The pressure exerted on a surface by electromagnetic radiation. As radiation carries momentum as well as energy, it exerts a force when it meets a surface, i.e. the photons transfer momentum when they strike the surface. The pressure is usually negligible on large bodies; for example, the pressure of radiation from the sun on the surface of the earth is of the order of 10−5 pascal. But on small bodies it can have a considerable effect, driving them away from the radiation source. The fact that the tail of a comet always points away from the sun is an example of this. Radiation pressure is also important in stars of very high mass.