Detecting the cooler objects in space requires a cold telescope. The detectors used in infrared telescopes are extremely sensitive to heat. They are designed to detect the infrared (or heat) radiation from space, which can come from objects as cool as about 10 degrees Kelvin (-442 degrees Fahrenheit). Anything which has a temperature, even a very low one, puts out infrared radiation, and the warmer an object is, the more infrared radiation it puts out.
Infrared detectors have to be kept in an environment which is as cold as possible. The colder the environment, the more sensitive the instruments are to infrared light. Any surrounding heat will create infrared signals that can interfere with signals from space. This includes heat radiation from the instruments and telescope, from the atmosphere (for ground based observatories), and from warmer objects in space like the Sun. To keep infrared detectors and instruments cold, cryogens such as liquid helium are used. The cryogen is usually contained in a chamber called a cryostat. Solar shields are also placed on infrared space telescopes to protect the instruments from the Sun's heat.

The lifetime of an infrared space mission, depends on how long the instruments can remain cold. Shortly after the cryogen (or coolant) runs out, the detectors will become useless and the data gathering portion of the mission will end.

The Spitzer Space Telescope's cryostat
The Spitzer Space Telescope's cryostat will keep its science instruments at temperatures as low as 1.4 degrees Kelvin (-457 degrees Fahrenheit) for up to 5 years. Spitzer's cryostat will keep the instruments cold by venting helium vapor from a liquid helium tank which will hold about 360 liters of superfluid helium.