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Cassini continues its exploration of the Saturn system with the 14.5-day Rev 230, which begins on January 7 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.90 million kilometers (1.18 million miles) from Saturn’s cloud tops. Rev 230 marks the transition from the second equatorial phase of the Cassini Solstice Mission to the second inclined phase. Over the next several orbits, Cassini will use encounters with Titan to gradually increase the inclination of its orbit. Twenty-six ISS observations are planned for Rev 230 with the majority focused on Titan and several of Saturn’s icy satellites.
For its first pair of observations of Rev 230, on January 9 and 10, ISS will observe the distant moon Narvi from a distance of 15.4 million kilometers (9.56 million miles). While Narvi will only appear as a point of light, these two observations can be used to measure the length of Narvi’s rotation as well the direction the north pole points. On January 11, Cassini will perform an observation of Titan from a distance of 2.78 million kilometers (1.73 million miles). This observation will be used to monitor clouds on Titan, in this case looking for clouds across the moon’s Senkyo region. Similar cloud monitoring observations will be acquired on January 12 and January 13. The closest of these observations will be taken on January 13 from a distance of 1.96 million kilometers (1.22 million miles), when ISS will be observing the sub-Saturn hemisphere of Titan.
On January 14 at 06:58 UTC, Cassini will reach periapse for Rev 230 at an altitude of 94,481 kilometers (58,707 miles) from Saturn's cloud tops, just outside the orbits of Janus and Epimetheus. During the periapse period, ISS will acquire observations as it makes a series of non-targeted encounters with several of Saturn’s icy satellites. First up is Enceladus. Cassini will pass by Enceladus at 72,200 kilometers (44,860 miles). During the encounter, ISS will acquire a three-frame mosaic of the moon’s leading hemisphere, a region of relatively youthful terrain. Next up, Cassini will encounter Mimas at a distance of 27,558 kilometers (17,123 miles). ISS will acquire a four-frame mosaic of Mimas’s trailing hemisphere, on the other side of Mimas from Herschel crater.
Afterwards, Cassini will pass a pair of small Saturnian satellites. First passing Daphnis at a distance of 20,300 kilometers (12,614 miles). Daphnis is a small moon, only 9.2 kilometers (5.7 miles) across and orbits within the Keeler Gap in Saturn’s outer A ring. In fact, it is the slight gravity of Daphnis that maintains the gap in the ring. ISS will acquire images from just before closest approach until a little over an hour later. The best images will have a resolution of 120 meters (400 feet) per pixel, making them the best images of Daphnis acquired by Cassini to-date. Cassini will then pass by Telesto, a Trojan moon that shares its orbit with Tethys, at a distance of 15,184 kilometers (9,435 miles). ISS will acquire images as Cassini approaches Telesto, with the best images having a resolution of 91 meters (300 feet) per pixel.
Before and after the Telesto observation, ISS will observe the south polar plume of Enceladus, monitoring its activity as Enceladus makes its way around Saturn. Afterwards, ISS will ride along with the Composite Infrared Spectrometer (CIRS) to observe a crescent Tethys from a distance of 340,000 kilometers (210,000 miles). Finally, on January 14, ISS will acquire another observation of distant Narvi.
On January 16 at 02:21 UTC, Cassini will perform at targeted encounter with Titan. This is Cassini’s 116th flyby of Titan, and the first one of 2016. The next encounter is scheduled for February 1. The T115 flyby has a close approach altitude of 3,817 kilometers (2,372 miles). This encounter will be used to increase the inclination of Cassini’s orbit, ending the second equatorial phase of the Cassini Solstice Mission and beginning the second inclined phase. On approach, Cassini’s instruments will view the Fensal-Aztlan hemisphere of Titan, and departing, it will observe a crescent Titan. The encounter starts with a far-infrared stare of Titan by CIRS followed by several scans of the moon by the Ultraviolet Imaging Spectrometer (UVIS). ISS will ride-along with both in order to look for methane clouds in Titan’s atmosphere.
For the rest of the encounter, CIRS will be prime, except for an observation by the Visual and Infrared Mapping Spectrometer (VIMS) at the end of the encounter. CIRS will do several limb-scans of Titan, measuring how temperature, gas and aerosol abundance varies with altitude in Titan’s atmosphere. T115 is an equatorial encounter, so limb views sounding the north and south poles are possible in the same encounter. By comparing this data set to each other and to those acquired on previous encounters, researchers hope to better understand how Titan’s atmosphere changes with its seasons. The south polar limb sounding is particularly important, as researchers hope to observe changes in the structure of the south polar vortex. During the outbound leg, VIMS will ride along with CIRS to observe specular reflection of Ligeia Mare and a lake near 73 N, 182 W. ISS will ride along with the VIMS observation at the end of the encounter and during many of the CIRS observations.
On January 18, ISS will acquire a pair of quick observations of Saturn using the Wide-Angle Camera (WAC). These observations are part of a series of “Storm Watch” 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 observations. They include blue, clear, two methane band, and one full-frame, continuum band filter images. Two more Storm Watch observations will be taken on January 19. Also on January 18, ISS will ride along with VIMS in order to observe the faint G ring.
On January 19, 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 as well as each other. Targets for this observation includes Polydeuces, Calypso, Helene, Telesto, Prometheus, Janus, and Methone. Also on January 19, ISS will observe a crescent Titan from a distance of 1.53 million kilometers (0.95 million miles). On January 21, ISS will acquire images of the distant moon Ijiraq, from a distance of 6.28 million kilometers (3.90 million miles).
On January 22, Cassini will reach apoapse, bringing Rev 230 to a close and starting up the next orbit, Rev 231. During the next orbit, Cassini will perform another targeted flyby of Titan.
Image products created in Celestia. Enceladus basemap by Paul Schenk. All dates in Coordinated Universal Time (UTC).