Another unexpected result was announced by Lyle Broadfoot, Principal Investigator for the ultraviolet spectrometer investigation. The scientists had expected to find very weak ultraviolet emissions on the sunlit side of Jupiter, caused by sunlight being scattered from hydrogen and helium in Jupiter’s upper atmosphere. “Instead we are seeing a spectacular auroral display. There are two features of the emission—the auroral emission which comes from the planet and a second type of emission which appears to come from a radiating torus or shell around the planet at the orbit of Io. The spectral content of these two radiating sources is distinctly different. What we find is that the auroral emission from Jupiter’s atmosphere is so strong that it completely dominates the emission spectrum even on the sunlit side of the atmosphere.” “Not since Mariner 4 carried its TV camera to Mars fifteen years ago have we been less prepared—have we been less certain of what we are about to see over the next two weeks,” said Bradford Smith, Imaging Team Leader. He mentioned the time-lapse “rotation movie,” in which the colorful planet spun through ten full Jupiter days; tiny images of the satellites passed across Jupiter’s face as though being whipped along by the rotation of the giant. A week or so earlier, when this film had been shown for the first time to the full Imaging Team, it provided an occasion for good-humored rivalry between planet people and satellite people, with jokes about the satellites getting in the way of the important studies of Jupiter. For the next few days, the imaging focus remained on Jupiter; it shifted to Io, Ganymede, and Callisto, as each was passed in turn after closest approach to the planet. Tuesday, February 27. (_Range to Jupiter, 7.1 million kilometers_). At a distance of 660 million kilometers from Earth, within 90 Jovian radii (R_J) of Jupiter, Voyager 1 was prepared to begin the encounter with the planet’s magnetosphere. On the previous day the spacecraft had crossed the point, at 100 R_J, at which Pioneers 10 and 11 had found the bow shock, the first indication of the magnetospheric boundary. The start of Voyager’s plunge into the Jovian magnetosphere was overdue, and scientists anxiously watched the data from the particles and fields instruments, looking for the first indication of disordered magnetic fields and altered particle densities that would mark the bow shock. Apparently, higher solar wind pressure, associated with increased solar activity since 1974, had compressed the magnetosphere, but no one could predict how strong this compression might be. ENCOUNTER DISTANCES FOR VOYAGER 1 Object Range to Center at Closest Best Image Resolution Approach (kilometers) (km per line pair)