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Cassini continues its extended tour of Saturn on April 4 with Rev108, the spacecraft's 109th orbit around the Ringed Planet. Cassini begins Rev108 early on April 4 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 1.14 million kilometers (708,000 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 encounters Titan on April 4 at 01:48 UTC for the 53rd time since 2004 and the third time of 2009. This flyby is the second of a pair of Titan flybys that will occur only eight days apart. The first took place on March 27. This second encounter is one of the more distant of Cassini's targeted Titan flybys, with a close approach distance of 4,150 kilometers (2,578 miles). This flyby, known as T52, will allow imaging of the southern trailing hemisphere of Titan outbound to the encounter, providing for the first time in a year a new region for Cassini's cameras to observe. This encounter is also the first flyby since May 2008 where Titan is more illuminated during the outbound segment of the encounter, so more surface features will be revealed to Cassini as the spacecraft leaves the moon after closest approach rather than as the spacecraft approaches the moon. On approach to Titan, the Composite Infrared Spectrometer (CIRS), the Visual and Infrared Mapping Spectrometer (VIMS), ISS, the Ultraviolet Imaging Spectrometer (UVIS), and RADAR will trade off control of spacecraft pointing and be considered the "prime" instrument at the time they control pointing. 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 wavelengths. 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. RADAR will scan Titan's far north in radiometry mode to fill in gaps in that instrument's brightness temperature map of the moon'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 T52, the Radio Science Subsystem will be prime. The RSS team will acquire a radio occultation and a radio bistatic observation during this encounter. The radio bistatic observation uses a signal from the high-gain antenna that is then bounced off a point on Titan's surface, which is then received back on Earth. The returned signal is analyzed to better understand the electrical properties of Titan's surface, which are affected by composition and roughness. For the radio occultation, Cassini's high-gain antenna will transmit a signal to Earth as Titan passes between Earth and the spacecraft. This will allow the radio science team to probe the atmosphere of Titan right down to the surface at high resolution by seeing how Cassini's signal is attenuated by the increasing density of the atmosphere as one approaches the surface. The signal can also be affected by changes in wind speed at different altitudes. This radio occultation is of high priority to the RSS team because it will allow them to examine the equatorial atmosphere of Titan near the surface for the first time since the Voyager 1 encounter in November 1980. RSS also will take two bistatic observations before and after the radio occultation. RSS will acquire swaths over the northern sub-Saturn hemisphere and the southern trailing hemisphere. This will allow the team to better understand the composition of the surface in the regions, particularly in the northern of the two sequences as this region is expected to be reasonably flat. Rough surfaces tend to complicate the interpretation of RSS bistatic data.
Following close approach, CIRS and ISS will trade off being prime. ISS in particular will acquire three mosaics during this outbound segment of the encounter. The first, REGMAP001, will consist of a seven-tile mosaic covering a region near 60 degrees south, 275 degrees west. REGMAP001 will allow the imaging team to fill in one of the remaining blank spots on the map of Titan at high resolution. GLOBMAP001, acquired from distances ranging from 103,000 to 171,000 kilometers (64,000-106,000 miles), will cover most of the visible surface (shown at right) using 26 footprints consisting of three images a piece. MONITORNA001 is a similar observation from farther out, taken from distances ranging from 242,000 to 269,000 kilometers (150,000-167,000 miles), and covering the visible surface of Titan using 11 footprints. These observations again will help fill in a low-resolution gap in our map of Titan in the southern trailing hemisphere. Among the features of particular interest is a dark albedo region to the east of Mezzoramia nicknamed "The Wolverine" (due to its shape resembling the head of a snarling animal). Mezzoramia and "The Wolverine" maybe dried up analogs to Kraken Mare in the northern hemisphere. 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 wavelengths.
For much of Rev108, Cassini imaging primarily will be focused on Titan and Saturn's other satellites with a few observations of rings mixed in. On April 5, 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. ISS also will observe two outer moons of Saturn, Narvi and Thrymr, as part of a campaign to better understand the photometry of the small satellites. Cassini can look at these moons at much greater phase angles than can be achieved from Earth. However, both moons are too small and distant from Cassini to see them as anything more than a point of light in these images. On April 6, Cassini again will observe Titan, this time observing the southern sub-Saturn hemisphere from a distance of 976,000 kilometers (607,000 miles). This should provide an excellent opportunity to observe Mezzoramia, which was much closer to the limb during the T52 encounter. On April 7, Cassini ISS will take a look at the shadows of Mimas and Tethys on the outer A ring.
Cassini reaches periapse, its closest point to Saturn on Rev108, late on April 8. At this point, Cassini will be 1.067 million kilometers (663,000 miles) from Saturn's cloud tops, between the orbits of Rhea and Titan. Shortly before periapse, ISS will image Titan's sub-Saturn hemisphere from a distance of 1.204 million kilometers (748,000 miles). The bright region Tsegihi should be near the center of the frame of the image. ISS also will acquire an astrometric observation of several of Saturn's small satellites including: Prometheus, Epimetheus, Methone, Janus, Pan, and Pandora. Astrometric observations are used to help provide better orbital calculations for some of these small rocks, particularly ones like Methone, which was discovered by Cassini only four years ago. Finally, ISS will take a look at the shadow of Mimas as it crosses Saturn's outer A ring.
ISS observations on April 9 are very similar to those taken the day before. ISS will observe Titan over the western part of Tsegihi from a distance of 1.027 million kilometers (638,000 miles). ISS will also acquire another astrometric observation, this time of Pallene, Prometheus, Methone, Helene, and Atlas, as well as another observation of shadows of Mimas and Tethys on the outer A ring. On April 10, Cassini will take yet another astrometric observation of Janus, Atlas, Epimetheus, Prometheus, and Pan, as well as a movie of spokes forming on the lit face of Saturn's B ring.
On April 11, Cassini will perform a non-targeted encounter of Titan, flying by Saturn's largest moon at a distance of 326,590 kilometers (202,933 miles). ISS will acquire three observation sequences to go along with this encounter. The day before the flyby, ISS will take CLOUD001, a four-image mosaic covering the entire visible surface of Titan. The mosaic's four images will be taken from distances ranging from 500,000 to 600,000 kilometers (310,000-372,000 miles). This observation is centered on Hotei Arcus, a possible cryovolcanic feature in the southeastern portion of Xanadu. The second observation, an eight-frame mosaic named CLOUD002, will be taken around closest approach and will cover the region shown in the image above-left. This mosaic will provide our highest resolution observations to date of the large impact basin Menrva as well as the region to the north and east of it. CLOUD003 is a single-frame observation taken shortly after the end of CLOUD002 and is designed to look for clouds north of Menrva as well as provide ISS with its best observation to date of Ganesa Macula, a RADAR-dark, circular feature once thought to be a volcanic dome, but which instead provides a cautionary tale of why it can be very difficult to interpret the geology of Titan surface features.
On April 12 and 13, Cassini ISS will take two more astrometric observations of Saturn's small moons including: Pallene, Enceladus, Telesto, Anthe, Prometheus, Epimetheus, Pan, Pandora, and Daphnis. On April 14, ISS will turn its cameras to Saturn's outer A ring to look at the shadow of Mimas. On April 15, Cassini will look at another outer satellite, this time Ijiraq. Finally, on April 16, ISS will observe a crescent Titan from a distance of 1.2 million kilometers (745,000 miles) as well as acquire a movie of Saturn's F ring.
Cassini reaches apoapse on April 16, bringing Rev108 to an end and starting Rev109. Rev109 will include Cassini's 54th flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).