How Cold is Space?

Fraser Cain

Published on Jul 1, 2013

In this short explainer, Universe Today publisher Fraser Cain researchers how cold space is. What temperature do astronauts experience? What about Pluto, or the depths of space. What’s the coldest possible temperature space can get?


If you could travel from world to world, from star to star, out into the gulfs of intergalactic space, you’d move away from the warmth of the stars into the vast and cold depths of the void.

Better pack a sweater, it’s going to get cold.

But, how cold? How cold is space?

Unlike your house, car, or swimming pool, the vacuum of space has no temperature.

So, how cold is space?

That’s a nonsense question.

It’s only when you put a thing in space, like a rock, or an astronaut, that you can measure temperature.

Remember there are three ways that heat can transfer:

conduction, convection and radiation.

Heat up one side of a metal bar, and the other side will get hot too; that’s conduction.

Circulating air can transfer heat from one side of the room to another; that’s convection.

But out in the vacuum of space, the only way heat can transfer is radiation.

Photons of energy get absorbed by an object, warming it up.

At the same time, photons are radiating away.

If the object is absorbing more photons than it emits, it heats up.

And if it emits more photons than it absorbs, it cools down.

There is a theoretical point at which you can’t extract any more energy from an object, this minimum possible temperature is absolute zero.

As we’ll see in a second, you can never get there.

Let’s look close to home, in orbit around the planet, at the International Space Station.

A piece of bare metal in space, under constant sunlight can get as hot as two-hundred-sixty (260) degrees Celsius.

This is dangerous to astronauts who have to work outside the station.

If they need to handle bare metal, they wrap it in special coatings or blankets to protect themselves.

And yet, in the shade, an object will cool down to below minus-one-hundred (-100) degrees Celsius.

Astronauts can experience vast differences in temperature between the side facing the Sun, and the side in shadow. Their spacesuits compensate for this using heaters and cooling systems.

Let’s talk a little further out.

As you travel away from the Sun, the temperature of an object in space plummets.

The surface temperature of Pluto can get as low as minus-two-hundred-forty (-240) Celsius, just thirty-three (33) degrees above absolute zero.

Clouds of gas and dust between the stars within our galaxy are only ten (10) to twenty (20) degrees above absolute zero.

And if you travel out far away from everything in the Universe, you can never get lower than a minimum of just two-point-seven (2.7) Kelvin or minus-two-hundred-sevety-point-four-five (-270.45) Celsius.

This is the temperature of the cosmic microwave background radiation, which permeates the entire Universe.

In space? It’s as cold as it can get.

Controlling the ISS


Published on May 1, 2013

How do you control the International Space Station, a ship the size of five hockey rinks? And from where? CSA Astronaut Chris Hadfield answers these questions from inside the heart of the matter. Credit: CSA/NASA

For more about the International Space Station, see:


NASA | Fermi’s Close Call with a Soviet Satellite


Published on Apr 30, 2013

NASA scientists don’t often learn that their spacecraft is at risk of crashing into another satellite. But when Julie McEnery, the project scientist for NASA’s Fermi Gamma-ray Space Telescope, checked her email on March 29, 2012, she found herself facing this precise situation.

While Fermi is in fine shape today, continuing its mission to map the highest-energy light in the universe, the story of how it sidestepped a potential disaster offers a glimpse at an underappreciated aspect of managing a space mission: orbital traffic control.

As McEnery worked through her inbox, an automatically generated report arrived from NASA’s Robotic Conjunction Assessment Risk Analysis (CARA) team based at NASA’s Goddard Space Flight Center in Greenbelt, Md. On scanning the document, she discovered that Fermi was just one week away from an unusually close encounter with Cosmos 1805, a dead Cold-War era spy satellite.The two objects, speeding around Earth at thousands of miles an hour in nearly perpendicular orbits, were expected to miss each other by a mere 700 feet.

Although the forecast indicated a close call, satellite operators have learned the hard way that they can’t be too careful. The uncertainties in predicting spacecraft positions a week into the future can be much larger than the distances forecast for their closest approach.
With a speed relative to Fermi of 27,000 mph, a direct hit by the 3,100-pound Cosmos 1805 would release as much energy as two and a half tons of high explosives, destroying both spacecraft.
The update on Friday, March 30, indicated that the satellites would occupy the same point in space within 30 milliseconds of each other. Fermi would have to move out of the way if the threat failed to recede. Because Fermi’s thrusters were designed to de-orbit the satellite at the end of its mission, they had never before been used or tested, adding a new source of anxiety for the team.
By Tuesday, April 3, the close approach was certain, and all plans were in place for firing Fermi’s thrusters. Shortly after noon EDT, the spacecraft stopped scanning the sky and oriented itself along its direction of travel. It then parked its solar panels and tucked away its high-gain antenna to protect them from the thruster exhaust.
The maneuver was performed by the spacecraft based on previously developed procedures. Fermi fired all thrusters for one second and was back doing science within the hour.
In 2012, the Goddard CARA team participated in collision-avoidance maneuvers for seven other missions. A month before the Fermi conjunction came to light, Landsat 7 dodged pieces of Fengyun-1C, a Chinese weather satellite deliberately destroyed in 2007 as part of a military test. And in May and October, respectively, NASA’s Aura and CALIPSO Earth-observing satellites took steps to avoid fragments from Cosmos 2251, which in 2009 was involved in the first known satellite-to-satellite collision with Iridium 33.


Chris Hadfield on getting sick in space


Published on Apr 21, 2013

Sometimes astronauts experience disorientation and nausea when they arrive in space. CSA Astronaut Chris Hadfield demonstrates how astronauts deal with space sickness. Credit: Canadian Space Agency/NASA

For more about living in space, visit:…