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Дата изменения: Wed Jun 15 19:38:40 2005 Дата индексирования: Sat Dec 22 11:49:59 2007 Кодировка: Поисковые слова: space shuttle discovery |
C. Biagetti
Space Telescope Science Institute, 3700 San Martin Drive,
Baltimore, MD 21218 USA
Keywords:
Upgrading the HST scientific capabilities with the installation of NICMOS and STIS involves planning that is driven by the instrument development, the second servicing mission (SM2), the observatory verification period following the mission, and the commencement of the science programs themselves. This paper will summarize the current plans for the servicing mission, the verification period, and the development of the science programs. It does not deal with the status and plans of pre-mission instrument development.
Launch of NASA's Space Shuttle ``Discovery'' is currently scheduled for 13 February 1997. Discovery is dubbed Orbiter Vehicle No. 103 and the Shuttle flight is labeled STS-82.
At the highest level, the mission has two primary goals: 1) the enhancement of HST's science capabilities by installation of two new orbital replacement instruments (ORIs), and 2) enhancement of HST's long-term scientific productivity through the replacement of failed or degraded orbital replacement units (ORUs) with new technology units.
The two new ORIs are of course NICMOS and STIS, both designed as HST axial instruments. NICMOS is an advanced infrared camera, developed by the University of Arizona, that will enable the HST to exceed the sensitivity and spatial resolution in the near-IR of both present and planned ground based facilities. STIS, developed by the NASA's Goddard Space Flight Center (GSFC), enhances HST's spectrographic capabilities by providing simultaneously wide spectral coverage and high spectral resolution, including long-slit imaging with an emphasis on the ultraviolet. More technical information on the design and scientific capabilities of these two new instruments is found elsewhere in these proceedings. NICMOS will replace the FOS in HST's instrument bay 2. STIS will replace the GHRS in bay 1.
Both NICMOS and STIS are equipped with their own special optics designed to correct for the spherical aberration of HST's primary optics and, as a result, will not require the correction provided by the Corrective Optics Space Telescope Axial Replacement (COSTAR), installed during the first servicing mission in December 1993. Therefore, after a nominal SM2, COSTAR's only purpose will be the optical correction for the Faint Object Camera (FOC). The FOC and the Wide Field Planetary Camera 2 (WFPC2), which also has its own corrective optics, along with NICMOS, STIS, and the Fine Guidance Sensor (FGS) system, will form the new complement of HST science instruments (SIs).
There are a variety of ORUs which provide the upgrades for failed or degraded units.
First, one of the FGSs will be replaced with an upgraded flight spare. All three FGSs have to date shown, to one degree or another, evidence of degradation that implies the need for eventual replacement of at least one of them in order to maintain the FGS system's astrometric and vehicle guiding capabilities. (At the time of this writing, the final decision as to which of the three FGSs to replace has not been made.) The flight spare will be upgraded with an adjustable mirror designed to compensate for the spherical aberration of HST's primary optics.
The DATA Interface Unit No. 2 (DIU-2) will be replaced with an upgraded version of the same device. The DIUs provide common command and data interface between the data management unit and other HST equipment. During SM1, DIU-2 suffered a Sida-A failure caused by an over-voltage state resulting from a power conditioning unit (PCU) relay failure. The replacement unit will have over-voltage protection circuitry to help prevent the recurrence of this type of problem.
The HST's data recording and playback systems will be extensively serviced. The current system consists of three Engineering/Science Tape Recorders (ESTRs), each with a capacity of about 1.2 gigabits, used to record and playback HST science and engineering telemetry. ESTR-1 has exhibited several tape-speed anomalies and ESTR-2 had experienced problems with reliability of track 2 recording. (Since the conference, ESTR-2 has suffered what appears to be a hard failure.) These anomalies, combined with the increased data demands during NICMOS and STIS operations, dictate an upgrade to the system. As of this writing, the plans are to replace ESTR-2 with a flight spare and ESTR-1 with a solid state recorder (SSR). The SSR is a prime example of a ``new-technology'' upgrade. It will have a 12-gigabit capacity and will offer the considerable operational advantages of simultaneous record and playback and simultaneous recording of science and engineering data to a single device.
HST has six gyroscopes, arranged in pairs called Rate Sensor Units (RSUs). Under normal operational conditions, four of the six gyros are configured to participate in the spacecraft's closed-loop pointing control system (PCU) by sensing and reporting vehicle accelerations. Four of the gyros (RSUs 2 and 3) were replaced on SM1 and RSU-1, as the oldest pair of gyros on-board, will be replaced with an identical unit in SM2 only if testing of RSU-1 prior to the mission returns negative results. As a result, while still appearing in the mission manifest, it may eventually be removed.
