The non-gravitational perturbations arise because outer space is not empty. First, planetary atmospheres extend to large altitudes, where they can be thin enough to allow for a satellite orbit but still generate a significant aerodynamic drag, given the high relative velocity of the spacecraft. As will be discussed in Chapter 16, there is interest in satellites orbiting as low as possible to determine high-order harmonics of the planetary gravity field. This may require propulsion to compensate for orbital decay and/or the use of on-board accelerometers to measure the amount of drag; that is, non-gravitational perturbations are critical in the design phase of the mission.

Second, outer space is pervaded by electromagnetic radiation: the light arriving directly from the Sun, reflected by the Earth, and by the other planets. The photons exchange momentum with spacecraft when they are absorbed and reflected; spacecraft themselves emit infrared radiation and electromagnetic waves carrying away some momentum. The resulting accelerations are small, but at the level of accuracy of current tracking systems they are not negligible, hence the need to model and/or measure them. Even small natural bodies, such as asteroids with diameters in the km range, have orbits affected by non-gravitational perturbations in a measurable way.

This chapter cannot be a full revision of the textbook by Milani et al. (1987), which should take into account all the new results accumulated in more than 20 years since its publication.