Cassini continues its exploration of the Saturn system with the 9.6-day Rev 245, which begins on October 8 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.34 million kilometers (0.83 million miles) from Saturn’s cloud tops. Rev 245 occurs toward the end 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. Eighteen ISS observations are planned for Rev 245 with the majority focused on Saturn’s rings.
For its first observation for Rev 245, on October 9, ISS will acquire a medium resolution, moderate phase movie of the Encke Gap in the outer A ring. The Encke Gap is carved by the gravity of the small moon Pan. On October 10, ISS will observe a half-phase Titan at a distance of 1.85 million kilometers (1.15 million miles). This observation will be used to monitor clouds across the northern sub-Saturn hemisphere of Titan. A similar observation will be acquired from a distance of 1.17 million kilometers (0.73 million miles), covering much of the same territory. Afterward, 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 which start next month.
Immediately after the astrometric observation on October 10, 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. Similar Storm Watch observations will be acquired on October 11 and October 17.
Late on October 10, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) to observe a stellar occultation of the red giant star Lambda Velorum, in the constellation Vela, by the rings. Stellar occultations can be used to probe the fine-scale structure of Saturn’s rings. Another stellar occultation by the rings will be observed by VIMS and ISS during the periapse period on October 12, this time using the red giant star Gamma Crucis. On October 13, VIMS will observe another stellar occultation using the red supergiant star Antares. On October 11, ISS will acquire a high-resolution, color scan of the main rings using a string of Narrow-angle Camera (NAC) pointings from the F ring to C ring.
On October 13 at 14:00 UTC, Cassini will reach periapse for Rev 245 at an altitude of 279,290 kilometers (173,540 miles) above Saturn's cloud tops, between the orbits of Tethys and Dione. On October 12 and 13, ISS will conduct a pair of surveys 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 used to keep track of previously discovered propellers, like Earhart and Bleriot. On October 14, ISS will observe the F ring and the outer A ring, and later the G ring arc, which surrounds the tiny moon Aegaeon. On October 15, ISS will observe the D ring at high phase angles, when it is easier to observe due to its dustiness. On October 16, ISS will observe several of Saturn’s faint, narrow rings, including the Pallene Ring and the Methone and Anthe ring arcs, at high phase angles. Like the G ring, these faint rings are replenished by micrometeorite impacts on small moons like Pallene and Methone.
On October 16, ISS will observe the small, distant moon Tarvos from a distance of 25.2 million kilometers (15.7 million miles). This observation will be used to estimate Tarvos’s shape and north pole direction by measuring its light curve. This observation will last for 17 hours. On October 17, ISS will observe a crescent Titan from a distance of 1.97 million miles (1.22 million miles). These crescent observations are useful for monitoring changes to Titan’s upper haze layers due to seasonal effects. Currently Titan and the rest of the Saturn system are in late northern spring.
On October 18, Cassini will reach apoapse, bringing Rev 245 to a close and starting up the next orbit, Rev 246.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).