Final answer:
The average radius of Mercury's orbit is computed by averaging its perihelion and aphelion distances, giving a semi-major axis of 58 million kilometers or 0.39 AU.
Step-by-step explanation:
To calculate the average radius of Mercury's orbit, we can use the method that takes into account the planet's perihelion and aphelion distances. By definition, the semi-major axis is the average distance from the planet to the Sun over one orbit, which, for elliptical orbits, is the arithmetic mean of the perihelion and aphelion distances.
Given the perihelion distance of 46 million kilometers and the aphelion distance of 70 million kilometers, the semi-major axis a can be calculated as follows:
a = (46 million kilometers + 70 million kilometers) / 2
a = 58 million kilometers or 0.39 AU
Therefore, the average radius of Mercury's orbit, which is the same as the semi-major axis, is 58 million kilometers or 0.39 AU.