Beyond Circles and Ellipses

THE first three chapters discussed orbits and the work of space mission analysts. However, there are a variety of complex space missions these days that use trajectories that do not conform to the predominantly circular or elliptical orbits that we have discussed so far. Some of these, such as hyperbolic swing-bys, could be described as ideal trajectories in the sense that they remain under the umbrella of Isaac Newton's theory. However, recalling the hyperbolas briefly discussed in Chapter 1, these trajectories represent open trajectories that in no way resemble circular or elliptic orbits. In addition, some spacecraft operate in places where the gravitational (and other) forces acting upon them do not approximate to Newton's inverse square law. In these cases the spacecraft's orbit may not resemble a circle or an ellipse at all, and can be described as completely non-Keplerian.

In this chapter, we look briefly at some of these unusual trajectories that are becoming more popularly used by spacecraft operators. In particular we look at swing-by maneuvers past planets, orbits around small irregularly shaped bodies such as asteroids or comets, and halo orbits around Lagrangian points.

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