The Cold Facts of the Spitzer Space Telescope

by Michael Werner, Project Scientist, Spitzer Space Telescope—Jet Propulsion Laboratory, California Institute of Technology 

Spitzer being prepared for thermal testing in 2003.

NASA’s Spitzer Space Telescope, launched in August 2003 and decommissioned in January 2020 after more than 16 glorious years of exploration of the universe at infrared wavelengths, was a technical and scientific marvel. Infrared astronomical studies at wavelengths longward of 1 micron, somewhat beyond the limit of human vision at around 0.7 microns, began in earnest around 1960. These early studies, carried out on ambient temperature telescopes within the atmosphere, advanced astronomical understanding of targets from planets within the solar system to distant and highly luminous galaxies. However, they could not disguise the fact that the earth is a hostile environment for infrared astronomy.

The reasons for this reside in Planck’s Laws of Blackbody Radiation, which tell us that both the earth’s atmosphere, at a temperature around 270 K, and a telescope within it at a similar temperature, will radiate copiously at infrared wavelengths longward of several microns. This bright foreground radiation swamps the faint infrared radiation from all but the brightest astronomical sources. Beginning with early rocket flights and culminating with the highly successful Infrared Astronomical Satellite (IRAS), which flew in 1983, scientists and engineers demonstrated and exploited the fact that a cold telescope in space operates in an environment where the sky brightness is a million times less than that encountered within the atmosphere. (Cold refers to truly cryogenic, with temperatures as low as 5 K or below.) This is about the factor by which the sky at midnight on a moonless night is darker than that at high noon. So, to work “at night” in the infrared, we use a cold telescope in space and can see the infrared sky in all its glory. As shown in Figure 1, the sky can look very different in the infrared than in visible light.

Figure 1. Visible (top) and infrared (center) views highlight the different appearance of the entire sky at different wavelengths. These images are presented so that the Milky Way, which is our view of the plane of our galaxies, runs horizontally through the center of each image. The infrared image is a signature product of the IRAS mission, discussed in this article. The bottom panel zooms in on Spitzer’s view of the central 2900 x 1100 light-years of the galaxy, including the Galactic Center, home to the recently imaged black hole, Sgr A*. The red glow represents radiation from the hydrocarbon molecules which abound in the galaxy. Credit: A. Mellinger - top; NASA/JPL-Caltech – middle; R. Arendt/AAS – bottom

Figure 1. Visible (top) and infrared (center) views highlight the different appearance of the entire sky at different wavelengths. These images are presented so that the Milky Way, which is our view of the plane of our galaxies, runs horizontally through the center of each image. The infrared image is a signature product of the IRAS mission, discussed in this article. The bottom panel zooms in on Spitzer’s view of the central 2900 x 1100 light-years of the galaxy, including the Galactic Center, home to the recently imaged black hole, Sgr A*. The red glow represents radiation from the hydrocarbon molecules which abound in the galaxy. Credit: A. Mellinger - top; NASA/JPL-Caltech – middle; R. Arendt/AAS – bottom


Several other cryogenic missions, most notably the European Space Agency’s Infrared Space Observatory (ISO), succeeded IRAS and advanced the science and technology of infrared space astronomy significantly. These programs led to the Spitzer Space Telescope, which was NASA’s (first) Great Observatory for infrared exploration of the universe. The gains of a cryogenic telescope in space are so great that, at wavelengths longward of 3 microns, the 85-cm-diameter Spitzer was at least 50 times more sensitive than the 8-m-diameter telescopes of the ground-based Gemini observatory. The use of monolithic detector arrays (similar in principle, if not in cost, to the megapixel cameras now found in every cell phone) in Spitzer’s three instruments assured that the sensitivity gain was achieved at many spatial or spectral resolution elements simultaneously. In addition, of course, operation in space provides access to all infrared wavelengths, many of which are obscured by Earth’s atmosphere. As a result of all these considerations, Spitzer had tremendous scientific power.

Spitzer’s scientific achievements are summarized in the book More Things in the Heavens: How Infrared Astronomy is Expanding our View of the Universe (Princeton University Press, 2019) by the author of this article and his NASA Jet Propulsion Laboratory colleague Peter Eisenhardt. Highlights of Spitzer science range from the discovery of a giant ring around Saturn (Figure 2); to the identification of seven Earth-sized planets orbiting a faint red star only 40 light-years from Earth (Figure 3); to comprehensive studies of nearby galaxies that reveal the full range of phenomena available for study in the infrared (Figure 4); and finally, to studying distant galaxies as they appeared when the universe was less than 3% of its current age and 7% of its current size.

Figure 2. The giant ring of Saturn found by Spitzer is shown on a scale that shrinks the planet and its previously known rings (shown enlarged in the circular inset) to a point. Note that the tilt of the giant ring differs from that of the previously known rings. The giant ring is attributed to material from Saturn’s outer satellite, Phoebe, which orbits within it. Credit: A. Verbiscer


Figure 3. Seven Earth-sized planets identified by Spitzer orbiting the small star, TRAPPIST-1, 40 light-years from Earth. This artist’s conception shows the expected state of water on the planets’ surfaces. Those closest to the star show steam; the farthest show ice. Those in the middle, where water would be liquid, are in the habitable zone and could be hospitable to life as we know it. Credit: NASA/JPL-Caltech/R. Hurt (IPAC).


