At its closest approach, a particular comet is about half an AU away from the Sun, and it's about 40 AU away at its farthest point. The semi-major axis of the comet's orbit is about

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Quizplus · Accepted Answer

The semi-major axis of an orbit is the average of the closest and farthest distances from the Sun. Therefore, (0.5 AU + 40 AU) / 2 = 20.25 AU, which is approximately 20 AU.

At Its Closest Approach, a Particular Comet Is About Half