An object 'm' in the gravitational field of another object 'M' can move on a bound (orbit) or unbound trajectory. Bound and unbound trajectories can be described by conic sections, and calculated using the concept of conservation of energy.

Conic Sections

In physics, the paths that can be taken by an object under gravitational attraction are called the conic sections. In mathematics, these curves are obtained by taking a slice from a cone at different angles. These shapes are the circle, the ellipse, the parabola and the hyperbola. The circle and ellipse are bound orbits (like planets around the sun), and the parabola and hyperbola are unbound (like a deflecting rocketship on a slingshot orbit).

Figure 1: Illustration of the four different conic sections.

Calculating Bound and Unbound Trajectories

To determine whether an object with mass 'm' will follow a bound or unbound trajectory, it is useful to apply conservation of energy and calculate the kinetic and gravitational potential energy of the object in the gravitational field of mass 'M' .

E_kinetic > E_potential : Object 'm' has enough energy to escape the gravitational pull of object 'M' and follows an unbound orbit, escaping to infinity.

E_kinetic < E_potential : The velocity of object 'm' is too small to escape the gravitational attraction of object 'M' and is stuck in freefall around object 'M'.

The kinetic energy E_kinetic and the gravitational potential energy E_potential of an object with mass 'm' in the gravitational field of mass 'M' can be described by equations in the escape velocity theory page.