Jupiter is the largest of the planets. It is more than twice as large as all the other planets combined. It is the fifth planet from the Sun. It is also the fourth brightest object in the sky (after the Sun, the Moon, and Venus). It was known in prehistoric times as the wandering star. In 1610, Galileo discovered Jupiter’s four large moons – Io, Europa, Ganymede and Callisto, known as the Galilean moons, and recorded their motions back and forth around Jupiter. They are visible today by telescope.
Galileo’s outspoken support of Copernicus’ theory gave him trouble with the Inquisition. Today, anyone can voice Galileo’s opinions without fear of retribution.
Jupiter has features very different from terrestrial planets. Remember that terrestrial planets are primarily composed of silicate rock. Jupiter’s composition is more like that of the stars. If it has any solid surface, it is hidden deep at its center. It is apparently composed entirely of gas and liquid. It also has an internal energy source and enormous magnetic fields.
Jupiter was first visited by Pioneer 10 in 1973, by Pioneer 11 in 1974, by Voyager 1 in 1979, and also by Voyager 2 in 1979. The spacecraft Galileo orbited Jupiter for eight years. It is still being observed regularly by the Hubble Space Telescope.
Jupiter is about 90% hydrogen and 10% helium. Data from Galileo indicate that there is much less water than expected.
The Great Red Spot has been seen by observers for more than 300 years. It is actually a swirling mass of clouds that are higher and cooler than surrounding ones. It is like a great hurricane, caused by tremendous winds that develop above the fast spinning planet. The winds blow counterclockwise at about 250 miles per hour. It is just one of several storms that rage on Jupiter.
Jupiter has been found to have 63 moons. When it is in the nighttime sky, Jupiter is often the brightest star in the sky, second only to Venus, which is seldom seen in a dark sky. Jupiter also has rings like Saturn. The four rings – Metis, Adrastea, Thebe, and Amalthea – are constantly being struck by meteoroid collisions that show Jupiter’s rings with debris and dust.
Aside from Mars, Jupiter’s system is the most likely place in the solar system to support life. The planet itself cannot support life, but the four moons discovered by Galileo offer some hope. Europe, particularly, is a prime candidate to support life. Also, Callisto and Ganymede offer an environment suitable for future human colonization.
The Romans named the planet after the Roman god Jupiter. NASA is planning a mission to study Jupiter in detail from a polar orbit. Named Juno, the spacecraft is planned to launch by 2011.