Space
Explainer
The cheapest way to change orbits
To reach a higher orbit, you first speed up, then speed up again.
Quick check · tap an answer to continue
Lesson notes
Key points
- A Hohmann transfer moves a spacecraft between two circular orbits using an ellipse that touches both of them.
- The first engine burn at periapsis, the closest point, raises the far side of the orbit to the target height.
- At apoapsis, the farthest point, the craft is moving too slowly to stay up there, so a second burn circularises the orbit.
- Total delta-v, the sum of both speed changes, is near minimal for many transfers, but the trip takes half an orbital period.
Definitions
- Apoapsis
- The point in an orbit farthest from the central body, where orbital speed is lowest.
Takeaway: Hohmann: two prograde burns joined by half an ellipse.
Quick check: Where is the second Hohmann burn performed?
- At periapsis
- At apoapsis
- Halfway along the ellipse
Hohmann transferPeriapsis burnCircularisationDelta-v
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