Hint: Cryogenic liquids are liquefied gases that are kept in their liquid state at very low temperatures. The word "cryogenic" means "producing, or related to, low temperatures," and all cryogenic liquids are extremely cold. All cryogenic liquids are gases at normal temperatures and pressures.
Complete answer: Cryogenic fuels are fuels that require storage at extremely low temperatures in order to maintain them in a liquid state. Over the years the term cryogenics has generally been used to refer to temperatures below approximately -150 C. According to the laws of thermodynamics, there exists a limit to the lowest temperature that can be achieved, which is known as absolute zero.These fuels are used in machinery that operates in space (e.g. rocket ships and satellites) because ordinary fuel cannot be used there, due to absence of an environment that supports combustion (on Earth, oxygen is abundant in the atmosphere, whereas in human-explorable space, oxygen is virtually non-existent) and space is a vacuum. Cryogenic fuels most often constitute liquefied gases such as liquid hydrogen.
Additional informationThe cryogenic engine gets its name from the extremely cold temperature at which liquid nitrogen is stored. Air moving around the vehicle is used to heat liquid nitrogen to a boil. Once it boils, it turns to gas in the same way that heated water forms steam in a steam engine.
Note: Cryogenics developed in the nineteenth century as a result of efforts by scientists to liquefy the permanent gases. One of the most successful of these scientists was English physicist Michael Faraday (1791–1867).