Keith Hansen, age 9, of St. Paul, Minnesota, for his question:

Which is the biggest satellite in the Solar System?

As satellites go, our moon is a big one. But four other satellites in the Solar System are bigger. They belong to the biggest planets, Jupiter and Saturn. Ganymede, Callisto and Io are Jupiter's largest moons, all three of them larger than the moon. Titan is Saturn's largest satellite and it too is larger than our moon. The biggest of them all is Ganymede. Latest measurements give its diameter as 3,480 miles    which is 1,320 miles wider than our moon. This largest satellite in the Solar System is 660,000 miles from Jupiter, the biggest planet in the Solar System.

Callisto's diameter is 3,231 miles. It is more than a million miles from giant Jupiter. Io is closer to Jupiter. Its diameter is 2,361 miles which is only 201 miles wider than our moon. Saturn's moon Titan is 3,100 miles wide, or about 940 miles wider than our moon. These four bigger than the moon satellites are very, very far away. Measuring their exact sizes was not easy. Some years ago, astronomers thought that Callisto was the largest satellite in the Solar System. But at present, as far as we know, Jupiter's G anymede rates as the largest satellite in our Solar System.