4
Short Report

During 2002, the Space Studies Board and its committees issued one short report, the main text of which this section presents.

4.1 Review of the Redesigned Space Interferometry Mission (SIM)

On September 12, 2002, Space Studies Board Chair John H. McElroy and Board on Physics and Astronomy Chair John P. Huchra sent the following letter report to Dr. Edward Weiler, NASA’s Associate Administrator for Space Science.

This letter responds to your request (attachment 1*) that the Committee on Astronomy and Astrophysics (CAA; see attachment 2*), a joint committee of the Space Studies Board and the Board on Physics and Astronomy, review the redesigned Space Interferometry Mission (SIM) to assess how its current scientific capabilities compare with what was originally envisioned by the 1991 Astronomy and Astrophysics Survey Committee (AASC) report1 and later reaffirmed by the 2001 AASC report.2 The CAA met in Washington, D.C., on May 1-2, 2002, was briefed by the Jet Propulsion Laboratory (JPL) project team on the redesigned SIM (see attachment 3*), and undertook a review of the redesigned mission’s scientific capabilities.3

SIM (originally AIM, the Astrometric Interferometry Mission) was prioritized in the 1991 AASC report as the fourth-ranked space program of moderate class. The science goals specified in the 1991 report were “. . . definitive searches for planets around stars as far away as 500 light-years through the wobbles of the parent star, trigonometric determination of distances throughout the galaxy, and the study of the mass distributions of nearby galaxies from stellar orbits” (p. 85). The report defined the wide-angle astrometric precision necessary for AIM to achieve the specified scientific goals: “The mission requirement would be to measure positions of widely separated objects to a visual magnitude of 20 with a precision of 30 millionths of an arcsecond; a more challenging goal would be to measure positions with a precision of 3 millionths of an arcsecond” (p. 85). The report also noted AIM’s technology demonstration potential for future space interferometry missions.

1  

Astronomy and Astrophysics Survey Committee, National Research Council. 1991. The Decade of Discovery in Astronomy and Astrophysics (Washington, D.C.: National Academy Press).

2  

Astronomy and Astrophysics Survey Committee, National Research Council. 2001. Astronomy and Astrophysics in the New Millennium (Washington, D.C.: National Academy Press).

3  

The CAA was not asked for, nor did it attempt to conduct, a technical assessment of the redesigned mission concept; however, two outside experts in interferometry (David Mozurkewich, Naval Research Laboratory, and Roger Angel, University of Arizona) were invited to the meeting to provide independent technical expertise during public discussions of the redesigned mission. The CAA was also briefed on the conclusions of a 2001 NASA external review board report (available at http://sim.jpl.nasa.gov/library/SIM_ERB.pdf) by Harold McAlister, Georgia State University.

*  

Attachments 1-3 are not reprinted in this annual report.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 101
4 Short Report During 2002, the Space Studies Board and its committees issued one short report, the main text of which this section presents. 4.1 Review of the Redesigned Space Interferometry Mission (SIM) On September 12, 2002, Space Studies Board Chair John H. McElroy and Board on Physics and Astronomy Chair John P. Huchra sent the following letter report to Dr. Edward Weiler, NASA’s Associate Administrator for Space Science. This letter responds to your request (attachment 1*) that the Committee on Astronomy and Astrophysics (CAA; see attachment 2*), a joint committee of the Space Studies Board and the Board on Physics and Astronomy, review the redesigned Space Interferometry Mission (SIM) to assess how its current scientific capabilities compare with what was originally envisioned by the 1991 Astronomy and Astrophysics Survey Committee (AASC) report1 and later reaffirmed by the 2001 AASC report.2 The CAA met in Washington, D.C., on May 1-2, 2002, was briefed by the Jet Propulsion Laboratory (JPL) project team on the redesigned SIM (see attachment 3*), and undertook a review of the redesigned mission’s scientific capabilities.3 SIM (originally AIM, the Astrometric Interferometry Mission) was prioritized in the 1991 AASC report as the fourth-ranked space program of moderate class. The science goals specified in the 1991 report were “. . . definitive searches for planets around stars as far away as 500 light-years through the wobbles of the parent star, trigonometric determination of distances throughout the galaxy, and the study of the mass distributions of nearby galaxies from stellar orbits” (p. 85). The report defined the wide-angle astrometric precision necessary for AIM to achieve the specified scientific goals: “The mission requirement would be to measure positions of widely separated objects to a visual magnitude of 20 with a precision of 30 millionths of an arcsecond; a more challenging goal would be to measure positions with a precision of 3 millionths of an arcsecond” (p. 85). The report also noted AIM’s technology demonstration potential for future space interferometry missions. 1   Astronomy and Astrophysics Survey Committee, National Research Council. 1991. The Decade of Discovery in Astronomy and Astrophysics (Washington, D.C.: National Academy Press). 2   Astronomy and Astrophysics Survey Committee, National Research Council. 2001. Astronomy and Astrophysics in the New Millennium (Washington, D.C.: National Academy Press). 3   The CAA was not asked for, nor did it attempt to conduct, a technical assessment of the redesigned mission concept; however, two outside experts in interferometry (David Mozurkewich, Naval Research Laboratory, and Roger Angel, University of Arizona) were invited to the meeting to provide independent technical expertise during public discussions of the redesigned mission. The CAA was also briefed on the conclusions of a 2001 NASA external review board report (available at http://sim.jpl.nasa.gov/library/SIM_ERB.pdf) by Harold McAlister, Georgia State University. *   Attachments 1-3 are not reprinted in this annual report.

