Magnetosphere of Jupiter By 1973 the magnetic moment was known within a factor of two, whereas the tilt was correctly estimated at about 10°. The modulation of Jupiter's DAM by Io (the so-called Io-DAM) was discovered in 1964, and allowed Jupiter's rotation period to be precisely determined. The definitive discovery of the Jovian magnetic field occurred in December 1973, when the Pioneer 10 spacecraft flew near the planet. As of 2009 a total of eight spacecraft have flown around Jupiter and all have contributed to the present knowledge of the Jovian magnetosphere. The first space probe to reach Jupiter was Pioneer 10 in
Exploration of Jupiter is the amount of radiation to which a space probe is subjected, due to the harsh charged-particle environment around Jupiter (for a detailed explanation see Magnetosphere of Jupiter). For example, when "Pioneer 11" made its closest approach to the planet, the level of radiation was ten times more powerful than "Pioneer"s designers had predicted, leading to fears that the probes would not survive. With a few minor glitches, the probe managed to pass through the radiation belts, but it lost most of the images of the moon Io, as the radiation had caused "Pioneer"<nowiki>'</nowiki>s imaging photo polarimeter to receive false