Finally, the Solar Array Drive Electronics No. 2 (SADE-2) will be replaced by the original SADE-1 unit, removed during SM1 and refurbished with heat sinks to prevent the recurrence of problems it experienced on orbit.
SM2, as currently planned, is a ten-day mission with a seven-person crew, and a launch scheduled early on 13 February 1997. A little less than two days after launch, the HST will be configured for capture while the Space Shuttle approaches for rendezvous, capture, and berthing. On flight day 4, the astronauts will begin the first of four scheduled six-hour extra-vehicular activities (EVAs), one per day, during which they exit to the Shuttle bay to go about the work of HST refurbishment. The first EVA is dedicated to the installation of both STIS and NICMOS, in that order. (Note that these activities include the removal of the GHRS and FOS during the same EVA.) The second EVA includes, among other things, the installation of the new FGS, thereby achieving the highest-priority mission items within the first two EVA periods. The other ORUs are to be installed in the remaining part of EVA 2 and in EVAs 3 and 4. As a result, all refurbishments are scheduled for completion by the end of flight day seven. The next day, HST is unberthed and, after the aperture door is re-opened, is released and separated from the Shuttle. NASA's Goddard Space Flight Center (GSFC) re-establishes control of the observatory while the Shuttle stays aloft for two more days, de-orbiting and landing about ten full days after launch. The mission thus designed allows for one contingency EVA and one deployment contingency. Figure 1 depicts NASA's current version (as of January 19, 1996) of SM2's proposed EVA scenario. It should be noted that this scenario is subject to change, as conditions warrant, between now and launch.
Figure: Proposed HST SM-2 EVA Scenario as of 19 January 1996.
The SMOV period begins, by definition, upon the Shuttle's release of HST on flight day eight of the SM2 (20 February 1997, under the nominal schedule). SMOV, which is performed by a team comprised primarily of people from the STScI, GSFC, and the instrument development teams (IDTs), has as its primary goal the timely recommissioning of the HST observatory for science operations. This goal results in four major categories of activities; 1) commissioning of the newly installed science instruments (NICMOS and STIS), 2) recommissioning of the existing science instruments (WFPC2 and FOC), 3) recommissioning of other observatory subsystems (which include the FGS and other ORUs), and 4) the performance of the Early Release Observations (EROs). At this point, the aforementioned team, having completed the SMOV requirements analysis phase, is working the detailed development of these recommissioning activities, including a determination of their durations, operational requirements, telescope pointing requirements, data processing and analysis requirements, and their scheduling interdependencies and constraints. This work is somewhat iterative in nature because our evolving understanding of the instruments' characteristics is a function of their development. It is resulting, therefore, in a progressively clearer picture of the SMOV program.
The two new SIs will be commissioned for science operations by means of a pre-defined series of tests. For each of the SIs, these tests can be categorized as engineering activation tests, optical alignment activities, target acquisition tests, aperture and slit locations, and initial performance characterizations and calibrations.
The engineering activation activities are designed to bring the new instruments out of their safe configuration, in which they will have been left following the Shuttle ``in-bay'' functional tests and to configure them mechanically and thermally for normal high-voltage operations. These initial engineering tests will also check out the proper functioning of the instruments' memories and data interfaces.
The optical alignment activities are iterative functions that determine the proper positioning of the corrective optics for these SIs that have experienced the rigors of launch and find themselves for the first time in a weightless environment. The iterations involve a cycle of observing the appropriate target, ground analysis of the optical properties of the resulting image, and determination of any necessary commanding to reposition the corrective optics. While the number of such iterations is hard to predict, conservative planning suggests, after factoring in the time for database and on-board table updates, several weeks of actual calendar time following release to achieve fine alignment.
The target acquisition activities involve verification of the flight software routines that are coupled with spacecraft attitude control algorithms to achieve the various types of target acquisitions. Both of the new instruments have a basic point source acquisition mode, but STIS in particular is capable of automatic acquisition of diffuse sources, bright sources, peak-ups, as well as coronographic acquisitions and associated peak-downs. In addition to target acquisition modes, the accuracies of centering targets in selected STIS slits are to be determined.
