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Io is the innermost of the Galilean moons orbiting Jupiter. Io is on average around 400 000 km away from Jupiter, which is approximately the same distance as our moon is from Earth. Jupiter looks 37 times larger than our full moon does in the night sky. Io faces great tidal forces from Jupiter, and the other three Galilean moons Europa, Ganymede and Callisto, which are tidally locked with respect to each other. At some places the tidal forces move the surface by as much as 100 meters up and down. This twists the moon inside out, but it is not because of this that the moon is the second hottest place (1 700 degrees C) in the solar system, next to the sun. Io's surface is being constantly bombarded by electromagnetic radiation from Jupiter. This radiation rips off about 1 000 kg of sulphur and other materials from the surface each second. This material (plasma) creates a physical bond between Jupiter and Io. As Io moves through Jupiter's magnetic field, it acts as a giant generator, inducing 400 000 volts and more than 3 000 000 amperes across Io's surface. Because of this bond, massive electrical discharges have been observed in Jupiter's atmosphere. More than 400 active volcanoes have been observed on Io, and the moon is known to change its' physical appearance over short periods (few years). An impact by an asteroid is seen on Jupiter's night side.