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Cassini continues its extended tour of Saturn with Rev113, the spacecraft's 114th orbit around the Ringed Planet. Cassini begins Rev113 on June 17 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 1.81 million kilometers (1.12 million miles) from Saturn's cloud tops. 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 Rev113 the day after apoapse. On June 18, ISS will take 97 images of the outer satellite Paaliaq over a period of nearly eight hours. This observation is designed to measure the light curve of this small satellite, which can be used to derive the rotational period of the small moon, and better understand the photometric properties of irregular outer satellites such as Paaliaq. On June 19 and 20, ISS will acquire two movie observations of the unlit face of Saturn's B ring, looking for spokes forming over the now quite faint ring system. ISS also will look at a crescent Titan from a distance of 1.3 million kilometers (810,000 miles). While this observation would not be good for observing surface features on Titan due to the high phase angle, these images would 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. Also on June 19, ISS will observe the shadow of Janus as it crosses Saturn's main ring system. On June 21, the Ultraviolet Imaging Spectrometer (UVIS) will observe Enceladus from a distance of 1.47 million kilometers (915,000 miles). UVIS is continuing to monitor the amount of water vapor that is released from Enceladus's south polar plumes.
Cassini encounters Titan on June 22 at 18:32 UTC for the 58th time and the second encounter this month. This flyby is also the sixth in a series of 10 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), the lowest safe altitude for a Titan flyby during the extended mission. This flyby, known as T57, will allow for imaging of the southern trailing hemisphere of Titan outbound to the encounter, similar to the area observed during the previous five encounters in this series. On approach to Titan, the Composite Infrared Spectrometer (CIRS), ISS, and UVIS teams will be controlling spacecraft pointing, or will be considered "prime." CIRS will focus on measuring Titan's atmospheric composition by mapping the visible disk of Titan in the far-infrared and conducting a far-infrared limb sounding observation over Titan's southern tropics, looking for minor chemical components in Titan's atmosphere. The ISS team will use their time observing the crescent Titan using the wide-angle camera, looking for cloud bands and changes in the haze layers over Titan's north polar region. UVIS will acquire a north-south oriented, spectroscopy scan in the far- and extreme-ultraviolet spectrum of Titan at high phase angles.
During closest approach on T57, the Radio Science Subsystem (RSS), Ion and Neutral Mass Spectrometer (INMS), and RADAR will be prime. Between 30 minutes and one hour before closest approach, RSS will perform a radio occultation of Titan's north polar atmosphere by beaming a signal to Earth through Titan's atmosphere. By measuring how the radio signal is affected by its passage through Titan's atmosphere to better understand its density structure. In the 40 minutes surrounding closest approach, INMS will measure the composition of Titan's ionosphere over the nightside southern hemisphere of Titan, one of only two prime opportunities for INMS. INMS measures the masses of ion and neutral molecules it encounters as Cassini flies through Titan's upper atmosphere, focusing on the different types of hydrocarbons observed. RADAR will be riding along with INMS during close approach, acquiring a short Synthetic Aperture RADAR (SAR) swath running northeast to southwest from southwestern Xanadu (around 15 degrees south, 150 degrees west), across the mid-latitude terrain south of Shangri-la, and ending in the south polar region near 75 degrees south, 200 degrees west. The features that RADAR will observe in this swath include two pairs of dark, crisscrossing lineaments named Bacab Virgae and Perkunas Virgae; an east-west dark lineament in Titan's southern mid-latitude terrain named Hobal Virga; and the northwest portion of Ontario Lacus, a large lake in Titan's south polar region first observed by ISS in June 2005. This swath parallels to the east and south the swath acquired during the previous encounter, T56. Following this SAR swath, INMS observation, and Titan closest approach, RADAR will acquire an altimetry swath near 55 degrees south, 295 degrees west as well as a HiSAR swath along 40 degrees south between 220 and 265 degrees west.
Following close approach, the RADAR, UVIS, ISS, Visual and Infrared Mapping Spectrometer (VIMS), and CIRS teams will trade off being prime. RADAR will acquire radiometry and scatterometry scans across the southern trailing hemisphere. UVIS also will acquire a north-south scan across the visible disk of Titan, measuring latitudinal variations in the far and extreme ultraviolet spectrum of Titan's atmosphere. During the CIRS observation time, the infrared spectrometer will measure Titan's atmospheric composition by mapping the visible disk of Titan in the far-infrared. Finally, VIMS and ISS will acquire a global mapping observation.
Following the T57 flyby, Cassini imaging will be focused on Saturn's ring and satellite systems. On June 24, ISS will acquire two radial scans of the main ring system using the narrow angle camera. This observation is designed to look for shadows of structure within the ring. Shadows of waves along the margins of several of the gaps in the main ring system, such as the Keeler and Huygens gaps, have already been observed, and this observation should provide more details on these types of structures. The gap margin waves are maintained due to eccentricity of the moons that help create the sharp edges of these gaps. Shortly before periapse, Cassini will observe Titan's sub-Saturn hemisphere on June 25 from a distance of 1.32 million kilometer (817,000 miles).
On June 25, Cassini reaches periapse, its closest point to Saturn on Rev113. At this point, Cassini will be 374,910 kilometers (319,560 miles) from Saturn's cloud tops, between the orbits of Dione and Rhea. During periapse, ISS will observe Enceladus's south polar plume at a medium phase angle. Like the UVIS observation earlier in the orbit, ISS will be looking for changes in the position and intensity of the plume compared to earlier observations. Following this observation, Cassini will turn its cameras to Mimas, taking a look at that moon's leading hemisphere from 595,000 kilometers (370,000 miles). Finally, VIMS and ISS will acquire a radial scan of the left ansa of the unlit face of the Saturn ring system to measure the composition of the rings.
On June 26, ISS will observe the shadows of Daphnis and Epimetheus as they cross the Saturnian ring system. The Daphnis observations also will allow ISS to observe the shadows of the waves along the edge of Keeler Gap in front of and behind the small satellite. On June 28, Cassini will turn its cameras to Titan, 2.36 million kilometers (1.47 million miles) away. This observation will allow ISS to observe cloud features across Titan's north polar region. ISS also will take an astrometric observation of several of Saturn's small satellites including Atlas, Prometheus, Pandora, Methone, and Pan. Astrometric observations are used to help provide better orbital calculations for some of these small rocks, which can be effected by gravitational interactions with the larger icy moons. Finally, ISS will take a look at the shadows of Atlas and Tethys as they cross Saturn's ring system.
Wrapping up ISS observations for Rev113, on June 30 and July 1 the camera system will acquire two astrometric observation of Saturn's small satellites, this time taking a look at Polydeuces, Calypso, Prometheus, Pandora, Epimetheus, Pallene, Janus, and Pan.
Cassini reaches apoapse on July 3, bringing Rev113 to an end and starting Rev114. Rev114 will include Cassini's 59th flyby of Titan, T58.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).