Compared with the Voyager 1 and 2 flybys of the planet in the early 1980s, the Cassini Jupiter flyby was leisurely and slow and nearly equatorial.|
Data collection began with the spacecraft 3.8 degrees above Jupiter's equator plane and approaching the planet from a phase (Sun-Jupiter-spacecraft) angle of 20 deg and a distance of 84.7 million kilometers. From this viewing geometry, Jupiter looked only slightly different than it did from Earth. By the middle of November, the phase angle dropped to 18 deg, and the distance decreased to the point where 4 narrow angle camera images were required to cover the planet. All throughout this period we made repeated observations of the atmosphere, and searched for previously undiscovered satellites in the region around Jupiter containing the Galilean satellites.
By the middle of December, the phase angle dropped to zero, repeated monitoring of the atmosphere ceased, and we began our observations of the rings and satellites. On December 18, we made our closest approach to Himalia, a small outer satellite of Jupiter. As Cassini swept through a large range of phase angle during the rest of the encounter, we monitored the light scattering behavior of the rings and Galilean satellites in a suite of spectral and polarimetric filters. (For a brief time surrounding closest approach, Jupiter was large enough to require 9 images to cover the planet.) And we were on the lookout for time-variability -- in the rings and in the expected diffuse glows from the tenuous atmospheres of Io, Europa and Ganymede as they passed into Jupiter's shadow. (The Galileo spacecraft first observed such glows, as well as high temperature hot spots, from volcanically active Io.)
Ring, satellite and occasional atmospheric observations continued through closest approach until January 15, at which point the spacecraft was looking back on a crescent Jupiter from a distance of 18 million km (11 million miles) and 3 degrees below the equator plane. At this time, we returned to repeated imaging of the planet as we departed. The last Jupiter images were taken on March 22, 2001.
The closest approach distance to the planet was not very close: 136 RJ, or 9.72 million kilometers (6.04 million miles). Thus, Cassini images did not have the exquisitely high resolution of either Voyager or Galileo images. But the slow pace of the flyby, the large data collection and downlinking capability of the spacecraft, and the wide spectral range and fine photometric precision of the Imaging Science System (ISS), made it possible to acquire high quality time-lapse CCD imagery of Jupiter's ever changing atmosphere extending over several months in a large suite of atmosphere-probing wavelengths, and to search for time-variability in other Jovian targets ... something no previous Jupiter-bound spacecraft has ever done before.