As you walk away from a campfire on a cool autumn night, you quickly feel colder. The same thing happens in outer space. As it spins, the sun continuously flings hot material into space, out to the furthest reaches of our solar system. This material, called the solar wind, is very hot close to the sun, and we expect it to cool quickly as it streams away. Satellite observations, however, show this is not the case—the solar wind cools as it streams out, but stays hotter than expected. There must be some additional way the solar wind heats up as it travels from the sun to Earth.