The telescopes of the National Aeronautics and Space Agency have given us unquestionably more information about the universe than we could ever have imagined.
Here is just a glimpse of Spitzer, one of the three telescopes now in service, Hubble, Chandra, and Spitzer; and one in the construction phase, James Webb.
To get around Earth’s atmosphere, astronomers in the1960s began attaching telescopes to huge balloons to carry them above the lower atmosphere’s obscuring effects. By the early 1970s, scientists attached small telescopes aboard high-flying Lear jets and sounding rockets, and discovered several thousand infrared sources.
None of these observatories, however, could get completely above the atmosphere, and by the early 1970s, astronomers began to consider the possibility of putting an infrared telescope into orbit. Most of the early ideas involved carrying the telescope on repeated flights on board the NASA Space Shuttle, but this idea was developed at a time when NASA thought that the Shuttle would be making routine flights every week, lasting 30 days or more. More importantly, it assumed that the contamination from the Shuttle (e.g., vapors, small particles, and heat interference) would not be a significant problem.
In 1979, the idea for a Shuttle Infrared Space Facility (SIRTF) was highly recommended in a report by the National Academy of Sciences, and in 1983, NASA announced a call for proposals to build instruments for a large Shuttle-based observatory, to remain attached to the Shuttle during the mission, and returned to the ground for refurbishment prior to re-flight. The first launch was expected to take place around 1990.
As NASA made this announcement, the first infrared telescope was launched into space by a collaborative team of the United States, the United Kingdom, and the Netherlands. The Infrared Astronomical Satellite (IRAS) was an Explorer-class satellite designed to conduct the first infrared survey of the sky. The 10-month mission was a resounding success, and led to huge interest in a follow-up mission from astronomers around the world. The impressive scientific returns from this free-flying observatory led NASA to revise their plans “to provide flexibility for the possibility of a [free-flyer] SIRTF mission.”
In 1984, NASA selected a team of astronomers to build the instruments and define the science program for a free-flying mission. The following year, in July 1985, this proved to be the correct decision, as the first Shuttle-based InfraRed Telescope (IRT) found that contamination problems from the Shuttle were considerable. The decision to proceed with a free-flyer observatory led to the first change for Spitzer, transforming the meaning of “SIRTF” to the Space Infrared Telescope Facility.
NASA’s Spitzer Space Telescope builds on a solid scientific and technical foundation established by two previous space infrared satellites. Both of these missions demonstrated the fundamental cryogenic technology and the considerable scientific benefit of liquid-helium-cooled telescopes and instruments in space.
The InfraRed Astronomical Satellite (IRAS), a NASA Explorer mission, conducted the first survey of the sky at thermal infrared wavelengths in 1983. A collaborative effort between the US, the Netherlands and the UK, IRAS opened a new chapter in astronomical exploration. Utilizing a 57-cm diameter telescope cryogenically cooled to a temperature of 4 K, IRAS circled the Earth in a 900-km polar orbit and operated for 10 months before its liquid helium was exhausted.
Ninety-six percent of the sky was mapped in four broad wavelength bands, centered at 12, 25, 60 and 100 microns. The hundreds of thousands of infrared sources detected by IRAS doubled the number of sources cataloged by astronomers. In the two decades since this path-breaking mission, scientists have published thousands of papers based on IRAS data, establishing the framework for all subsequent infrared observatories.
IRAS discovered disks of dust circling nearby stars, now thought to be an evolutionary step in the formation of planetary systems. The satellite also noted the presence of “infrared cirrus,” or dust grains, throughout the Milky Way Galaxy. IRAS identified a class of “starburst” galaxies, whose luminosity is due primarily to the birth of countless young and massive stars. The familiar winter constellation of Orion presents a spectacular contrast between the visible-light view (left) and the appearance as seen by IRAS (right).
The Infrared Space Observatory (ISO), a cornerstone European Space Agency mission, was launched in late 1995. ISO employed a suite of four scientific instruments to study the cosmos at wavelengths extending from 2.5 to 240 microns. Employing a cryogenic 60-cm diameter telescope and the first infrared detector arrays in space, ISO provided astronomers with a significant improvement in capabilities. Operating in a highly elliptical orbit, ISO circled the Earth once a day during its 30-month mission. Unlike IRAS, the ISO satellite was used primarily to observe individual targets, conducting 28,000 separate observations.
While the satellite no longer functions, ISO archival research continues to this day. Among the most important of the ISO findings is the discovery that water is abundant throughout the Galaxy. Its unprecedented spectroscopic capabilities allowed ISO to discover and characterize many new interstellar molecules. Moreover, ISO confirmed and extended many of the IRAS discoveries, including the existence of planet-forming circumstellar dust disks.
Other airborne (Kuiper Airborne Observatory) and space-based experiments (COBE/FIRAS, IRTS, MSX) have made important contributions to the field of infrared astronomy research. Spitzer is the next-generation leap in infrared astronomy, providing order-of-magnitude improvements in astronomical capabilities beyond previous and current observatories.
Spitzer has gone well beyond the scope of any previous cryogenic infrared space mission by making extensive use — for both imaging and spectroscopy — of the large-format infrared detector arrays now coming into wide use for astronomical applications.
Following the resounding success of the Infrared Astronomical Satellite (IRAS) mission and the increasing eagerness of the astronomical community for a follow-up observatory, a report was commissioned to recommend the most important new ground- and space-based missions for the coming decade.
The resulting report, called the Bahcall report, was published in 1991. It referred to the 1990s as “the decade of the infrared”, and listed that an infrared space telescope be “the highest priority for a major new program in space-based astronomy” for the next decade. This telescope would eventually become NASA’s Spitzer Space Telescope.
In a comparison with the other highest-rated infrared facilities being proposed (SOFIA and Gemini), the report described Spitzer as follows:
“[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 [Gemini]. 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.”
Spitzer was envisaged as the fourth and final element of NASA’s family of Great Observatories, along with the Hubble Space Telescope (HST), the Chandra X-Ray Observatory (CXO) and the Compton Gamma-Ray Observatory (CGRO), each of which was to observe the Universe in a different wavelength.
Shortly after the publication of the Bahcall report, however, NASA’s budget was dramatically revised. This led to the cancellation of some planned missions, and the re-design of many more, Spitzer included. Spitzer underwent two massive revisions in just five years, changing from a massive observatory with development costs in excess of 2.2 billion dollars to a modest-sized (but still powerful) telescope, with costs of less than 0.5 billion dollars.
An independent report on the redesign, released in April 1994, concluded that “…despite reductions in scientific scope that have resulted from NASA’s current cost ceiling for new science missions, Spitzer remains unparalleled in its 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.”
A significant factor in maintaining the scientific integrity of Spitzer, despite the budget cuts and dramatic redesign, was a series of clever and innovative engineering decisions, including a warm-launch, and a unique choice of orbit.
In December 2003, four months after its launch, NASA formally gave the Spitzer Space Telescope its new name, finally retiring the old SIRTF acronym.”
Some views from Spitzer
The Helix nebula was photographed by the Spitzer Space Telescope
Rosette Nebula taken by Spitzer
The dusty, star-studded arms of M81, a nearby spiral galaxy
similar to our own, are illuminated in unprecedented detail.