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Cassini continues its extended tour of Saturn on April 16 with Rev109, the spacecraft's 110th orbit around the Ringed Planet. Cassini begins Rev109 late on April 16 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 1.26 million kilometers (780,000 miles) from Saturn. The spacecraft remains in a high-inclination orbit, providing an opportunity to study the rings and the polar regions of Saturn and its satellites.
Cassini starts its observations for Rev109 the next day, on April 17. Cassini will turn its cameras and Ultraviolet Imaging Spectrometer (UVIS) to a crescent Enceladus 1.5 million kilometers (926,000 miles) away. The observation is designed to help monitor the eruption of water vapor and ice particles from the satellite's south polar jets, looking for changes over time. Next, Cassini will take an 8-hour movie observation of ringlets in the Encke Gap in the outer A ring. These ringlets are generated from ejecta from micrometeorite impacts on Pan, the small moon that orbits within the Encke Gap. Finally, Cassini will observe the passing the shadows of Mimas and Tethys on the A and B rings. On April 18, the UVIS and ISS teams will calibrate their instruments by observing the blue giant star, Spica.
Cassini encounters Titan on April 20 at 00:21 UTC for the 54th time and the second time this month. This flyby is also the second in a series of 10 flybys between April and August that will be spaced 16 Earth days, or one Titan day, apart. This encounter is one of the more distant of Cassini's targeted Titan flybys with a close approach distance of 3,600 kilometers (2,237 miles). This flyby (known as T53) will allow for imaging of the southern trailing hemisphere of Titan outbound to the encounter, similar to the area observed during the previous encounter on April 4. On approach to Titan, the Composite Infrared Spectrometer (CIRS), the Visual and Infrared Mapping Spectrometer (VIMS), ISS, and the UVIS will trade off control of spacecraft pointing, or be considered "prime." CIRS will focus on understanding the composition of Titan's stratosphere by performing nadir and limb integrations of the satellite's atmosphere in the far- and mid-infrared. The CIRS instrument will point straight down at Titan to perform the nadir integrations, staring at particular spots. During the limb integrations, Cassini will observe the atmosphere at the edges of the visible disk of Titan. ISS and VIMS will look for clouds in Titan's far north, which will be visible as a crescent. VIMS in particular will examine the high-altitude ethane cloud that it is has been observed over the north polar region. This cloud appears to be transparent at the 938 nanometer near-infrared wavelength ISS uses to observe Titan's surface. Finally, as Cassini nears Titan, UVIS will acquire a stellar occultation, using the star Alpha Erdiani to probe Titan's upper atmosphere.
During closest approach on T53, CIRS and UVIS will be prime. CIRS will perform several scans of the limb of Titan both before and after closest approach. These scans will allow CIRS to measure the composition of Titan's atmosphere at different altitudes, particularly looking at aerosols in Titan's stratospheric haze layers. CIRS also will measure the temperature of Titan's atmosphere at different altitudes, acquiring a temperature profile of Titan's nightside atmosphere. At closest approach, UVIS will acquire a solar occultation, observing how the Sun's ultraviolet light is dimmed by Titan's atmosphere. From this data, the UVIS team will be able to look for fine-scale details in Titan's haze layers.
Following close approach, VIMS, RADAR, ISS, and CIRS will trade off being prime. VIMS will acquire three observations at different distances from Titan. VIMS will acquire a high resolution observation of the margin of a bright region with lobate edges. This bright region surrounds a 300-kilometer wide dark region near 40 degree south latitude, 280 degrees west longitude. This dark feature resembles many of the dark lakes seen at Titan's polar regions. The fact that two features with morphologic characteristics similar to Titan's polar lakes have been found on Titan's trailing hemisphere near 40 degrees south Latitude suggests a possible connection with the mid-latitude convective cloud features seen earlier in the Cassini mission. It is not known if surface liquids at these features help increase the methane "humidity" levels at this latitude, or if these features are fed by rainfall from the clouds. ISS and VIMS will observe this feature in the higher altitude mosaic observations. CIRS again will look at Titan to measure the composition of the stratosphere by observing the satellite's atmosphere in the mid- and far-infrared. RADAR also will perform several radiometry scans of Titan.
