Jim Shaylor, age 11, of Hughesville, Pa., for his question:
HOW BIG ARE JUPITER'S MOONS?
Giant Jupiter has a family of at least a dozen moons. They come in assorted sizes and orbit at different distances above the surface of their massive parent planet. Three of them are larger than our moon and two are larger than the planet Mercury. This family of supersatellites is possible because of Jupiter's enormous size and stupendous mass.
A 12th Jovian satellite was discovered as recently as 1951, and possibly there may be others still unfound. It was a very small moon, perhaps less than 10 miles wide, and no doubt other additions also would be very small. The four largest members of Jupiter's family were spotted by the great Galileo in 1610. They were and still are visible through a small telescope that magnifies only about 30 times.
The smallest of these four Galilean satellites is named Europa, which orbits at an estimated distance of 417,000 miles above the planet's surface. Europa's diameter is estimated to be 1,926 miles, which is a mere 234 miles less than the diameter of our own moon.
Next in size among these four satellites comes Io, orbiting at a distance of 263,000 miles. Io's estimated diameter is 2,361 miles, which makes it about 201 miles wider than our own golden moon. Still larger is Callisto, in an enormous orbit at an average distance of 1,169,000 miles above Jupiter's mysterious clouds. Its diameter is 3,231 miles, which is more than 1,000 miles wider than our moon.
Jupiter's fourth orbit is occupied by Ganymede, largest of the Jovian satellites. Its diameter is estimated to be 3,480 miles, which is 1,320 miles wider than our moon. What's more, these two whopping Jovian satellites also outrank the smallest of the sun's planets. Callisto is about 131 miles wider than the planet Mercury and the diameter of Ganymede is about 380 miles greater than the diameter of Mercury.
Of the other eight Jovian moons, one is estimated to be about 100 miles wide and another is about 80 miles wide. One is about 30 miles and three are about 20 or so miles. The final two are midgets, with estimated diameters of 10 miles or so. Numerous moons have been observed and photographed by the planetary spacecraft Voyager I, Voyager II and The Galileo. There is still a lot to learn about the moons of Jupiter.
Satellites, as we know, are held in orbit by a planet's gravity which makes it possible for Jupiter to have the largest family of moons in our solar system. The strength of gravity depends on mass, which is the amount of matter packed into a certain volume. And, after all, giant Jupiter is by far the largest and most massive of the sun's planets.