| Recent History
The Decade of Infrared
By the late 1980s, the impressive scientific heritage from the 10-month Infrared Astronomical Satellite mission -- and the promise of infrared astronomy from space -- was increasingly obvious to the scientific community. In 1989, the National Research Council of the National Academy of Sciences commissioned the Astronomy and Astrophysics Survey Committee (AASC) to recommend the most important new ground- and space-based initiatives for the coming decade. This committee of astronomers and astrophysicists represented the entire research community, and established fifteen advisory panels to represent the various wavelength disciplines, as well as solar, planetary, theoretical and laboratory astrophysics. These panels were responsible for canvassing views from a wide cross-section of the astronomy and astrophysics community. After the two-year study was completed, more than 15 percent of all U.S. astronomers had played an active role in some aspect of the decade survey.
The AASC committee has come to be known as the Bahcall Committee, named after Chair John Bahcall (Institute for Advanced Study, Princeton University). The survey results and recommendations were published in 1991 by the National Academy Press in The Decade of Discovery in Astronomy and Astrophysics, often referred to as the Bahcall Report.
Citing the fundamental importance of the 1-1000 micron infrared and submillimeter portion of the electromagnetic spectrum for studying some of the most compelling problems of astrophysics, the Bahcall Report referred to the 1990s as the "Decade of the Infrared." This proclamation was also based on the revolutionary advances that had been made in infrared detector technologies.
In this report, Spitzer was listed as the "highest priority for a major new program in space-based astronomy" for the next decade. The Executive Summary of the report stated:
[Spitzer], which would complete NASA's Great Observatory program, would be almost a thousand times more sensitive than earth-based telescopes operating in the infrared. Advanced arrays of infrared detectors, pioneered in the United States, would give [Spitzer] the ability to map large complex areas and measure spectra a million times faster than any other space-borne infrared telescope. Two successful Explorer missions [IRAS and COBE] provide an excellent technical heritage for [Spitzer].
Spitzer was envisioned as the fourth and final element of NASA's family of Great Observatories. The intention was to launch Spitzer early enough to permit a scientifically valuable overlap with the Hubble Space Telescope and the Chandra X-Ray Observatory (previously known as the Advanced X-ray Astrophysics Facility). The Bahcall Report listed some of the scientific contributions Spitzer was expected to make in each of four areas of research deemed to be of "currently particular significance."
Formation of Planets and Stars
Comets, Primordial Solar System;
Planetary Debris Disks;
Brown Dwarf Surveys
Origin of Energetic Galaxies and Quasars
Spectra of Luminous Galaxies to z = 5
Distribution of Matter and Galaxies
Deep 10 to 100 Micron Surveys;
Galactic Halos and Missing Mass
Formation and Evolution of Galaxies
In comparing the attributes of the three highest rated infrared facilities being recommended -- Spitzer, SOFIA, and an infrared-optimized 8-meter ground-based telescope (now known as Gemini Observatory)-- the Bahcall Report stated:
[Spitzer] has the highest sensitivity for photometry, for imaging, and for low- to moderate-resolution spectroscopy. Between 3 and 20 microns, [Spitzer] will be 10 to 40 times more sensitive than the infrared-optimized 8-m telescope. Despite advances in ground-based telescope design and detector technology, [Spitzer] will maintain fundamental advantages in sensitivity longward of 3 microns. [Spitzer] will also have the uninterrupted spectral coverage from 2 to 200 microns needed to detect important molecular and atomic spectral features.
Shortly after the Bahcall Report was published, the budget outlook for astronomy and astrophysics changed dramatically. Severe pressures on NASA's budget led to the cancellation of some missions and to the re-design of others -- including Spitzer. In fact, Spitzer underwent two significant "descoped" designs within five years, changing from a massive observatory with development costs in excess of 2.2 billion dollars to a modest-sized (but still powerful) observatory with comparable costs of less than 0.5 billion dollars. The various versions of Spitzer (then called SIRTF) were named after the NASA rocket that would be used to launch the observatory.
Following the last Spitzer re-design in the mid 1990s, and in recognition of the changed fiscal environment, the Committee on Astronomy and Astrophysics, a joint activity of the National Research Council's Space Studies Board and the Board on Physics and Astronomy, established a Task Group on SIRTF [Spitzer] and SOFIA (TGSS). The group was asked by NASA to conduct an independent assessment of the scientific capability of the re-scoped Spitzer (and SOFIA).
In a report released in April 1994, the TGSS concluded that:
... despite reductions in scientific scope that have resulted from NASA's current cost ceiling for new science missions, Spitzer remains unparalleled in it potential for addressing the major questions of modern astrophysics highlighted ... in the Bahcall Report. The TGSS is unanimous in its opinion that Spitzer still merits the high-priority ranking it received in the Bahcall Report.
Naturally, some science capabilities have been omitted during the de-evolution of Spitzer designs. Despite a dramatic 80 percent reduction in development costs, however, the scientific vitality and integrity of Spitzer has been maintained. How? Largely through clever and innovative engineering decisions.
In December 2003, four months after its launch, NASA formally gave the Spitzer Space Telescope its new name, finally retiring the old SIRTF acronym.