Callisto

Jupiter · moons · Solar System · space exploration

Giant moon of Jupiter. Callisto is the second-largest moon orbiting Jupiter and the third-largest moon in the entire Solar System. It is nearly as big as the planet Mercury and about a third larger than Earth's Moon.

Discovered long ago. Callisto was one of the four Galilean moons discovered in 1610 when telescopes first let astronomers see the night sky in detail. These four large moons were named after Galileo, the scientist who spotted them.

Visible from Earth. You can actually see Callisto from Earth using just binoculars or a small telescope. It orbits Jupiter at a distance of about 1.883 million kilometers, which is roughly five times farther out than our Moon orbits Earth.

Heavily cratered world. Callisto's surface is covered with impact craters from billions of years of collisions with asteroids and meteorites. Unlike some other moons, Callisto has not been geologically active, so its scars from space remain as a record of the Solar System's early history.

Massive in size. With a diameter of 4,821 kilometers, Callisto is a true giant among moons. It is so large that if you could stand on its surface, you would experience about one-seventh of Earth's gravity.