OCR for page 101
In the intervening decade, the discovery of extrasolar planets opened up an entirely new and exciting scientific field, providing the impetus for a more ambitious narrow-angle astrometry capability on SIM that would allow the detection of planets with masses as low as a few times that of Earth. As presented to the 2001 AASC by the JPL project team during the AASC’s deliberations in 1999, SIM would have had a capability for wide-angle astrometry (10 microarcseconds requirement; 4 microarcseconds goal) and for narrow-angle astrometry (3 microarcseconds requirement; 1 microarcsecond goal), an astrometry magnitude limit of 20, full synthesis imaging plane coverage from 1 to 10 meters, and nulling capability (the last two being precursor technology demonstrations for TPF, the Terrestrial Planet Finder, which was one of the recommended new major missions in the 2001 report). As the scientific capabilities were enhanced, and as the technical understanding of the mission matured, the estimated mission cost increased so that the mission graduated from the astronomy surveys’ moderate mission category (with an estimated mission cost of ~$250 million in FY1990 dollars) to the major mission category (with an estimated mission cost of ~$1,000 million in FY2000 dollars). The 2001 AASC report reaffirmed the recommendation of the 1991 AASC by endorsing the completion of SIM, although the 2001 report did not explicitly mention the increased estimated cost of SIM, which had the effect of moving it from the decadal surveys’ moderate-mission category to the major-mission category. The scientific capabilities explicitly called for by the 2001 AASC were “. . . [enabling] the discovery of planets much more similar to Earth in mass and orbit than those detectable now, and . . . [permitting] astronomers to survey the Milky Way Galaxy 1,000 times more accurately than is possible now” (p. 20). The report emphasized the “particular attraction” of the dual capability of the new SIM, noting that this capability would enable “. . . both narrow-angle astrometry for detecting planets and wide-angle astrometry for mapping the structure of the Milky Way and other nearby galaxies” (p. 111). The report of the 2001 AASC’s Panel on Ultraviolet, Optical, and Infrared Astronomy from Space (UVOIR Panel),4 which contains more detailed and explicit statements about SIM and its scientific goals than those included in the main AASC report, stressed that “the primary scientific objective of the SIM mission is ultrahigh-accuracy astrometry” (p. 337). SIM’s capabilities for making distance measurements of “1 percent accuracy to distances of several kiloparsecs and of 10 percent accuracy throughout the Galaxy” would address the astrophysical goals of “providing a firm foundation for the understanding of stellar astrophysics . . . [making] luminosity determinations for key classes of stars . . . [to] reduce the calibration uncertainties in the cosmological distance scale . . . [and refining] our understanding of galactic structure, and in particular the structure of the Galactic halo, thus tracing the distribution of dark matter” (p. 338). The UVOIR Panel stated that “[s]earching for planets near stars in the solar neighborhood is the most ambitious of SIM’s goals . . . [because the search] should generate a preliminary survey of the local planetary population and a more extensive survey of the Jovian-mass planets . . . and . . . [thereby] not leave any ambiguity about the masses of extrasolar planets” (p. 338). To contain rising costs, NASA’s Office of Space Science directed in October 2000 that SIM be redesigned. The simpler (shared-baseline) design selected in May 2001—precisely one year after the recommendations of the AASC were made public—omits the nulling and synthesis imaging intended as technology demonstrations for the future Terrestrial Planet Finder mission, thus lowering SIM’s estimated cost and significantly reducing the technical risk. Loss of synthesis imaging translates to a loss of SIM’s capability to study certain high-surface-brightness structures, a capability that would have permitted, for example, measurement of the rotation of the inner parts of galaxies and the diameters of supernova remnants. Loss of nulling translates to loss of SIM’s so-called “nulling-imaging” capability that would have enabled the study of objects with a large dynamic range in brightness such as astrophysical processes occurring near a star, for example, winds of Wolf-Rayet stars, circumstellar dust disks of main sequence stars, and disks of young stellar objects. These imaging and nulling studies are interesting in their own right, but they are not integral to the core astronomical goals described in the 1991 and 2001 AASC reports, such as, for example, mapping of galactic structure, calibration of the cosmological distance scale, and detection of extrasolar planets. At the May 1, 2002, CAA meeting, the SIM project team from JPL reported that current component performance suggests the project’s ability to meet the 2001 UVOIR Panel report’s stated wide-angle requirement of 10 microarcseconds and to exceed the goal (3.2 microarcseconds, compared to the goal of 4 microarcseconds), making it possible for SIM to address the fundamental wide-angle astrometric science goals emphasized by the 4   Panel on Ultraviolet, Optical, and Infrared Astronomy from Space, Astronomy and Astrophysics Survey Committee, National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports, pp. 327-374 (Washington, D.C.: National Academy Press).

