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Cassini continues its exploration of the Saturn system with the 13-day Rev 226, which begins on November 17 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.89 million kilometers (1.17 million miles) from Saturn’s cloud tops. Rev 226 occurs during the second Equat¬¬orial phase of Cassini’s extended-extended mission. During this 10-month phase, Cassini will orbit within the orbital plane of Saturn’s rings, allowing for frequent encounters with Saturn’s icy satellites. Eleven ISS observations are planned for Rev 226 with the majority focused on Saturn’s atmosphere.
For Cassini’s first observation of Rev 226 on November 18, ISS will acquire a quick observation of Saturn using the Wide-Angle Camera (WAC). This observation is part of a series of “Storm Watch” observation sequences designed to take advantage of short, two-minute segments when the spacecraft turns to point the optical remote sensing (ORS) instruments back at Saturn, as a waypoint between other experiments’ observations. These sequences include blue, clear, two methane band, and one full-frame, continuum band filter images. During this observation, Dione will appear in front of the middle of Saturn’s disk. Immediately afterward, ISS will acquire an astrometric observation of Saturn’s small inner moons. Astrometric observations are used to improve our understanding of the orbits of these small satellites, which can be influenced by Saturn’s larger icy moons. Next, ISS will acquire an observation of the distant outer moon Bestla for the last time in the Cassini mission. By measuring the brightness of Bestla in the 213 images planned for these observations, researchers hope to calculate how long its day is, where its north pole points, and estimate its shape. The first observation will be acquired from a distance of 10.2 million kilometers (6.34 million miles). At only 7 kilometers (4 miles) across, Bestla will appear as a faint point of light in the constellation Leo in these images.
On November 23, ISS will ride along with a Visual and Infrared Mapping Spectrometer (VIMS) observation of Saturn’s northern hemisphere. Afterward, ISS will ride-along with an Ultraviolet Imaging Spectrometer (UVIS) scan of Saturn. ISS will acquire photometric and polarimetric observations of Saturn during this observation. At 21:57, Cassini will perform a non-targeted encounter with Tethys, at an altitude of 17,518 kilometers (10,885 miles). Cassini will ride along with the Composite Infrared Spectrometer (CIRS) to acquire an eight-frame color mosaic of Tethys’ anti-Saturn hemisphere. CIRS will then perform scans of Tethys, including during closest approach. Afterwards, ISS will ride along again with CIRS while it performs a pair of observations of Saturn’s limb. Shortly before the second limb observation, on November 24 at 03:14 UTC, Cassini will reach periapse for Rev 226 at an altitude of 92,478 kilometers (57,463 miles) from Saturn's cloud tops, just outside the orbits of Janus and Epimetheus. After the CIRS observations, ISS will perform its own scan of Saturn’s limb hazes.
On November 25, ISS will observe a half-phase Titan from a distance of 1.39 million kilometers (0.87 million miles). This will allow for haze and cloud monitoring over Titan’s sub-Saturn hemisphere. On November 26, ISS will observe the faint, dusty Phoebe ring. The ring is so broad, faint, and distant that it will use Saturn’s shadow on the ring to detect it. After this observation and one last downlink, Cassini will enter solar conjunction, when Cassini and Saturn are on the other side of the Sun from Earth. Between November 27 and December 2 (after the start of the next orbit), Cassini will have very limited communications with Earth due to interference from the Sun’s atmosphere, so very few science observations will be acquired (mostly lower bitrate fields-and-particles observations). No ISS observations are planned for this six day period.
On November 30, Cassini will reach apoapse, bringing Rev 226 to a close and starting up the next orbit, Rev 227, which will include non-targeted encounters with Epimetheus, Atlas, and Prometheus.
Image products created in Celestia. Tethys map by Steve Albers. All dates in Coordinated Universal Time (UTC).