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
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
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
Lightweight telescopes that can be collapsed for launch and reassembled in space are under
intense development for the James Webb Space Telescope
and Terrestrial Planet Finder
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.