In vacuum, which mode of heat transfer dominates?

• A. Conduction
• B. Convection
• C. Radiation
• D. Evaporation

Answer: (C)

In vacuum, a heat transfer mechanism must work without any matter present. Conduction needs molecules in contact to pass energy along, and convection requires a fluid to move heat; both disappear without a medium. Radiation, on the other hand, uses electromagnetic waves to carry energy and does not need a surrounding material, so it can move heat across empty space. Objects emit thermal radiation based on their temperature and surface properties, and they can exchange energy with their surroundings through these electromagnetic waves—think how sunlight warms planets and spacecraft in space. Evaporation involves a phase change and latent heat, which is not the general method for transferring heat through a vacuum, so it doesn’t dominate.