Pioneer

A polar projection of Pioneer 10's route through the Jovian system, contrived so that it would be occulted by Io.

try a gravitational slingshot to send Pioneer 11 on to Saturn. The slingshot technique was demonstrated on 5 February 1974 by Mariner 10, which used a close fly-by of Venus to become the first - and so far only - mission to Mercury, the innermost of the planets.

Although Saturn orbits the Sun farther out than Jupiter, it would be on the far side of the Solar System when Pioneer 11 reached Jupiter in 1974. To reach Saturn, the vehicle would have to fly sufficiently close for the tremendous gravitational field to virtually double the trajectory back on itself to redirect the spacecraft right across the Solar System. Such a deflection would involve making a much closer fly-by than before, but it was evident from the intensity of the radiation close to Jupiter that the spacecraft would not survive such a deep penetration. However, the charged particle radiation was concentrated in the equatorial zone, and Pioneer 10 had suffered such a dosage because its trajectory was near the Jovian system's equatorial plane. Ames therefore decided to have Pioneer 11 pass 'beneath' the most intense radiation on the near side of the planet, skim over the south pole and be deflected around the far side so that it would race through the radiation in the equatorial belt on the far side on a steeply inclined trajectory. If all went well, Pioneer 11 would be subjected to a lower dosage than its predecessor. As a bonus, this fly-by would also provide a different perspective on the inner magnetosphere. The spacecraft would emerge in an elliptical solar orbit that was steeply inclined to the ecliptic and would meet Saturn five years later. Without Pioneer 10's reconnaissance, the navigational requirements for such

The 'spin-scan' photopolarimeters of Pioneer 10 (left) and Pioneer 11 (right) built up images of Jupiter's disk, revealing its atmospheric structures in unprecedented detail.

an ambitious two-planet mission would not have been practicable, so this was an excellent use of the notional 'backup' spacecraft.

On 2 December 1974, as Pioneer 11 skimmed a mere 43,000 kilometres over the Jovian cloud tops, its imagery not only documented the high-latitude zones, but the closer fly-by gave an even better view of the intricate structure of the atmosphere.

The navigation on the way in had been excellent, and the spacecraft, now dubbed 'Pioneer Saturn', emerged on course for Saturn. Its particles and fields instruments reported on the state of the solar wind during the cruise through uncharted territory far above the ecliptic. In early 1976, some 150 million kilometres 'above' the orbit of Mars, the trajectory was at its greatest departure from the ecliptic. This information was a significant secondary objective as all the previous spacecraft in the series had sampled the wind in the plane of the ecliptic. In fact, this part of the mission was to stimulate a proposal to fly a pair of particles and fields spacecraft out to Jupiter on trajectories that would send them over opposite poles, to emerge in emerge in elliptical solar orbits almost perpendicular to the ecliptic, one above and the other below, and report on the solar wind emanating from the Sun's polar regions. However, in the event, only one spacecraft for this International Solar Polar Mission was built; it was launched in 1990 as Ulysses.

Despite having employed a trajectory designed to minimise the radiation dosage, Pioneer 11 did not escape Jupiter unscathed. It began to suffer spurious commands, and after months of analysis it was realised that the Meteoroid Detector had also been damaged, so it was switched off. As the instruments had, in turn, been temporarily deactivated during testing to isolate the fault, the Plasma Analyser suffered from chill and initially failed to restart, but after many attempts, it was reactivated in late 1977.

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