The Emergence of Infrared Astronomy

The Earth and its atmosphere emit infrared radiation in all direc – tions,9 swamping any signals from space. It’s analogous to trying to see stars in the noontime sky. For a faint, cool source to stand
out against a warmer backdrop, an infrared telescope must lower thermal backgrounds. Which is to say, the longer the wavelength of light being observed, the colder a telescope and its detector need to be to see an infrared source from space. By the 1960s, infrared astronomy took off with the development of a new type of solid-state detector that had greater sensitivity than old-style radiometers.10 Another obstacle to detecting infrared radiation from cosmic sources is the fact that those waves are absorbed by water vapor in the air we breathe. Astronomers initially chose high mountaintops for their observations, to be above as much of the infrared-absorbing atmosphere as possible. For example, some of the best infrared observations are made 4,200 meters high atop the dormant volcano Mauna Kea in Hawaii, and new telescopes are being constructed in the arid Atacama Desert in Chile, includ­ing a major new millimeter array on the Chajnantor plateau at an altitude of 5,000 meters. An extreme version of this strategy was the Kuiper Airborne Observatory, which operated from 1974 to 1995. This 36-inch telescope in a converted Air Force C141 jet made observations for a few hours at a time at an altitude of 45,000 feet. The successor to this facility is a 2.5-meter telescope in a converted Boeing 747-SP, the Stratospheric Observatory for Infrared Astronomy.11 High-altitude balloons and rockets carrying small telescopes have been used to reach even higher altitudes of over 100,000 feet.

It proved essential to cool infrared telescope detectors to liquid nitrogen or even liquid helium temperatures so that radiation from the detector wouldn’t wash out signals from the sky. On ground – based telescopes observing the near infrared, two to six times the wavelength of visible light, liquid nitrogen is used to cool the de­tector to 77 Kelvin or -177°C. Though seemingly exotic, liquid nitrogen is basically liquefied air and costs no more than milk. For observing at the mid-infrared, 20-200 times the wavelength of visible light, liquid helium is used to cool the detector to 4.2 Kelvin or -269°C, a hair’s breadth from absolute cold. But the Earth environment emits so much radiation at these wavelengths that astronomical signals are swamped by background radiation and deep mid-infrared observations can only be made from space.

Space infrared astronomy was kicked into gear with the very successful NASA Infrared Astronomical Satellite (IRAS) mission,

which in 1983 surveyed the entire sky in the infrared for the first time. In less than a year, IRAS detected about 350,000 infrared sources. Around the same time, ESA launched the Infrared Space Observatory, which had higher sensitivity and much broader wave­length coverage.12 In 1997, an infrared camera was added to the instrument suite of the Hubble Space Telescope. This camera has been very successful but it has a small field of view and shares time with four other instruments, so infrared astronomers had always planned for their own facility.

Spitzer, from its earliest inception, was especially designed for infrared astronomy and is sensitive enough to detect infrared sig­natures of stars and galaxies billions of light-years away. The space telescope has been instrumental in unveiling small, dim objects like dwarf stars and exoplanets and can even determine the tem­perature of their slender atmospheres. Originally proposed in the late 1970s as NASA’s Space Infrared Telescope Facility, the Spitzer Space Telescope suffered from uncertainty, a delay after the loss of the space shuttle Challenger, near-cancellation, congressional limbo, budget cuts, and “descoping.” Nevertheless, in 2003 the telescope was finally launched, after being renamed subsequent to a public opinion poll conducted by NASA. The last of NASA’s four Great Observatories, the $800 million telescope was named after Lyman Spitzer, an early advocate of the importance of or­bital telescopes.13 After launch, the spacecraft took about 40 days to cool to its operating temperature of 5 Kelvin. Once cooled, it took just an ounce of liquid helium per day to maintain its detec­tors at their operating temperature. A solar panel facing the Sun serves to gather power and protect the telescope from radiation. On the anti-solar side, a series of concentric shells and a black shield radiate heat into space. Spitzer’s most precious resource was its liquid helium, used to cool the telescope and its instruments to the phenomenal temperature of just 1.2 Kelvin, within an iota of the temperature where atoms and molecules have no motion. This extra cold relative to the liquid helium operating temperature comes from venting the helium into the vacuum of space, much as your skin would cool if a liquid evaporated from it. Background radiation is reduced by a factor of a million relative to a similar­sized telescope on the Earth’s surface. The telescope’s cryostat held 350 liters at launch, as much as the gas tank of a large minivan. Helium evaporates and is boiled away by tiny heat inputs from the instruments and from the telescope structure, so the act of observ­ing steadily depletes the cryogen.

The goals of cooling the telescope and preserving the liquid he­lium as long as possible dictated the telescope’s orbit and design. Careful design minimized these heat inputs, which are typically measured in milliwatts. As a reference, your fingertip radiates 25 milliwatts. The Spitzer facility is 4.5 meters tall and 2.1 meters in diameter and weighs about as much as a minivan. Previous space observatories had used either a low Earth orbit to be serviceable by the Space Shuttle (such as the Hubble Space Telescope) or a high Earth orbit with a period of one to two days (like the Chan­dra X-Ray Observatory). Spitzer is in an unusual Earth-trailing solar orbit. The big benefit is in escaping from the Earth’s heat and being situated in a good thermal environment where the tele­scope can cool in the vacuum of space. The orbit also avoids the Earth’s radiation belts, and there’s excellent efficiency of observing because the Earth and Moon rarely intervene. But one disadvan­tage is that Spitzer’s radio signals are getting weaker as it moves away from Earth. Spitzer slips away from Earth at about 10 mil­lion miles per year and is now farther away than the Sun. Only the huge 70-meter dishes of the Deep Space Network are sensitive enough to gather its precious data. Mission planners estimate that they will lose touch with Spitzer in early 2014.