[For trouble viewing the images/movies on this page, go here]
Cassini continues its extended tour of Saturn on with Rev116, the spacecraft's 117th orbit around the Ringed Planet. Cassini begins Rev116 on August 3 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.15 million kilometers (1.34 million miles) from Saturn's cloud tops. During Rev116 on August 11, the Saturn system reaches vernal equinox. During this orbit, with equinox so close, observations only possible during this unique time (the last equinox occurred 15 years ago, in 1995) are taking on a greater importance. These observations include imaging of shadows of eccentric features within the ring system, the ring system becoming more illuminated by Saturn than it is by the Sun, and the thinning ring shadow on Saturn.
Cassini ISS starts its observations for Rev116 the day after apoapse. On August 4 through 7, ISS will take a movie observation each day using the wide-angle camera of Saturn's nightside and the two ansae of the main ring system. These movies are designed to look for periodicities in the spokes that form over Saturn's B ring. The imaging covering the left ansa (if Saturn's north pole is taken as up) will be designed to ensure that Saturn is avoided due to the faintness of the rings 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. More illumination on the rings is coming from Saturnshine rather than directly from the Sun. On August 7, ISS will look at a crescent Titan over the satellite's leading hemisphere from a distance of 1.53 million kilometers (953,000 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 of Titan's position in Saturn's magnetosphere.
On August 8, ISS will acquire several north-to-south scans across Saturn's nightside. With ringshine on Saturn's atmosphere at a minimum, this observation will be used to search for lightning in Saturn's atmosphere.
Cassini encounters Titan on August 9 at 14:04 UTC for the 61st time and the first encounter this month. This flyby is also the ninth 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 970 kilometers (603 miles), close to the lowest safe altitude for a Titan flyby during the extended mission. This flyby (known as T60) will allow for imaging of the southern trailing hemisphere of Titan outbound to the encounter, similar to the area observed during the previous eight encounters in this series. Due to the sequences needed for the equinox observations coming shortly after the flyby, this encounter is relatively short, with observations ending only three hours after the encounter. On approach to Titan, the Composite Infrared Spectrometer (CIRS), ISS, Ultraviolet Imaging Spectrometer (UVIS), and RADAR teams will be controlling spacecraft pointing, or will be considered "prime". With Cassini approaching over the nightside, northern anti-Saturn hemisphere, CIRS will map atmospheric temperature and composition over Titan's north polar region looking for seasonal variations in the north polar vortex. ISS, with the Visual/Infrared Mapping Spectrometer (VIMS) riding along, will observe the crescent of Titan focusing on the haze structures over the north polar region. Following this observation, UVIS will acquire a north-to-south extreme and far ultraviolet spectroscopy scan of Titan's nightside. Like during the ISS observation, UVIS will be focused on Titan's upper haze layers.
During closest approach on T60 and the five hours leading up to it, RADAR will be prime. During the approach to Titan, RADAR will run through many of its various observation modes, such as radiometry, which passively measures the 2.2-centimeter wavelength brightness of Titan's surface, and scatterometry, which measures the large-scale roughness of it. Following these observations, RADAR will acquire a HiSAR, low-resolution imaging swath in central Shangri-la, an albedo feature. This swath is designed to bridge the gap between the T8 and T13 swaths. Such a swath should be helpful for defining Titan's shape. At close approach, RADAR will acquire an altimetry swath that runs along the length of the SAR swath the instrument acquired during T58. This altimetry swath has several purposes. First, it helps to confirm the altimetry-from-SAR results derived from the earlier SAR data, similar to how the altimetry swath in T30 was used. Second, it provides another opportunity to acquire altimetry data over Ontario Lacus, a hydrocarbon lake in Titan's south polar region, discovered by ISS in June 2005. Altimetry can provide height information for Titan's landforms, and, by examining the shape of the individual pulse returns, the RADAR team can look for specular reflection from smooth surfaces such as a lake. Third, the RADAR team would like to use the swath to help refine their global shape results. Following this altimetry swath, RADAR will acquire a short SAR swath over several bright streaks observed by ISS and VIMS on Titan's trailing hemisphere. The VIMS team has interpreted these features as mountain ranges, and this SAR data should be able to confirm or refute that theory. Following RADAR's prime observation, ISS will finish up observations for T60 by acquiring an eight-frame mosaic over a bright-dark boundary in southwestern Senkyo, another of Titan's albedo features. This includes the potential to directly observe dunes in this area. These dunes wrap around topographic structures in this area to form numerous streamlined forms.
