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

Only one way works for long. Heat comes in from several places and has just one way out.

1. Heat in

   Every watt a spacecraft’s electronics use ends up as heat. Its crew, if it has one, adds more, and so does any sunlight it soaks up.

2. No air

   On Earth, air or water flows past a hot surface and carries heat off. In space there are almost no molecules, so nothing can. A vacuum is an insulator; it is the gap between the two walls of a vacuum flask.

3. Light out

   Every warm surface glows in infrared, and that light crosses a vacuum. A radiator is a big panel that glows the heat away into the cold, dark sky.

A loop of pumped fluid carries the heat from the electronics to the radiator. On the space station, water loops collect heat inside and hand it to two loops of liquid ammonia outside, which carry it to the radiators, NASA’s overview says. Spacesuits and the Space Shuttle could also let water evaporate into space to dump heat, but that uses up water, so it suits short jobs, not a station that stays up for years.

Because the glow grows as the fourth power of temperature. The Stefan–Boltzmann law says a surface sheds power in proportion to T⁴, with T measured from absolute zero, in kelvin. Double the temperature and the glow is 2⁴, sixteen times stronger. In Space Radiator Lab a square metre of white panel, radiating from both faces, sheds about 840 W at 300 K (27 °C) and about 13,400 W at 600 K (327 °C).

So why not run radiators hot? Because heat only flows from hot to cold, a radiator has to be cooler than the things it cools, and electronics cook at around 100 °C: Intel says its processors’ limits are usually between 100 and 110 °C. That keeps radiators down on the flat end of the curve, where each square metre sheds a few hundred to about a thousand watts. To dump the same heat at half the absolute temperature takes sixteen times the panel.

And the −270 °C? It is the temperature of the faint microwave glow that fills the sky, 2.725 K as NASA’s COBE satellite measured it. It matters because that sky sends almost nothing back: a radiator facing it keeps nearly all of the light it sheds. Near Earth, the planet fills much of the sky and sends back far more.

Big. NASA’s External Active Thermal Control 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 with Z-93, a paint that glows well in the infrared but takes in little sunlight, and in daylight they are turned edge-on to the Sun; the ammonia comes back from them as cold as −40 °C. Its cousin AZ-93 absorbs about 15% of sunlight and has an infrared emittance of 0.91, the values the tool uses.

The station’s six offer about 475 m² a face, about three times what Space Radiator Lab asks for 70 kW at an assumed −15 °C. The model leaves out Earth: in low orbit the warm planet fills much of each panel’s sky with its own infrared and reflected sunlight, and the panels are sized for the hottest orbits.

Data centers in orbit make the problem bigger. In the model, a 1 GW data center with radiators at 27 °C needs about 1.2 km² of panel. In a debate on X in early November 2026, after Utrecht University professor Jan Broersen called space data centers nearly impossible to build, Elon Musk wrote that about 1,300 W/m² is easily achievable for an aluminum radiator pointed away from the Sun, and that solar panels would need about five times the radiator’s area. In the model, 1,300 W per square metre means a panel near 62 °C radiating from both faces, which cuts the gigawatt to about 0.77 km². Either way, the radiator is as big a part of the design as the power. Orbit Chip Lab shows what happens when one is too small.

1. 1 See

   Open Space Radiator Lab and tap Server rack · 40 kW. At 40 °C, Each m² of panel sheds reads 991 W, so Radiator needed reads 40 m², about 3 parking spaces.

2. 2 Change

   Drag Radiator temperature to 600 K (327 °C), or drag along the curve. Each m² of panel sheds reads 13,400 W, Compared with 27 °C reads ×16, and the radiator shrinks to 3.0 m², about 2 doors. Electronics couldn’t run that hot.

3. 3 Understand

   Tap Server rack · 40 kW again, then Facing the Sun. The white paint soaks up 204 W per square metre, the panel sheds 786 W net, and the rack needs 51 m². Then tap 1 GW data center: about 1.2 km², 168 soccer fields.

Space Radiator Lab is one flat panel at one temperature, radiating from both faces into deep space. Real radiators cool along the path of their coolant, so they are warmer where the fluid comes in and colder where it leaves. The model has no Earth below the panel, which is why it asks for less area than the space station carries, and it sees the Sun only when you turn the panel toward it.

It also leaves out what it takes to move the heat: the pumps, pipes and heat exchangers between the electronics and the panel, and any heat pump that would let a radiator run hotter than the electronics, at the cost of more power and so more heat. It ignores the solar panels that power everything.

The film uses short NASA footage of the space station, filmed from the departing Crew-2 Dragon in 2021, and two thermal-camera clips by @ivy_doll_too on TikTok, credited on screen; the spacecraft, the air, the radiator and the numbers are drawn from the model. The spacecraft’s warmth gauge in the film is illustrative.