The spacecraft that have visited the outer planets have all flown close to inner planets (Venus, for example), often more than once, as part of their journeys. What was the primary reason for this?

1) The outer planets happened to have been on the far side of the sun, so the shortest route passes the inner planets

Explanation:

All the planets revolve around the sun in a circular orbital. The orbits of Mercury, Venus, Earth, and Mars are tightly held with sun as they are very close to the sun and revolve faster in comparison to outer planet Jupiter, Saturn, Uranus, and Neptune.

Because of the shortest distance between the sun and inner planets, the spacecraft is able to visit the inner planet more than once while outer planets are very far from the sun and revolve around a large orbit, so the spacecraft visited the outer planets less than the inner planet.

Hence, the correct answer is "1".

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