The Single Aperture Far-InfraRed observatory (SAFIR) is a large cryogenic space telescope which could be launched as early as 2015. "Single Aperture" refers to the telescope's single primary mirror, distinguishing it from multi-mirror interferometry missions.

SAFIR will study the earliest phases of forming galaxies, stars, and planetary systems at wavelengths where these objects are brightest and which contain a wealth of unique information: from 20 microns to one millimeter. Most of this portion of the electromagnetic spectrum is not accessible from the ground because it is absorbed by moisture in Earth's atmosphere.

SAFIR's primary mirror is expected to be 5-10 meters in diameter, quite large for a space-based telescope. For comparison, SAFIR's predecessor, the Spitzer Space Telescope (Spitzer), launched in 2003, has a primary mirror only 0.85 meters in diameter.

The SAFIR telescope will be cooled to a temperature of about 5 K (-451 F), just five degrees Celsius above absolute zero. (Technology advances may allow a temperature as low as 4 K.) The combination of large mirror size and cold temperature will make SAFIR more than 1000 times more sensitive than the currently planned Spitzer and Herschel Space Observatory -- approaching the ultimate sensitivity limits at far-infrared and submillimeter wavelengths. SAFIR's sensitivity will be limited only by the irreducible noise of photons in the astrophysical background, rather than by infrared radiation from the telescope itself.

In consideration of its enormous scientific potential and technological feasibility, the mission was recommended for technology development by the National Academy of Sciences Astronomy 2000 Decadal Committee as "the next step in exploring this important part of the spectrum."

What makes this part of the spectrum so important is that, while far-infrared and submillimeter light can penetrate dust clouds, half or more of the optical and ultraviolet light produced in the universe is absorbed by dust and reradiated in the far-infrared and submillimeter. Even in our local area of the universe, many galaxies are so dusty that they radiate mainly at those wavelengths.


This has two important consequences: First, to accurately measure the energy output and structure of objects that are obscured by dust, far-infrared continuum emission (emission across a broad band of wavelengths) must be included. Second, spectroscopy at these wavelengths makes the best probe of conditions in the vast clouds of dust and gases that lie between stars, known as the interstellar medium (ISM). These general features apply on all scales from the formation of stars and planetary systems in our corner of the Milky Way to the earliest galaxies that formed when the universe was only 10% to 20% of its current age (link to "An Infrared Search for Origins" multimedia flash).

Looking back to even earlier times, when the universe was only 1% of its present age, SAFIR will be able to observe how the very first stars formed without benefit of the cooling agents that are critical to the development of all the stars that followed.

Much of the key technology that will make SAFIR possible has been or is being developed for other missions.

Lightweight telescopes that can be collapsed for launch and reassembled in space are under intense development for the James Webb Space Telescope (JWST) and Terrestrial Planet Finder (TPF) programs. While the newest architecture for SAFIR minizmizes deployment complexity, such folding technologies are highly enabling for any future large telescope.

Cryogenic systems with multiple-stage closed-cycle coolers, which provide base temperatures of a few degrees above absolute zero, are being developed and will be used by the European Planck and the NASA JWST missions. This technology will enable a robust multi-year cryogenic mission. Ultra-cold temperatures are necessary to keep the telescope from emitting its own confusing infrared radiation (which all warm bodies do), and because far-IR and submillimeter detectors work only at such temperatures.

Also critical for SAFIR are large arrays of high-sensitivity direct detectors for wavelengths from 50 microns (µm) to 1 mm (one micron = 0.001 mm). These devices have seen substantial improvements in the last decade. They are now being produced with high sensitivity in arrays with hundreds of elements, and scientists envision significant further improvements in array size and sensitivity over the next 10 years.