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
Jupiter Magnetospheric Orbiter a payload up to 10 kg, equivalent to 2 to 3 scientific instruments. In order to conduct observations never made before, the spacecraft would have carried an X-ray imaging spectroscope, featuring different wavelengths from instruments onboard NASA's Juno probe. In November 2013, Japan's JAXA scientists stated that ″it is difficult for JAXA to launch the outer planet mission by itself to meet the time constraint for cooperative observation with JUICE. Therefore, direct collaboration with European groups/team should be necessary for Japanese scientists to participate in this attractive Jupiter system mission.″ The outcome was the selection of four Japanese instrument development