The instruments used in infrared telescopes must be kept extremely cold to
detect the faint infrared radiation from space.
The operating temperature of infrared
detectors is very close to absolute zero (the lowest temperature possible), which
occurs at 0 degrees Kelvin (-273.15 degrees Celsius or -460 degrees Farenheit).
Previous infrared space telescopes were kept cold through the use of
huge amounts of liquid helium. The more liquid helium that an infrared space
observatory could carry, the longer the telescope could be kept cold, and the
longer the mission would last. Carrying the amount of
cryogen needed for a
successful infrared mission, meant a costly increase in both the size and weight of
the observatory. This in turn, required a more powerful (and expensive)
rocket to launch the observatory into space. The design of an infrared
observatory to carry vast amounts of cryogen is called "cold-launch architecture".
Unlike previous infrared space missions, the
Spitzer Space Telescope
will, for the first time, use a
unique warm-launch architecture which will greatly reduce the cost of the
mission while still allowing for a high quality, long duration mission.
Instead of the entire telescope being enclosed in liquid helium, this new design
requires only the telescope's instruments to be kept cold. This means that much
less cryogen needs to be used, reducing the size and cost of the observatory.
Spitzer will use only 360 liters of liquid helium for a five year mission. In
IRAS, launched in 1983, used 520 liters of cryogen during its 10-month mission,
and ISO, launched in 1995,
carried 2140 liters for 2.5 years.
A smaller and lighter observatory means that a smaller, less powerful rocket can
be used for launch. Spitzer will be launched on a modified Boeing Delta-II rocket
rather than a larger (and more costly) Titan or Atlas rocket.
Another benefit to Spitzer's warm-launch architecture is that it simplifies and
reduces the cost of pre-launch testing. Since the entire observatory does not
need to be enclosed in a cryostat, it is easier to test portions of the observatory
prior to launch.
The Spitzer Space Telescope
was launched at ambient temperature and cooled to just below 40 degrees Kelvin
(-233 degrees Celsius or -388 degrees Fahrenheit) a few weeks after launch.
Spitzer was able to radiatively cool to such a low temperature because of its
unique orbit - a trailing Earth orbit which places it
far away from the Earth in a cooler region of space.
Once the telescope reached the same temperature as the space surrounding it, the
liquid helium began to produce a vapor that cooled Spitzer's instruments
to their operating temperature of 5.5 degrees Kelvin
(-268 degrees Celsius or -450 degrees Fahrenheit).