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Cassini continues its exploration of the Saturn system with the 7.2-day Rev 263, which begins on February 25 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 263 is the 13th 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. Ten ISS observations are planned for Rev 261 with the majority focused on Saturn’s atmosphere and rings.
On February 25, ISS will perform its first observation of Rev 263 by imaging the limb of Saturn. This observation will be useful for monitoring Saturn’s high-altitude haze layers. On February 26, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to image Saturn’s north polar region while UVIS examines the planet’s aurorae. A similar observation will be performed on February 27. Also on February 26, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) again to map Saturn’s north polar region.
On February 28 at 14:31 UTC, Cassini will reach periapse for Rev 263 at an altitude of 87,328 kilometers (54,263 miles), near the orbits of Janus and Epimetheus. Most of the observations during the periapse period focus on Saturn’s atmosphere and magnetosphere. Late on February 27, the camera system will ride along with a UVIS observation of Dione as it occults the star Beta Crucis. Dione will be 505,000 kilometers (314,000 miles) from Cassini during the occultation. UVIS will use the observation to detect and measure the density of Dione’s exosphere. Afterward, ISS will observe Titan from 1.71 million kilometers (1.06 million miles) away. This observation will be used to monitor Titan’s upper haze layers and to look for clouds across Titan’s northern trailing hemisphere. Shortly before periapse, ISS will acquire high-resolution images of the A ring including the propeller belt and the inner edge of the A ring. During periapse, the Magnetospheric Imaging Instrument (MIMI) will be prime.
Outbound from periapse, ISS will ride along with the Composite Infrared Spectrometer (CIRS) as it observes Enceladus’s south pole for 13 hours. This observation will be taken from 203,120 kilometers (126,210 miles) to 490,000 kilometers (304,500 miles) away. Afterward, on March 1, ISS will ride along with VIMS as it observes several faint rings including the C and G rings while Cassini is in the shadow of Saturn. ISS will be looking for meteor impacts in the C ring. On March 3, ISS will image Titan from 909,000 kilometers (565,000 miles) away to monitor clouds across its northern sub-Saturn and leading hemispheres.
On March 4, Cassini will reach apoapse, bringing Rev 263 to a close and starting up the next orbit, Rev 264. Twenty-one ISS observations are planned for this orbit, with most covering Titan and Saturn’s atmosphere. On March 5, Cassini will perform a non-targeted encounter with Titan at a distance of 489,903 kilometers (304,412 miles). ISS will acquire five mapping mosaics covering Titan’s north polar region and northern mid-latitudes. These observations will also include stares at Titan’s northern mid-latitudes for cloud tracking and at the north pole to allow for CIRS mid-infrared measurements.
On March 7 at 18:31 UTC, Cassini will reach periapse for Rev 264 at an altitude of 86,726 kilometers (53,889 miles), near the orbits of Janus and Epimetheus. Most of the observations during the periapse period focus on Saturn’s atmosphere. Late on March 6 and early on March 7, ISS will ride along with UVIS and VIMS to observe Saturn’s north polar region. Afterward, ISS will ride along with a VIMS observation of a stellar occultation by the rings of the star Gamma Crucis. Occultations like this one are used to map the fine-scale structure of the rings. Also on March 7, ISS will ride along with CIRS to observe the limb of Saturn. Four similar observations will be acquired later on March 7 and on March 8.
Near periapse, ISS will observe the small ring-moon Pan, which orbits within the Encke Gap in the outer A ring. Cassini will approach within 22,210 kilometers (13,800 miles) of Pan during this non-targeted encounter. Fifty-four images are planned of Pan, though some will likely miss the satellite. The closest planned image will be taken from 24,572 kilometers (15,268 miles) away and will have a resolution of 147 meters (484 feet) per pixel, though Pan will likely be close to the edge of the Narrow-Angle Camera (NAC) field-of-view. These will be closest images ever taken of Pan and will help to characterize its shape and geology. On March 8, ISS will acquire a pair of observations of the B and C rings at very high-phase angles. ISS will also ride-along with UVIS to observe Saturn’s south polar aurorae.
On March 9, ISS will acquire a distant observation of Enceladus’s south polar plume from 950,000 kilometers (590,000 miles). On March 10, ISS will observe a crescent Titan from 1.92 million miles (1.19 million miles) away. Afterward, ISS will observe the E ring at high phase angles, a lighting geometry that’s useful for observing dust in the Saturn system. Finally, on March 11, ISS will observe Iapetus from 2.5 million kilometers (1.55 million miles) away. Iapetus will be at very low phase angles during this observation. By observing how Iapetus’s brightness changes as it approaches full phase, various properties of its fine-scale surface structure, like roughness, can be mapped.
On March 11, Cassini will reach apoapse, bringing Rev 264 to a close and starting up the next orbit, Rev 265.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).