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.
Spitzer was launched with three cryogenically cooled instruments. In a new and innovative launch configuration, the telescope was launched warm and passively cooled during the first 3 months of the mission. The instruments were cooled directly with liquid helium. The liquid helium also vented into the telescope, passively cooling the entire system.
The mission cryogenic lifetime was designed for a minimum of 2.5 years but lasted more than two times longer, with the cryogen being depleted after 5.5 years on May 15, 2009. The cryogenic instruments were:
- Infrared Array Camera (IRAC) - simultaneous imaging at 3.6, 4.5, 5.6, and 8.0 microns
- Infrared Spectrograph (IRS) – high- and low-resolution spectroscopy at 5 – 40 microns plus 16 micron imaging
- Multiband Imaging Photometer – imaging at 24, 70, and 160 microns plus low resolution spectroscopy from 50 – 100 microns
After the depletion of the cryogen on May 15, 2009, Spitzer began a new ‘warm’ mission, at 28K, in July 2009. The IRAC the 3.6 and 4.5 micron cameras are still working at peak performance. This is the near- to mid-infrared portion of the spectrum. Astronomers use IRAC to study everything from comets and asteroids in our solar system, planets orbiting other stars, to the most distant galaxies in the Universe.
Each IRAC detector images 5 x 5 arcminute area of the sky. IRAC can quickly and efficiently map large areas of the sky and conversely can star at a single point on the sky for tens of hours with the only interruptions caused the by necessity to point at the earth every 24 – 48 hours to downlink the observations.
In the current warm mode of operations, Spitzer can continue to operate until late in this decade, providing a science mission of 10-15 years.