Mechanical waves cannot travel through vacuum. Mechanical waves require a medium to propagate, such as air, water, or solids, to transmit the mechanical energy from one point to another.

In a vacuum, there are no particles or molecules present to transmit the energy, so mechanical waves cannot propagate. Therefore, sound waves, which are mechanical waves, cannot be heard in space. Sound waves require air molecules to propagate, so they dissipate in the absence of air, such as in the vacuum of space.