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Cassini continues its exploration of the Saturn system with the 7.2-day Rev 259, which begins on January 27 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 259 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. Twenty-one ISS observations are planned for Rev 259 with the majority focused on Saturn’s satellites.
For its first observation of Rev 259, on January 27, ISS will acquire a distant observation of Paaliaq, one of Saturn’s three dozen outer satellites. This observation will be taken from 10.3 million kilometers (6.42 million miles) away. By measuring how its apparent brightness changes over the course of this observation, Paaliaq’s rotational period and rotational axis can be estimated. On January 28 and 29, three more lightcurve observations will be taken of Saturn’s outer satellites, one each of Albiorix, Paaliaq, and Tarqeq. Another Tarqeq lightcurve observation will be taken on January 31, after periapse. On January 29, ISS will observe Titan from 1.67 million kilometers (1.04 million miles) to monitor clouds across Titan’s northern mid-latitudes, in this case on the sub-Saturn hemisphere.
On January 30 at 21:24 UTC, Cassini will reach periapse for Rev 259 at an altitude of 88,352 kilometers (54,900 miles), near the orbits of Janus and Epimetheus. Most of the observations during the periapse period will focus on Saturn’s moons. Early on January 30, ISS will acquire color filter images of the north polar region of Enceladus from 498,000 kilometers (310,000 miles) away. Afterward and again later in the day, ISS will observe Tethys. The earlier observation will be taken from 577,000 kilometers (359,000 miles) and will cover the northern sub-Saturn hemisphere. Twelve hours later, ISS will observe Tethys again from 355,000 kilometers (221,000 miles) away, again covering the northern sub-Saturn hemisphere. This observation will be used to map red streaks seen up-close, elsewhere on Tethys.
At 21:02 UTC, Cassini will perform a non-targeted encounter with Epimetheus, an irregularly-shaped moon of Saturn orbiting between Mimas and Saturn’s main rings, at a distance of 3,542 kilometers (2,200 miles). ISS will acquire 60 images during the hour and a half before the encounter, focusing on Epimetheus’s northern hemisphere. The closest image will be taken from 6,133 kilometers (3,811 miles) away and have a resolution of 37 meters (120 feet) per pixel. During periapse for Rev 259, after the Epimetheus encounter, ISS will acquire an observation of Mimas’s south polar region from 41,230 kilometers (25,620 miles) away. Afterwards, ISS will ride along with a Composite Infrared Spectrometer (CIRS) observation of the south polar region of Enceladus, acquiring several color filter images from 225,000 kilometers (140,000 miles) away.
On February 1 at 19:53 UTC, Cassini will perform a non-targeted encounter of Titan at a distance of 219,409 kilometers (136,334 miles). ISS and CIRS will acquire a series of observations designed to monitor changes in Titan’s atmosphere as Titan approaches equinox in May. Early in the segment, ISS will observe the southern sub-Saturn hemisphere, while CIRS will focus on the growing, high-altitude winter hood over the south pole. Outbound, ISS will observe a crescent Titan to monitor Titan’s high-altitude haze layers.
On February 3, Cassini will reach apoapse, bringing Rev 259 to a close and starting up the next orbit, Rev 260. Twelve ISS observations are planned for this orbit, with most covering Saturn’s rings. On February 5, ISS will acquire a 17-hour, time-lapse movie of the narrow F ring to monitor features in the ring created by the gravity of nearby moons and ring clumps.
On February 7 at 01:47 UTC, Cassini will reach periapse for Rev 260 at an altitude of 89,498 kilometers (55,611 miles), near the orbits of Janus and Epimetheus. Most of the observations during the periapse period focus on Saturn’s rings. Late on February 6, ISS will acquire a pair of high-resolution ring observations of the outer A and F rings as well as the middle and outer C ring. These observations will also include a few quick images of Epimetheus from 356,270 kilometers (221,380 miles) away. Afterward, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) and the Visual and Infrared Mapping Spectrometer (VIMS) to observe the lit side of the A ring on approach and the unlit side of the rings on departure. Between the two observations, RADAR will acquire high-resolution radiometry of the rings. RADAR will perform similar scans after the VIMS observation. Finally, 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.
Late on February 7, ISS will acquire a mosaic of Saturn’s faint rings while Cassini is in Saturn’s shadow. Such lighting geometry is useful for mapping dust in Saturn’s rings. On February 8, ISS will ride along with a VIMS stellar occultation observation of the rings. During the observation, the rings will pass in front of the red supergiant star Betelgeuse. By measuring how the star’s light dims as the rings pass in front of it, the fine-scale structure of the rings can be observed.
Later on February 8, ISS will acquire an astrometric observation of several Saturn’s smaller moons. Astrometric observations are important for fine-tuning our understanding of the orbital motions of these moons, which are influenced by the gravitational pulls of Saturn’s larger icy satellites. Next, ISS will observe Enceladus’s south polar plume from 1.16 million kilometers (0.72 million miles) away, monitoring changes in the amount of dust in the plume as the moon orbits Saturn. Afterward, ISS will observe the Cassini Division as the spacecraft approaches the ring plane. On February 10, ISS will observe the limb of Saturn to monitor its upper haze layers.
On February 10, Cassini will reach apoapse, bringing Rev 260 to a close and starting up the next orbit, Rev 261.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).