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Cassini continues its exploration of the Saturn system with the 13-day Rev181, which begins on February 7 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 1.66 million kilometers (1.03 million miles) from Saturn's cloud tops. Rev 181 occurs nine months into the first inclined phase of the Cassini Solstice Mission, a phase which lasts until March 2015. The inclined phase will allow for polar views of Saturn and Titan as well as better vistas of Saturn's rings than those Cassini had while in the earlier, equatorial phase of the Solstice Mission. Twenty-two ISS observations are planned for Rev 181 with the majority focused on Saturn's atmosphere and rings, as well as the T89 flyby of Titan.
On February 9, ISS begins its observations for Rev 181 two days after apoapse with a quick observation of Saturn using the wide-angle camera (WAC). These observations are part of a series of "Storm Watch" observation sequences designed to take advantage of short, two-minute segments when the spacecraft turns the optical remote sensing (ORS) instruments back to Saturn as a waypoint between other experiments' observations. These sequences include blue, clear, two methane band, and one full-frame, continuum band filter images. Six more observations are planned between February 9 and 11. On February 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. Finally, ISS will acquire a movie of the F ring, observing its various channels and streamers created by the interaction between the ring material and the nearby moon, Prometheus.
On February 11, ISS will acquire a series of images of Saturn rings using the WAC. These images will be tracking spokes -- a ring phenomenon Cassini has monitored throughout the mission -- over the B ring. With Cassini over the unlit side of the rings and with Saturn at a high phase angle, the spokes, if visible, will be brighter than the dark B ring. On February 12, ISS will acquire a NAC movie of the Encke Gap, which includes the small moon Pan.
On February 14 at 12:50 UTC, Cassini will reach periapse for Rev 181 at an altitude of 387,760 kilometers (240,940 miles) from Saturn. On February 13, ISS will acquire a high-resolution movie of the inner D ring at high phase angles. This movie will be used to assess the density of dust in the inner portion of the ring system, a region Cassini will pass through in its final orbits in 2017. Next, ISS will ride along with a Visual and Infrared Mapping Spectrometer (VIMS) observation of the F ring, measuring its brightness at high phase angles. Afterward, ISS will ride along with VIMS again to look for small, cometary impacts in the C ring as Cassini is eclipsed by Saturn, followed by another photometry observation of the F ring with VIMS. Early on February 14, ISS will monitor the south polar plume of Enceladus from a distance of 290,000 kilometers (180,000 miles). Afterward, ISS will perform a long observation of the A ring. The A ring is host to thousands of propellers, which are gravitationally-formed voids created by large ring particles. This observation will be used to search for these propellers in the region between the Encke Gap (itself carved by the small moon Pan) and the Keeler Gap (a narrower gap carved by Daphnis). Of particular importance for this observation is to re-image previously observed propellers to better measure their sizes and orbits. Finally, ISS will ride along with a VIMS stellar occultation observation by the ring system of the red giant star W Hydrae (one of the brightest stars in the night sky, in the near-infrared).
Three days after periapse, Cassini encounters Titan on February 17 at 10:22 UTC for the 90th time. This is the first of nine Titan flybys planned for 2013, with the next encounter scheduled for April 5 during Rev185. T89 is a relatively high altitude flyby with a close-approach altitude of 1,978 kilometers (1,229 miles). This flyby will allow for imaging of the Adiri region and the southern anti-Saturn hemisphere of Titan outbound from the encounter. For the 12 hours before and after closest approach, the Radio Sub-System (RSS) will be used to measure Titan's gravity field. The High-gain antenna will be pointed at Earth during the encounter, and the effect of Titan's gravity on the spacecraft will be measured by looking at the Doppler Effect on Cassini's signal. This information will be used (along with previous and later RSS gravity passes) to refine our understanding of Titan's internal structure. ISS will ride along with observations by the Composite Infrared Spectrometer (CIRS), taken before and after the RSS pass.
As the spacecraft departs from Titan and nears apoapse, ISS will ride along with a pair of CIRS observations of Titan, each observation will last around 24 hours and are designed to monitor and track cloud patterns across Titan's southern hemisphere while the spacecraft is near apoapse.
On February 20, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev182. The next orbit includes a close, non-targeted encounter with Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).