"We are very enthusiastic about having taken delivery of the SIRTF cryogenic telescope assembly and its three high performance science instruments," said Tom Dougherty, Lockheed Martin SIRTF program manager. "We have completed the careful process of integrating the payload and spacecraft, and are beginning to perform environmental and other tests that will verify that SIRTF is ready for launch early next year."

SIRTF is a cryogenically-cooled space observatory that will conduct infrared (IR) astronomy during a two and one-half-to-five year mission beginning in 2003. SIRTF completes NASA's family of Great Observatories that also includes the Hubble Space Telescope, the Chandra X-Ray Observatory and the Compton Gamma Ray Observatory. The SIRTF program, a cornerstone of NASA's Origins Program, is managed by the Jet Propulsion Laboratory in Pasadena, Calif. for NASA's Office of Space Science in Washington DC.

The spaceborne SIRTF observatory comprises a 0.85-meter diameter telescope and three scientific instruments capable of performing imaging and spectroscopy in the 3-180 micron wavelength regime. Incorporating the latest in large-format infrared detector array technology, SIRTF will offer orders-of-magnitude improvements in sensitivity over previous IR missions. Cornell University, University of Arizona, and the Harvard-Smithsonian Center for Astrophysics have provided the instruments for SIRTF.

An important feature of the SIRTF mission is the adoption of a solar orbit. To reach this orbit, the spacecraft will be launched on a Delta 7920 launch vehicle with slightly greater than terrestrial escape velocity. The resulting orbit will have SIRTF trailing the Earth in its orbit around the Sun. This orbit makes better use of launch capability than does a conventional Earth orbit, and it permits excellent, uninterrupted viewing of a large portion of the sky without the need for Earth-avoidance maneuvers. In addition, the absence of heat input from the Earth provides a stable thermal environment and allows the exterior of the telescope to reach a low temperature via radiative cooling.

A one meter-diameter transmitting antenna fixed to the bottom of the spacecraft will be used twice each day to transmit 12 hours of stored science data to stations of NASA's Deep Space Network. In this manner, an adequate average data rate of 85 kbps -- corresponding to one image from SIRTF's largest array every 10 seconds -- can be maintained over the lifetime of the mission.

SIRTF's scientific potential is rooted in four basic physical principles that define the importance of infrared investigations for studying astrophysical problems:

Infrared observations reveal cool states of matter: Solid bodies in space -- ranging in size from sub-micron-sized interstellar dust grains to giant planets -- have temperatures spanning the range from 3K to 1500K (above which nearly all solids evaporate). Most of the energy radiated by objects in this temperature range lies in the infrared part of the spectrum. Infrared observations are therefore of particular importance in studying low-temperature environments such as dusty interstellar clouds where stars are forming and the icy surfaces of planetary satellites and asteroids.

Infrared observations explore the hidden Universe: Cosmic dust particles effectively obscure parts of the visible Universe and block our view of many critical astronomical environments. This dust becomes transparent in the infrared, where observers can probe optically invisible regions such as the center of our Galaxy (and other galaxies) and dense clouds where stars and planets may be forming. For many objects -- including dust-embedded stars, active galactic nuclei, and even entire galaxies -- the visible radiation absorbed by the dust and re-radiated in the infrared accounts for virtually the entire luminosity.

Infrared observations access unique spectral features: Emission and absorption bands of virtually all molecules and solids lie in the infrared, where they can be used to probe conditions in cool celestial environments. Many atoms and ions have spectral features in the infrared that can be used for diagnostic studies of stellar atmospheres and interstellar gas, exploring regions that are too cool or too dust-enshrouded to be reached with optical observations.

Infrared observations reach back to the early life of the cosmos: The cosmic redshift which results from the general expansion of the Universe inexorably shifts energy to longer wavelengths in an amount proportional to an object's distance. Because of the finite speed of light, objects at high redshift are observed as they were when the Universe and those objects were much younger. As a result of the expansion of the Universe, most of the optical and ultraviolet radiation emitted from stars, galaxies, and quasars since the beginning of time now lies in the infrared. How and when the first objects in the Universe formed will be learned in large part from infrared observations.

Apart from a few windows at short wavelengths, all of the infrared radiation emitted by the above objects is absorbed by Earth's atmosphere. Worse, the infrared emission of the atmosphere itself blinds astronomers peering through those windows. Hence the need for a cooled space-based infrared observatory with high sensitivity -- SIRTF.

NASA's Origins Program follows the chain of events that began with the birth of the Universe at the Big Bang. It seeks to understand the entire process of cosmic evolution from the formation of chemical elements, galaxies, stars and planets, through the mixing of chemicals and energy that cradles life on Earth, to the earliest self-replicating organisms and the profusion of life. In short, Origins hopes to answer the fundamental question: Are we alone in the Universe?

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