Question

Why do astronauts on the moon have to communicate by radio even when close to one another?

Answer 1

We all know that sound can only travel through a medium (like air, water, solid or glass) as it can be heard when particles in the air vibrate and pass on the vibration. It can't travel through vacuum. However radio waves can pass through vacuum as well.

The moon has no air and hence no particles to pass the vibrations and so WE CANNOT HEAR SOUND ON THE MOON.

Astronauts have a device which transmit radio waves and be heard by another person. Hence astronauts have to communicate by radio even if they are very very close to each other.

Hope it'll help. :-D