A computer-enhanced image of the non-illuminated side of the ring system by Pioneer 11 on 31 August, at a range of just under 1 million kilometres. This 'discovery' image revealed not only detail in the 'A' ring but also the presence of the 'F' ring beyond, and what appeared to be a new satellite. The 'B' ring is opaque. The 'C' ring and Cassini's Division appear bright because there is fine material within them that is efficiently forward-scattering sunlight.
ticked away, the engineers, scientists and managers in the control room fell silent, all listening anxiously to the signal, wondering whether it would be abruptly terminated. The scene was broadcast live by the TV networks. The signal continued through the predicted moment. Then, as David Morrison, Tom Young's deputy, later reflected, there were "scattered cheers and many sighs of relief". There was jubilation at JPL, because the Grand Tour trajectory had been shown to be viable. Some of the Ames researchers later voiced "a few rueful comments'' lamenting the lost opportunity of the 'inner' option.
A few minutes after the ring-plane crossing, at 2.53 radii, the particles and fields instruments noted Pioneer ll's passage through a significant magnetospheric wake. As Saturn's inner magnetosphere rotates with the planet, it sweeps across the more slowly travelling satellites and so the magnetic anomalies lead, rather than follow the satellites. In fact, the data suggested that the spacecraft had passed a few thousand kilometres directly in front of an object 200 kilometres in diameter. Could this be a second new moonlet? The orbital period of an object at this distance from Saturn would be 17 hours. It was soon established that the moonlet that had been imaged earlier was not only orbiting at the same distance, but would have been exactly where the second object was encountered at the time Pioneer 11 was present; it was
therefore the same moonlet, and the spacecraft had almost collided with it! If it had struck the moonlet and fallen silent, the JPL planners would have been obliged to reconsider their plan to use this trajectory for the gravitational slingshot to Uranus. In a sense, therefore, Pioneer 11's most significant achievement in the Saturnian system was to survive.10 Prior estimates of its chance of passing safely through the rings had varied from a mere 1 per cent (the pessimists) to 99 per cent (the optimists). The radar observations11 of rocky material beyond the 'A' ring had been particularly worrisome.
It had been predicted that charged particles flowing back and forth along Saturn's magnetic field lines would be absorbed where the lines were intersected by the main ring system. The peak intensity for low-energy magnetospheric electrons had been encountered at about 7 radii, mid-way between the orbits of Rhea and Dione.12 The predictions were accurate: as Pioneer 11 continued to close on the planet after passing through the ring plane, the counts of charged particles of all energies fell to zero at 2.292 radii, as it passed into the 'magnetic shadow' of the 'A' ring, and for a while it documented the most benign radiation environment in the Solar System.13
Half an hour after crossing the ring plane, Pioneer 11 had closed to 1.35 radii, its closest point of approach. Although as it flew 20,880 kilometres above the cloud tops the imagery confirmed the atmosphere to be subdued in comparison to Jupiter, it showed considerably more structure than had ever been glimpsed telescopically. The Infrared Radiometer mapped the temperature field. The mean temperature of 93K at the cloud tops was warmer than could be accounted for by insolation, so Saturn, like Jupiter, has an 'energy budget' sufficient for it to radiate slightly more energy than it receives from the Sun.
One surprise was that the axis of Saturn's magnetic field is virtually coincident with the planet's rotational axis; the offset being only 0.7 degrees.14 The fields of the Earth and Jupiter are tilted by 11.7 and 9.6 degrees respectively, and so it had been presumed that this must be an inevitable consequence of the generation mechanism. This and the fact that the centre of activity is displaced 2,400 kilometres northward of Saturn's centre sent the theorists scurrying back to their 'drawing board'. Unlike the Jovian moons, therefore, Saturn's satellites are not subjected to an environment in which the polarity of the field flips as an inclined rapidly rotating magnetosphere sweeps past them. Because Saturn's magnetic field is not as strong as Jupiter's, its magnetosphere is neither as large nor as able to resist the gusts in the solar wind, and it dramatically inflates and deflates in response. In fact, its boundary washes back and forth over Titan's orbit, at 20 radii, so this satellite is often exposed to the solar wind when passing sunward of the planet; Hyperion, Iapetus and distant Phoebe are always in the solar environment except when passing through the magnetotail. The aurorae result from charged particles flooding in through the magnetosphere's polar cusps to excite the ionosphere, and so are similar to the terrestrial phenomenon.
Two hours after the fly-by, Pioneer 11 slipped back through the ring plane, this time at 2.78 radii, where its particles and fields instruments recorded evidence of yet another tenuous ring (later designated the 'G' ring). It had been a remarkable initial reconnaissance. Having survived the rings unscathed, the spacecraft was able to head for its final objective.
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