One of the most common questions from students when presented with an introduction to in-space applications of cryogenics is “What temperature is space?” While the askers of this question are usually enthusiastically interested, thinking space to have tactile temperatures just like the playground outside the school, this is actually a bit of a trick question. Let’s explore.

The first, most correct and most literal answer of the question is 2.725 +/- 0.002 K.[1] In 2006, Dr. John Mather of Goddard Space Center and Dr. George Smoot of UC Berkeley won the Nobel Prize in Physics for their work on measuring the cosmic microwave background residual from the Big Bang as measured by the Far Infrared Absolute Spectrophotometer (FIRAS) instrument on the Cosmic Background Explorer (COBE) satellite.[2] Their work showed that not only is the cosmic microwave background of the universe a perfect blackbody spectrum, but it also investigated disturbances in that background caused by galaxies and other celestial bodies.

However, this is not really the question that the student is asking. They are really asking what temperature are “things” in space. This requires a much more detailed answer that is usually somewhat shortened to avoid the technical details. Given that most of “space” is a vacuum or a bunch of “nothing,” it can’t have a temperature because to have a temperature, that “something” must contain atoms that are vibrating. Wavebands of energy, solar or infrared, don’t produce thermal energy in a vacuum until they interact with a surface. Of course, this is a non-sequitur answer that just annoys the student so is usually not given.

The second, also mostly correct and somewhat literal, is that “it depends.” Well, what does it depend on? It depends on where the object of interest is (relative to various celestial bodies), what sources of energy (generally radiation, but also local heaters on the spacecraft) are heating the spacecraft, how much energy (also in the form of radiation) is the spacecraft emitting and other similar parameters. In an orbit near to Earth (orbits less than 2,000 km from the surface of Earth are called Low Earth Orbit), the predominant heating source is the sun with secondary heating coming from Earth. The energy from the sun at the nominal distance from the Earth (approximately 150 million kilometers) averages 1361 W/m2, ranging between 1317 W/m2 and 1408 W/m2.[4]
This energy from the sun is spread out across a spectrum of different wavelengths, including microwaves, infrared, ultraviolet and x-rays; of which only the visible portion of the spectrum can be detected by our eyes.[10] Outside of the protection of Earth’s atmosphere, all of these different wavelengths provide energy which is converted by the outer surface of the spacecraft to heat.

The energy from the Earth is both in reflecting solar energy (albedo, approximately 30% of the energy from the sun on the Earth is reflected) or infrared radiation coming off of the Earth itself. The Earth emits energy from it as a blackbody of approximately 254 K.[3] The farther away from the Earth that the orbit gets, the less energy, both reflected and emitted, that the Earth imparts on that satellite.

The spacecraft also emits energy to deep space (the technical thermal engineering term for that 2.725 K microwave background). As surfaces have different propensities to radiate energy (they can reflect, store, or emit energy), different materials can be selected during the design of the spacecraft to help to control the temperature. A spherical object at the same distance from the sun as the Earth, but without any energy from the Earth, with equal propensity to absorb and emit radiative energy (i.e. it’s solar absorptivity and IR emissivity are the same) will be approximately 279 K (see Figure 2). Coating the sphere with different materials with different solar absorptivities and infrared emissivities (taken from the Sheldahl Redbook)[8] yields spheres of a range of temperature between 160 K and just over 500 K (see Figure 3). Recently, NASA has been developing coatings that absorb even less of the sun’s energy and could produce temperatures much lower (less than 100 K) in the examples shown.[9] Additionally, the geometry of the satellite can be adjusted to be such as a 3:1 cylinder with spherical end caps, where the cross-section facing the sun is the same as the sphere but the length is extended three times the sphere’s diameter, or a 1:3 cylinder with spherical end caps, where the same cylinder is turned broadside to the sun. Figure 3 shows that the 3:1 cylinder temperature drops as the shape yield significantly more area for the satellite to radiate energy from it with the same solar input whereas the 1:3 cylinder only slightly increases in temperature due to the change in relative heat transfer areas.

A real satellite has multiple different thermal environments it travels through, including periods of eclipse where the Earth blocks all of the sunlight from the satellite. This causes many temperature swings that the exterior surfaces of the satellite experience. As a result, most satellites are insulated with multilayer insulation and the interior of the satellite is maintained at approximately 300 K. This keeps the electronics on board the satellite working within their design temperatures. Some observatories and orbital telescopes use specially designed sunshields to insulate their optical elements from both solar and Earth-based radiation such that a cryocooler can keep them cold.

However, there are many interesting locations in the solar system that are at a wide variety of temperatures. In the craters of the Moon’s south pole with rims that are angled such that sunlight never gets into them where NASA’s Lunar Reconnaissance Orbiter measured temperatures as low as 33 K while nighttime temperatures get as cold as 110 K.[5] To further explore these cold regions where the possibilities for many unique elements and molecules to be trapped is one of the main scientific reasons for NASA’s Artemis program.[6] The surface of Titan, one of the moons of Saturn, is known to contain lakes of methane-based liquids as evidenced by images from the Cassini spacecraft.[7] Average surface temperatures of Neptune and Pluto are also very cold due to their large distances from the sun.

Images:

1. Figure 1: COBE/FIRAS image. Credit: NASA

2. Figure 2: Energy balance on a sphere in space. Credit: NASA

3. Figure 3: Temperature for sphere and cylinders with spherical end caps at 1 AU with different material properties. Credit: NASA

4. Figure 4: Electromagnetic spectrum.[10] Credit: NASA Mission Directorate

References

[1] https://lambda.gsfc.nasa.gov/product/cobe/

[2] https://www.nobelprize.org/prizes/physics/2006/popular-information/

[3] https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html

[4] ASTM-E490-22, Standard Solar Constant and Zero Air Mass Solar Spectral Irradiance Tables, 2022

[5] https://spacemath.gsfc.nasa.gov/news/7Page42.pdf

[6] https://www.nasa.gov/mission/artemis-iii/

[7] https://www.esa.int/Science_Exploration/Space_Science/Cassini-Huygens/Titan_has_liquid_lakes

[8] Sheldahl Redbook, https://sheldahl.com/wp-content/uploads/2023/07/RedBook.pdf

[9] Youngquist, R.C., Nurge, M.A., Johnson, W.L., et. al., Cryogenic Deep Space Thermal Control Coating, J. Spacecraft and Rockets, 2018.

[10] National Aeronautics and Space Administration, Science Mission Directorate. (2010). Introduction to the Electromagnetic Spectrum. Retrieved July 3, 2024, from NASA Science website: http://science.nasa.gov/ems/01_intro