Sky & Telescope - USA (2020-01)

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qSPITZER’S JOURNEY Spitzer trails behind Earth in its heliocentric orbit,

moving farther from us with time. The space telescope operated with three

instruments during its six-year cryogenic mission. During the subsequent

“warm” and “beyond” phases of its mission, it has stayed cool by hiding in

its solar panels’ shadow, but it could only use one instrument at this higher

temperature (about 27K). As the craft has pulled farther from Earth, the

angle between its observing orientation and the one for pointing its antenna

toward Earth for data downlinks has become more severe. The solar panels

must now tilt away from the Sun beyond what mission planners designed

the craft to handle.

skyandtelescope.com • JANUARY 2020 19

Legacy

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10th Anniversary
Aug. 25, 2013
Cryogen depleted,
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May 15, 2009
Spitzer launch
Aug. 25, 2003
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11 billion years, but Spitzer has detected galaxies seen as
they were more than 13 billion years ago, just a few hundred
million years after the Big Bang. In short, Spitzer has greatly
advanced our understanding of the universe.
Cosmic Chill
Spitzer originally observed mid- to far-infrared wavelengths,
from 3.6 to 160 microns. For sensitive infrared observa-
tions from space, it is necessary to cool the telescope and
the detectors to within shouting distance of absolute zero.
Previous infrared instruments had launched cold, but the
team took a different approach with Spitzer: SIRTF launched
with most of the telescope at room temperature, then turned
the spacecraft so that its solar panels shielded the telescope
from sunlight and let it cool to less than 40 kelvin (−233°C)
by radiating its heat into cold space. Radiative cooling is very
effective in a heliocentric orbit, far from Earth’s glow; only
after this initial cooling did the liquid helium cryogen kick in
to take the detectors to less than 2K.
After the cryogen supply was exhausted in 2009, radia-
tive cooling enabled Spitzer to continue observing in its two
shortest wavelength bands, at 3.6 and 4.5 microns, with no
loss of sensitivity. This second phase is known as Spitzer’s
warm mission.
The inherent sensitivity of a cryogenic telescope in space,
which provides access to the entire infrared spectrum free of
Rechristened
The Shuttle Infrared Telescope Facility’s name
changed to Spitzer when the first scientific results
were announced in December 2003. The name
honors astrophysicist Lyman Spitzer, Jr., who in 1946
was one of the first to propose putting a telescope in
space, and who tenaciously lobbied both NASA and
Congress for a space telescope’s development.