On August 10, following the Titan flyby, ISS will take a 15-frame mosaic along the boundary of Saturn's shadow in the faint D, F, G, and E rings. Also, ISS will look for shadows of structures within the A and B rings. Such eccentric ring features include vertical structures stirred up by moons such as Daphnis and large ring particles like those that create propellers. The very low solar incidence angle makes the appearances of these "topographic" structures more prominent. Finally, VIMS and ISS will search for spokes on the B ring.
On August 11, Cassini reaches periapse, its closest point to Saturn on Rev116. At this point, Cassini will be 116,650 kilometers (72,483 miles) from Saturn's cloud tops, just outside the orbit of the G ring. Earlier in the day, at 00:15 UTC, Saturn reaches a milestone of its own, as the system reaches vernal equinox. Equinox, when the sun shines directly over the equator of Saturn, occurs twice each Saturnian year (or about once every 15 Earth years). This equinox marks the beginning of spring in the northern hemisphere of Saturn and its major satellites, and fall in the southern hemisphere. Also during this time, the shadow of the rings on Saturn's atmosphere, a dominant fixture in Cassini's earlier images of the planet, effectively disappears as the rings appear narrower than the sun if viewed from the planet's equator (looking up from Saturn's equator, the sun would appear with a width of 1/20th of a degree across). From this point on, until 2029, the shadow of Saturn's rings will appear over Saturn's southern hemisphere.
For the first day of spring on Saturn's northern hemisphere, Cassini ISS will acquire two observations. First, ISS will acquire ride-along observations of the ring system, while CIRS acquires temperature scans of the system. Second, ISS again will look for shadows of structures within the rings now that the solar incidence angle is at its minimum. This will allow even small-scale deviations above or below the ring plane, such as warping of rings, to be visible.
The next day, August 12, ISS will acquire a 19-frame, wide-angle-camera mosaic of Saturn's northern hemisphere and the lit face of the main ring system, the geometry of which is depicted at right, though due to limitations in the visualization software used, the rings appear much brighter than they will appear in ISS images during this observation. In this observation, the rings should appear quite dark with only one day having passed since equinox. The primary source of illumination at this point will be Saturn, so there will be a gradient in ring brightness from the Saturn sub-solar point on the left side of the mosaic toward the anti-solar point on the right. The ring shadow will appear as a faint dark line along Saturn's equator as the rings still appear narrower than the Sun from that vantage point. In addition, ISS will observe Titan's sub-Saturn hemisphere from a distance of 2.18 million kilometers (1.35 million miles) as well as some of the larger propellers within the Saturnian ring system. Propellers are voids in Saturn's rings created by the gravitational interaction between 100-meter-wide ring particles and smaller particles in their vicinity.
On August 14, Cassini ISS will observe Titan's sub-Saturn hemisphere from a distance of 2.6 million kilometers (1.62 million miles). In addition, ISS will take an astrometric observation of several of Saturn's small satellites including Telesto, Janus, Pallene, Atlas, and Pan. 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. On August 14 to 16, ISS will take an observation each day of the shadows of some of the structures found within the ring system.
On August 17, ISS again will acquire a north-to-south scan using the narrow-angle camera across the nightside of Saturn, again looking for lightning in the planet's atmosphere. Also, VIMS will look for spokes on Saturn's B ring.
Cassini reaches apoapse on August 19, bringing Rev116 to an end and starting Rev117. Finishing up Rev116, ISS will observe Saturn using the narrow- and wide-angle cameras and in numerous filters. The observation will provide another great look of Saturn's northern hemisphere and the thin shadow of the ring system. ISS also will look at shadows of features in the ring system.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).