Moons of Uranus from Uranus outward, and this finally stuck. In 1852, Herschel's son John Herschel gave the four then-known moons their names. No other discoveries were made for almost another century. In 1948, Gerard Kuiper at the McDonald Observatory discovered the smallest and the last of the five large, spherical moons, Miranda. Decades later, the flyby of the "Voyager 2" space probe in January 1986 led to the discovery of ten further inner moons. Another satellite, Perdita, was discovered in 1999 after studying old "Voyager" photographs. Uranus was the last giant planet without any known irregular moons, but since 1997 nine distant
Stability of the Solar System essentially the distance from sun to earth). These are now known as the Kirkwood gaps. Some asteroids were later discovered to orbit in these gaps, but their orbits are unstable and they will eventually break out of the resonance due to close encounters with a major planet. Another common form of resonance in the Solar System is spin–orbit resonance, where the period of spin (the time it takes the planet or moon to rotate once about its axis) has a simple numerical relationship with its orbital period. An example is our own Moon, which is in a 1:1 spin–orbit resonance