Understanding Apoapsis From Orbit
Apoapsis is the farthest point of an elliptical orbit. It is measured from the center of the main body. Many users also need apoapsis altitude. That value subtracts the body radius from the apoapsis radius. This calculator supports both views. It can work from semi-major axis and eccentricity. It can also work from periapsis and eccentricity. A third path uses semi-major axis with periapsis data.
Why It Matters
Apoapsis helps describe orbit size and mission reach. A satellite with a high apoapsis spends more time far from the body. That changes speed, lighting, communication range, and observation windows. In games, classroom tasks, and planning notes, the same geometry applies. The calculator keeps the main values together, so the orbit can be checked quickly.
Inputs And Units
The body radius is important when altitude is used. Earth, Moon, Mars, Jupiter, and Sun presets fill common values. A custom option lets you enter another radius. Distances may be entered in kilometers, meters, or miles. The tool converts them internally. The output unit can also be changed. This helps compare results without rewriting the inputs.
Reading The Results
Apoapsis radius is the distance from the center of the body. Apoapsis altitude is the height above the surface. Periapsis radius shows the nearest orbital distance. The semi-major axis is the average of apoapsis and periapsis radii. Eccentricity shows the orbit shape. Values near zero are almost circular. Values close to one are very stretched.
Advanced Notes
When a gravitational parameter is available, the calculator also estimates orbital period and speeds. These numbers come from standard two-body equations. They ignore drag, thrust, uneven gravity, and third-body forces. Therefore they are best for ideal orbit study. Real mission design should add safety margins and detailed simulation. Use exported CSV or PDF files to save results.
Good Practice
Check that eccentricity is below one for a closed ellipse. Use positive distances only. Confirm whether your periapsis value is a radius or an altitude. Mixing those two ideas creates large errors. If the apoapsis altitude is negative, the path intersects the body. That means the orbit is not physically clear. Record every unit choice, because later reviews depend on consistent distance labels and notes.