A double asteroid is a system consisting of two celestial bodies that orbit each other. The primary body is usually larger, while the secondary body, or moonlet, is smaller. These systems can provide valuable insights into the formation and evolution of asteroids in our solar system. One well-known example of a double asteroid is Didymos, which has a smaller companion called Dimorphos. Scientists study double asteroids to understand their dynamics and potential impact on Earth, as well as to explore their composition and structure for future space missions.