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Exploration of Jupiter a European "Jovian Europa Orbiter" mission were also studied, but were superseded by the "Europa Jupiter System Mission". The "Europa Jupiter System Mission" (EJSM) was a joint NASA/ESA proposal for exploration of Jupiter and its moons. In February 2009 it was announced that both space agencies had given this mission priority ahead of the "Titan Saturn System Mission". The proposal included a launch date of around 2020 and consists of the NASA-led "Jupiter Europa Orbiter", and the ESA-led "Jupiter Ganymede Orbiter". ESA's contribution had encountered funding competition from other ESA projects. However, the "Jupiter Europa Orbiter" (JEO), NASA's contribution, was