The performance characterizations and calibrations for each of the instruments are designed to validate and/or update the initial pre-launch calibration database and to demonstrate the basic on-orbit ability to perform complete calibrations. However, these complete calibrations are not temporal prerequisites for normal science operations and therefore are not planned as part of SMOV. They will be designed for and scheduled as part of HST's Cycle 7 science program.
For NICMOS and STIS combined, there are in all 55 discrete SMOV activities, over half of which involve external targeting and therefore dedicated telescope pointing. As of this writing, NICMOS is expected to require approximately 90 dedicated orbits for SMOV pointing, and STIS approximately 125. Note that, because of the interactive nature of the activities described above and other scheduling constraints to be described below, these 200 or so orbits cannot be scheduled contiguously, but rather will be spread over something on the order of 1000--1500 HST orbits (i.e., over several months from the start of SMOV).
Because of the amount of time required to carry out all of SMOV's required activities, the team will determine the earliest possible time for scheduling of the ERO observations, and for declaring each of the instruments ready to initiate science operations.
The two existing cameras are not scheduled for servicing and therefore their performance characteristics are not expected to be significantly affected by SM2. We are planning to take advantage of this circumstance to satisfy the high-level SMOV goal of timely recommissioning of the observatory for science operations; WFPC2 and FOC will resume their respective Cycle 6 science programs (which will have been initiated in July 1996), while we proceed with the more elaborate and extensive SMOV activities for NICMOS and STIS. As a result, within two weeks of the end of SM2, the Observatory will be performing productive science operations interleaved with the ongoing NICMOS and STIS commissioning prerequisites.
The SMOV activities for WFPC2 include only a routine decontamination procedure to eliminate any effects of the servicing environment and step-wise cool-down to operating temperature. This cool-down will be contingent on the results of periodic observations of a standard star at UV and Lyman alpha wavelengths to monitor any potential detector contamination resulting from new-instrument outgassing or other servicing effects. Following the achievement of operating temperature, which is expected within a week of release, and following a short set of calibrations, WFPC2 science operations can resume.
Likewise, the SMOV activities for FOC are relatively simple and quick. However, the FOC does require the re-deployment of the COSTAR, which will have been retracted prior to the start of SM2. The COSTAR re-deployment and the recommissioning of FOC for Cycle 6 science operations are expected to be achieved within two weeks of the start of SMOV.
SMOV activities for the new FGS will include calibrations and reference frame alignments that will result in its commissioning as an astrometer as well its incorporation into HST's Pointing Control System (PCS). These activities, as well as those for the check-out of the other serviced ORUs (e.g., the SSR, the DIU, and the SADE) are designed to be performed independently of the SMOV commissioning of the science instruments, and therefore are not SMOV schedule-drivers.
As in the original HST commissioning and following SM1, the ERO program will consist of several observations that together will demonstrate to the astronomy community, to the news media, and to the public at large, HST's new scientific capabilities afforded by the installation of NICMOS and STIS. The ERO program is as yet undefined in any detail. However, the observations will be carried out and the results released to the public as soon as technically feasible during SMOV.
While there is no hard evidence of serious contaminants involved in servicing or subsequent outgassing of newly-installed ORIs and ORUs, prudence nevertheless dictates that SMOV include anti-contamination measures as well as the monitoring of their effectiveness. In particular, there is concern, albeit unsubstantiated, that following servicing and during post-servicing outgassing, some molecular species that are subject to polymerization by high UV flux could be present in the HST's hub area. While observations of targets at UV wavelengths are not expected to present UV flux of dangerous proportions, such flux could result from the telescope's routine exposure to the sunlit portion of the Earth, thereby polymerizing such contaminants on the SI external optics and resulting in the potential reduction in UV sensitivities.
To avoid this possible scenario, the HST Project has decided that during SM2, STIS shall not be exposed to Earth-reflected sunlight, and that after release of HST, for a period of one to two weeks, HST will avoid occultations and slews involving the Earth's bright hemisphere. This condition may constrain the choice of early SMOV targets for all of the instruments, old and new, but is not expected to be a serious impact to overall SMOV scheduling unless evidence for loss of UV sensitivity is actually detected.
The feasibility of further precautionary measures are being studied. One such possible measure is the reduction of bright-Earth occultations and slew-crossings at the target-scheduling level. Since, during the early SMOV period, Observatory efficiency will be maintained, as described above, by scheduling WFPC2 and FOC Cycle 6 science programs interleaved with NICMOS and STIS SMOV activities, these science programs could be constrained to be those Cycle 6 programs that are in the celestial hemisphere centered on the Sun at the time of early SMOV, thereby reducing the average amount of bright-Earth exposure during target occultations.
