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Cassini continues its new mission with Rev 77, its 78th orbit around Saturn and the third full orbit of the Cassini Equinox Mission. During this orbit, Cassiniís observations focus on Saturnís rings and several of the icy satellites. Cassini begins Rev77 on July 17 at its farthest distance from Saturn, called apoapsis. At this point, the spacecraft is 1.25 million km (779,000 mi) from Saturn. The first observations of the orbit involve Saturnís atmosphere. On July 18 and 19, the Ultraviolet Imaging Spectrometer (UVIS) performs several scans of the north polar region of Saturn, looking at the aurora there. ISS will ride along on both of these observations, acquiring wide-angle camera images of the planetís high northern latitudes.
Starting early on July 20 and running through July 24, Cassiniís instrument pointing will be controlled by the Rings Targeting Working Team, so most of the observations during this period will focus on the ring system. The first of these observations involves a radial scan of the unlit side of the rings by the Visual and Infrared Mapping Spectrometer (VIMS) followed by a Composite Infrared Spectrometer (CIRS) point-and-stare. Late on July 20 and early on July 21, VIMS and UVIS will acquire observations during three stellar occultations of the rings, the first with VIMS using Gamma Crucis (part of the Southern Cross) and the other two with UVIS using Beta Centauri. ISS will also acquire a partial azimuthal scan of the Columbo ringlet in the inner C ring. On July 21, Cassini reaches periapse, its closest point to Saturn on Rev77. At that point, the spacecraft will be 162,214 km (100,795 mi) from Saturnís cloud tops. Near periapse, Cassini will quickly pass high over the north polar region of Saturn before descending below the ring plane 22 minutes before periapse. Before periapse, VIMS and ISS will observe the ring system at high phase, including a high-resolution observation of the F ring. ISS will also observe the sub-Saturn hemisphere of Dione from a distance of 290,000 km (180,000 mi). Shortly after periapse, ISS will perform a high-resolution radial scan of the ring system from the C ring to just past the edge of the A ring. Following a downlink, ISS will again observe the F ring at high resolution. CIRS will also perform two radial scans of the ring system, including one of the area within the shadow of Saturn. On July 23, Cassini will acquire two wide-angle camera mosaics of the entire ring systemís sunlit side. Both mosaics will include visible-color images, making true-color (Blue-Green-Red) mosaics possible. Cassini will also perform several observations of Saturnís small, inner satellites on that date. The observations are designed to study the orbits of these objects and how they might evolve over short periods due to perturbations from the other satellites in the system. VIMS will acquire a ring occultation observation using the red giant variable star R Leo. Finally, ISS will make two observations of the outer A ring at the left ansa.
On July 24, ISS and UVIS will acquire observations of the northern auroral zone on Saturn.
Cassini begins Rev78 early on July 25. Rev78 includes a targeted flyby of Titan, as well as distant observations of Tethys, Rhea, and Saturnís rings.
Image products created in Celestia. Dione basemap by Steve Albers.