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Cassini continues its exploration of the Saturn system with the 6.5-day Rev 279, which begins on June 13 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 279 is the 9th of 22 proximal orbits that will take place between April 2017 and the end of the mission in September. During these orbits, Cassini’s closest approach to Saturn occurs between the ring system and Saturn’s atmosphere. Seven ISS observations are planned for Rev 279.
On June 14, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to image a crescent Saturn using the Wide-Angle Camera (WAC) while UVIS is acquiring extreme- and far-ultraviolet scans of the planet. On June 15, the Visual and Infrared Mapping Spectrometer (VIMS) will map a crescent Saturn and its nightside at near-infrared wavelengths. Afterward, ISS will image a crescent Titan from 1.99 million kilometers (1.24 million miles) away to continue monitoring seasonal changes to its upper haze layers.
On June 16 at 23:56 UTC, Cassini will reach periapse for Rev 279 at an altitude of 3,283 kilometers (2,040 miles) above Saturn’s cloud tops. Inbound on this periapse pass, UVIS will perform several scans across Saturn’s north polar region to map the planet’s entire auroral oval, with the Composite Infrared Spectrometer (CIRS) riding along. During closest approach, the Ion and Neutral Mass Spectrometer (INMS) will be prime, acquiring in situ measurements as the spacecraft passes between the main rings and Saturn of the ring “rain” of water molecules and ions like O2+ falling on Saturn’s atmosphere. ISS will ride along to acquire a WAC scan across the main rings as they pass through the camera’s field-of-view. Outbound, UVIS will acquire an auroral observation of Saturn’s south polar region. Afterwards, UVIS will observe a solar occultation by Saturn’s rings, measuring the transmission of extreme ultraviolet light through the rings. This allows for high-resolution mapping of the main ring’s structure.
On June 17, ISS will image a crescent Titan from 1.94 million kilometers (1.20 million miles) away, again to monitor its upper haze layers. On June 18, ISS will stare at Enceladus’s south polar plume for 14 hours from 990,000 kilometers (615,000 miles) away, measuring variations in the brightness of the plume as the moon orbits Saturn and experiences slight changes in tidal forces on its surface. On June 19, ISS will observe the C and D rings, the innermost of Saturn’s main rings, at high phase angles, useful for observing these dusty rings. On June 20, ISS will ride along with UVIS to observe a stellar occultation by the rings of the B-type star Saiph or Kappa Orionis.
On June 20, Cassini will reach apoapse, bringing Rev 279 to a close and starting up the next orbit, Rev 280. At this point, Cassini is 1.21 million kilometers (0.75 million miles) from Saturn’s cloud tops. Thirteen ISS observations are planned for this orbit, with most taking place during a non-targeted encounter with Titan. On June 20, ISS will observe a crescent Titan from 1.79 million kilometers (1.11 million miles) away. Afterwards, ISS will acquire a set of dark current calibration images with the shutter closed. These are used to measure changes to the background charge of the Narrow-Angle Camera (NAC)’s charge-couple device (CCD) sensor. Dark current images are used during the calibration of ISS science images. On June 21, UVIS will also acquire flat-field calibration data while observing the star Spica. ISS will ride along to also acquire calibration data. On June 22, ISS will ride along with CIRS to observe a half-phase Dione from 361,000 kilometers (224,000 miles) away. ISS will be mapping color variations across Dione’s north polar region. CIRS will acquire temperature data of Dione’s summer pole, which will be useful for understanding the fine-scale structure of the moon’s shallow sub-surface.
On June 23 at 10:58 UTC, Cassini will reach periapse for Rev 280 at an altitude of 3,340 kilometers (2,075 miles) above Saturn’s cloud tops. 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 measure the mass of Saturn’s rings. RSS will then conduct a radio occultation of Saturn’s rings. The Cosmic Dust Analyzer (CDA) will also be used during this pass to measure the composition of dust in falling inward from Saturn’s main rings. Outbound, ISS will ride-along with CIRS as it observes the unlit side of Saturn’s A and F rings.
On June 24 at 22:29 UTC, Cassini will perform a non-targeted encounter with Titan at an altitude of 297,970 kilometers (185,150 miles). ISS will acquire five mosaics covering Titan’s sub-Saturn hemisphere. The last two mosaics include stares at Titan’s northern mid-latitudes. Images from these stares will be used to track any clouds visible south of Kraken Mare and north of Senkyo. Between the ISS mosaics, UVIS will observe a pair of stellar occultations by Saturn of Epsilon Orionis and Zeta Orionis. Both star systems are part of Orion’s Belt. Separate observations will cover the ingress and egress segments of both occultations. These occultations will be used to measure the structure of Saturn’s upper atmosphere, both near the south pole, currently experiencing polar winter, and near the equator. CIRS will also acquire a compositional map of Titan between the second and third ISS mosaics of Titan.
On June 26, ISS will observe the distant, outer moon Bebhionn from 9.47 million kilometers (5.88 million miles) away. Afterward, ISS will ride along with UVIS to observe a stellar occultation by the rings of the B-type star Saiph or Kappa Orionis. Shortly after the UVIS observation, Cassini will reach apoapse, bringing Rev 280 to a close and starting up the next orbit, Rev 281.
Image products created in Celestia. Dione basemap by Steve Albers. All dates in Coordinated Universal Time (UTC).