Voyager 2 reached Jupiter on July 9, 1979, after Voyager 1

Voyager 2 made its closest approach to Jupiter on July 9, 1979, passing within about 350,000 miles of the planet’s cloud tops, four months after its twin, Voyager 1.

Why “number 2” arrived second

The numbering trips people up. Voyager 2 actually launched first, on August 20, 1977, about two weeks before Voyager 1, which rode a faster trajectory, overtook it and reached Jupiter in March 1979. Neither was Jupiter’s first visitor: Pioneer 10 and 11 had flown past in 1973 and 1974, but the Voyagers carried more sophisticated instruments for deeper study.

Voyager 2 also made the trip on a spare radio. In April 1978 its primary receiver failed, and it has run on its backup receiver ever since.

On the way in it flew past four of Jupiter’s moons, coming within about 38,600 miles of Ganymede and 127,900 miles of Europa. Europa’s images showed extensive irregular markings, possibly the sign of a frozen crust floating on a deep ocean. On the way out it ran a 10-hour volcano watch of Io, confirming Voyager 1’s discovery that the moon has active volcanoes.

Looking back at the sunlit-from-behind planet, it confirmed Jupiter’s thin ring, showed the Great Red Spot to be a complex storm system, and found a new moon just outside the rings, later named Adrastea. By the time observations ended on August 5, it had sent home about 17,000 images.

Jupiter’s gravity then bent its path toward Saturn, and from there on to Uranus and Neptune, exploiting a planetary alignment that recurs only once every 175 years. Voyager 2 is still the only spacecraft to have visited those two ice giants, and on November 5, 2018 it crossed the heliopause into interstellar space.