Hohmann Transfer Orbit

A Hohmann transfer orbit is a method of getting a spacecraft or satellite from one circular orbit to a lower or higher circular orbit. It involves using thrusters at particular points in the orbits to either speed up or slow down the craft.

The reasoning behind using this type of orbit is that for circular orbits, the speed of the craft does not change as it is a constant distance from the center of the orbit (the center of the planet or moon that it is orbiting). A Hohmann transfer orbit is a semi-elliptical orbit with the focus of the ellipse at the center of the system.

To go from a lower orbit to a higher orbit, the spacecraft must increase its orbital speed. This increase will put it on the elliptical orbit that will transfer the craft to the higher orbit. Once the craft reaches the desired orbit, the craft must then again fire its engine to speed the craft up to the correct speed for the orbit, putting it into a new circular (higher) orbit. This is the method in which spacecraft dock with the International Space Station.

The reverse also works, but in this case, the craft's engines slow it down to get to the lower orbit, again slowing the craft at both the higher orbit (to transfer) and the lower orbit (to insert it into the new orbit).

In the picture below, the blue orbit is the lower circular orbit, the yellow/green dashed path is the Hohmann transfer orbit, and the red orbit is the higher orbit. At point A, the engines are fired to increase the speed of the craft and at point B, they are fired again to insert it into orbit. To transfer from the higher to the lower, the engines are retro-fired at B to slow the craft down and again at A to insert it into the lower orbit.