Overview

Galileo's discovery | Big moons | Small moons

Jupiter has at least 16 moons. The four largest -- Io, Europa, Ganymede and Callisto, shown here from top to bottom -- are called the Galilean moons, after their discoverer. In this collage of photos, the moons are shown in their relative sizes. (This image also shows Jupiter's Red Spot.)

Galileo's discovery

Galileo Galilei aimed a crude device with glass lenses mounted in a pipe organ -- it wasn't even called a telescope yet -- toward Jupiter in 1610. He saw three points of light around the planet and assumed they were distant stars.

As Galileo continued watching the "stars" each night, he noticed that they moved, appearing sometimes on one side of the planet, sometimes on the other. And the number of them changed. Soon, Galileo realized he was seeing four moons orbiting Jupiter. The names were given later.

The four big moons

All four Galilean satellites orbit Jupiter in the same plane, called the equatorial plane. In size and mass, Io and Europa resemble our own Moon. Ganymede and Callisto are similar to Mercury.

Each of the moons orbits in a near circle, and like Earth's Moon, they all keep the same face pointed toward their home planet at all times.

Explore Io, Europa, Ganymede or Callisto

The smaller moons

Four smaller moons -- named Adrastea, Metis, Thebe and Amalthea -- orbit very close to Jupiter and in the same plane as the Galilean moons. Three of these (all but Amalthea) are so small and close to Jupiter that they weren't discovered until 1979 when the Voyager spacecraft spotted them.

Even Amalthea, an irregular-shaped object that looks a little like an asteroid, is difficult to spot, partly because the reflected sunlight from Jupiter overpowers the scant light reflected from this tiny moon.

Eight other satellites, as small as 5 miles across, circle Jupiter in irregular orbits inclined to the equatorial plane (explore more about inclination), bringing the total number of known moons to 16.

Four of these -- Leda, Himalia, Lysithea and Elara -- orbit in the same direction of the others. But another group, the four that are furthest from Jupiter, orbit in the opposite direction. They are Ananke, Carme, Sinope and Pasiphae. ez

Explore Io, Europa, Ganymede or Callisto

Explore Jupiter

Explore Why Planets (and moons) Orbit

Photographs: NASA
Graphics: explorezone.com
Text by Robert Roy Britt

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