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Cassini continues its exploration of the Saturn system with the 13-day Rev 228, which begins on December 13 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 228 occurs toward the end of the second Equatorial phase of the Cassini Solstice 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. Twenty-seven ISS observations are planned for Rev 228 with the majority focused on Saturn’s atmosphere and its various icy satellites.
For the first observation of Rev 228, on December 13, 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. Between December 16 and 18, ISS will acquire four more Storm Watch observations, and between December 22 and 25, it will take four more.
On December 14, ISS will observe the distant, outer moon Skathi at a distance of 15.7 million kilometers (9.77 million miles). While the 7-kilometer- or 4-mile-wide moon is too far away for Cassini to observe any surface details, measurements of its apparent brightness in the 269 images, along with those from earlier observations of Skathi, will be used to estimate the small moon’s rotation rate, pole axis direction, and shape. Similar observations of the distant moons Hati and Erriapus will be acquired on December 20 and December 25. On December 16, 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. ISS will acquire a similar observation on December 25.
On December 17, ISS will perform a non-targeted encounter of Titan, passing the large moon at an altitude of 148,668 kilometers (92,378 miles). ISS will acquire three observations during this encounter. Early on December 16, at a distance of 613,000 kilometers (381,000 miles), ISS will observe Titan’s leading hemisphere in search of clouds. On December 17, ISS will acquire a pair of observations with several mosaic designs and stares. During the first, ISS will acquire an eight-frame mosaic across the illuminated part of Titan, which includes portions of the equatorial dark terrain, Shangri-La. ISS will then stare at an area covering eastern Dilmun and northern Shangri-La, including Oahu Facula, Nicobar Faculae, and Crete Facula. During the second observation, ISS will acquire a four-frame mosaic across Dilmun and Shangri-La, a long-staring observation of Titan’s north pole, and an eight-frame, true-color mosaic of a crescent Titan.
On December 19 at 14:09 UTC, Cassini will reach periapse for Rev 228 at an altitude of 92,509 kilometers (57,482 miles) from Saturn's cloud tops, just outside the orbits of Janus and Epimetheus. The periapse period for this orbit will allow Cassini’s various instruments to acquire observations of a few of Saturn’s icy moons. First, the Composite Infrared Spectrometer (CIRS) will observe Rhea in the far infrared. At 12:59 UTC, Cassini will perform a non-targeted encounter with Aegaeon, at a distance of 2,518 kilometers (1,565 miles). Aegaeon is the source of Saturn’s G ring and is surrounded by a denser arc of dust produced by micrometeorite impacts on the moon’s surface. Aegaeon is very small and dark. At 1.4 x 0.5 x 0.5 kilometers (0.9 x 0.3 x 0.3 miles) in size, approximately the size of the fictional Battlestar Galactica, it is the smallest known moon in the solar system and one of the most oblong. Despite orbiting close to a number of very bright moons like Enceladus and Mimas, Aegaeon is very dark, with an albedo similar to the dark terrain on Iapetus. ISS will first acquire several clear filter and true color images of Aegaeon from a distance of 11,580 kilometers (7,195 miles) to 3,931 kilometers (2,442 miles). Then, around closest approach, ISS will alternate between images of Aegaeon and images of the G ring arc with the moon outside of the field of view of the Narrow-Angle Camera (NAC). Each footprint (except the last) will consist of three clear-filter images at different exposure times to ensure that properly exposed images are taken of both Aegaeon and the G ring arc. The best images will have a resolution of 15 meters (50 feet) per pixel and Aegaeon will appear 83 x 26 pixels in size.
On December 19 at 17:49 UTC, Cassini will perform a targeted encounter with Enceladus at a distance of 4,999 kilometers (3,106 miles). Inbound, ISS will ride along with CIRS to observe Enceladus’s trailing hemisphere, including a pair of global color observations. CIRS will then observe a stellar occultation by Enceladus of the red giant star Arcturus. Shortly before closest approach, ISS will ride along with CIRS to acquire a four-frame mosaic covering al-Medinah crater, Samarkand Sulci, and Anbar Fossae. At closest approach, CIRS will acquire a temperature scan along the length of Damascus Sulci, one of the “tiger stripe” fractures that cut across Enceladus’s south polar region. CIRS will also perform a series of scans across the anti-Saturn side of Enceladus’s south polar region. Outbound, ISS will acquire a mosaic of a crescent Enceladus while riding along with CIRS, as well as a pair of plume movies designed to monitor variations in activity from the many jets that make up the inner part of the south polar plume. Afterward, on December 20, ISS will ride along with CIRS to observe a crescent Dione from a distance of 374,750 kilometers (232,850 miles).
On December 21, ISS will acquire a photometric observation of Saturn’s ring system, while Cassini is near the ring plane and the rings have a high phase angle, emphasizing inclined, dusty material in the ring system. Similar observations will be taken on December 22 and December 24 while riding along with the Visual and Infrared Mapping Spectrometer (VIMS). On December 23, ISS will acquire a photometric and polarimetric observation of Jupiter from a distance of 11.02 astronomical units (1.65 billion kilometers or 1.02 billion miles). Even at that great distance, Jupiter will appear 11 pixels across. The observation will cover a full Jovian day of nearly 10 hours.
On December 26, Cassini will reach apoapse, bringing Rev 228 to a close and starting up the next orbit, Rev 229.
Image products created in Celestia. Enceladus map by Paul Schenk. All dates in Coordinated Universal Time (UTC).