Voyager 1's view of Mimas. The top set were taken at ranges between 800,000 and 400,000 kilometres, and show the giant crater in the centre of the leading hemisphere. North is towards the top. The bottom set were taken at ranges between 300,000 and 127,000 kilometres as the spacecraft passed the moon, showing the heavily cratered south polar region. North is to the left in this sequence, and the south pole is on the terminator on the rightmost image.
intensely cratered. The closest approach occurred about an hour and a half before the Saturn slingshot, but at that time the moon was 416,000 kilometres away, on the far side of the planet. The trailing hemisphere of Dione was recorded from 620,000 kilometres at a resolution of 5 kilometres per pixel. The wispy and filamentary character of the streaks was shown to be rather different to the sulci of Ganymede. The network radiates out from a large dark patch and projects across the entire hemisphere. The imagery of Rhea, from 720,000 kilometres, established that the Saturn-facing hemisphere is an intensely cratered plain. By this time, most of the wispy feature was beyond the terminator, indicating that it is concentrated between 180 and 270 degrees longitude. L.A. Soderblom said of such streaks: "I don't believe that these can possibly have been produced by impact processes.'' Their extent and connectedness suggested that they were created by internal processes. For bodies so small, so lightweight, and so cold, this was a surprising discovery.
The encounter sequence included imaging the co-orbiting Janus and Epimetheus, which were mere specks of light in telescopes. Both are irregular with their primary axes aligned towards Saturn. Neither exceeds 200 kilometres on its longest dimension, and Janus is slightly the larger. Both have cratered surfaces and they looked to be fragments of a single progenitor. Helene ('Dione-B') is no more than 30 kilometres across. Tethys was confirmed to have a pair of co-orbiting moonlets, with 'Tethys-B' (Telesto) leading and 'Tethys-C' (Calypso) trailing.64 However, a suspected moonlet co-orbiting with Mimas was not confirmed. The recently discovered 'A' ring and 'F' ring shepherds could not be inspected in detail because their orbits were insufficiently defined to aim the scan platform.
An image of the 'F' ring from a range of 750,000 kilometres, with a resolution of 15 kilometres per pixel, indicated that this feature actually comprised a pair of thin bright stands and a fainter diffuse band about 100 kilometres towards Saturn. The thin strands were distorted and in places intertwined, and the distribution of material was rather 'clumpy'. The 'braiding' of the strands ''seemed to defy all of the laws of celestial mechanics'' an amazed B.A. Smith told journalists. In fact, the braiding is the result of gravitational interactions with the shepherding moonlets Prometheus and Pandora, which are very close to the ring. The material ahead of one of the moonlets will be retarded and will descend, and the material behind will be accelerated and will rise, thereby distorting the ring. These distortions will travel with the moonlets, and may well interact with one another when the inner shepherd overtakes its outer companion.65,66,67,68,69,70
At 15:45 on 12 November, Voyager 1 flew 123,500 kilometres above Saturn's cloud tops. When passing Titan, it had crossed onto the far side of the ring system, so during its final approach to the planet it was able to view the non-illuminated face. The trajectory had been constrained by the requirement to inspect Titan on the way in, and then to set up the Saturn occultation so that the spacecraft would emerge (as viewed from Earth) between the limb and the ring system. The planetary occultation began at 19:08, and lasted an hour and a half. Within a few minutes of re-emerging, the spacecraft started the highly prized ring occultation. The way in which the signal fluctuated served to measure the density of the material in a 'radial' across the entire system.71 This showed that there are many more fine ringlets than
The kinked, clumped and multiple 'F' ring imaged at high resolution on 12 November 1981 by Voyager 1. Note that the faint inner band is unperturbed by the force that is distorting the main pair. It is possible that they are not physically intertwined, but in slightly different planes and the overlapping is a perspective effect.
could be seen in even the highest-resolution imagery.72 The size of the particles ranges from a few microns to a few tens of metres. The tenuous 'C' ring evidently comprises particles which are typically 1 metre in diameter: ''boulders flying around Saturn'', explained G.L. Tyler, the leader of the radio science team. The data also finally established that the 'B' ring, which is the visually brightest and most opaque section of the system, is no more than 100 metres thick and comprises particles ranging in size from a few centimetres up to 1 metre. The 'A' ring material ranges from a few microns up to 10 metres. The outer edge of the 'A' ring is 10 metres thick. If the largest particles are towards its periphery, it is only one 'particle' thick.73 Given its overall span, the ring system is the flattest object in the Solar System.74
The rings received much of Voyager 1's attention because, apart from the spinscan longshots by Pioneer 11, this was the first opportunity to inspect their detailed structure. Some telescopic observers had reported fine structure in the 'B' ring, but others had only reported a smooth feature. Clearly, if such structure was real, it was of a transient character, and it had been speculated to be due to spiral density waves propagating through the system. The Voyager imagery established that there are indeed such waveforms. A search for moonlets embedded in the ring system proved to be frustrating, implying either that the finer structure is not induced by such rocks, or they are smaller than the camera's resolution.75,76 Nevertheless, a detailed analysis of the density waves in the 'A' ring subsequently prompted the prediction of a moonlet within Encke's Division, and thus Pan was discovered. The historical variability of Encke's Division had prompted the belief that it was only an intermittent thinning out of the 'A' ring particles rather than a clear zone. Now the task for the theoreticians was to explain how, with a moonlet present, the gap could seem to close. Furthermore, as Voyager 1 closed in, it became evident that this division, which was just 270 kilometres wide at the time, hosted several ringlets that were both discontinuous and 'kinky'.
Once Voyager 1 was able to view the rings forward-scattering sunlight, it saw a region of tenuous material extending in from the 'C' ring to about 3,200 kilometres of the cloud tops. In 1969, when the ring system was opening up, Pierre Guerin had reported material in this zone, and it had been designated the 'D' ring. However, the feature found by the spacecraft was too tenuous to have been seen telescopically by reflected sunlight - indeed, it had not been visible even an hour before the spacecraft crossed the ring plane. As O.W. Struve had speculated in 1851, material is spiralling onto the planet. Perhaps the flow rate is variable, and Guerin had been fortunate and seen a clump spiralling in.
Although Voyager 1 could not modulate its carrier wave to transmit data during the planetary and ring occultations, it had continued to make observations and stored the data on tape for later replay. As Saturn's gravitational field bent the spacecraft's trajectory, it flew within 88,000 kilometres of Mimas an hour after closest approach to the planet, recording the illuminated part of the moon's south polar region. An hour later it viewed Enceladus's trailing hemisphere from 201,000 kilometres. This was not a particularly favourable fly-by, but it showed that much of the surface is remarkably smooth and devoid of craters, at least down to the 12-
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