the most massive of the moons are the four galilean moons, which were independently discovered in 1610 by galileo galilei and simon marius. they were the first objects found to orbit a body that was neither earth nor the sun. the galilean moons are by far the largest and most massive objects to orbit jupiter. the other 75 known moons and the rings together make up just 0.003% of the total orbiting mass. the four are io, europa, ganymede and callisto. they are roughly the same size as earth's moon, some are a bit bigger, some are smaller.