The question of “What is the temperature in space?” does not have a single, simple answer because the environment is far from uniform, and the definition of temperature changes in a vacuum. On Earth, temperature measures the average kinetic energy of moving particles, like molecules in the air. In the near-perfect void of space, the sparsity of matter means this thermodynamic definition often becomes meaningless. The temperature experienced by an object depends entirely on its location, nearby energy sources, and how it exchanges heat.
Defining Temperature in a Vacuum
Temperature on Earth is typically transferred through conduction and convection, processes that rely on countless particle collisions within a dense medium. Conduction moves heat through direct contact, while convection moves heat through the circulation of fluids. Space is an ultra-low-pressure environment where particles are too spread out to support these energy transfers effectively.
In the vacuum of space, the primary mechanism for heat transfer is radiation, which uses electromagnetic waves to carry energy. An object absorbs energy from radiation sources, such as starlight, and simultaneously loses energy by emitting its own thermal radiation. The temperature of the vacuum itself is better described by the energy flux of the radiation that fills it, rather than the kinetic movement of the few atoms present. This distinction between the temperature of matter and the temperature of radiation is fundamental to understanding the space environment.
The Baseline Temperature of Deep Space
When scientists discuss the absolute temperature of space, they refer to the Cosmic Microwave Background (CMB) radiation. This faint, uniform glow permeates the entire universe and is the cooled, remnant energy from the Big Bang. This radiation acts as a thermal bath, setting the lowest possible temperature throughout deep, intergalactic space.
The temperature of the CMB has been precisely measured at approximately 2.725 Kelvin (K), equivalent to about -455 degrees Fahrenheit or -270.4 degrees Celsius. The Kelvin scale is an absolute thermodynamic scale where 0 K represents absolute zero, the point at which all particle motion ceases. The CMB’s precise temperature supports the Big Bang theory, showing the universe has cooled as it expanded. Any object shielded from all other heat sources would eventually cool to this baseline temperature, achieving equilibrium with the background radiation.
How Objects Acquire Temperature in Space
Solid objects like satellites experience temperatures vastly different from the frigid CMB because they constantly exchange energy with their surroundings through radiation. An object’s temperature in orbit is determined by the balance between the energy it absorbs and the energy it radiates away. The sun is the dominant heat source, bathing the sunlit side of a spacecraft with intense solar radiation.
The side of a satellite facing the sun can heat up dramatically, sometimes reaching temperatures well over 250 degrees Fahrenheit. Conversely, the shaded side, radiating heat into the deep, cold void and receiving almost no solar energy, can plummet below -250 degrees Fahrenheit. This massive thermal gradient requires engineers to use specialized thermal control systems, including reflective materials and multilayer insulation, to prevent structural damage and equipment failure. Since there is no air to transfer heat away via convection or conduction, a spacecraft relies on radiators to emit infrared radiation into the cold sink of space.
Temperature Variation Across the Solar System and Galaxy
The temperature of space is highly variable across the solar system and the galaxy, depending on proximity to energy sources and local matter density. Close to the sun, the solar wind and the outer atmosphere, known as the corona, exhibit extremely high kinetic temperatures. The plasma in the solar corona can reach millions of degrees Kelvin, but its low density means a spacecraft passing through it would not absorb much heat energy.
In the interstellar medium (ISM) between stars, the gas is extremely diffuse and exists in phases with very different kinetic temperatures. Warm neutral gas might be around 6,000 K, while the hottest phase, the tenuous coronal gas, can reach a million Kelvin. These differences illustrate that high kinetic temperature does not necessarily mean high thermal energy content due to low particle density. Dense molecular clouds—the nurseries of stars—can be among the coldest places, with internal temperatures dropping to only 10 to 20 Kelvin, slightly warmer than the CMB.