Following the T53 flyby, Cassini imaging will be focused primarily on Titan and Saturn's ring system. On April 21, Cassini will turn its cameras to Titan for a cloud monitoring observation, however, Titan will not be visible because it will be in Saturn's shadow at the time. CIRS will examine the lit side of Saturn's ring system, measuring the thermal emission of the rings. This is part of an observation campaign by CIRS to measure how Saturn's rings respond thermally to the much lower sun angle as the Saturn system approaches equinox in August. On April 22, Cassini will image Saturn's B ring, looking for the formation of dust spokes. On April 23, ISS will take an astrometric observation of several of Saturn's small satellites including: Epimetheus, Prometheus, and Atlas. 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. Finally, ISS will perform a wide-angle mosaic observation of the entire ring system, looking at the lit face of the rings.
Cassini reaches periapse, its closest point to Saturn during Rev109 early on April 24. At this point, Cassini will be 962,000 kilometers (598,000 miles) from Saturn's cloud tops, between the orbits of Rhea and Titan. Shortly after periapse, ISS will observe Titan's sub-Saturn hemisphere from a distance of 1.186 million kilometers (748,000 miles). The bright region Tsegihi should be near the center of the frame. Next, ISS will take an 11-hour movie of the F ring. The observation is designed to study the evolution of channels and streamers created by Prometheus when it is at apoapse in the narrow ring. These effects are predicted to become more pronounced as Prometheus dives deeper into the F ring as the apoapse of Prometheus's orbit and periapse of the F ring become aligned. This alignment will occur in December. ISS also will take another astrometric observation of several of Saturn's small satellites including: Prometheus Pandora, Janus, Daphnis, Epimetheus, and S/2008 S 1. The next day, on April 25, CIRS will acquire another temperature map of the lit face of the ring system.
On April 27, Cassini will perform a non-targeted encounter of Titan, flying by Saturn's largest moon at a distance of 695,883 kilometers (432,401 miles). ISS will acquire six observation sequences to go along with this encounter spread out over three days. Each of these observations consists of a single frame over the center of the visible disk of Titan, rather than the more elaborate mosaic designs used during the non-targeted encounter in the previous orbit. This will allow for cloud tracking between observations and filling in gaps in the current map of Titan, particularly over the northern leading hemisphere. During the encounter, roughly half of Titan's visible surface will be illuminated by the Sun with the phase angle increasing as the flyby progresses. The view of Titan from Cassini starts out over Hotei Arcus and western Tsegihi at the start of the encounter, then progresses northward over western Fensal, east of the large impact basin Menrva, and finally over the northern trailing hemisphere. In addition to these observations of Titan, ISS also will take some time to observe Rhea as Cassini, 779,000 kilometers (484,000 miles) away, crosses the ring plane. The goal of this observation is to search for albedo markings on Rhea's leading hemisphere along its equator. These markings would be related to the ring system in orbit around the icy satellite.
Following the non-targeted encounter of Titan, on April 29, ISS will take another astrometric observation, this time taking a look at Prometheus, Janus, Methone, Anthe, and Pan. ISS also will take a look at a crescent Titan, 1.199 million kilometers (749,000 miles) away. ISS will be examining the haze layers over Titan's north polar region. Finally, ISS will observe the shadow of Tethys as it cross the A and B rings of Saturn. VIMS also will search for spokes on the unlit face of the B ring.
On the last two full days of Rev109, ISS will acquire two more astrometric observations, observing the small satellites Pallene, Atlas, Daphnis, Prometheus, Methone, Pan, Epimetheus, Janus, and Pandora. ISS also will observe the shadow of Mimas as it crosses the A and B rings. Finally, on May 1, UVIS and ISS will perform more instrument calibrations using the star Spica.
Cassini reaches apoapse on May 2, bringing Rev109 to an end and starting Rev110. Rev110 will include Cassini's 55th flyby of Titan, T54.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).