Here's why:

* Heat Transfer Mechanisms: Heat transfer occurs through three main mechanisms: conduction, convection, and radiation.

* Conduction and Convection: These methods require a medium (like a solid, liquid, or gas) for heat to transfer. A vacuum has no medium, so conduction and convection are impossible.

* Radiation: Radiation, however, does not require a medium. It involves the transfer of energy through electromagnetic waves. This is how the sun's heat reaches Earth through the vacuum of space.

Therefore, while heat transfer through a vacuum is impossible via conduction or convection, it can occur through radiation.