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Cassini continues its exploration of the Saturn system with the 32-day Rev 235, which begins on April 18 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 3.32 million kilometers (2.06 million miles) from Saturn’s cloud tops. Rev 235 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. Fifty-three ISS observations are planned for Rev 235 with the majority focused on Saturn’s atmosphere, its system of rings, and Titan.
For its first observation of Rev 235, on April 19, ISS will take a look at Titan’s northern trailing hemisphere from a distance of 3.65 million kilometers (2.27 million miles). ISS will be looking for clouds across Titan’s north polar region and at northern mid-latitudes during this sequence. Five similar cloud monitoring observations will be acquired between April 21 and May 2. The closest of these will be acquired on April 23 from a distance of 2.17 million kilometers (1.35 million miles) and will be focused on the northern mid-latitude region, north of Shangri-La and Xanadu. Immediately after the Titan cloud observation on April 19, 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. Ten more Storm Watch observations will be taken between April 20 and May 2. Another three will be taken between May 13 and May 19.
On April 21, 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 April 23, April 27, and May 2. Two more will be acquired on the outbound leg of this orbit on May 14 and May 19. On April 25 and 26, ISS will spend nearly two days staring at Saturn, watching as clouds evolve and move in the planet’s atmosphere over the course of three Saturn days using the Wide-Angle Camera (WAC). On April 27 and 28, ISS will track the Anthe ring arc for 19 hours.
Tarvos and Skathi will only appear as point of lights, these two observations can be used to measure the length of their days as well the where their north poles point. On May 1, ISS will acquire a movie of the narrow F ring and the Keeler Gap ringlet, lasting nearly thirteen hours and consisting of 110 frames. On May 3, ISS will observe several propellers in Saturn’s 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 used to keep track of previously discovered propellers, like Earhart and Bleriot.
On May 4 at 16:56 UTC, Cassini will reach periapse for Rev 235 at an altitude of 442,380 kilometers (274,882 miles) above Saturn's cloud tops, just inside the orbit of Rhea. Most of the observations during the periapse period focus on Saturn’s atmosphere. On May 3, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) to acquire cloud maps of Saturn’s northern and southern hemisphere. On May 4, after periapse, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to monitor Saturn’s south polar aurorae. A similar auroral observation will be taken a few hours later. Between the two, ISS will image the bright limb of Saturn to better understand its upper haze layers.
On May 6 at 17:01 UTC, Cassini will perform a targeted encounter of Titan. This is Cassini’s 120th flyby of Titan, the fifth of eleven planned for 2016. The next encounter is planned for the next orbit on June 7. The T119 flyby has a close approach altitude of 971 kilometers (603 miles). This encounter will increase the inclination of Cassini’s orbit from 27.8 degrees to 35.3 degrees. 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, particularly near the south pole. ISS will acquire an 11-frame mosaic of Titan’s surface using near-infrared images as well as red-green-blue images for viewing Titan’s atmosphere at visible wavelengths. This observation will be taken from a distance of 254,630 kilometers (158,220 miles).
At closest approach, the Ion and Neutral Mass Spectrometer (INMS) will acquire direct measurements of gas abundances in Titan’s upper atmosphere. This encounter was a high priority for INMS due to its unique geometry, where Cassini will be passing over local midnight on Titan while Titan is over the nightside of Saturn. As Cassini emerges from the shadow of Titan, the Radio Science Sub-System (RSS) will perform a radio occultation observation of Titan’s atmosphere, probing its thermal structure at mid-northern latitudes. Afterwards, RSS will perform a bistatic scattering observation of Titan’s north polar region, covering the small lakes on the anti-Saturn side of the north pole. Bistatic scattering observations provide information of surface reflectivity, dielectric constant (which provides a constraint on surface composition and phase state), and roughness. Outbound, VIMS will map the north polar region, looking for clouds and specular reflections off the lakes and seas in the region. CIRS will also perform atmospheric compositional measurements.
On May 7 and 8, ISS will acquire several observation of Titan’s north polar region, looking for clouds. On May 8, the camera system will observe the large, diffuse Phoebe ring. Due to its distance and faintness, the ring is best observed with Saturn’s shadow on the ring. On May 9, ISS will take a movie of the F ring, lasting 15 hours. On May 16 and 19, ISS will acquire distant, cloud monitoring observations of Titan.
On May 20, Cassini will reach apoapse, bringing Rev 235 to a close and starting up the next orbit, Rev 236. During the next orbit, Cassini will perform another targeted flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).