why does the sky appear dark instead of blue to an astronaut

The sky appears dark to an astronaut because there is essentially no atmosphere in space to scatter sunlight, so almost no light is redirected toward their eyes from the surrounding “sky.”

On Earth: Why the sky looks blue

On Earth, sunlight passes through a thick blanket of air filled with molecules and tiny particles. These very small particles scatter shorter-wavelength light (like blue and violet) much more than longer wavelengths, a process called Rayleigh scattering. The scattered blue light comes to us from all directions, so when you look up during the day, you see a bright blue dome rather than the blackness of space.

In space: What’s different for an astronaut

Once you leave the atmosphere, that blanket of air and particles is essentially gone. With no air, there is nothing around to scatter sunlight into your eyes from the sides, above, or “behind” you. As a result, you see the Sun as an intense, bright source against a background that is almost completely black, even when you are in full daylight in orbit.

Why it still looks bright near Earth

An astronaut in low Earth orbit still feels sunlight and sees Earth brightly lit, because the Sun’s rays are directly hitting objects and reflecting off them. The blue disk of Earth is visible because its atmosphere is still scattering sunlight, so Earth itself looks blue and bright from above. But when the astronaut looks away from the planet and the Sun, there is no surrounding air to glow, so the rest of the sky appears as deep black space.

One-line summary

To sum up: the sky is blue on Earth because our atmosphere scatters sunlight, but to an astronaut in (nearly) empty space, with no atmosphere to scatter light, the sky has nothing to “glow,” so it appears dark instead of blue.