English: NASA's Galileo spacecraft has found Jupiter's volcanic moon Io to have a huge iron core that takes up half its diameter. The spacecraft's 899 kilometer (559-mile) flyby of Io on December 7, 1995 is depicted in this computer graphics painting. Galileo also detected a large "hole" in Jupiter's magnetic field near Io, leading to speculation about whether Io possesses its own magnetic field. If so, it would be the first planetary moon known to have one. The mission is conducted for NASA by JPL.