Jupiter and Its Moons

By JONATHAN CORUM

Jupiter’s four largest moons were discovered by Galileo in 1610 and named after mythological lovers of Zeus. NASA’s Juno spacecraft arrived at Jupiter on July 4.


Callisto

Ganymede

Europa

Io

Jupiter
Spin the globe
10,000 MILES

Large craters are shown in roman type and REGIONS in italic caps.

Auroras

Jupiter’s intense magnetic field generates the solar system’s largest and brightest auroras, which are 1,000 times larger than Earth’s Northern Lights. Juno’s polar orbit will allow the spacecraft to study the auroras in detail.

NASA, ESA, J. NICHOLS
Jupiter’s aurora
NASA, ESA

Galileo’s Discovery

Using his improved telescope in 1610, Galileo Galilei spotted four stars in a line near Jupiter.

Siderius Nuncius

Observations over the following weeks convinced Galileo that the stars were moving in orbit around the planet. The four Galilean moons were the first objects discovered to be orbiting another planet. His discovery was published in “Sidereus Nuncius,” the Starry Messenger.

Sidereus Nuncius
HISTORY OF SCIENCE COLLECTIONS, UNIV. OF OKLAHOMA LIBRARIES

Naming the Moons

Galileo called his discovery the Medicean Stars, after his patron. The names we use today were chosen by Simon Marius, a German astronomer who discovered the moons independently around the same time as Galileo. A diagram of the four moons appears over his shoulder in “Mundus Iovialis,” the World of Jupiter.

Simon Marius
HISTORY OF SCIENCE COLLECTIONS, UNIV. OF OKLAHOMA LIBRARIES

Approaching Jupiter

The Juno spacecraft took this image of Jupiter and the four Galilean moons on June 21, from 6.8 million miles away.

Jupiter and moons
Io
Europa
Ganymede
Callisto
NASA; JET PROPULSION LABORATORY; SOUTHWEST RESEARCH INSTITUTE

Sources: NASA; Jet Propulsion Laboratory; Southwest Research Institute. Maps by NASA’s Cassini mission (Jupiter), the U.S. Geological Survey (Io) and Björn Jónsson based on data from the Voyager and Galileo missions (Europa, Ganymede, Callisto).