Figure 4. Spitzer images of the nearby spiral galaxy, M81, at wavelengths from 3.6 to 160 microns. The large image at the top is a composite of the 3.6, 8, and 24 micron images. In this composite, old stars in the center of the galaxy dominate at the shortest wavelength (blue), while emission from hydrocarbon molecules (green) and interstellar dust (red) become dominant at longer wavelengths. At 224 microns, regions where stars are forming are strung out along the spiral arms like ornaments on a Christmas tree. Credit: S. Willner & K. Gordon/AAS.


Even though Spitzer is no longer taking data, its scientific legacy is assured by a readily accessible data archive, which interested readers can access at www.irsa.ipac.caltech.edu/Missions/spitzer.html. More than half of the over 10,000 papers which have been published using Spitzer data included archival data, even as new observations were ongoing and new data were entering the archive. Thus, Spitzer is a gift that keeps on giving.

The cryogenic and thermal performance of Spitzer may be of particular interest to readers of Cold Facts. Spitzer orbited the sun rather than the earth, and thus was far from the substantial heat load attributable to Earth’s own infrared radiation. In this solar orbit, it was possible to keep Spitzer oriented so that the sunlight was always incident on the fixed solar panel, and this facilitated the simple, robust thermal architecture illustrated and discussed in Figure 5. Spitzer was launched with a tank, located within the outer shell identified in Figure 5, which, at the start of scientific observations, contained 42.4 liters of superfluid liquid helium that cooled the telescope and the detectors (also located within the outer shell). The outer shell was always shaded from direct sunlight by the solar panel while it cooled by radiation into the vast refrigerator of deep space, facilitated by the black coating on its antisolar side. Careful thermal design, tests and fabrications of this system, and thoughtful choices of the thermal properties and coatings of the elements shown in Figure 5 allowed the outer shell to cool to below 36 K entirely by radiative cooling; the telescope and instruments were cooled well below 36 K by the liquid helium cryogen. At 36 K, the power carried to the interior of the outer shell, either by conduction or by radiation, was virtually negligible. This meant that the main heat source, which boiled away the liquid helium, had the small amount of electrical power required to operate the detector arrays.

Figure 5. Spitzer being prepared for thermal testing in 2003 at Lockheed Martin in Sunnyvale, Ca. The tall structure on the right is the solar panel, which is always kept oriented towards the sun. To the left of the solar panel, the outer shell, which contains the telescope, the instruments, and the cryogen tank, is seen atop the spacecraft. These are shielded by the solar panel from direct sunlight. The black anti-sun hemi-cylinder of the outer shell is always oriented towards deep space and radiates away heat which leaks inward from the solar panel or the spacecraft. The structures to the left of the outer shell/spacecraft stack are part of the test configuration. Lockheed Martin provided the solar panel and the spacecraft, while Ball Aerospace provided the outer shell, the telescope, the cryogenic system and two of the three instruments; the third was built by NASA’s Goddard Space Flight Center. Credit: NASA/JPL-Caltech


To summarize, before the cryogen boiled away almost six years after launch, the average heat load into the cryogen was only 5.1 mw, and the original 42.4 kg of liquid helium lasted over 2,000 days. By contrast, IRAS started scientific observations with 73 kg of liquid helium, which lasted for 10 months, or 300 days. This comparison shows that the Spitzer cryogenic system was an order of magnitude more efficient than IRAS; the difference is largely due to the fact that IRAS operated in low Earth orbit. The earth’s infrared radiation warmed the exterior of the spacecraft (equivalent to Spitzer’s outer shell) to several hundred kelvins, so that much more “parasitic” heat was conducted into the helium tank than was the case for Spitzer.

Following the exhaustion of the liquid helium supply in 2009, the outer shell warmed up by only about a degree, so the heat conducted into the helium tank from the outer shell remained small. Consequently, the temperature of the telescope and the instruments equilibrated at a value of about 26 K, with the cooling due entirely to radiation out of the open aperture of the outer shell. This was cold enough for efficient operation of Spitzer’s two shortest wavelength detector arrays at 3.6 and 4.5 µm. Spitzer continued to observe with these two arrays for almost 11 years. During this Warm Spitzer mission, extremely important scientific results, including the detection of the Trappist-1 planets shown in Figure 2, were gathered, reduced, published and added to Spitzer’s archives.

As is always the case for NASA missions, success builds upon success, and Spitzer’s successor, the James Webb Space Telescope (JWST), has been launched and deployed successfully and is within a few months of extending Spitzer’s results with higher sensitivity and spatial and spectral resolution. JWST’s primary mirror has 50 times the collecting area of Spitzer’s, and its detector arrays have hundreds of times as many pixels; so the gains which it will bring over Spitzer will be marvelous, but we should not lose sight of the fact that Spitzer’s scientific and technical results (the latter particularly in the cryothermal area) have motivated the design and scientific use of JWST in many ways.

Further details on Spitzer’s thermal/cryogenic system and other characteristics of the mission can be found in papers by R. Gehrz et. al. (Review of Scientific Instruments, Vol. 78, 2007, p.1) and M. Werner et. al. (Journal of Astronomical Telescopes, Instruments, and Systems, Vol. 8, id. 014002, 2002).

This paper is based on work carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. ©2022. California Institute of Technology. Government sponsorship acknowledged.

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