Jupiter's Moons

"Jupiter's Moons" with Jane Houston Jones at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California.


Jupiter has 63 confirmed moons, giving it the largest retinue of moons with "reasonably secure" orbits of any planet in the Solar System.

The most massive of them, the four Galilean moons, were discovered in 1610 by Galileo Galilei and were the first objects found to orbit a body that was neither Earth nor the Sun.

From the end of the 19th century, dozens of much smaller Jovian moons have been discovered and have received the names of lovers, conquests, or daughters of the Roman god Jupiter, or his Greek equivalent, Zeus.

The Galileans are far and away the largest objects in orbit around Jupiter, with the remaining 59 moons and the rings together comprising just 0.003 percent of the total orbiting mass.

Jupiters largest moons were first seen 400 years ago in early 1610. On the seventh of January, 1610 in Padua, Italy, Galileo looked up above the constellation Orion. He aimed his telescope at the well-known starry wanderer, the planet Jupiter, which was near Orion that night.

What he saw through his telescope startled him and marked the beginning of modern astronomy.Jupiter was not just one object, as he wrote and drew in his journal. There are three stars in the heavens moving about Jupiter, as Venus and Mercury around the sun, he wrote.

Galileos January 7 observation showed three stars. The one star to the west was Ganymede. And to the east there were two objects.One was the moon Callisto. And the other was a tight pairing of Io and Europa. Io and Europa appeared so close together they looked like one object in Galileos modest telescopic view.

On January 8 he saw a different lineup altogether. There were three stars on one side of the planet. Io was the moon closest to the planet, followed by Europa and Ganymede. Two cloudy nights and two additional observations later, on January 13 Galileo identified a fourth object orbiting Jupiter.

The arrangement this night turned out to be Europa on the east and Ganymede, Io and Callisto on the west. On January 15 all four stars were seen on one side of the planet. Everyone who aims a modest telescope, or even binoculars, at Jupiter will see the same view that Galileo did.

The views of tiny moons orbiting the king of the planets will surprise and delight all who look up. [ Jupiter is hard to see in the evening sky this month. But northern hemisphere observers may see Jupiter and Venus close together, low on the southwestern horizon, on Valentines Day.

Then it will be a few months wait until Jupiter becomes visible in the morning sky. By August you can once again view Jupiter and the four Galilean moons after dinner or as soon as the sun sets and the stars come out. NASAs Galileo Mission, which ended in 2003, changed the way we look at our solar system.

It found evidence of subsurface saltwater on Europa, Ganymede and Callisto and intense volcanic activity on Io. NASAs JUNO Mission will launch in 2011 on a mission to study Jupiter. And the Europa-Jupiter System Mission, a joint mission of the European Space Agency and NASA, is slated to launch in 2020.It will primarily study Jupiters moons Europa and Ganymede and Jupiters magnetosphere.

• www.nasa.gov