Report of the HST-JWST Transition Panel
Posted: August 14, 2003
We were charged "to evaluate the scientific impact of the current NASA plan for ending HST operations and beginning JWST operations," and we were asked to "provide conclusions that NASA may consider in order to maximize the overall scientific contribution."
Based upon the material that we have read, the oral presentations that we have heard, and the discussions we have had with scientific and engineering experts, the panel has reached a number of conclusions.
Our conclusions and recommendations primarily concern how to achieve the best science program for the Hubble Space Telescope (the HST), consistent with our understanding of the constraints and the uncertainties.The HST has made enormous contributions to science and human culture and the James Webb Space Telescope (the JWST) has the potential to make a similarly large impact. Our goal is to be helpful to NASA in the complex process in which NASA weighs all of the factors and makes a final plan.
Our principal conclusion is that the options for achieving the HST-JWST transition can be prioritized as follows, with the most preferred option listed first.
1. Input to the panel
Rarely has a scientific panel's activities attracted so many valuable contributions from such a wide range of interested scientists and non-scientists. We thank all those who have given so generously of their time to educate us.
We are grateful to distinguished colleagues who agreed to serve as anonymous referees on extremely short notice. Their comments significantly improved the clarity and focus of this report.
The panel is especially grateful to the NASA Office of Space Science Division Director of Astronomy and Physics, Dr. Anne Kinney, for sharing her deep knowledge and understanding of the facts and constraints related to the transition from the HST Observatory to the JWST Observatory. We are also deeply grateful to Dr. Eric Smith, the JWST Program Scientist and the HST Program Scientist for innumerable important services to the panel in organizing and facilitating our work.
We believe that we speak for the entire astronomical community when we express our admiration for the effective and articulate manner in which the Associate Administrator for the Office of Space Science (OSS), Edward Weiler, has enabled and implemented the scientific priorities of the scientific community as articulated in the Astronomy and Astrophysics Decade Survey Reports. Dr. Weiler has provided valuable insights to this panel.
2. The scientific importance of the HST and the JWST
Because it can be serviced by the Shuttle, the HST's suite of instruments can be regularly updated to take advantage of fast moving technology. It has thereby maintained an unflagging rate of cutting-edge scientific discovery over an extended lifetime.
By any standards the HST has been a spectacular successčone of the most remarkable facilities in the entire history of science. The HST has enabled thousands of astronomers in the U.S. (and also in Europe through partnership with ESA) to study an immense variety of cosmic phenomena. Moreover its marvelous images have captivated the imagination of a broad public, especially offering scientific inspiration to young people.
As often happens in science, each advance raises new questions that are tantalizingly beyond present capabilities. A fundamental new challenge is to probe still deeper into space and further back in time --to the era when the very first groups of stars, the 'ancestors' of galaxies like ours, started to form. This is the prime mission of the JWST, a major new telescope which goes beyond the capabilities of the HST, as well as being in some respects complementary to it. The 6-meter mirror of JWST will have 5.8 times as much collecting area as the HST's. Moreover, it will be optimized for the infrared, the waveband in which light from highly-redshifted distant objects reach us. The JWST's suite of instruments will offer unprecedented sensitivity from 0.6 microns to almost 30 microns. (It will not, however, have the HST's capability in the shorter wavelength visual or ultraviolet part of the spectrum.). Placed in a location permanently in Earth's shadow, the JWST will be about 1.5 million kilometers away. JWST is a collaboration with Canada and ESA. Several European nations contribute additionally to one instrument.
The JWST will have optimal properties for studying the cooler and more distant object within our Galaxy, and for probing regions shrouded by dust. Its unprecedented sensitivity and infrared imaging will be directed at asteroids, protostars, and planets as well as objects at the furthest reaches of the cosmos.
We conclude that the HST isčand that JWST will beča great observatory whose achievements will be written large in the history of science and whose impact will enrich human culture.
3. The life cycle of the HST
Like any device, HST's capabilities gradually diminish with age. Fortunately, there is no evidence for degradation of the optics to date. The instruments on the HST have design lifetimes of 5 years, but experience has shown that this is generally a conservative lower limit to their lifetimes. Most of the spacecraft systems, with the exception of the gyroscopes, have also demonstrated long lifetimes. Four of the six gyroscopes are currently operating. Except for the gyroscopes, only the thermal insulation and one of the Fine Guidance Sensors are presently degraded. SM4 is scheduled to repair the thermal insulation and replace the faulty Fine Guidance Sensor. Informed engineering estimates show that the HST must be revitalized by the Shuttle every 3 to 5 years in order to maintain its scientific capabilities.
