The question of “how hot is space” is complicated because it confuses the concepts of heat and temperature. Heat is the transfer of thermal energy, while temperature measures the average kinetic energy of molecules. In the near-perfect vacuum of space, there are virtually no molecules to transfer energy or measure temperature in the way we do on Earth. Therefore, the temperature of space itself is fundamentally different from the temperature an object reaches when exposed to the Sun.
How Heat Moves in a Vacuum
On Earth, heat moves primarily through conduction and convection, both of which rely on matter. Conduction is the transfer of energy through direct contact, such as vibrating atoms bumping into neighbors. Convection involves the movement of fluid, like circulating warm air or water, which carries thermal energy.
In the vacuum of space, both conduction and convection are impossible due to the absence of a medium, leaving radiation as the only significant method of heat transfer. Radiation is the transfer of energy via electromagnetic waves, such as visible light and infrared waves, which do not require particles to travel. This is how the Sun’s energy travels 93 million miles to Earth.
Any object with a temperature above absolute zero constantly emits this thermal radiation into the surrounding environment. This means that a spacecraft in orbit is continuously radiating energy away from its surface into the cold vacuum. The object’s final temperature is determined by the balance between the solar radiation it absorbs and the thermal radiation it manages to emit.
Measuring the Ambient Temperature of Space
While the vacuum itself cannot have a temperature in the conventional sense, scientists can define a baseline temperature for the universe. This measure is based on the faint, uniform energy permeating all of space, known as the Cosmic Microwave Background (CMB) radiation. The CMB is the leftover thermal energy from the Big Bang, which has cooled as the universe expanded over billions of years.
This radiation is the temperature of the universe’s remnant energy. It is remarkably consistent across the entire sky, measuring approximately 2.7 Kelvin. This translates to an extremely cold -270.4 degrees Celsius (about -454.8 degrees Fahrenheit).
This background temperature is what objects in deep space, far from any stars or planets, will eventually cool down to. However, this measure is largely disconnected from the experience of an object near a star, which is constantly bombarded by solar radiation. The CMB provides a theoretical floor for the temperature of the cosmos, but direct sunlight dramatically overrides it.
The Temperature of Objects in Sunlight
When an object like a satellite or an astronaut’s suit is exposed to direct solar radiation, its temperature is not fixed; it is a dynamic balance of energy transfer. This thermal equilibrium—where absorbed solar energy equals radiated thermal energy—is highly dependent on two intrinsic material properties: albedo and emissivity.
Albedo is a measure of how reflective a surface is, indicating the fraction of incoming sunlight that is immediately bounced away. A material with a high albedo, like a white surface, reflects most of the energy and stays cooler. Emissivity describes a surface’s efficiency at radiating absorbed heat away as infrared light.
A spacecraft’s temperature can vary wildly, with the sunlit side absorbing intense energy and the shaded side radiating heat into the cold vacuum. The sunny side of the International Space Station (ISS), for example, can soar to temperatures exceeding 121 degrees Celsius (around 250 degrees Fahrenheit). Conversely, the shaded side of the same structure can simultaneously plummet to temperatures as low as -157 degrees Celsius (around -250 degrees Fahrenheit).
Space engineers manage these extremes using specialized thermal control systems and surface coatings. Black surfaces, which have low albedo and high emissivity, absorb and radiate heat quickly. Highly reflective, multi-layered insulation shields sensitive equipment, ensuring internal temperatures remain within a narrow, life-sustaining range.