Independent Science Review:
NICMOS Cryocooler

September 10 - 12, 1997

1. Background

The Independent Science Review is a mechanism that permits NASA, AURA, and the Space Telescope Science Institute to seek independent scientific, technical, and managerial advice on high-level issues related to the Hubble Space Telescope Project. Previous reviews were convened in July 1996 and May 1997. These concentrated, respectively, on issues relating to the Second Servicing Mission (SM2) and an anomaly that developed shortly after SM2 in the Near Infrared Camera and Multi-Object Spectrometer (NICMOS). The reviews were conducted by senior scientists with experience with the Hubble Space Telescope or other space missions and their implementation and management.

The present Independent Science Review (ISR) was recommended by the May 1997 ISR and convened by AURA at the request of the HST Project. The Review was asked to consider the implications of mounting a cryocooler on NICMOS during the Third Servicing Mission (SM3), with the aim of prolonging the life of the instrument. Review Team members were George R. Carruthers, Judith G. Cohen, Robert A. E. Fosbury, Fred C. Gillett, Richard J. Harms, Martin Harwit (chair), Jeffrey Linsky, Stephan Price, and Richard J. Wainscoat.

2. The Charge to the NICMOS Cryocooler Independent Science Review Team

NICMOS, a cryogenically cooled instrument, was originally designed to operate for a period of 4 to 4.5 years. A thermal short, that developed in its dewar soon after installation on orbit, now indicates that the instrument will have a useful lifetime of only about 1.6 years, unless the short breaks -- an eventuality now considered remote. Cryogenics systems experts within GSFC's Engineering Directorate have proposed an approach to cooling NICMOS to acceptable operating temperatures with a Reverse Brayton-Cycle Cryocooler, a new, high-technology mechanical cooler.

The Cryocooler Independent Science Review (CC-ISR) was charged with conducting a high-level review of the cooler as soon as possible after the completion of a Critical Design Review that was held at GSFC on September 4 and 5, 1997. The purpose was to:

3. Two Basic Issues

The ISR decided to break down its task into two separate and independent questions:

A. The first concerns itself with the Creare Corporation's Reverse Brayton-Cycle Cryocooler alone. It asks whether the capabilities the cooler could provide the astronomical community are of sufficient significance for NASA to pursue the goal of preparing the cooler for flight and conducting an orbital test.

The ISR team was unanimous in its assessment that a reliably working, space-qualified cryocooler has long been a priority of the space community, and should be energetically pursued. It has many applications in space science, both in earth-viewing and astronomical missions. In infrared astronomy alone, cryocoolers have been considered in preliminary planning for major missions such as NGST, where a mechanical cooler could permit operations beyond the 5 micron wavelength limit, and in designs for FIRST, where mechanical coolers could provide similar advantages.

While cryocoolers for space operations have long been considered important, vibration, mechanical wear, and low efficiencies, precluded their widespread use. The Brayton cryocooler now proposed for NICMOS shows great promise. In the laboratory it has been working continuously for thirteen years, with thousands of on-off cyclings showing it to be reliable and robust.

The HST Project at GSFC is enthusiastic about pursuing the goal of conducting a test of a cryocooler aboard the Shuttle, in 1998. They plan to test the cooler's performance under zero-gravity conditions, and determine its cooldown rates, maximum performance, and ability to control temperature within strict bounds. This so-called "HOST" mission will also test the performance of a Capillary Pump Loop and all flight electronics components, and will assess vibration levels and general compatibility with HST systems.

These activities, though currently on an exceptionally fast track, carry the promise of providing the space community, and in particular the astronomy community, with a long-awaited capability of genuine importance. We endorse the approach and judge the risks involved worth the potential yields.

B. The second, more complex question asks whether this cryocooler should be installed on the NICMOS instrument on the SM3 changeout mission in 1999. To answer this in sufficient depth, the ISR examined

These respective issues are addressed in the next three sections.

4. Astronomy with NICMOS: An Exciting Program with Great Potential

The NICMOS instrument has a number of unique capabilities which ground-based instruments will not match in the foreseeable future. Early-release observations (ERO) now in hand point to the promise of the instrument. Its current capabilities on orbit exceed a number of pre-flight expectations. It has demonstrated diffraction-limited performance at all wavelengths and significantly low scattered light levels.

A key strength of NICMOS is the low background it sees between wavelengths of 1 and 2 microns. In contrast to ground-based instruments, whose broad-band observations are hampered by the atmosphere's irregularly undulating OH airglow, the NICMOS on-orbit background is a hundred times fainter and limited primarily by zodiacal light. This difference should permit Camera 3 with its broad-band filters at 1.1 and 1.6 microns to study extremely faint extended structures 0.2 to 10 arc seconds in size, to a depth 3 to 4 times fainter than possible from the ground.