The two primary factors that limit the lifetime of the HST are the gyroscopes and orbital decay. The HST carries a complement of 6 gyroscopes, and at present 3 are needed for successful operation of the telescope (although operation with 2 gyroscopes is anticipated). These gyroscopes are among the most sensitive ever built, but in the past they have lost their accuracy within a few years. The net result is that recent engineering estimates show that the HST has only a 50% chance of being able to carry out scientific observations more than 5 years after the 6 gyroscopes have been replaced.
Even at an altitude of more than 300 miles, Hubble feels the drag of the atmosphere. Without a reboost by the shuttle, its orbit will decay and it could return to Earth as early as 2013. A re-boost during SM4 could extend the in-orbit lifetime of the mission by a number of years; were it to be reboosted around 2010, then its orbital lifetime could be extended to 2020 or beyond.
We conclude that the HST, while in principle capable of a very long lifetime, requires for its continued scientific viability servicing missions by the Space Shuttle. HST servicing missions, like all shuttle missions, are expensive and risky.
4. Revised End of Mission Scenarios
We conclude that it is prudent for NASA to be prepared for a range of possibilities from the most pessimistic (no future Shuttle servicing missions) to the most optimistic (two future servicing missions). We therefore describe our further conclusions in a series of options that depend upon the availability of the Shuttle and the development of a practical and approved de-orbiting strategy.
We urge NASA to maintain flexibility in the development of plans for the transition from the HST to the JWST since the facts and the context in which decisions are made may change in the future.
6. No SM4?
In this worst case scenario, the HST project should be prepared to maximize the scientific return from the observatory in the absence of new servicing missions. This might require changes in the operational or scientific strategies.
Moreover, a decision to discontinue servicing would make available two outstanding science instruments that have already been built and tested, COS and WFPC-3, as well as significant undesignated funding in the years 2004-2009. NASA could consider a dedicated new mission featuring a fast-track 2-meter class telescope with COS and/or WFPC-3 in the focal plane, instruments that are built and could be adapted at minimum cost to a stand-alone mission. Such a mission could conceivably be launched into geo-synchronous orbit by 2010, in time to overlap significantly with JWST.
We conclude that the STScI and NASA should develop backup plans to cover the worst-case scenario that there are no further servicing missions to the HST.
7. Only SM4
We conclude that an inter-disciplinary group should be formed from experts at the GSFC, the STScI, and the MSFC, augmented as appropriate by contractors, astronauts, European representatives, other outside experts, etc., to explore all plausible approaches to lengthening the scientific lifetime of the HST within realistic mission and budgetary constraints. If possible, SM4 should occur in 2005 and no later than 2006. Timely servicing and installation of the new instruments are essential to take full scientific advantage of SM4.
8. The JWST Launch date
In the original NASA transition plan, a three year overlap in HST and JWST operations was scheduled. In the present OSS plan, a gap of one year is scheduled, with a planned cessation of HST operations in 2010 and the launch of JWST in 2011.
The panel was impressed by the progress that has been made by the JWST team and was encouraged by the fact that there are no recognized problems that would prevent meeting the 2011 launch deadline.
A significant milestone in a space science mission is the Announcement of Opportunity (AO) for proposals for instruments, which signifies that the scientific goals and the optical design of the telescope are well enough established that more detailed design of instruments should be undertaken. The AO to launch intervals for the four Great Observatories are: the Compton Gamma Ray Observatory (14 years), the Hubble Space Telescope (13 years) , the Chandra X-Ray Observatory(16 years) , and the Space Infrared Telescope Facility (19 years).
The JWST AO for instruments was released in November 2001. If JWST takes as long from AO to launch as the average Great Observatory, then the JWST launch date would be in 2017. We are impressed by the care and detail which has led to the more optimistic schedule determined by the JWST project and recognize that JWST may succeed in meeting schedule milestones better than previous observatories of comparable complexity.
We conclude that the JWST launch date might be delayed substantially beyond 2011. Appropriate planning should take into account the possibility that the JWST launch could be later than indicated by the current schedule.
9. Servicing and End of Mission
The continued vitality of the HST science program requires Shuttle service missions. Full exploitation of the HST would require two service missions during the period before the end of mission, SM4 in about 2005 (or not later than 2006) and SM5 in about 2010.
The SM4 mission is presently being considered for 2005 and is necessary for normal servicing of the HST, but especially for replacement of the gyros and for installing improved instruments. At the conclusion of SM4, Hubble's complement of scientific instruments will consist of two spectrographs and three cameras (as well as a Fine Guidance Sensor useful for astrometry). This will produce a vigorous science program prior to a possible final servicing mission.