UV-sensitivity monitoring by means of periodic observations of a UV standard will be a routine part of SMOV for both STIS and WFPC2.
Each of the SIs will be separately enabled for science at the earliest possible time in their respective SMOV programs. As indicated above, this means that for WFPC2 and FOC, Cycle 6 will be resumed, having already been started several months prior to SM2, within the first two weeks of SMOV. The initiation of HST science programs for NICMOS and STIS resolves itself into three components: 1) the ERO program, which consists of several observations by each of the new instruments, and which, as mentioned above, is as yet undefined in any detail; 2) Guaranteed Time Observer (GTO) programs, which are a set of priority programs and allotted orbits of observing time allocated to the Instrument Development Teams (IDTs); and 3) the HST's Cycle 7 science programs, which will be defined per the schedule discussion below. Estimated times for the initiation of each of these types of science programs is given below, as is a schedule that correlates at a high level all the major activities discussed in this paper.
The foregoing discussion indicates that there is still some uncertainty in the overall time required to reach the latter stages of SMOV and in particular to the specific activities whose results will be used to enable each type of science.
EROs: These observations, because of the intense interest there will be in experiencing HST's enhanced capabilities, will be performed at the earliest technically feasible time in SMOV. Though yet undefined, the programs most likely can be performed with an instrument that is optically aligned but only in a rudimentary state of calibration. Conservative estimates at this time put the STIS ERO observations at 9 to 12 weeks after SM2 and the NICMOS EROs at 10--13 weeks. The STIS period is driven primarily by the iterative target acquisition tests and slit-centering accuracies discussed above. This of course suggests that an ERO using a suitably simple operating mode (e.g., slitless spectroscopy using the CCD in a ``point-and-shoot'' mode and therefore not requiring automated target acquisition routines or precise slit location knowledge) could be done much earlier. The NICMOS period is driven primarily by the uncertainty in the time required to reach passive thermal equilibrium. Likewise, an ERO not requiring the accuracies afforded by fine optical alignment after thermal equilibrium can be done much earlier in SMOV. In any case, as we proceed in our refinement of the SMOV program, we retain as a goal the start of these ERO programs at six to eight weeks from completion of SM2.
GTO Programs: These programs are currently being developed by the IDTs and, depending on their requirements on the state of their respective instrument, are expected to be enabled at roughly the same time or soon after the start of the ERO program. One would expect, however, that many GTO observations would require a fully aligned and relatively well characterized instrument, and therefore would not be enabled until later in SMOV. Again, our conservative estimates put these times at 10 to 13 weeks for STIS and 12 to 15 weeks for NICMOS, with a goal, as with the EROs, of six to eight weeks after SM2.
Cycle 7 Programs: The start of HST's Cycle 7 is driven not only by the SM2 and the SMOV Program, but also by the STScI's science program planning cycles. The current plans are for a Cycle 7 call for proposals (CP), which will involve all the on-board instruments, in June 1996. Prospective General Observers (GOs) will have until September 1996 to submit a Phase 1 proposal for HST observing time. A Time Allocation Committee (TAC) will be formed, as usual, to evaluate the proposals and, in November 1996, will announce the accepted proposals and the awarded observing times. February 1997 will be the deadline for submission of the Phase 2 versions of the accepted proposals, after which the STScI's PRESTO group will begin the process of flight preparation of the programs. These plans result in the initiation of Cycle 7 in July 1997.
Figure: HST Observing Schedule---Cycles 6 and 7.
Figure 2 is a schedule that maps together all the planned activities discussed in this paper which lead to the full scientific utilization of HST in the NICMOS and STIS era. Note that Cycle 6 resumes shortly after SM2, but at that point consists only of WFPC2 and FOC observations. As described above, this plan serves to maintain Observatory efficiency while the more elaborate and time-consuming SMOV programs for NICMOS and STIS proceed to their commissioning for science. EROs and GTO programs start well before the completion of SMOV and Cycle 7 starts in July 1997, at a point when the SMOV program will have fully configured and sufficiently characterized the new instruments for normal science scheduling.
We are grateful to John Campbell, Associate Director for Flight Projects for HST, for providing SM2-related information and to Rud Moe, Servicing Mission Manager/GSFC for permission to use the proposed SM-2 EVA scenario.
GSFC 1995, STR-45 Hubble Space Telescope Servicing Mission Level II Requirements
GSFC 1995, SMR-2021 Hubble Space Telescope Servicing Mission Observatory Verification Plan (Draft)