Space is cold. Very cold. The average temperature in space is around -270°C or 3 Kelvin (-454°F or 3 degrees above absolute zero). This frigid temperature is a result of the lack of atmosphere in space which allows heat to radiate away unimpeded. Understanding the extreme temperatures of space and how spacecraft and astronauts deal with these conditions is an important part of space exploration.
Why is space so cold?
Space appears as a black void, but it is not actually empty. Outer space is filled with some particles and radiation, as well as minimal amounts of gas. However, the density of gas molecules in space is extremely low, around 5 atoms per cubic centimeter in interstellar space, compared to around 10^19 molecules per cubic centimeter in Earth’s atmosphere at sea level. This near-vacuum condition means there is no atmosphere in space to retain heat or insulate objects from temperature extremes.
On Earth, the atmosphere acts like a blanket, trapping some of the heat from the Sun and keeping temperatures moderate. The greenhouse effect of gases like carbon dioxide and water vapor prevent daytime heating from quickly radiating out into space at night. In space, with no atmosphere, there is no greenhouse effect or insulation, so heat from the Sun can radiate directly out into the emptiness.
The complete lack of atmosphere also means that the temperature in space is not driven by the heat capacity or circulation of atmospheric gases. Instead, the primary determinant of temperature is radiation – how much sunlight or thermal radiation is being absorbed or emitted by an object.
Radiative cooling
In the shade or shadow of an object in space, with no atmospheric conduction or convection of heat, temperatures can plummet thanks to radiative cooling. All objects above absolute zero (-273°C) emit thermal radiation, and in space that radiation can escape into the void without being absorbed by air molecules. In this way, heat is efficiently radiated out to space, causing rapid cooling.
The Hubble Space Telescope uses radiative cooling to help keep its sensitive instruments chilled. The telescope has large sunshields to block light and prevent heating from the Sun, allowing the outer layers to radiate heat away into the abyss of space. Hubble’s instruments are cooled to around -400°F (-240°C) using this radiative cooling in the shadows of its sunshields.
Equilibrium temperature
For objects in direct sunlight, like spacecraft or space suits exposed to the Sun, the primary factor determining temperature is the amount of radiation absorbed from the Sun versus the amount emitted as thermal radiation. Over time an equilibrium is reached where energy in equals energy out.
With no atmosphere, the equilibrium temperature in direct sunlight in Earth orbit is around 120°C (250°F). On the Moon, with its slower rotation and longer day/night cycle, temperatures range from 107°C (225°F) during the day to -153°C (-243°F) at night.
The planet Mercury has essentially no atmosphere, so despite being the closest planet to the Sun, temperatures on the night side can plunge to -173°C (-280°F). Venus on the other hand has a very dense CO2 atmosphere causing a runaway greenhouse effect and temperatures around 462°C (863°F).
Effects of extreme cold
The extreme cold of space creates challenging conditions for spacecraft and astronauts. Materials become brittle, liquids freeze, and chemical reactions and human physiology can be impaired. NASA and other space agencies have developed special technologies and procedures to counteract the effects of the cold vacuum of space.
Spacecraft insulation
Without atmospheric insulation, spacecraft have to be carefully shielded from the temperature extremes of space. Insulation like multi-layered blankets are wrapped around spacecraft to trap heat inside. Sunshields or heatshields made of special materials are used to reflect or absorb sunlight depending on heating needs.
Some electronics and batteries need to be kept warm to function properly so will have internal heating elements. Other instruments like infrared cameras need to be kept very cold, so will have additional radiators to help cool them.
Liquid fuel and water
Onboard liquid fuel and water supplies can freeze in space. This could damage pipes and tanks and prevent engines from firing properly. To keep liquids usable in space, special precautions are taken such as insulation and heating to prevent freezing.
The Space Shuttle’s large external fuel tank had aerodynamic foam insulation to limit ice formation. Prior to launch any ice was melted using heaters on the launch pad. The Space Shuttle itself also had heaters on its water lines to prevent freezing once in orbit.
Materials and moving parts
In the cold of space, the contraction of materials can cause them to become brittle. Rubber or plastic components will stiffen and could potentially crack under stress. Lubricants also become more viscous which can impede the operation of mechanical systems.
Spacecraft parts are tested extensively on Earth to ensure they withstand temperature cycles without failing. Components are made from special materials and alloys suitable for the temperature range in space. Any moving parts are lubricated accordingly, and heaters can keep critical mechanisms warm.
Astronaut spacesuits
Like spacecraft, astronauts’ spacesuits have to provide protection from temperature extremes. Spacesuit insulation includes up to 10 layers of materials including Mylar, Dacron, Kapton, and Nomex to retain heat. Liquid cooling garments worn under the spacesuit circulate water to transfer excess heat away from the body.
Heaters warm oxygen flows to prevent valves and regulators from freezing up. Batteries and other electronics are temperature regulated, and astronauts’ exhaled breath condenses rather than freezing on visors. Even an astronaut’s skin is protected with special liners and gloves to prevent direct contact with the cold metal and plastic outer layers.