10. Options for Ending the Mission
In both methods, the propulsion module installation can either be followed by continuing science operations or be the final mission to terminate the HST. The conceptual plans for implementing either method are preliminary. We believe strongly that attaching the propulsion module to the HST should not result in the premature termination of the science operations. This strategy requires either a propulsion device that can be attached to HST without degrading essential HST operational capabilities or the installation of a propulsion device after science operations are terminated. Preliminary discussions suggest to us that a technical solution is achievable in which a propulsion device is installed on HST without disrupting science operations provided appropriate tradeoffs are made between some science and engineering requirements.
The Shuttle could be used to attach the de-orbiting device and at the same time provide an additional servicing mission, SM5. This combined propulsion and service mission could greatly extend HST useful science lifetime. The committee prefers a Shuttle mission for this purpose because the same mission will also improve the performance of the HST before it is brought down to Earth.
In order to solve the technical issues that would allow the continued science operations the trade-offs of requirements and what is achievable technically must be reconciled. There is no demonstrated evidence that the robotic solution would be less expensive and the opposite might be the case. Either the robotic or shuttle option could enable termination, but only the shuttle method would also provide the benefit of full scientific exploitation of the HST.
In a related point, we note the very significant cost of maintaining a full servicing staff at GSFC to support the potential for future servicing missions of the HST. To minimize this cost, we think innovative ways should be sought to try to involve some of this staff in the support of other missions, such as the JWST. We understand that it may be difficult to reassign contract personnel at GSFC who are expert at tasks related to servicing missions to other tasks, but we think that this issue merits thorough study.
We conclude that it would be desirable to form a joint team of experts in propulsion engineering, spacecraft operations, and science operations in order to develop the best solution for servicing and ending the mission of HST, while taking into account safe termination, servicing requirements, and the possible trade-off of requirements to continue the science program. This team would presumably include scientists from the STScI, HST servicing and engineering experts from GSFC , and propulsion experts at MSFC, and might include astronauts or other relevant experts.
11. Is an additional servicing mission for the HST scientifically worthwhile?
Although there are qualitative descriptions of attractive science programs that could be done with an extended HST mission, there is no definitive proposal which the panel could evaluate.
The panel has great confidence in the Office of Space Science peer review system and great admiration for what this system has produced in the past.
We therefore conclude that the best way to determine the value of a future science-enabling SM5 is for NASA to issue as soon as possible a fast-track AO permitting scientists to propose a science program, which could include new instrumentation, to be performed with the HST dedicated to performing the observations described in the proposal. In the AO, prompt release of the data to the community should be encouraged, as well as robust and simple science operations, and the relevant experts at STScI and GSFC should assist in drawing up the AO.
If more than one competing SM5 proposal is submitted, NASA should have a peer reviewed competition to select the best HST proposal or proposals within total budgetary, programmatic and operational constraints. The successful proposal, which may include a new instrument, should then be peer evaluated in competition with other comparably sized new scientific proposals such as those within the Explorer or Discovery programs. In this way, no already approved science project would be adversely affected. It is our intention that this process should maintain the relative priorities of the Decade Surveys.
We understand that it will be difficult to release an AO and select an instrument in time for a service mission in about 2010. However, the Office of Space Science is highly respected for its ability to create original solutions for new problems. We note the possibility of a pre-AO public meeting, or a written information release, that might allow prospective proposers to begin work in the near future. We also note that there may be science proposals that do not include any new instruments, but instead rely on the post-SM4 instruments. In any event, only proposals that meet the time constraints should be considered for the peer review competition.
A fast-track AO will permit a science program that, for example, could provide spectacular images in the ultraviolet, optical, and infrared, coupled with information-laden spectroscopic measurements.
If the combined propulsion and service program is successful in the peer review competition, and if the NASA Administrator approves an additional Shuttle flight for this purpose, then we believe that the Administrator should find a way to fund the required Shuttle-related costs out of the entire NASA budget (not just out of the OSS budget). This special treatment of the Shuttle costs seems appropriate to us because of the enormous outreach benefits of HST science.
Appendix A: Panel Membership
Appendix B: Charter
The panel should produce a brief report summarizing their findings with respect to the charge and providing conclusions that NASA may consider in order to maximize the overall scientific contribution. NASA will provide the following items as initial input to the panel for their consideration:
During their discussions the panel may seek additional technical details or clarifications through the OSS and the HST Program Office. On July 31, 2003 the panel will hold an open meeting (Washington, DC [additional details]) at which interested parties can provide input related to the HST-JWST transition plan. The panel will additionally consider comments from the community received via a web site (http://HST-JWST-transition.hq.nasa.gov/).
Panel Composition: The Panel shall consist of 6 members, inclusive of the panel chair
Panel Products: The panel should produce a brief report for the Office of Space Science summarizing their findings with respect to the charge and providing conclusions that NASA may consider in order to maximize the overall scientific contribution
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