Report of the HST-JWST Transition Panel 

Executive Summary 

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. Two additional Shuttle servicing missions, SM4 in about 2005 and SM5 in about 

2010, in order to maximize the scientific productivity of the Hubble Space Telescope.

The extended HST science program resulting from SM5 would only occur if the HST 

science was successful in a peer-reviewed competition with other new space 

astrophysics proposals. 

2. One Shuttle servicing mission, SM4, before the end of 2006, which would include 

replacement of HST gyros and installing improved instruments. In this scenario, the 

HST could be de-orbited, after science operations are no longer possible, by a

propulsion device installed on the HST during SM4 or by an autonomous robotic 

system. 

 

3. If no Shuttle servicing missions are available, a robotic mission to install a propulsion

module to bring down the HST in a controlled descent when science is no longer 

possible. 

 

We describe in the following eleven sections the principal facts and reasoning that led to 

this prioritized list of transition options and also present additional conclusions. We

include in appendices the panel’s membership and charter. 

1. Input to the panel 

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The panel has received and studied engineering and science data from NASA sources and 

from the Space Telescope Science Institute. Among the items received by the panel (prior to

the public meeting) were 55 thoughtful, individual emails from members of the scientific 

community, as well as 40 well-reasoned and fact-filled documents from NASA sources and 

other informed groups. In addition, the panel heard important oral testimony from a number

of experts including the Director of the Astronomy and Physics Division at NASA (Anne 

Kinney), the Director of the Space Telescope Science Institute (Stephen Beckwith), the 

James Webb Space Telescope (JWST) Senior Project Scientist (John Mather), the Director

of the SIRTF Science Center (Tom Soifer), the Hubble Space Telescope (HST) Propulsion 

Module Study Manager (David Stephenson), Associated Universities, Inc. President and 

former Director of the Space Telescope Science Institute (Riccardo Giacconi), and NASA 

Associate Administrator for Space Science (Edward Weiler). The panel also benefited from

volunteered oral presentations containing significant relevant information and stimulating

insights. Those contributing in this way included: C. Beichman, R. Fienberg, H. Ford, J.

Green, J. Grunsfeld, J. Huchra, B. McCandless, J. Morse, G. Rieke, and R. Thompson. 

Nearly all of the oral presentations were accompanied by carefully reasoned documents 

made available for study by the panel well before our public meeting. The panel reviewed 

about 200 email messages from the general public submitted through the panel’s website. 

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 

The HST has been NASA's flagship for more than a decade. It has spectacularly extended 

our knowledge of the cosmos -- from nearby planets to the most distant galaxies. From the 

HST’s vantage point above the absorption and blurring of Earth's atmosphere, this unique 

‘space observatory’ has attained unprecedented sensitivity and yielded brilliantly sharp

images. 

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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 success—one 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 is—and that JWST will be—a 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

Servicing missions have continually rejuvenated Hubble since its launch in 1990. After the 

first servicing mission in 1993 corrected a flaw in the optics, subsequent missions have 

maintained the health of the spacecraft and provided ever more powerful instruments to 

study the cosmos. 

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 

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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

The Columbia tragedy has raised new questions about astronaut safety, and the original 

plan to return the HST to Earth is no longer considered an option by NASA. Instead, plans 

are being developed to deorbit the HST in a controlled fashion to ensure that it lands 

safely in the ocean 

We note that recent events have initiated a revision of end of mission scenarios, and 

we conclude that implications for continued HST science operations should be 

examined. This report is an initial step in the re-examination. 

5. Uncertainties 

Formulating a strategy for the HST-JWST transition is difficult because of uncertainties

regarding the availability of the Space Shuttle, the JWST launch schedule, the need for 

developing a mode for de-orbiting the HST with at present unknown characteristics and 

time of availability, and because one cannot be sure when vital components of the HST 

will fail.

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. 

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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?

