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NASA Spitzer Space Telescope • Jet Propulsion Laboratory
• California Institute of Technology
• Vision for Space Exploration
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Frame Frame About the Spitzer Space Telescope Frame Frame
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Fast Facts
 
Current Status
 
Spitzer History
 
— Early History
 
— Recent History
 
— Innovations
 
—— Technology
 
—— Orbit
 
—— Cryogenics
 
—— Telemetry
 
—— Management
 
— Heritage
 
Spitzer Technology
 
Spitzer Science
 
Lyman Spitzer, Jr.
 

Innovations: Clever Choice of Orbit

Artist depiction of Spitzer in solar orbit
Spitzer in Orbit
NASA/JPL-Caltech

An important breakthrough in the redesign of Spitzer was to abandon the idea of placing the observatory into Earth orbit and instead to insert it into an Earth-trailing heliocentric orbit. In other words, the Observatory will be launched into an orbit where it will simply drift behind Earth as it circles the Sun.

Spitzer will drift away from Earth at the rate of ~ 0.1 AU/year. [An AU, or Astronomical Unit, is the average distance between the Sun and Earth, or about 150 million kilometers]. Since the Observatory must be cooled to within a few degrees of absolute zero, this orbit choice offers a more benign thermal environment than any geocentric orbit. Earth not only reflects visible light from the Sun, but it also emits warm infrared radiation. Any satellite in a reasonable geocentric orbit would therefore be bathed in temperatures exceeding 250 Kelvin (K). The drifting heliocentric orbit places Spitzer in "deep space," where the temperature of the telescope without any active cooling is about 30 to 40K. By using Nature to assist in cooling the Observatory, Spitzer can carry much less liquid helium cryogen than it would need in an Earth orbit.

A consequential benefit of the solar orbit is that Spitzer will have a large instantaneous view of the celestial sky. Sensitive observatories such as Spitzer and the Hubble Space Telescope must avoid looking at (or anywhere near) extremely bright objects such as the Sun, Earth, and Moon. Spitzer's view of the sky will be limited by only two pointing constraints (see Figure below). First, the Observatory cannot point closer than 80 degrees in the direction of the Sun, in order to minimize the thermal heating of the telescope by solar radiation. Second, it cannot point more than 120 degrees away from the direction of the Sun, because of need to illuminate the solar panels and produce electricity to power the Observatory.

Spitzer sky viewing geometry
Spitzer Sky Viewing Geometry
NASA/JPL

Spitzer's window of visibility on the celestial sky forms an annulus, perpendicular to the ecliptic plane, of 40-degree width. A region of the sky will be minimally visible to the Observatory twice a year, for about 40 days each period (at the ecliptic equator). The visibility periods increase to about 120 days per year at an ecliptic latitude of 30 degrees, to about 200 days at latitudes of 60 degrees, and reach constant viewing at the ecliptic poles. About a third of the sky will be instantaneously visible to Spitzer at any given time. This broad window on the sky will simplify scheduling and operations of Spitzer, and will allow it to achieve very high astronomical observing efficiency.

Spitzer Sky Visibility in ecliptic (top), equatorial (middle), and Galactic (bottom) celestial coordinates.
Spitzer Sky Observability
NASA/JPL

More Innovations



The Spitzer Space Telescope is a NASA mission managed by the Jet Propulsion Laboratory. This website is maintained by the Spitzer Science Center, located on the campus of the California Institute of Technology and part of NASA's Infrared Processing and Analysis Center. Privacy Policy

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