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Cassini continues its exploration of the Saturn system with the 19-day Rev 238, which begins on July 11 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 2.50 million kilometers (1.55 million miles) from Saturn’s cloud tops. Rev 238 occurs during the second inclined phase of the Cassini Solstice Mission. Over the next several orbits, Cassini will use encounters with Titan to gradually increase the inclination of its orbit. Thirty-five ISS observations are planned for Rev 238 with the majority focused on Titan and Saturn’s rings and atmosphere.
For its first observation for Rev 238, on July 11, ISS will take a look at Titan’s northern trailing hemisphere from a distance of 1.98 million kilometers (1.23 million miles). ISS will be looking for clouds across Titan’s north polar region and at northern mid-latitudes during this sequence. Two more cloud monitoring observations will be taken on July 14 and July 16. The closer of the two, taken on July 14, will be taken from a distance of 1.88 million kilometers (1.17 million miles) and will cover the anti-Saturn hemisphere, the north polar region and the northern mid-latitudes of Titan. Immediately after the Titan cloud observation on July 11, 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. Careful measurements of the positions of these moons are important for later imaging of them at much closer distances during the F ring orbits later this year and early next year. Three more astrometric observations will be acquired on July 15, July 16, and July 29.
After the astrometric observation on July 11, ISS will acquire a quick observation of Saturn using the Wide-Angle Camera (WAC). This observation is 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. Four more Storm Watch observations will be taken between July 12 and July 17. Another three will be taken between July 28 and July 30. On July 13, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) as it performs a slewing calibration observation involving the star Spica (Alpha Virginis). On July 19, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) as it observes a stellar occultation by Saturn’s rings of the star Antares (Alpha Scorpii).
On July 23 at 03:36 UTC, Cassini will reach periapse for Rev 238 at an altitude of 586,340 kilometers (364,335 miles) above Saturn's cloud tops, outside the orbit of Rhea. ISS and the other remote sensing instruments will spend most of the periapse period observing Saturn’s atmosphere. On July 20 and 21, ISS will track cloud features on Saturn as the planet rotates, observing the same features at low, medium, and high emission angles. On July 20, ISS will observe Mimas at very low phase angles from a distance of 1.08 million kilometers (0.67 million miles). On July 22, ISS will ride along with UVIS as they observe Saturn’s south polar aurorae. On July 23, VIMS will map the south hemisphere of Saturn. Later that day, the Radio Science Sub-System (RSS) will perform a radio occultation of Saturn’s atmosphere and rings. On July 24, ISS will acquire a cloud monitoring observation of Titan from a distance of 491,100 kilometers (305,100 miles), focusing on Titan’s southern sub-Saturn hemisphere. Afterward, ISS will observe Saturn’s sunlit limb.
On July 25 at 10:05 UTC, Cassini will perform a targeted encounter of Titan. This is Cassini’s 122nd flyby of Titan, the seventh of eleven planned for 2016. The next encounter is planned for August 10 during Rev 239. T121 flyby has a close approach altitude of 976 kilometers (606 miles). This encounter will increase the inclination of Cassini’s orbit from 42.3 degrees to 48.7 degrees and will shave off eight days from the spacecraft’s orbital period. On approach, the Composite Infrared Spectrometer (CIRS) will view the sub-Saturn hemisphere of Titan, acquiring nadir and limb sounding observations at mid- and far-infrared wavelengths. Many of these are designed to measure seasonal changes in atmospheric temperatures and trace gas abundances, as well as map surface temperatures. VIMS will observe a stellar occultation of the red giant star 30 Herculis. This and similar stellar occultation are used to track changes in the composition of Titan’s atmosphere. ISS will ride along the CIRS limb observation and VIMS’s stellar occultation.
At closest approach, RADAR will acquire altimetry and SAR swaths across portions of Titan’s leading hemisphere. The SAR imaging swath will run from near 30 degrees South Latitude, 55 degrees West Longitude (northwest of Mezzoramia) northwest to cover portions of Hotei Arcus. RADAR will then switch from right look to left look to cover western Tui Regio, the Xanadu/Shangri-La boundary near Kerguelen Facula, parts of Shangri-La, and Dilmun before ending around 25 degrees North Latitude, 195 degrees West Longitude. This is the last RADAR SAR swath to cover high-southern latitudes on Titan. Outbound, CIRS will perform additional atmospheric compositional measurements. VIMS will look for clouds across the north polar region. Late on July 26, ISS will acquire a series of cloud tracking observations covering Titan’s north polar region.
On July 27, Titan will be observed during a calibration observation of the polarizer filters on ISS. On July 29, Titan will be observed again, this time for a haze monitoring observation. That observation will be taken from a distance of 1.76 million kilometers (1.10 million miles). On July 30, ISS will observe sun-like star 16 Cygni B for absolute calibration of irregular moon observations. 16 Cygni B is host to an extra-solar planet with 2.38 times the mass of Jupiter.
On July 30, Cassini will reach apoapse, bringing Rev 238 to a close and starting up the next orbit, Rev 239, which includes a flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).