Beyond the region of the minor planets is Jupiter, the largest planet in the Solar System. Like the other gaseous planets, Jupiter does not have a solid surface. When Jupiter is looked at through even a small telescope cloud belts can be seen crossing the planet. There is also the famous great Red Spot which is thought to be a gigantic storm taking place in Jupiter's atmosphere. Jupiter has a very short day, taking less than ten hours to spin once on its axis. There are a total of sixteen moons in orbit around Jupiter. The four largest of these can be seen through binoculars or a small telescope. One of these is Io, which has a large number of active volcanoes on its surface. Callisto, another of the satellites, has a surface which is almost completely covered in craters.