Earth

200,000 km

Voyager 2's route through the Saturnian system.

Immediately thereafter, the spacecraft was to document the non-illuminated side of the ring system and secure close-in mosaics of the terminators of Enceladus and Tethys. Unfortunately, as the scan platform was slewed, its azimuth actuator began to fail, and at 23:40 it seized, so this section of the encounter sequence was lost. The fact that the spacecraft had been in Saturn's shadow at the time prompted some suspicion that the gear train's lubricant may have frozen, so a command was sent to activate a heater, but to no avail. In fact, it took almost three days of thermal control to coax the mechanism back to life. After a detailed analysis, the engineers concluded that the sheer intensity of the slewing had driven the lubricant from the gear train, and it was not released until the heat enabled the components to separate sufficiently for the lubricant to re-enter the mechanism. While some data had been lost, E.C. Stone assured the disappointed reporters that science was measured in terms of increased understanding rather than the number of data bits, and when considered in this way Voyager 2 had achieved ''200 per cent'' of its science programme.

''We found so many things to do with Voyager 2 after the Voyager 1 fly-by; that may have led to the scan-platform problem, because we had the thing swinging back and forth in the sky so fast and so furiously that we think that's what drove the lubricant out of the gears and caused the gear train to seize,'' reflected E.D. Miner of JPL, Voyager's assistant project scientist.

Voyager 1 had shown little detail on Enceladus, but it had appeared tantalisingly smooth. Voyager 2's trajectory enabled it to document the northern hemisphere at a resolution of a few kilometres per pixel. Imagery from 120,000 kilometres shortly before the ring-plane crossing showed a section of grooved plain transecting the cratered terrain, a landform strongly reminiscent of the sulci on Ganymede. The hint of geological activity on Dione had stirred up interest, but Enceladus was amazing. ''The oldest terrains on Enceladus are similar in crater density to the least cratered plains of Dione,'' L.A. Soderblom told reporters. The sparse cratering of the smooth plains meant that they are even younger, perhaps forming within the last 100 million years, or as Soderblom expressed it, ''at a time when dinosaurs roamed the Earth''. There was evidence of repeated episodes of fluid flooding across its surface. Even with the possibility of cryovolcanic activity having been predicted by analogy with Io,116 and with the broad hints provided by Voyager 1, E.M. Shoemaker joyfully admitted that Enceladus had turned out to be ''far beyond my wildest expectations''.

Voyager 2 dramatically improved on its predecessor's coverage of Tethys. There are areas that are slightly darker than the norm. The 'strange' feature on the trailing hemisphere is indeed a crater (named Penelope). Its western rim is juxtaposed with a dark band and so, despite the full illumination, had given the appearance of shadow. As the range reduced and the resolution improved, the feature which had previously been the focus of attention paled into insignificance as, firstly, a 400-kilometre-diameter crater (Odysseus) was revealed on the leading hemisphere, one so vast in proportion that its floor has 'risen' to match the moon's radius of curvature, and then the moon was found to be girdled by a system of canyons (Ithaca Chasma). The fact that these canyons trace out a 'great circle', one of whose 'poles' coincides with

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