Temperature in outer space depends on many factors: distance from a star or other cosmic event, whether a point in space is in direct light or shade and if it is subject to a solar flare or solar wind. Variation in the temperature of space near the Earth is primarily based on location and time: Temperatures are drastically different on the light and shaded sides of the planet, which gradually change minute to minute based on the planet's rotation on its axis and its revolution around the sun.

NASA has great resources describing the temperatures of outer space and the gradient of temperatures moving throughout Earth’s atmosphere. The temperature of outer space does not change a lot in the solar system, but the temperature of individual planets varies greatly. Planetary temperatures depend on distance from the sun and their specific material composition.

Near Earth

The average temperature of outer space around the Earth is a balmy 283.32 kelvins (10.17 degrees Celsius or 50.3 degrees Fahrenheit). This is obviously a far cry from more distant space's 3 kelvins above absolute zero. But this relatively mild average masks unbelievably extreme temperature swings. Just past Earth's upper atmosphere, the number of gas molecules drops precipitously to nearly zero, as does pressure. This means there is almost no matter to transfer energy - but also no matter to buffer direct radiation streaming from the sun. This solar radiation heats the space near Earth to 393.15 kelvins (120 degrees Celsius or 248 degrees Fahrenheit) or higher, while shaded objects plummet to temperatures lower than 173.5 kelvins (minus 100 degrees Celsius or minus 148 degrees Fahrenheit).

The International Space Station travels in this near Earth region of space, so they have to deal with extreme heat and extreme cold constantly. This is part of the reason why spacesuits and other protection is extremely important for protecting astronauts and equipment.

The key defining characteristic of outer space is emptiness. Matter in space concentrates into astronomical bodies. The vacuum of space between these bodies is truly empty – technically a near-vacuum where individual atoms may be many miles apart. Heat is the transfer of energy from atom to atom. Under outer space conditions, almost no energy is transferred because of the vast distances involved. The average temperature of empty space between celestial bodies is calculated at 3 kelvins (minus 270.15 degrees Celsius or minus 457.87 degrees Fahrenheit). Absolute zero, the temperature at which absolutely all activity stops, is zero kelvins (minus 273.15 degrees Celsius or minus 459.67 degrees Fahrenheit). This makes space one of the ‘coldest places’ in the universe, although scientists and physicists have actually created colder conditions in laboratories on Earth.

Spacecraft that travel into deep space (like Voyager, the Cassini Saturn probe, or the Pluto New Horizons mission) have to protect against extreme cold to preserve their instruments, but they also have to deal with overheating! Because there is so little in space, heat cannot be transferred through conduction or convection as on Earth, instead it is exchanged through emission/radiation. This is extremely slow, so spacecraft can sometime struggle with instruments and technology overheating without the ability to cool down.

Radiation is energy transferred from an object or event out into space. Part of astrophysics studies the cosmic microwave background radiation (CMB) – a remnant of energy released during the Big Bang – which is calculated at almost 2.6 kelvins (minus 270.5 degrees Celsius or minus 455 degrees Fahrenheit). This accounts for most of empty space's temperature of 3 kelvins. The rest comes from constant solar energy emitted from stars, intermittent energy from solar flares and intermittent blasts from cosmic events such as supernovas.

Distance from stars determines the average temperature of specific points in space. Whether a specific point is fully exposed to light or partially or fully shaded determines its temperature at a specific time. Distance and light exposure are the prime temperature determinants for all objects and points that lack atmosphere and are suspended in near-vacuum. While the intensity of light diminishes with distance from a light source, the individual pockets of energy from light, called photons, will actually travel forever through space until they collide with an object and interact (or they get sucked in by a black hole).