Innovations: Cryogenic Architecture
Previous infrared telescopes in space have been enshrouded within a giant cryostat containing the super-fluid liquid helium necessary to refrigerate the telescope to operating temperatures near absolute zero. This system configuration has come to be known as the "cold-launch" architecture (below left). Unlike IRAS and
ISO, however, Spitzer adopts an innovative "warm-launch" cryogenic architecture (see Figure below). The Observatory is launched at ambient temperature and radiatively (or passively) cooled in the deep recesses of space. Only the focal-plane instruments and the liquid helium cryostat are
enclosed in a vacuum shell containing the cryostat.
|The "warm-launch" cryogenic architecture adopted by Spitzer (right) substantially reduces mass and development costs. Note that the size of the telescope, depicted in red, is identical in the two designs. The reduction in the size of the cryogenic "packaging" (in blue) enables the entire Observatory to be vastly reduced in size.
It is important to recognize that the warm-launch architecture is enabled by the choice of orbit. With Spitzer inserted into a relatively cold space environment (i.e., away from the warm Earth), the ambient temperature of deep space passively cools the Observatory to less than 40 K within a few weeks of launch. At that time, the telescope is thermally de-coupled from the cryogenic telescope assembly
(CTA) outer shell. The boil-off of liquid helium produces a vapor that cools the CTA to the operational temperature of 5.5 K.
This innovative launch architecture, combined with 360 liters of liquid helium, yields an estimated mission lifetime of about five years. For the sake of comparison, IRAS used 520 liters of cryogen during its 10-month mission and ISO used 2140 liters for to achieve a mission lifetime of nearly 2.5 years.
The most obvious benefit of this architecture is the substantial reduction in the size of the Observatory, and its resultant launch costs. Spitzer will be launched on a modified
Delta-II rocket rather than a larger (and more costly) Titan or Atlas rocket.
A secondary benefit of the warm-launch architecture is that it simplifies pre-launch ground-based testing and integration. Without the need to enclose the entire Observatory in a cryostat, it is easier and less costly to test various sub-systems during the integration of the Observatory prior to launch.
The warm-launch architecture and passive cooling in deep space is the future of space-based infrared observatories. This cost-effective approach is also being adopted, with variations, in the design of future large-aperture infrared telescopes, such as the
James Webb Space Telescope.