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Spitzer’s Cold Look at Space

To get a clear view of infrared emissions from celestial objects, the Spitzer Space Telescope has been cryogenically cooled—and what sights it has seen

Michael Werner

In astrophysical observations, more is more—imaging across multiple wavelengths leads to richer information. One electromagnetic band in which most celestial bodies radiate is the infrared: Objects ranging in location from the chilly fringes of our Solar System to the dust-enshrouded nuclei of distant galaxies radiate entirely or predominantly in this band. Thus, astrophysicists require good visualization of these wavelengths. The problem, however, is that Earth is a very hostile environment for infrared exploration of space, as the atmosphere also emits in the infrared spectrum and additionally absorbs much of the incoming signal. Even heat produced by a telescope itself can degrade its own clarity.

infrared image Milky Way galaxyClick to Enlarge ImageStarting at the end of the 1950s, a number of pioneering groups confronted this challenge and carried out increasingly exciting infrared investigations from ground-based, airborne and balloon-borne observatories. This work continues in parallel with space-based exploration; infrared capabilities form an integral component of current and planned ground-based telescopes with apertures of 10 to 30 meters in diameter.

But the best solution is to send into space a telescope that’s cooled by liquid helium to temperatures just a few degrees above absolute zero. NASA’s Space Infrared Telescope Facility (SIRTF) was proposed in the early 1970s and finally launched in August 2003. It was renamed the Spitzer Space Telescope in honor of the late Lyman Spitzer, Jr., an astrophysicist who was one of the first to propose the idea of placing a large telescope in space, and was also the driving force behind the Hubble Space Telescope. Therefore it’s very appropriate that Spitzer is a member of NASA’s multi-spectral family of Great Observatories satellites, which also includes Hubble, the Chandra X-ray Observatory and the recently launched Fermi Gamma-ray Space Telescope.

Spitzer dwarfs techniciansClick to Enlarge ImageThe Spitzer Space Telescope, about 4.5 meters tall and 2 meters in diameter, weighed 861 kilograms at launch. It is in orbit around the Sun, allowing its solar panels to always face their energy source while simultaneously shielding the craft from the Sun’s heat. Behind its solar array, a passive radiative-cooling system of reflective and emitting shells and shields allows the telescope’s outer shell to cool to about 34 kelvins. Onboard, helium vapor does the rest of the job of getting the payload down to about 5 kelvins or less, just slightly above absolute zero.

Spitzer is not the first cryogenically cooled infrared observatory in space, but it goes beyond its two successful and pioneering predecessors, the Infrared Astronomical Satellite (IRAS) that lasted for 10 months in 1983, and the Infrared Space Observatory (ISO) that operated from 1995 to 1998. For one thing, Spitzer uses a new generation of infrared detectors comprised of large arrays of sensors, whereas the previous missions used single detectors or small arrays. The quality of Spitzer’s arrays and its slightly larger aperture give it typically about 10 to 100 times the sensitivity of these earlier observatories. In addition, Spitzer’s arrays have 10 to 100 times more pixels than previously available on a cryogenic telescope in space.

Since its launch, Spitzer has provided the scientific community the most powerful tool yet available for astronomical explorations at wavelengths between 3.6 and 160 micrometers, known as mid-to-far infrared. The telescope uses an 85-centimeter primary mirror to direct infrared radiation to three main instruments: a short-wavelength infrared array camera, a long-wavelength multiband imaging photometer (which, like the camera, measures the intensity and spatial distrubution of radiation) and an infrared spectrograph (which can identify the type and amount of chemical compounds present by the specific spectrum they emit).

infrared image Whirlpool galaxyClick to Enlarge ImageA cooled telescope reduces the background infrared brightness of the sky by about six orders of magnitude; this is about the factor by which the sky brightness at visible wavelengths drops from high noon on a sunny day to midnight on a moonless night. The effects of the background reduction are so powerful that Spitzer, with its relatively small mirror, is more sensitive for many infrared observations than even the largest ground-based telescopes.

Although infrared wavelengths are invisible to human eyes and instead we perceive them as heat, it is ironically the colder objects, too frigid and with too little energy to glow visibly, that emit primarily in the infrared. And happily, infrared signals can be seen through much of the dust that enshrouds objects of interest, such as stellar nurseries and very energetic galactic nuclei. Spitzer has made remarkable progress in studying such objects, and also has been incredibly successful at directly measuring light from exoplanets (planets orbiting other stars) for the first time.

The first phase of Spitzer’s rich scientific life came to an end as the last milligrams of its onboard liquid helium evaporated into space on May 15, 2009—several years beyond the contractually required end of the cryogenic mission, as the onboard cooling system’s efficiency exceeded expectation. But there are still great prospects for the telescope’s rebirth as the Warm Spitzer mission; it may now be limited by heat, but it still has powerful capabilities for continuing exploration of the universe.

With 40,000 hours of observations already under its belt, resulting so far in well over 1,500 published research papers from astronomy teams all over the world, Spitzer’s scientific returns already go far beyond what can be discussed in a single article. But even just a few examples of Spitzer’s most significant results are illustrative of the range of science that the telescope has produced.

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