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Cassini continues its exploration of the Saturn system with the 28-day Rev 234, which begins on March 21 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 2.77 million kilometers (1.72 million miles) from Saturn’s cloud tops. Rev 234 occurs near the beginning of 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 ISS observations are planned for Rev 234 with the majority focused on Saturn’s atmosphere, its system of rings, and Titan.
For its first observation for Rev 234, on March 22, ISS will ride along with an extreme- and far-ultraviolet scan of Saturn’s atmosphere by the Ultraviolet Imaging Spectrometer (UVIS). Similar observations will be taken on March 25 and March 29. On March 23, ISS will observe a half-phase Titan from a distance of 1.99 million kilometers (1.24 million miles). This observation is part of a campaign to monitor Titan’s cloud activity and changes to its upper haze layers. Cloud monitoring observations will be taken on March 29, March 31, and April 1. Also on March 23, ISS will ride along with the Composite Infrared Spectrometer (CIRS) to observe Saturn’s atmosphere. A similar ride along observation will be taken on March 29. On March 24, ISS will acquire a pair of Saturn cloud track observations, each lasting five hours. These observations will track three areas in Saturn’s mid-northern latitudes as they rotate around the planet. By measuring the changes in the positions of these clouds, wind speeds can be calculated. Four more cloud tracking observations will be taken between March 26 and March 28.
On April 2 at 21:13 UTC, Cassini will reach periapse for Rev 234 at an altitude of 308,160 kilometers (191,480 miles) above Saturn's cloud tops, just inside the orbit of Dione. Most of the observations during the periapse period focus on Saturn’s rings. On March 31, ISS will acquire a movie of the narrow F ring, lasting a little over six hours and consisting of 75 frames. On April 2, ISS will acquire a high-resolution, color radial scan across Saturn’s C ring. This is followed by a scan along the outer edge of the A ring. An hour after periapse, ISS will observe several propellers in Saturn’s A ring. A similar observation will be acquired a few hours later on April 3. 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. Between the two propeller observations, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) to acquire a color radial scan of the C and D rings.
On April 4 at 19:48 UTC, Cassini will perform a targeted encounter of Titan. This is Cassini’s 119th flyby of Titan, the fourth of eleven planned for 2016. The next encounter is planned for the next orbit on May 6. T118 has a close approach flyby has a close approach altitude of 990 kilometers (615 miles). This encounter will increase the inclination of Cassini’s orbit from 20.6 degrees to 27.8 degrees. On approach, CIRS will view the sub-Saturn hemisphere of Titan, acquiring nadir and limb sounding observations. UVIS will acquire several extreme- and far-ultraviolet scans across Titan. ISS will then take a high-resolution, three-frame mosaic across eastern Shiwanni Virgae, a set of dark lineaments between the bright region Tsegihi and the dark, dune-filled region Aztlan.
At closest approach, UVIS will observe a solar occultation by Titan’s atmosphere, with ingress and egress points near 58 degrees South latitude and 59 degrees North latitude. These points are near the edge of the polar vortices, where unusual gas abundances and temperatures have previously been observed by CIRS. The Ion and Neutral Mass Spectrometer (INMS) will sample Titan’s upper atmosphere near UVIS’s ingress measurements, allowing for the only opportunity for INMS and UVIS to measure Titan’s upper atmospheric density simultaneously. Previous measurements by the two instruments have been hard to reconcile due to atmospheric variations at different latitudes and times. Outbound, ISS will ride-along with CIRS, UVIS, and VIMS observations of a crescent Titan. VIMS will be looking for specular reflection across the lakes in Titan’s north polar region as well as clouds.
On April 6, ISS will acquire a movie of the Cassini Division lasting nine hours. On April 7, ISS will observe a crescent Titan from a distance of 1.61 million kilometers (1.00 million miles). Immediately after the Titan haze observation, 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. Eight more Storm Watch observations will be taken between April 8 and April 17.
Also on April 7, ISS will acquire a movie of the Encke Gap in the outer A ring lasting nearly 13 hours. On April 9, 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. Additional astrometric observations will be taken on April 14 and April 16. On April 16, ISS will observe a half-phase Titan from a distance of 4.40 million kilometers (2.73 million miles). On April 17 and 18, ISS will observe the distant moon Ijiraq for 30 hours. The observation will be taken from a distance of 11.5 million kilometers (7.14 million miles). While Ijiraq will only appear as a point of light, these two observations can be used to measure the length of its day as well the direction the north pole points.
On April 18, Cassini will reach apoapse, bringing Rev 234 to a close and starting up the next orbit, Rev 235. During the next orbit, Cassini will perform another targeted flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).