Why does the sky appear dark instead of blue to an astronaut?
The sky appears blue on Earth because the atmosphere contains air molecules and fine particles. These particles are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end.
However, an astronaut is located in space where there is no atmosphere. In the absence of an atmosphere, there are no particles to scatter the sunlight. Therefore, no scattering of light occurs, and the sky appears dark instead of blue to the astronaut.
Explanation
The answer is derived directly from the textbook section '10.6.2 Why is the colour of the clear Sky Blue?'. The text explains that scattering requires a medium (atmosphere). It explicitly states, 'If the earth had no atmosphere, there would not have been any scattering. Then, the sky would have looked dark.' Since an astronaut is in space (no atmosphere), this condition applies.