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Cassini continues its exploration of the Saturn system with the 6.5-day Rev 273, which begins on May 6 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.21 million kilometers (0.75 million miles) from Saturn’s cloud tops. Rev 273 is the 3rd of 22 proximal orbits that will take place between April 2017 and the end of the mission in September, where Cassini will approach Saturn between the ring system and Saturn’s atmosphere. Nine ISS observations are planned for Rev 273 with the majority focused on Titan.
On May 7 at 20:32 UTC, Cassini will perform a non-targeted encounter with Saturn’s moon Titan at an altitude of 496,000 kilometers (308,000 miles). ISS will acquire a series of time-lapse sequences to look for clouds and monitor their motion and evolution. These observations will be focused on Titan’s north polar region and the northern mid-latitudes on the sub-Saturn and leading hemispheres of Titan. Images in the last year have revealed numerous cloud streaks and minor storms across this part of the northern mid-latitudes. If no clouds are visible, these observations will be useful for filling in a gap of the global map of Titan near 40 degrees North Latitude, 0 degrees West Longitude. The Composite Infrared Spectrometer (CIRS) will also acquire a mid-infrared temperature scan across Titan as well as temperature and compositional measurements of Titan’s north polar region. On May 8, ISS will acquire a four-frame mosaic of Titan, when the moon is at very low phase angles at a distance of 575,000 kilometers (357,000 miles). On May 9, after periapse, ISS will observe a half-phase Titan, this time from 969,000 kilometers (602,000 miles) away. This observation will focus on Titan’s sub-Saturn hemisphere.
On May 9 at 06:16 UTC, Cassini will reach periapse for Rev 273 at an altitude of 2,670 kilometers (1,659 miles) above Saturn’s cloud tops, the third of 22 periapses where Cassini will pass between the rings and Saturn to provide unique observations of Saturn’s internal structure. On this pass, Cassini will transmit a signal from the High-Gain Antenna (HGA) to Earth to provide higher resolution information about Saturn’s internal structure by measuring variations in Saturn’s gravity field and to conduct a radio occultation of Saturn’s rings. Later on May 9, after the ISS Titan observation, the Visual and Infrared Mapping Spectrometer (VIMS) will observe a stellar occultation of the red supergiant star Betelgeuse by Saturn. Early on May 10, the Ultraviolet Imaging Spectrometer (UVIS) will acquire several ultraviolet imaging slews of Saturn’s south polar aurorae. On May 11, ISS will acquire a lightcurve observation of the small, distant moon Bebhionn from 9.85 million kilometers (6.12 million miles) away.
On May 12, Cassini will reach apoapse, bringing Rev 273 to a close and starting up the next orbit, Rev 274. At this point, Cassini will be 1.21 million kilometers (0.75 million miles) from Saturn’s cloud tops. Seven ISS observations are planned for this orbit, with the majority covering Saturn’s rings. On May 13 and 14, ISS will acquire a pair of time-lapse movies of Saturn’s narrow F ring. These movies are designed to monitor features in the ring created by the gravity of nearby moons and ring clumps. On May 13, VIMS will observe a stellar occultation of the A-type star Sirius by first Saturn’s atmosphere and then its rings. On May 14, ISS will observe a crescent Titan from 1.81 million kilometers (1.13 million miles) away. This observation will be used to monitor changes to the moon’s outer atmospheric haze layers resulting from seasonal changes. Northern summer begins on Titan (and the rest of the Saturn system) on May 24 during the next orbit. On May 15, 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.
On May 15 at 16:45 UTC, Cassini will reach periapse for Rev 274 at an altitude of 2,616 kilometers (1,625 miles) above Saturn’s cloud tops. This is Cassini’s closest approach to Saturn until Rev 288 in August. Like the previous pass, Cassini will transmit a signal from the High-Gain Antenna (HGA) to Earth to provide higher resolution information about Saturn’s internal structure by measuring variations in Saturn’s gravity field and to conduct a radio occultation of Saturn’s rings. On May 17, ISS will ride along with CIRS to observe the F ring. Afterwards, ISS will observe the faint and dusty E ring as Cassini crosses above the ring plane. On May 18, ISS will observe the bright limb of Saturn to monitor changes in the planet’s upper haze layers.
On May 19, Cassini will reach apoapse, bringing Rev 274 to a close and starting up the next orbit, Rev 275, which will include a non-targeted encounter with Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).