Glossary / Space
Hohmann transfer
Moving between two circular orbits by riding an ellipse tangent to both, with just two burns — a fuel-efficient way to transfer.
In 1925 the German engineer Walter Hohmann showed that an ellipse touching both the starting and target orbits allows a transfer with only two burns, one at each end, and that this is usually the least fuel-hungry route[1].
The cost is time, and departure dates are fixed by the relative positions of the planets. That is why missions to Mars can launch only about every 26 months.
Related terms
- Kepler's lawsThree laws of planetary motion: planets move on ellipses with the Sun at one focus, sweep faster when closer to it, and have orbital periods tied to orbit size.
- Gravity assistUsing a planet's gravity and orbital motion to change a spacecraft's speed or direction without fuel as it flies past.
- Rocket equationThe equation stating that a rocket's gain in speed depends on its exhaust velocity and on the ratio of its starting mass to its burnt-out mass.
On the timeline
Sources
- W. Hohmann, “Die Erreichbarkeit der Himmelskörper,” München: R. Oldenbourg (book) (1925).