.
The computer systems utilised for data reduction and analysis are, in the Case of EXOSAT, largely stand-alone and fully dedicated to the programme. However, capital expenditure for upgrading and/or replacement of certain items must be foreseen together with, of course, the necessary consumables. Some use is made of the ESA main frame computers.
A budgetary estimate is given below for the activities of the Observatory Team over the period from TO to TO+4. Some uncertainties exist, coming from precise dates of staff redeployment, use of main frame, level of staff support and associated overheads etc.
The financial situation of the scientific programme (ESA/SPC(85)6 of 10 June 1985 under headings 'EXOSAT' and 'D/Sci Planning Reserve Extensions') foresaw EXOSAT operated fully through to mid-1987 with a nominal run-down, corresponding to and limited to the immediate clearing of backlogs etc. through to mid-1988. The budget earmarked internally was 4.75 MAU. With EXOSAT operations terminating earlier, it is the Executive's proposal to use these funds to support the post-operational phase over the four years as described herein.
The Executive proposes the course of action described here to support EXOSAT in
its post-operational phase and invites the SPC to approve the funding as
EUROPEAN SPACE AGENCY
The Astronomy Working Group supports the proposals by the Executive for the post operational phase of the EXOSAT mission as described in ESA/SPC(86)3. The AWG considers it absolutely essential that the invaluable resource represented by the EXOSAT data base be preserved and worked on and the scientific community supported in its use in the years after orbital operations are completed. (The AWG pointed to the intensive ongoing Use of the Einstein data base and the excellent science now coming out. The same could be expected for EXOSAT.) The AWG considers that such post-operational phase support should be foreseen by tile Executive for IUE (currently still operational) and ISO (currently at the beginning of the development phase).