FROM:
DATE:
Rodger Doxsey,
Denise Taylor, Bill Workman, Ian Jordan
August 18, 2004
SUBJECT: HST orbit availability after the STIS Suspend
3700 San Martin Drive
Baltimore, MD 21218
FAX (410) 338-4767
Following the STIS suspend on August 3, 2004, the anticipated unavailability of STIS in the remainder of Cycle 13 implies a dearth of orbits in the remainder of the Cycle 13 HST long range plan. This document provides an accounting of the orbits allocated to HST and an updated estimate of the number of orbits which would need to be filled by supplementary means. No discussion of ancillary issues, such as transition to Two-Gyro mode and adequacy of the tail, is discussed in this memo.
Table 1: Summary Recommedation for post-STIS TAC Orbit Re-Allocation
Recommendation Considerations
Total est_orbits until C13/C14 boundary (44 weeks)
“Tail” to maintain efficiency (10 weeks)
Subtotal orbits to plan for
Assumed HOPRs (2.5%)
Remaining “In house” orbits
Total available for re-allocation:
80 orbits/wk 85 orbits/wk
+3520
+800
4320
-108
+3740
+850
4590
-115
-3158 +/- 90 -3158 +/- 90
1054 +/- 90 1317 +/- 90
The total number of orbits which remain to be planned for is based upon the number of weeks
Operated by the Association of Universities for Research in Astronomy, Inc., for the National Aeronautics and Space Administration

which are planned to be filled. A “tail” is used to allow smooth and efficient transition between
Cycles and adjust for inability to schedule all visits within Cycle boundaries due to constraints on some programs. The tail used here is the same tail length specified in the February 24 pre-TAC
ODM orbit recommendation memo. Two different conceivable execution rates are specified.
2
Weeks remaining in current cycle (44 weeks)
“Tail” weeks at end of cycle (10 weeks):
Subtotal
Table 2: Weeks available within Cycle 13 Boundary ( < 2005.185)
80 est_orbits/ wk
3520
800
4320
85 est_orbits/ wk
3740
850
4590
This section details the orbit totals which have already been reserved for all known commitments..
Table 3: Pre-Allocated/Reserved From Cycle 12
Type
Planned (with plan windows)
Unplanned (without plan windows)
Calibration orbits (Phase 2s not in) non-STIS Chandra allocated orbits (Phase 2s not in)
Engineering (2-Gyro Tests)
Directors Discretionary (100 orbits minus 7 orbits in-house)
Target of Opportunities
STIS --> non-STIS transfer subtotal
Orbits
2300
123
126
38
40
93
328 +/- 50
110 +/- 40
3158 +/- 90
A number of uncertainties exist in recommendations on the number of orbits which should be substituted for the loss of STIS fromthe Cycle 13 TAC runners-up. Explicitly, those uncertainties are:

3
• HST est_orbit execution rate may or may not increase with the loss of STIS.
• Target of Opportunity activations are uncertain.
• Number of STIS orbits transferrable to other instruments has not been finalized.
• DD allocation rate may change in a post-STIS environment.
This document assumes that all DD time will be used. Also assumed is that all activations in the
PANS ToO program will be used (10339 & 10340) and a significant fraction of the remaining non-STIS ToOs will be used (60%-100% of remainder used). A guestimate range for the number of STIS-to-non-STIS transfers is loosely based on a triage performed by the STIS team.
Approximately 1286 orbits of STIS visits with plan windows will be lost, and another 34 orbits of unplanned STIS (not including Target of Opportunity visits), for an approximate total of 1320 orbits. Note that this number is comparable to the number of orbits computed available in the summary section based upon the higher execution rate.