Final answer:
In 1986, Voyager 2 discovered ten new moons orbiting Uranus during its 'Grand Tour' of the outer solar system.
Step-by-step explanation:
In 1986, Voyager 2 discovered ten new moons orbiting the planet Uranus. This was part of Voyager 2's groundbreaking 'Grand Tour' of the outer planets, which utilized gravity-assisted flybys to alter its trajectory and make it possible to visit Jupiter, Saturn, Uranus, and Neptune in a single mission. This alignment of planets occurs approximately once every 175 years and allows for efficient exploration of the outer solar system. The close-up images and data obtained by Voyager 2 drastically improved our understanding of the outer planets and their moons.