If space is so cold, why is cooling a spacecraft so hard?

Deep space is about −270 °C: the faint glow left over from the early universe, measured by NASA’s COBE satellite, matches a temperature of 2.725 K. Yet a spacecraft can overheat out there. Its electronics turn all the power they use into heat, and sunlight adds more, and the space station carries six big white radiators to get rid of it. If space is so cold, why is cooling a spacecraft so hard?

▶ Try it yourself in Space Radiator Lab: change the radiator’s temperature, the heat it has to dump and the sunlight on it, and watch the panel it needs grow or shrink.

Nothing to hand the heat to

On Earth, heat rarely has to leave on its own. Air molecules bump into a hot surface, pick up energy and carry it off, and a fan or a pump keeps cool air or water flowing past. In space there are almost no molecules, so neither works. A vacuum is an insulator: it is the gap between the two walls of a vacuum flask, which is why coffee stays hot in one for hours.

Only light gets out

Every warm surface glows in infrared, and that light carries heat away even through a vacuum. How much depends steeply on temperature: the Stefan–Boltzmann law says the power grows as the fourth power of the absolute temperature. A square metre of white panel, radiating from both faces, sheds about 840 W at 27 °C (300 K). Double the temperature above absolute zero, to 600 K or 327 °C, and it sheds about 13,400 W: 2⁴, sixteen times as much.

So radiators run cool, and big

Hot radiators could be small. But electronics cook above about 100 °C (processor limits are usually 100–110 °C), and heat only flows from hot to cold, so a radiator has to run cooler than what it cools. Down there each square metre sheds only a few hundred to about a thousand watts, so the panels get big. NASA’s system on the space station rejects up to 70 kW through six radiators, each eight panels long and 23.3 × 3.4 m, about as long as a tennis court. They are painted white so sunlight warms them little, and turned edge-on to the Sun in daylight; the ammonia inside comes back as cold as −40 °C.

A radiator problem as much as a power problem

Scale that up and the radiator becomes a structure of its own. In this model, a 1 GW data center in orbit, with its radiators at 27 °C and edge-on to the Sun, needs about 1.2 km² of panel. Running warmer helps: in a debate on X in early November 2026, Elon Musk wrote that about 1,300 W/m² is easily achievable for an aluminum radiator pointed away from the Sun. In this model that is a panel near 62 °C radiating from both faces, which cuts the gigawatt to about 0.77 km²; he also said the solar panels would need about five times the radiator’s area. Either way, a data center in orbit needs its radiators as surely as its solar panels. For one company’s plan in detail, see Orbit Chip Lab, on why Google’s orbiting chip can only run in bursts.

The tool is a simple model: one flat panel at one temperature, radiating to deep space, with no Earth below it. Near Earth the planet fills much of each panel’s sky and sends back its own infrared and reflected sunlight, so real radiators need more area: the station’s six offer about 475 m² a face, about three times this model’s figure for 70 kW at −15 °C. The film uses short NASA footage of the station, filmed from the departing Crew-2 Dragon, and two thermal-camera clips by @ivy_doll_too on TikTok, credited on screen; the spacecraft, air, radiator and numbers are drawn from the model.

Sources: NASA, Active Thermal Control System (ATCS) Overview; Intel, Information about temperature for Intel processors (junction limits usually 100–110 °C); NASA Goddard, CMB intensity (COBE FIRAS: 2.725 K); Modus Advanced, AZ-93 white thermal control paint; Wccftech, Elon Musk defends space AI satellites (3 Nov 2026).