Temperature is a measure of the average kinetic energy of a substance, typically expressed in Kelvin by astronomers. Kelvin is a unit not expressed in degrees as in Farenheit or Celsius, so the proper denotation, for example, is 25K not 25 degrees K.

In outer space, temperatures vary greatly depending on exposure to the kinetic energy in the form of radiation. Heat energy on the side of a spacecraft exposed to the sun will be a much higher value than the side opposite the sun (shade side).

The coldest temperature in space is on average approximately 2.725 Kelvin and is called the Cosmic Background Radiation, which is the energy remaining from the beginning of the universe. This means the majority of the universe is about three Kevin above absolute zero (0 Kelvin) – the temperature at which molecules themselves stop moving. That’s almost -270.425 degrees Celsius, or -454.761 degrees Fahrenheit.

The hottest temperature in space depends upon the nature of the molecule and the energy that gave it its state of excitation or amount of kinetic energy imparted to the molecule while in space). For example, a molecule ejected from a star can be hundreds to millions in Kelvin temperature and will maintain that temperture as it travels through space. It has no way to reduce its temperature to its natural temperature state (equilibrium) because there is no other molecule nearby so it can become cooler by contacting with another molecule and pass along some of its energy. Space is a great insulator preventing this molecule from changing its temperature condition.

Engineers designing spacecraft must consider materials to use to protect equipment and crew from solar heating and non-solar cooling effects. One popular method was to place the satellite or spacecraft in a roll called the Bar-B-Que (BBQ) roll to evenly distribute the temperature difference to the spacecraft exterior while it is exposed to the sun.

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