OCR for page 101
Table 1 Key SIM Parameters Defined by AASC Reports and JPL’s SIM Project Team   Wide-angle Astrometry Narrow-angle Astrometry   Concept Requirement (µs) Goal (µs) Requirement (µs) Goal (µs) Magnitude Limit (V) Nulling? Synthesis Imaging? 1991 AASC (AIM) 30 3 – – 20 No No 2001 AASC (SIM) 10 4 3 1 20 Yes Full UV plane from 1 to 10 m 2002 JPL redesign plan 30 4 3 1 20 No 10-m baseline only (plus rotation) Reported current performance 3.2 1.7   UVOIR Panel and the 1991 and 2001 AASC reports. SIM’s predicted narrow-angle accuracy performance will meet the UVOIR Panel report’s recommended requirement of 3 microarcseconds and is within a factor of two of the performance goal (1.7 microarcseconds, compared to the goal of 1 microarcsecond). Table 1 summarizes the key AASC and corresponding JPL expected mission parameters. SIM’s predicted narrow-angle capability will allow the detection of planets with masses five times that of Earth out to a distance of 10 parsecs. Progress toward the narrow-angle performance goal is continuing, and additional improvement in narrow-angle precision will lead to greater capability in an area where the SIM contribution will be unique: the measurement of masses and orbits for extrasolar planets with masses a few times that of Earth. However, the CAA notes that the size of the sample of such planets that SIM will detect and accurately characterize is very uncertain. The CAA was pleased to hear the JPL team’s report that excellent progress has been made in meeting SIM’s technical goals. The committee concludes that the core astronomical capabilities motivating the original recommendation for SIM in the 1991 AASC report, and the enhanced astronomical and planet-finding capabilities endorsed by the 2001 AASC report, have been preserved in the current rescope. The CAA reaffirms the scientific excitement of the 2001 AASC for the important new planet-finding narrow-angle science capability of SIM. Nevertheless, the CAA is concerned by the JPL redesign plan’s stated wide-angle accuracy requirement of 30 microarcseconds— which was the level recommended by the 1991 AASC but not by the 2001 AASC. The 2001 recommendation is the benchmark to which a redesigned SIM should be compared. Accordingly, the CAA reaffirms the following statement made in the 2001 UVOIR Panel report: “The majority of exciting astrophysical goals already pose a challenge under the floor requirements currently proposed: 3 microarcseconds for narrow-angle astrometry and 10 microarcseconds for wide-angle astrometry. Relaxing these floor requirements might eliminate large fractions of the important science. If SIM is unable to meet these requirements within its presently planned budget and schedule, it should be reevaluated by the scientific community to determine if it should remain a high priority as a major mission” (p. 338). Specifically, the CAA notes that not meeting the wide-angle astrometry requirement recommended by the 2001 AASC report would preclude the ability to accurately measure the distances to, and ages of, globular clusters, and to pursue almost all of the fundamental galactic structure science, such as the accurate measurement of the distance from Earth to the center of the galaxy, the shape of the galaxy’s outer rotation curve, and other basic properties of the Milky Way. In conclusion, the CAA finds that the scientific capabilities (based on the current component analysis) of the redesigned SIM meet the core capabilities originally envisioned by the 1991 AASC report and reaffirmed by the 2001 AASC report. In making this assessment, the CAA compared the reported astrometric performance of the redesigned SIM with the performance requirements and goals stated in the report of the 2001 AASC’s UVOIR Panel, and not with the less exacting wide-angle requirement level stated in the decade-old 1991 AASC report as was apparently expected by the JPL project team based on its May 2002 presentation to the CAA. This distinction should not matter in practice since (1) the JPL team reported that the wide- and narrow-angle capabilities are coupled in such a way that the wide-angle requirement/goal will be met on the road to meeting the more demanding narrow-angle requirement/goal, and (2) the JPL team reported an expected performance better even than the performance goal accuracy recommended by the 2001 AASC. Nevertheless, if the developing SIM project fails at some future time to meet the 2001 AASC performance requirements, it should be reevaluated by the scientific community to determine if it should remain a high-priority astronomy and astrophysics mission.

OCR for page 101
The CAA notes that it did not conduct a thorough assessment of the impact of current and future ground- and space-based astrometry missions on the science enabled by SIM, so no discussion of these other capabilities is included in this letter. In addition, while the current assessment has focused on an evaluation of the extent to which the mission can still meet the scientific objectives considered by the 1991 AASC and assumed by the 2001 AASC, the CAA did not address the question of whether the redesigned SIM would be given high ranked-priority in the context of the major missions recommended by the 2001 AASC report. As a result, neither a previous decadal survey committee nor the CAA has ever explicitly priority-ranked SIM against other major (as opposed to moderate-cost) astronomy and astrophysics missions. Finally, since the CAA conducted a scientific, not a technical, assessment of the mission, the committee expects that the SIM project will continue to be peer reviewed by nonadvocate panels—as is standard NASA practice. Signed by John H. McElroy Chair, Space Studies Board John P. Huchra Chair, Board on Physics and Astronomy