Spitzer's Unique Orbit
Most space telescopes are placed in orbit around the Earth. However, the Spitzer Space Telescope will be launched into a unique solar orbit - trailing the Earth as it moves around the Sun. To reach this orbit, Spitzer will be launched on a Delta rocket, which will travel at a speed slightly greater than the velocity required to escape the Earth's gravity. Spitzer will move farther away from the Earth with time, while still trailing the Earth as it orbits the Sun. It will recede from Earth at the rate of 9.3 million miles (15 million kilometers) per year. For example, about 2.5 years after launch, Spitzer will trail the Earth in its orbit by about 28 million miles (~ 45 million kilometers). Five years into the mission, Spitzer will be about twice this distance away (56 million miles or 90 million kilometers). At these distances, communication with Spitzer will require the use of NASA's Deep Space Network.
Since the Earth will not be in the way, Spitzer will be able to view larger portions of the sky without interruption. Also, there will be less need for Earth-avoidance maneuvers. An additional benefit is the absence of heat from the Earth. Infrared is primarily heat radiation and the detectors on infrared telescopes are extremely sensitive to heat. To detect the faint infrared signals from space, Spitzer's instruments must be kept as cold as possible to operate. Spitzer will be in a much cooler space environment away from the Earth. This will allow the telescope to reach a very low temperature by simple radiative cooling. The venting of liquid helium after the telescope has cooled radiatively, will lower the temperature even further. Because, Spitzer will be launched into a colder space environment, less cryogen (liquid helium), needs to be carried. This significantly reduces the size and weight of the observatory as well as the cost of the mission.