The congress or the executive branch could decide for policy reasons that there will be no 

further service missions to the HST, ruling out the possibility of implementing the plans 

for SM4. If there is not an SM4 servicing mission, the HST science program will be

aborted prematurely with a major loss of science opportunity. Moreover, citizens 

worldwide would be deprived of an important source of inspiration and of cultural 

enrichment. Outside support for NASA could be weakened. 

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 geosynchronous 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

The panel is impressed with the careful scientific and engineering preparations for the 

repair mission SM4. We urge continuing efforts to maximize the productive scientific 

lifetime of the HST under the assumption that SM4 will be the last HST servicing mission. 

In this connection, we applaud the efforts to make possible scientific operations with only 

two gyroscopes and to extend operation with a reboost. Since SM4 is likely to be delayed 

significantly, we believe that a study should be commenced as soon as possible to 

investigate the likely increase in operational lifetime from installing a complete set of six 

gyroscopes that are produced with the best technology presently available. Current 

estimates suggest that the expected duration of science operations is limited by the

robustness of the gyro design. Because of the need to consider possible robotic de-orbiting

of the HST, a careful study should be initiated to consider the possibility of attaching a

device to the telescope during SM4 that would make robotic de-orbiting easier. 

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 

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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

Many astronomers have written to the panel emphasizing the importance of an overlap in the 

operation of the HST and the JWST. We agree that this would be desirable since there will 

likely be a number of objects studied by the JWST for which it would be important to obtain 

optical and uv images with the HST. If there is a gap between the cessation of HST 

observations and the beginning of the JWST, it should be kept to a minimum. Even though 

financial support may be available during this time, a substantial gap in which no orbiting 

space telescope is available could erode scientific teams and decrease interest in the design 

of equipment that is essential for progress in the field. 

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

In the absence of a re-boost, the HST orbit will decay and the vehicle will re-enter and 

breakup in an uncontrolled manner as early as 2013 to 2020, depending on solar activity. A 

re-boost would postpone the reentry date by a few years. In any event, NASA policy is that 

the HST will not be allowed to reenter in an uncontrolled manner. Therefore, plans for the

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HST science program must include an intervention that allows an acceptable end to the 

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 

There are two options under consideration for enabling the end of mission. (The return to 

earth via the shuttle ,the original plan, is no longer under consideration.) These two 

options are: 

• Installion of a propulsion module on the HST during a Shuttle mission; 

• Robotic installation of a propulsion module. 

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, 

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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? 

The panel has received much insightful advice on this question. Two conclusions are clear. 

First, extraordinary science could be enabled by an additional servicing mission, SM5, with 

the Shuttle. Two, the resources that would be used for the support of a science-enabling 

SM5 could instead support other extraordinary space science projects. 

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 scienceenabling 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. 

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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. 

John N. Bahcall, Insitute for Advanced Study (Chair) 

Barry C. Barish, Linde Professor of Physics, California Institute of Technology 

Jacqueline N. Hewitt, Professor of Physics and Director, Center for Space Research, MIT 

Christopher F. McKee, Chair, Physics Department, UC Berkeley 

Martin Rees, Royal Society Research Professor, U. Cambridge 

Charles Townes, Professor in the Graduate School,UC Berkeley 

Appendix B: Charter 

The Office of Space Science (OSS) charters the HST-JWST Transition Plan Review 

Panel to evaluate the scientific impact of the current NASA plan for ending HST 

operations and beginning JWST operations. The panel should solicit input from the 

scientific community and other experts. In the course of its general review, the panel 

should present answers to the following questions: 

• Does the current plan provide for the best scientific use of unique HST abilities in

the context of the overall NASA program? 

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• Is there sufficient flexibility in the plan to respond to, for example, unforeseeable 

failures of HST instruments, for limitations on shuttle servicing missions, and for 

possible delays in the JWST program?

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: 

OSS Plan for HST – JWST transition

The HST Post-SM4 Science Review Panel Report

HST Propulsion Module Study Report (Interim) 

Origins Roadmap 2003

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-JWSTtransition.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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