Jupiter's moon Io is the most volcanically active world in the solar system, with hundreds of volcanoes, some erupting lava fountains dozens of miles (or kilometers) high. Io is caught in a tug-of-war between Jupiter's massive gravity and the smaller but precisely timed pulls from two neighboring moons that orbit farther from Jupiter—Europa and Ganymede.

Io’s volcanoes are at times so powerful that they are seen with large telescopes on Earth, and Io even has lakes of molten silicate lava on its surface. Only slightly larger than Earth’s Moon, Io is about one-quarter the diameter of Earth itself.

We’re still getting to know Io, though it’s powerful volcanoes have captured imaginations since their discovery decades ago. Io plays a memorable role in the sequel to "2001: A Space Odyssey" – "2010: The Year We Make Contact" – in which astronauts make a dangerous spacewalk above Io’s volcanoes to board an abandoned spacecraft.