SIRTF (Spitzer, SST)

SIRTF [NASA]

The SIRTF (Space Infrared Telescope Facility) is a 1 meter class, cryogenically cooled space telescope operated as an observatory for infrared astronomy in the 3 - 180 micron range. The observatory is the final element of NASA's four Great observatories. The main objectives of the mission are:

• physical studies of the planetary system;
• detailed study of cold circumstellar dust clouds;
• a search for the enigmatic brown dwarfs;
• extension of IRAS studies of forming stars to lower temperatures and luminosities;
• identification and study of powerful infrared galaxies; and,
• infrared measurements of all presently catalogued quasars.

The spacecraft consists of an octagonal bus structure, and a solar array to power the science instruments. The pointing contron subsystem employs a celestial-inertial, three-axis stabilized control system. SIRTF is planned to have an Earth-trailing Heliocentric orbit.

Unlike IRAS (Infrared Astronomical Satellite), which swept rapidly across the sky, SIRTF will be a true observatory, carrying a variety of focal plane instruments. The SIRTF telescope is a lightweight reflector of Ritchey-Chretien design.

Spitzer carries three instruments on-board:

• Infrared Array Camera (IRAC), a four-channel infrared array camera imaging which operates simultaneously on four wavelengths (3.6 µm, 4.5 µm, 5.8 µm and 8 µm). Each module uses a 256×256-pixel detector—the short wavelength pair use indium antimonide technology, the long wavelength pair use arsenic-doped silicon impurity band conduction technology. The flight hardware was built by NASA Goddard Space Flight Center.
• Infrared Spectrograph (IRS) providing high- and low-resolution spectroscopy at mid-infrared wavelengths. It is an infrared spectrometer with four sub-modules which operate at the wavelengths 5.3–14 µm (low resolution), 10–19.5 µm (high resolution), 14–40 µm (low resolution), and 19–37 µm (high resolution). Each module uses a 128×128-pixel detector—the short wavelength pair use arsenic-doped silicon blocked impurity band technology, the long wavelength pair use antimony-doped silicon blocked impurity band technology. The flight hardware was built by Ball Aerospace.
• Multiband Imaging Photometer for Spitzer (MIPS), an imaging photometer, with three detector arrays in the far infrared (128 × 128 pixels at 24 µm, 32 × 32 pixels at 70 µm, 2 × 20 pixels at 160 µm). The 24 µm detector is identical to one of the IRS short wavelength modules. The 70 µm detector uses gallium-doped germanium technology, and the 160 µm detector also uses gallium-doped germanium, but with mechanical stress added to each pixel to lower the bandgap and extend sensitivity to this long wavelength. The flight hardware was built by Ball Aerospace.

The instrument sensitivity is expected to be increased by a factor of 100 to 1000 over that of IRAS, and the spatial resolution will be at least a factor of 10 times finer than IRAS.

SIRTF has been renamed Spitzer Space Telescope (SST) in December 2003.

Spitzer ran out of liquid helium coolant on 15 May 2009, which stopped far-IR observations. Only the IRAC instrument remains in use, and only at the two shorter wavelength bands (3.6 µm and 4.5 µm). The telescope equilibrium temperature is now around 30 K (-243 °C), and IRAC continues to produce valuable images at those wavelengths as the "Spitzer Warm Mission".

Spitzer will be deactivated on 30 January 2020, effectively ending its mission more than 16 years after its launch. The mission is terminated because Spitzer is drifting far enough away from the Earth to complicate spacecraft operations, and making it impossible to operate it effectively. Much of the infrared astronomy that Spitzer had been doing will be continued by the upcoming JWST and NGRST (Nancy Grace Roman Space Telescope, WFIRST) observatories.