It isn’t. Satellites and space shuttles do indeed get cold up there, but it’s not because it’s cold up there.
First of all, there’s really no such thing as cold, no matter what the penguins tell you. Cold is a linguistic concept, not a scientific one. Our caveperson ancestors needed a word for “not hot,” and “cold” (or its grunt equivalent) is what they came up with. It’s like light and dark, wet and dry. Light and water are tangible things, but dark and dry denote the lack of light and water. They’re negative adjectives, if grammarians will permit me.
Heat is energy. It’s the energy that an object’s molecules have by virtue of the fact that they’re in motion. Why are they in motion? Because around 12 billion years ago an incomprehensible amount of energy arose in the void (or whatever) via the Big Bang, that mind-boggling blast that scientists believe ignited the universe, and all the atoms are still quivering. Some, the hotter ones, are quivering more than others; we refer to those others as colder.
Some forty years ago, when we left the cuddly atmosphere of our native planet to venture into the vast beyond, we encountered for the first time an environment in which there is no heat to compare anything with because there are no (or precious few) molecules up there to quiver, and the word “cold” became even more meaningless. Space can be neither hot nor cold in any sense of the words, because it is empty of matter.
Then why do satellites and spacecraft get so … frigid? Parts of NASA’s space shuttle do get down to a couple of hundred degrees below zero Fahrenheit (around -130 degrees Celsius).
Here’s what’s happening. A space shuttle or any other object can gain or lose heat not only by being in contact with stuff that’s hotter or colder, and that’s out because there’s no stuff up there, but also by radiation. The sun and stars are putting out all sorts of radiation, waves of pure energy, both visible to the human eye (light) and invisible (ultraviolet, infrared and others).
This radiation travels through space without being diminished because there’s nothing there to absorb it. But when it strikes an object, for example a space shuttle, some of it will bounce off and continue on its way in a different direction. Some of it will be absorbed, however, and its energy will dissipate into heat. Thus, the space shuttle is receiving radiated heat from the sun and stars. The sun, of course, is by far the chief heat radiator because it is so much closer than the other stars.
But at the same time the shuttle, still carrying its burden of earthly warmth, is radiating some of its own energy away, because anything that has any warmth at all sends out infrared radiation, “heat radiation”. That’s how night-vision devices can “see” people in the dark: by the infrared radiation they’re sending out. And that’s how old-fashioned radiators work: They radiate heat into the room, rather than blowing hot air around the house.
The shuttle, then, is receiving lots of radiated heat on the side facing the sun while radiating heat rapidly away on the other side, which then gets exceedingly cold.
Note, then, that the shuttle itself can be said to be cold because it is a real object, but the environment it is flying through is not cold, either semantically or physically.
It’s not cold in outer space.