Jupiter is the most massive planet in our solar system; with four large moons and many smaller moons it forms a kind of miniature solar system. In fact, Jupiter resembles a star in composition, and if it had been about 80 times more massive, it would have become a star rather than a planet.
On 7 January 1610, using his primitive telescope, astronomer Galileo Galilei saw four small "stars" near Jupiter. He had discovered Jupiter's four largest moons, now called Io, Europa, Ganymede, and Callisto. These four moons are known today as the Galilean satellites.
Newly discovered moons of Jupiter are reported by astronomers and acknowledged with a temporary designation by the International Astronomical Union; once their orbits are confirmed, they are included in Jupiter's large moon count. Not including the "temporary" moons, Jupiter has 50 total.
Galileo would be astonished at what we have learned about Jupiter and its moons, largely from the NASA mission named after him. Io is the most volcanically active body in our solar system. Ganymede is the largest planetary moon and the only moon in the solar system known to have its own magnetic field. A liquid ocean may lie beneath the frozen crust of Europa, and icy oceans may also lie beneath the crusts of Callisto and Ganymede. Jupiter's appearance is a tapestry of beautiful colors and atmospheric features. Most visible clouds are composed of ammonia. Water vapor exists deep below and can sometimes be seen through clear spots in the clouds. The planet's "stripes" are dark belts and light zones created by strong east-west winds in Jupiter's upper atmosphere. Dynamic storm systems rage on Jupiter. The Great Red Spot, a giant spinning storm, has been observed since the 1800s. In recent years, three storms merged to form the Little Red Spot, about half the size of the Great Red Spot.
The composition of Jupiter's atmosphere is similar to that of the sun -- mostly hydrogen and helium. Deep in the atmosphere, the pressure and temperature increase, compressing the hydrogen gas into a liquid. At depths of about a third of the way down, the hydrogen becomes metallic and electrically conducting. In this metallic layer, Jupiter's powerful magnetic field is generated by electrical currents driven by Jupiter's fast rotation. At the center, the immense pressure may support a solid core of rock about the size of Earth.
Jupiter's enormous magnetic field is nearly 20,000 times as powerful as Earth's. Trapped within Jupiter's magnetosphere (the area in which magnetic field lines encircle the planet from pole to pole) are swarms of charged particles. Jupiter's rings and moons are embedded in an intense radiation belt of electrons and ions trapped by the magnetic field. The Jovian magnetosphere, comprising these particles and fields, balloons 1 to 3 million km (600,000 to 2 million miles) toward the sun and tapers into a windsock-shaped tail extending more than 1 billion km (600 million miles) behind Jupiter as far as Saturn's orbit.
Discovered in 1979 by NASA's Voyager 1 spacecraft, Jupiter's rings were a surprise: a flattened main ring and an inner cloud-like ring, called the halo, are both composed of small, dark particles. A third ring, known as the gossamer ring because of its transparency, is actually three rings of microscopic debris from three small moons: Amalthea, Thebe and Adrastea. Data from the Galileo spacecraft indicate that Jupiter's ring system may be formed by dust kicked up as interplanetary meteoroids smash into the giant planet's four small inner moons. The main ring probably is composed of material from the moon Metis. Jupiter's rings are more easily visible when backlit by the sun but have been captured by Hubble Space Telescope images.
In December 1995, NASA's Galileo spacecraft dropped a probe into Jupiter's atmosphere, which made the first direct measurements of the planet's atmosphere. The spacecraft then began a multiyear study of Jupiter and the largest moons. As Galileo began its 29th orbit, the Cassini-Huygens spacecraft was nearing Jupiter for a gravity-assist maneuver on the way to Saturn. The two spacecraft made simultaneous observations of the magnetosphere, solar wind, rings, and Jupiter's auroras.
NASA launched a mission named Juno in 2011 to conduct an in-depth study of Jupiter from a polar orbit. Juno will examine Jupiter's chemistry, atmosphere, interior structure, and magnetosphere.
How Jupiter Got its Name
The largest and most massive of the planets was named Zeus by the Greeks and Jupiter by the Romans; he was the most important deity in both pantheons.
- 1610: Galileo Galilei makes the first detailed observations of Jupiter.
- 1973: Pioneer 10 becomes the first spacecraft to cross the asteroid belt and fly past Jupiter.
- 1979: Voyager 1 and 2 discover Jupiter's faint rings, several new moons and volcanic activity on Io's surface.
- 1994: Astronomers observe as pieces of comet Shoemaker-Levy 9 collide with Jupiter's southern hemisphere.
- 1995-2003: The Galileo spacecraft drops a probe into Jupiter's atmosphere and conducts extended observations of Jupiter and its moons and rings.
- 2007: Images by NASA's New Horizons spacecraft, on the way to Pluto, show new perspectives on Jupiter's atmospheric storms, the rings, volcanic Io, and icy Europa.
- 2009: On July 20, almost exactly 15 years after fragments of comet Shoemaker-Levy slammed into Jupiter, a comet or asteroid crashes into the giant planet's southern hemisphere.
- 2011: Juno launches to examine Jupiter's chemistry, atmosphere, interior structure, and magnetosphere.