This capability will be demonstrated in the very deep observations to be made in December 1997 of the Hubble Deep Field, where NICMOS is expected to reveal the nature of faint, distant galaxies only slightly larger than 0.2 arc seconds across, at sensitivity levels never matched from the ground. Other expected results are faint detection limits on -- or potentially the discovery of -- brown-dwarf halos around galaxies, extended structures around quasars, stellar bridges between galaxies, and faint structures in star-forming regions, detected to unprecedented depths.

The grism mode of NICMOS offers unique advantages that cannot be achieved from the ground. Slitless spectroscopy at a spectral resolution of 200 becomes possible on orbit and appears to yield good spectra on H = 20 mag. objects in observations lasting only a single orbit. The absence of OH emission features endows NICMOS with a significant multiplex advantage in this spectroscopic mode, compared to ground based instruments restricted to the use of a slit along which at best one or two objects can be placed. The parallel mode NICMOS grism survey currently under way illustrates the potential of this capability for red shift determinations of faint field-galaxies, for low resolution spectroscopy, and for isolating samples of peculiar objects for more detailed study.

Unlike ground-based adaptive optics systems, NICMOS provides a stable, fully diffraction-limited point-spread-function. This is crucial for observations requiring a high dynamic range, and has already led to the detection of faint companions of bright stars and features around luminous quasars. Studies of surface brightness fluctuations that permit measuring cosmic distances also benefit from a stable, well-characterized point-spread-function.

The earth's atmosphere blocks observations of the spectral lines of several important atoms and molecules, making their detection difficult or impossible from the ground. Among the most important of these are the Paschen-alpha line at 1.88 microns and many water vapor features. Water vapor observations of solar system objects, evolved stars, and interstellar shocks and star-forming regions, are of particularly high interest.

The original 4 to 4.5 year lifetime foreseen for NICMOS would have lent itself naturally to a program of follow-up observations. As with any new capability, early observations often provide unexpected results that need to be verified or more deeply explored. This is only possible given enough time to carefully calibrate the instrument to fully understand its characteristics and limitations in each of its many observing modes. The reliable reduction of astronomical data cannot proceed until such instrumental parameters are well established. A thoughtfully crafted program of follow-up observations requires operation for a sufficiently long time.

It is, therefore, not only the wealth of new observations that would be increased by extending the NICMOS lifetime. An added benefit would be the validation and systematic exploration of unexpected new results made possible by an orderly sequence of follow-up observations. An extended lifetime will permit a systematic investigation of highlighted new discoveries and help the community to gain the astronomical insights this mission was intended to provide.

NICMOS promises a rich program of exciting observations that will not be matched by ground-based instruments nor, for the next few years, by any other instrument to be launched into space. The instrument's relatively recent launch has not yet permitted a complete assessment of its various capabilities, although its performance, to date, appears to match or exceed preflight expectations. A more detailed assessment should become available over the next twelve months and permit a critical analysis of the instrument's full scientific promise.

5. Costs to the HST Program

The ISR was given a presentation on the costs of preparing a Creare cryocooler for a HOST flight aboard the Shuttle and for the SM3 installation of the cryocooler on NICMOS.

Funds directly chargeable to the HST program were presented as roughly $6M. The source of these monies, the ISR was told, were contingency funds earmarked for the years 2002 and rephased by NASA Headquarters. Use of these funds was presented as having no impact on operations, flight hardware, or data analysis. Additional funding was being provided by the Department of Defense which, in past years, was the major funder in the development of space cryocoolers.

The project pointed out that the cited costs were modest compared to the overall cost of NICMOS -- $105M.

The ISR accepted these figures as presented. Time did not permit a deeper analysis, but the HST Project appeared sincere in its desire to minimize the costs of a mechanical cryocooler and any financial impact on HST science.

6. Uncertainties Surrounding a Cryocooler Mounted on NICMOS

A substantial number of uncertainties persist. Many come about from the ongoing evolution of the cryocooler concept. Others arise from a lack of information on the structural changes that may have taken place in the NICMOS dewar. The sum of these uncertainties has to be weighed against the scientific value of maintaining NICMOS operational after cryogen exhaustion. The undefined factors fall into two classes:

A. Impact on the Overall Scientific Yield of HST

  1. Thermal impacts on the Aft Shroud and on the other astronomical instruments on HST;
  2. Operational restrictions resulting from the cryocooler's high power-requirements -- particularly the possible restriction of operating several instruments in parallel;
  3. Impact on the HST Pointing Control System, including jitter and potential thermal snapping' of the external radiator;
  4. Electromagnetic interference and noise;
  5. Contamination;
  6. Costs in the development of operational software and additional costs of operating NICMOS beyond the originally anticipated lifetime;
  7. Risks to HST hardware during the SM3 extravehicular activities, which will be on a tight time schedule;
  8. Necessary reduction of contingency funding up to and beyond SM3. This could come at a cost, e.g. if the Advanced Camera System were to run into unanticipated problems.
B. Impact on the Long-Term Scientific Yield of NICMOS
  1. Current uncertainties about the degree to which the NICMOS dewar has been stressed;
  2. Difficulties, to date, in determining whether the focal plane might be even further displaced after cryogen runs out and the system warms up before again being cooled -- this time by the cryocooler. Related questions concern the degree to which vignetting through focal plane misalignment and displacement of the coronagraph occulting disk might affect the scientific yield of a re-cooled NICMOS.
  3. Currently unresolved questions about parasitic heat losses that might make it difficult to reach the low temperature and temperature stability required for detectors and filters if NICMOS is to continue to provide the highest quality observations.
Most of these points were addressed in depth with the HST Project but, of necessity, remain uncertain. Without exception these factors are in flux, often varying on a day by day basis.

7. Recommendation Relating to a Cryocooler on NICMOS

Given the numerous remaining uncertainties, the ISR submits four recommendations:
  1. Preparations for flight testing the Creare cryocooler on the HOST mission should proceed.
  2. If a cryocooler is mounted on NICMOS it will most probably have detectors operating at a temperature roughly ten degrees higher than today. The NICMOS instrument PI team, STScI, and the HST project, should try to assemble whatever data can be gathered around the time of NICMOS dewar warm-up to determine the extent to which higher temperatures can be tolerated, and the extent to which an optimum set of operating temperatures might be defined as a target for later, mechanically cooled operations.
  3. The ISR agrees with the May 1997 ISR's recommendation that STScI should allocate NICMOS an increased fraction of HST observing time -- along lines that the Institute has already implemented. However, we further recommend that the selection of NICMOS observing programs should keep in mind that results obtained with the instrument now will serve as a guide to instrument designers for future missions, such as NGST. We, therefore, recommend that the Director of STScI provide discretionary time to observations that particularly stretch the envelope of capabilities and have the greatest potential for providing both new astronomical and instrumental insights. The more NICMOS can teach us about its findings through particularly challenging observations, the better informed we will be in designing successful future astronomical missions.
  4. Following flight testing on the HOST mission in the early fall of 1998, a further Independent Science Review, whose membership should include the Principal Investigators of ACS, COS, NICMOS, STIS, and WFPC2, should be convened to reassess the scientific justification for flying the cryocooler on NICMOS. By this time, we should have learned far more than we know today about the status of the NICMOS dewar and optics. We should know how well the performance of NICMOS has held up in forefront astronomical observations, particularly in view of continuing advances in ground-based techniques that might conceivably reduce the advantages of observing with NICMOS from space. By the late summer or early fall of 1998, most of the outstanding engineering uncertainties -- in particular any uncertainties about the ability of the Aft Shroud Cooling System (ASCS) to maintain sufficiently low temperatures for optimal functioning of all instruments aboard HST -- should have been removed. We should then have a far clearer understanding of technical concerns and potential risks surrounding the attachment of a cryocooler on NICMOS.
By the late summer of 1998, a comprehensive collection of the major observations and discoveries made by NICMOS will also be in hand. These should be made available to the ISR to be convened, independent of proprietary data rights held by individual NICMOS investigators; only with access to these data will the 1998 ISR be able to evaluate the full merit and promise of NICMOS observations and contrast these to capabilities that will be available with ground-based instruments in the post-1999 period and to the quality of observations yielded by other instruments on HST.

With more specific information than currently in hand, an Independent Science Review convened in 1998 will be in a position to formulate a necessary, informed judgment and reasoned recommendation on the overall scientific merits of flying a cryocooler on the HST.

Summary

1. The ISR recommends that the HST Project pursue the flight testing of a Reverse Brayton-Cycle Cryocooler on the HOST mission planned for 1998.

2. Because of the speed with which preparations for this flight test have had to be pursued, and the detailed approaches that still need to worked out, the ISR was unable to reach a reasoned decision on whether the merits of mounting a cryocooler on NICMOS outweighed the potential scientific risks to other HST instruments. The ISR, therefore, strongly recommends that a further Independent Science Review, whose membership should include the Principal Investigators of ACS, COS, NICMOS, STIS, and WFPC2, be convened shortly after the HOST mission is flown. The data available at that time will need to be sufficiently complete to remove current uncertainties and persuade the ISR that the scientific risks are sufficiently low and the scientific promise sufficiently high to go forward with flying a cryocooler on NICMOS aboard HST.

Martin Harwit, Chair
Independent Science Review
September 18, 1997