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Cassini continues its exploration of the Saturn system with the 7.2-day Rev 254, which begins on December 22 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.22 million kilometers (0.76 million miles) from Saturn’s cloud tops. Rev 254 is one of 20 F-ring orbits that will take place between November 2016 and April 2017 where Cassini will approach Saturn just outside the main ring system. Four ISS observations are planned for Rev 254 with the majority focused on Saturn’s satellites.
For its first observation of Rev 254, on December 24, ISS will acquire a distant observation of Erriapus, one of Saturn’s three dozen outer satellites. This observation will be taken from 10.7 million kilometers (6.66 million miles) away. By measuring how its apparent brightness changes over the course of this observation, Erriapus’s rotational period and rotational axis can be estimated. A similar observation covering around 24 hours of rotation will be acquired on December 29 into Rev 255. On December 25, ISS will observe a crescent Titan from 1.66 million kilometers (1.03 million miles) away to monitor Titan’s upper haze layers.
On December 26 at 1:52 UTC, Cassini will reach periapse for Rev 254 at an altitude of 88,533 kilometers (55,012 miles), near the orbits of Janus and Epimetheus. ISS will ride along with a Composite Infrared Spectrometer (CIRS) observation of Enceladus’ south polar region, currently nearing the middle of polar night. The ISS images will be taken from 169,000 kilometers (105,000 miles) away. Other Cassini observations during the periapse period include a Cosmic Dust Analyzer (CDA) observation of dust near Saturn’s rings, a solar occultation observation of the rings by the Visual and Infrared Mapping Spectrometer (VIMS), and CIRS compositional measurements of Saturn’s A ring.
On December 29, Cassini will reach apoapse, bringing Rev 254 to a close and starting up the next orbit, Rev 255. Twelve ISS observations are planned for this orbit, with the majority covering Saturn’s rings and Titan. On December 31, Cassini will perform a non-targeted encounter with Titan at an altitude of 679,094 kilometers (421,969 miles). ISS will be prime during the entire, 26-hour encounter, acquiring a series of mosaics and stares covering Titan’s northern leading and sub-Saturn hemisphere as well as the north polar region. These will be used to map Titan’s northern mid-latitudes and to monitor and track clouds across the northern hemisphere of Titan. Some stares are included in the ISS observations to allow CIRS to measure temperatures near Titan’s north pole.
On January 2 at 05:48 UTC, Cassini will reach periapse for Rev 255 at an altitude of 88,292 kilometers (54,862 miles). During the periapse period, Cassini’s various instruments will be focused on acquiring observations of Saturn’s rings. ISS will acquire a color mosaic of portions of the C and B rings on January 1. Early on January 2, ISS will ride along with VIMS as that instrument acquires a compositional scan across the B ring. After closest approach, RADAR will acquire several radiometry scans of the rings, which provide a useful constraint on composition. Afterwards, ISS will ride along with CIRS as they observe Enceladus’ south polar region, just as they did during Rev 254.
Later on January 2, ISS will conduct a survey of the propellers in the outer A ring. Propellers are voids in the ring created by the gravity of large, 100 – 1000-meter (328 – 3280 foot) ring particles. Due to the influence of the rings on their motion, these observations are needed to keep track of previously discovered propellers, like Earhart and Bleriot. Afterwards, ISS will observe the edge of the A ring and the F ring at high resolution. On January 4, ISS will observe the G ring at high phase angles and low-elevation, good for looking at the vertical structure of dust in the ring.
On January 5, Cassini will reach apoapse, bringing Rev 255 to a close and starting up the next orbit, Rev 256.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).