When the actor William Shatner returned to space, he said it had reminded him of death.
“I saw a cold, dark, black emptiness. It was unlike any blackness you can see or feel on Earth. It was deep, enveloping, all-encompassing,” Shatner said. “I turned back toward the light of home. I could see the curvature of Earth, the beige of the desert, the white of the clouds and the blue of the sky. It was life.”
Space is always dark. Despite being relatively close to the Sun, it’s profoundly black. That’s how directors portrayed it in cinema long before Star Trek, and that’s what we’ve seen in countless launches and space walks. Why doesn’t the Sun illuminate space? A child once asked astronauts on the International Space Station the same question several years ago.
It’s a good question. This is the answer.
An Intense Source of Light
The Sun, like any star, is an intense source of light. It liberates much of its energy through electromagnetic radiation, which it emits in all directions in circular form.
Most of the radiation it emits is visible, such as a stream of light. We know that the intensity of solar light decreases based on the square of the distance. In other words, an object that’s twice as far from the Sun compared to Earth will receive one-fourth of its light. This is because the Sun projects a finite stream of light over a continuously growing spherical area. As such, Mars receives less light than Earth but more than Saturn, which is even further away.
Let’s look at it another way. Although light decreases rapidly with distance, the interplanetary space near Earth receives a stream of radiation like the one our planet receives. The Sun does shine light in space, but it doesn’t illuminate uniformly like it does with the sky on Earth.
The Void Doesn’t Reflect Light
To understand why the Sun doesn’t illustrate space, all you have to do is turn on a light bulb in an open area and another in a room painted white. The light bulb barely manages to illuminate a small circle of its surroundings in the open space but manages to light up the closed room in a more or less uniform way.
Illuminating a space doesn’t only require a source of light, it also needs something to disperse or reflect it, such as the white walls of a room. A better example would be to point a flashlight to the sky. If there’s a lot of humidity, you can see the light beam reach a certain distance. However, if there’s not a lot of humidity, it’ll be hard to see any change.
On the side of Earth that’s illuminated, the light disperses uniformly through the atmosphere, which is made up of small particles. Space, in turn, is practically empty: It doesn’t have particles to disperse light. There’s light there, but there’s nothing to disperse or reflect it to illuminate space like the Sun does on Earth.
It’s the same reason why the sky on Earth is blue, the sky on Mars is a gray-ish red, and why the Moon doesn’t have a sky. Distinct atmospheres, or the absence of them, disperse light in different ways (or don’t disperse it). In space there’s a lot of light, but that light is only visible when you look at its source or at an object that reflects it.
Images | NASA
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