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Figure 7.27. Disk-integrated irradiance spectra of the giant planets recorded by both ISO/SWS and ISO/LWS, plotted together on a log-scale. Jupiter: solid line; Saturn: dotted line; Uranus: dashed line; Neptune: dot-dashed line. The SWS Uranus spectrum has been divided by 105 in order to distinguish it from the SWS Neptune spectrum. The spectra can be seen to be in good agreement with the synthetic spectra shown previously in Figure 7.8.

cryogenically cooled telescope that forms the final element in NASA's Great Observatory Program. The four major scientific objectives of Spitzer are: (1) to study the early universe; (2) to search for and study brown dwarfs and superplanets; (3) to study ultraluminous galaxies and active galactic nuclei; and (4) to discover and study protoplanetary and planetary debris disks.

The telescope incorporates a 0.85 m primary mirror cooled to 5.5 K by liquid helium and at launch weighed 950 kg (Figure 7.28). The telescope was placed in an Earth-trailing, heliocentric orbit, where the background radiation levels are low and where it may observe chosen targets for long integration times, uninterrupted by orbital considerations, as was the case for the Infrared Space Observatory (ISO) (Section 7.7.2). The low thermal radiation environment means that Spitzer's optics naturally cool to low temperatures without the need for a very large supply of cryogen, and the cryogens that are carried are only used to cool the mirror and detectors to their final operating temperatures. Hence, the lifetime of Spitzer will be much longer than it was for ISO, and Spitzer is expected to remain operational until April 2009.

Spitzer has three instruments and performs imaging, photometry, and spectrometry from 3 ^m to 180 ^m, as will now be described.

Figure 7.28. An artist's impression of the Sp/Zzer Space Telescope, launched in 2003. Courtesy of NASA.

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