Cryogenics is the study of how to get to low temperatures and of how materials behave when they get there. Besides the familiar temperature scales of Fahrenheit and Celsius (Centigrade), cryogenicists use other temperature scales, the Kelvin and Rankine temperature scales.
Here at the Cryogenics and Fluids Branch of Goddard, we concentrate on ways to cool spacecraft. Although the apparatus we use for spacecraft is specialized, some of the general approaches are the same as used in everyday life. To point out the similarities, I've made a short table of cooling comparisons.
One interesting feature of materials at low temperatures is that the air condenses into a liquid. The two main gases in air are oxygen and nitrogen. Liquid oxygen, "lox" for short, is used in rocket propulsion. Liquid nitrogen is used as a coolant. Helium, which is much rarer than oxygen or nitrogen, is also used as a coolant. For more information on liquid air and liquid helium, see:
Liquid Helium
Once you're familiar with liquid helium in general, you can check out its space uses in:
Liquid Helium in Space
To reach temperatures even colder than liquid helium, we use the adiabatic demagnetization refrigerator (ADR). We have an introduction to ADR's.
The X-Ray Spectrometer (XRS) was a satellite payload with a cooling system that operated down to sixty thousandths of a degree above absolute zero. For info, see the Introduction to XRS.
One idea that keeps showing up in science fiction is: "Wouldn't it be neat if we could freeze people and then revive them?" That idea was used, for instance, in the Stanley Kubrick/Arthur C. Clarke movie "2001: A Space Odyssey". There is a field devoted to freezing people, called cryonics. It's currently used for freezing people who die of diseases that, they hope, will be curable by the time scientists learn how to revive people. At present, though, reviving people has been successful only in science fiction. And not even all the time there, if you remember what happened in "2001"...
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