Human exposure
Without proper protection, exposure to the near-absolute zero cold of space would be fatal within minutes. Water on the skin or in the lungs would vaporize, causing tissue damage. Lack of air pressure would lead to hypoxia within seconds. Body fluids could boil once exposed to vacuum conditions.
Astronauts have to wear pressurized spacesuits when doing spacewalks. If an accidental suit puncture occurred, the lack of oxygen and extreme cold would rapidly incapacitate and potentially kill the astronaut unless quickly resolved. Procedures are in place to re-pressurize spacesuits and get astronauts to safety during an emergency.
Measuring space temperatures
Remote sensing instruments aboard spacecraft allow the temperature of planets, moons, asteroids and more to be determined from a distance. Infrared cameras and spectrometers can measure the thermal radiation emitted by objects to determine their temperature.
Here are some average temperature readings taken around the Solar System:
| Object | Average Temperature |
|---|---|
| Mercury | -173 to 427°C |
| Venus | 462°C |
| Earth | 14°C |
| Mars | -63 to 20°C |
| Jupiter | -108°C |
| Saturn | -140°C |
| Uranus | -195°C |
| Neptune | -200°C |
| Pluto | -225 to -235°C |
Temperature readings in space can vary depending on whether it is in sunlight or shadow. Measurements also detect only surface temperatures, without accounting for internal heat sources which keep planets like Earth warm inside.
Cosmic Microwave Background
The cosmic microwave background (CMB) is electromagnetic radiation left over from the early universe shortly after the Big Bang. It fills all space and represents the baseline temperature of space at 2.725 Kelvin (-270.4°C). The CMB is extremely uniform, with tiny fluctuations revealing the seeds of early galaxy formation.
Maintaining habitable temperatures
Creating habitable environments in space requires maintaining temperatures suitable for humans and technology to operate safely. Spacecraft and space stations use a variety of methods to maintain ambient temperatures within comfortable ranges.
Insulation
As with protective spacesuits, layers of insulating materials are used extensively aboard crewed spacecraft and habitats. Multilayer insulation (MLI) blankets cover the exterior to hold in heat and protect from temperature swings. Internal insulation prevents conductive heat transfer between warm and cold sections.
Heaters
Electric heaters warm up critical components and living spaces. Heaters may be used to prevent freezing or keep systems and crew areas within operational temperature limits. Spacecraft computer systems monitor temperatures and automatically activate heaters as needed.
Coolant loops
Liquid cooling loops transport excess heat from electrical systems and warm areas to radiators. Pumps circulate fluid through cold plates and heat exchangers to collect waste heat which is then radiated into space or used to warm up coolant returning from the radiators.
Radiators
Radiators with large surface areas emit heat in the form of infrared radiation into space. They can be positioned to avoid solar heating and utilize the cold of shade. Radiators emit thermal radiation until equilibrium is reached with internal coolant loops and external temperatures.
Electric heaters
Along with pumps and valves to manage liquid coolant, spacecraft thermal control systems also utilize electric heaters. Heaters activate as needed to maintain equipment, crew compartments, and water lines within habitable temperature ranges.
Solar heating
Solar energy can be harvested using collectors to provide heating. This renewable energy source can supplement electric heaters and coolant loops. Thermal mass and phase change materials store solar heat for gradual release during dark periods.
Effects on crew health
Without adequate thermal controls, the extreme hot and cold temperatures of space could rapidly kill or incapacitate astronauts. Crew health and performance requires suitable ambient temperatures.
Thermoregulation
In the normal temperature range, the human body can maintain thermal homeostasis by sweating or shivering. But in extreme heat or cold, dangerous conditions like hyperthermia, hypothermia, frostbite, and burns can occur if core temperature leaves the tolerable range.
Comfort
A comfortable working temperature helps astronauts stay alert and productive on long missions. Shared crew areas are kept at around 21°C (70°F). Individuals can adjust personal spaces or what they wear to customize comfort levels.
Sleep
Sleep is crucial for physical and mental health. Cooler sleep temperatures around 15-18°C (59-64°F) have been found optimal for most people. Adjustable or water-filled temperature pads allow astronauts to regulate their sleep surface.
Exercise
Astronauts follow rigorous exercise routines to counter bone and muscle loss in microgravity. Controlled temperatures help prevent overheating and make workouts comfortable so exercise discipline can be maintained.
Morale
Stable, comfortable ambient temperatures improve mental well-being. Thermal comfort helps astronauts feel relaxed and happy during long space missions, lifting morale when isolated from Earth.
Conclusion
Space is an extremely cold environment with temperatures approaching absolute zero. This is caused by the near-vacuum condition which allows heat to radiate away rapidly without atmospheric insulation. Specialized technology protects spacecraft and astronauts from these harsh temperatures. Maintaining equipment and crew areas within a habitable temperature range is essential for successful space missions.