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Cassini continues its extended tour of Saturn with Rev115, the spacecraft's 116th orbit around the Ringed Planet. Cassini begins Rev115 on July 18 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.06 million kilometers (1.28 million miles) from Saturn's cloud tops. Although the last few Titan flybys have greatly reduced Cassini's orbital inclination, the spacecraft remains in a high-inclination orbit, providing an opportunity to study the rings and the polar regions of Saturn and its satellites. The Saturn system is nearing vernal equinox, which will occur on August 11 during Rev116. With equinox so close, observations only possible during this unique time (the last equinox occurred 15 years ago, in 1995) are taking on greater importance. These observations include satellite mutual events where one moon eclipses the sun as seen from another, moon shadows cast onto the main ring system, the ring system becoming more illuminated by Saturn that it is by the Sun, and the thinning ring shadow on Saturn.
Cassini ISS starts its observations for Rev115 two days after apoapse. On July 20, ISS will use the wide-angle camera to take two movie observations of Saturn's night side and the right (assuming Saturn's north pole is up) ansa of the main ring system. These movies are designed to look for periodicities in the spokes that form on Saturn's B ring. On July 21, ISS will look at a crescent Titan over the satellite's leading hemisphere from a distance of 1.35 million kilometers (838,000 miles). While this observation would not be good for observing surface features on Titan due to the high phase angle, these images will help in characterizing any changes in the haze layers in the moon's upper atmosphere as the result of the approaching equinox or Titan's position in Saturn's magnetosphere. In addition, ISS will take another wide-angle movie observation of the right ansa of Saturn's ring system in order to observe spokes on the B ring. A similar movie, this time of the left ansa, will be taken on July 22. But this time Saturn will be avoided in the framing of the images because the rings are faint compared to Saturn's sunlit atmosphere. As the illumination of the ring system becomes more glancing with the Saturn system approaching equinox, the rings are becoming fainter and more illumination is coming from Saturnshine rather than directly from the sun.
On July 23, ISS will take a 16-frame, wide-angle-camera mosaic of the entire Saturn system out to the orbit of Enceladus. This mosaic will include not only a crescent Saturn, but the faint, unlit side of the ring system and the icy satellites Mimas, Enceladus, and Tethys.
Cassini encounters Titan on July 24 at 15:34 UTC for the 60th time and the second encounter this month. This flyby is also the eighth in a series of ten flybys between April and August 2009 that will be spaced 16 Earth days, or one Titan day, apart and occur as Cassini is inbound toward Saturn. The close approach distance is only 955 kilometers (593 miles), close to the lowest safe altitude for a Titan flyby during the extended mission. This flyby (known as T59) will allow for imaging of the southern trailing hemisphere of Titan outbound to the encounter, similar to the area observed during the previous seven encounters in this series. On approach to Titan, ISS, the Composite Infrared Spectrometer (CIRS), Visual/Infrared Mapping Spectrometer (VIMS), and Cassini Plasma Spectrometer (CAPS) teams will be controlling spacecraft pointing, or will be considered "prime." VIMS and ISS will observe the crescent of Titan, looking for clouds over the north polar region and at the haze structures over this area. CIRS will map atmospheric temperature and composition over Titan's north polar region looking for seasonal variations in the north polar vortex, as well as acquire limb integrations to measure compositional variations at different altitudes.
During closest approach on T59, CAPS, the Ion and Neutral Mass Spectrometer (INMS), and CIRS will be prime. Between 19 and 90 minutes before closest approach, CAPS will map the interaction between Saturn's magnetosphere and Titan's ionosphere. CAPS will be looking particularly for pickup ions as the magnetosphere strips material from the ionosphere. CAPS also can be used to remotely observe the composition of Titan's ionosphere. In the 40 minutes surrounding closest approach, INMS will be prime, measuring the composition of the nightside ionosphere at southern latitudes. Combined with the results of the T57 encounter, the INMS team hopes to better understand how the ionosphere responds to changes in solar illumination and position in Saturn's magnetosphere. During this INMS observation, RADAR will be riding along, acquiring a short Synthetic Aperture RADAR (SAR) swath running east to west across the trailing hemisphere side of the south pole. This RADAR swath is complementary to one taken during T39 (December 20, 2007), which took at look at the leading hemisphere side of the pole. As it did during that swath, RADAR will be searching for lakes in this region, like Ontario Lacus seen during T58 and lakes in the north polar region earlier in the Cassini mission. This swath parallels to the east and south the swath acquired during the previous encounter, T58. Following this SAR swath, CIRS will acquire several limb scans over the south polar region (55 degrees south), acquiring aerosol and composition profiles over this region. Following the close approach period, CIRS will continue to be prime until the T59 data is downlinked to Earth. CIRS will acquire temperature and composition scans across Titan's sub-Saturn hemisphere in the far-infrared.
On July 25, following the Titan flyby, ISS will look for shadows within the main ring system, like those along the margins of some ring gaps. The next day, Cassini will acquire an azimuthal scan along the outer margin of the B ring. On July 26, Cassini reaches periapse, its closest point to Saturn on Rev115. At this point, Cassini will be 175,000 kilometers (109,000 miles) from Saturn's cloud tops, just inside the orbit of Enceladus. During periapse, ISS and CIRS will observe Janus, an irregularly shaped moon between the F and G rings, from a distance of 94,000 kilometers (58,000 miles). This observation will focus on Janus's southern trailing hemisphere.
Immediately following, ISS will observe the icy plume over Enceladus' south pole. During this observation, Enceladus will be half lit by the Sun. Normally, at this phase angle, Enceladus is so bright that it causes internal reflections in the camera to show up in these types of observations, making them less useful for studying Enceladus' plume. However, for a few minutes during this sequence, Enceladus will experience a solar eclipse as Rhea passes between it and the sun. This will hopefully allow Cassini ISS to observe the plume without having reflected sunlight from Enceladus contaminating the images. On July 27 and 28, Cassini will take a look at the shadows of Pan and Daphnis on the ring system, as well as structures like streamers and channels in the F ring which are created by the influence of Prometheus's gravity.
On July 29, Cassini ISS will observe Titan's Fensal-Aztlan region from a distance of 2.72 million kilometers (1.69 million miles). In addition, ISS will take an astrometric observation of several of Saturn's small satellites including Atlas, Pandora, Epimetheus, Pan, and Janus. Astrometric observations are used to help provide better orbital calculations for some of these small rocks, which can be affected by gravitational interactions with the larger icy moons. ISS also will observe the shadow of Atlas on the Saturnian ring system.
On July 30, another astrometric observation will be acquired of several of Saturn's small satellites, including Prometheus, Polydeuces, Methone, Calypso, and Pallene. ISS also will acquire a movie of the F ring as it stares at the F ring opposite Saturn's sub-Solar point. This observation is designed to look for shadows of structures in the F ring.
Cassini reaches apoapse on August 3, bringing Rev115 to an end and starting Rev116. Finishing up Rev115, ISS will observe Saturn using the narrow- and wide-angle cameras and in numerous filters. The observation will provide a great look at Saturn's northern hemisphere and the thin shadow of the ring system as the planet approaches equinox, by then only one week away.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).