Question

How is the semi-major axis found if perihelion and aphelion distance are known?

For example, if an asteroid has a perihelion distance of 2.0 and an aphelion distance of 4.0 AU, what is the semi-major axis?

Answer 1

The semi major axis distance is half the sum of the perihelion and aphelion distances.

Explanation:

Given that the orbit is an ellipse with semi major axis a and eccentricity e, then the aphelion distance is `a(1-e)` and the aphelion distance is `a(1+e)`. Adding the two together gives `2a`.
Therefor the semi major axis distance is half the sum of the perihelion and aphelion distances.