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Cassini's journey at Saturn continues with Rev 48, its 49th orbit of the ringed planet. Cassini has a full plate during this month-long orbit, as the spacecraft observes Titan's trailing hemisphere, Saturn's moons (notably Helene) and ring system, and the planet itself. Cassini begins Rev48 on July 9 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 2.8 million km (1.7 million mi) from Saturn. The first week of Rev48 is filled with observations of Saturn's small satellites (to refine their orbits), faint rings, and atmosphere. Several observations of the E- and G-rings occur between July 13 and 17. These observations are designed to study the sizes of the particles that make up these two faint rings. Such studies are preformed by measuring the brightness of a ring at different phase angles (whether the Sun is behind, to the left or right, or in front of the spacecraft with respect to the ring in question). The brightness of a ring can change depending on the sizes of the particles that make up the ring.
Over the next few orbits, Cassini's path will lie close to Saturn's ring plane. With the exception of the E- and G-ring studies, such orbits are not optimal for observing the ring system. These orbits are often dedicated to the study of Saturn's icy moons. During orbit 49, numerous distant observations are planned for the mid-sized icy satellites, in order to study their brightness at different phase angles. Like the earlier rings observations, these are designed to improve scientists' understanding of particle sizes on the surfaces of these moons and to see how the phase-angle dependent response changes across the surface of the moons due to differences in composition and grain size.
The observations during this first week include distant images of the outer satellite Erriapus and several of Saturn's inner satellites. Cassini reaches periapse, the closest point in its orbit, on July 20 when the spacecraft is 320,000 km (200,000 mi) above Saturn's cloud-tops. Shortly before periapse, Cassini will encounter the small satellite Helene at a distance of 36,500 km (22,700 mi). This is the closest encounter with this 32-km (20-mi) wide moon during the primary mission, which runs through June 2008; although an even closer pass of only 1,500 km (930 mi), is being planned for the extended mission, on March 3, 2010. Helene is a Trojan satellite, sharing Dione's orbit but staying 60 degrees or 400,000 km (250,000 mi) ahead of the much larger moon. Images taken in February and August 2006 revealed an irregularly shaped moon with a relatively smooth surface. Images with a resolution as high as 215 meters per pixel, spectra in the near-infrared and ultraviolet, and thermal observations are planned for this small satellite during the July encounter. Cassini encounters Titan for the 35th time on July 19, with a closest approach distance of only 1,332 km (830 mi). Unlike the last several encounters with Titan, this flyby (known as T34) will allow for imaging of the Saturn-facing portion of Titan's trailing hemisphere, centered near the dark, equatorial region named Senkyo. ISS and the Visual and Infrared Mapping Spectrometer (VIMS) will be observing Titan throughout the encounter, from 16 hours prior to 9 hours after closest approach. Prior to the encounter, Cassini will observe Titan at high phase angles. The VIMS instrument will observe the surface and how it changes in brightness at different phase angles; ISS will do the same for the atmosphere and will also attempt some surface observations (although these are not as optimal as the lower phase angle images planned for after closest approach). VIMS will be controlling spacecraft pointing during the first couple of hours after closest approach. Several high-resolution VIMS image "cubes" (or datasets) are planned for the period around closest approach. Several observations are planned for Titan's dark, equatorial terrain, in northwestern Shangri-la and Belet. With pixel scales approaching 300 m/pixel for VIMS, that team hopes to resolve individual longitudinal dunes that make up Titan's dark terrain and measure the spectra of these dunes (hopefully, to determine their composition). Fits to data taken during an encounter in October 2006 suggest that the sand-sized grains that make-up the dunes may be made of benzene.
ISS will also perform high resolution observations starting around an hour after closest approach. These observations will examine a 630-km wide bright region in the north-central part of Senkyo. RADAR SAR observations acquired in July 2006 revealed several river-like channels within this bright region, and these ISS observations are designed to examine the possibility of observing such features in the near-infrared. The VIMS instrument will be making similar observations. Beginning five hours after closest approach, Cassini will acquire a global mosaic designed to map Titan's surface, search for surface changes in the six months since the previous ISS observations of this region, and look at changes in Titan's weather patterns as the moon approaches its August 2009 equinox.
The only major exception to ISS' almost continuous observations of Titan will be during a one-and-a-half hour period just before closest approach, when the radio science team will perform a bistatic observation of Titan's surface. The bistatic experiment involves using the high-gain antenna to bounce a radio signal off the surface of Titan, which will then be received by radio telescopes on Earth. The experiment is used to search for specular reflections on Titan's surface and to study its electrical properties. (Specular reflection is indicative of a very smooth surface, and is often caused by liquids.) The region examined by the radio science team during this orbit is in central and western Adiri, in an area observed by RADAR SAR in October 2005 and by ISS during the previous orbit. The RADAR team found this region to be quite rough, with several linear mountains and an area that is pockmarked with numerous circular features, all around 3-5 km across (likely too small to be impact craters). While the chances of finding specular reflections would seem slim based on the SAR observations, studying the same terrain as SAR will provide the radio science team with a useful calibration point for interpreting their data. Following its closest approach to Saturn on July 20, Cassini will spend much of its time observing Saturn's various icy satellites from a distance, as well as the planet itself. The next few orbits will provide an opportunity to observe Saturn's atmosphere at low to moderate phase angles. The observations made during this period will be turned into movies examining cloud motions in Saturn's atmosphere. The movies can also be used to study wind patterns and convection in the upper layers of the atmosphere.
Cassini begins the following orbit, number 50 (Rev49), on August 9, during which it will encounter several of Saturn's satellites. Most notably, Cassini will perform its only close encounter with the perplexing, two-toned moon Iapetus. Cassini will examine a number of the odd features found at this satellite, including a series of ridges and mountains that lies along the moon's equator, dark material centered on its leading hemisphere, and several large impact basins. As a sort of preview image, near the end of Rev48, Cassini will image Iapetus from more than 7 million km (4.4 million mi) away, from across Saturn's north pole. During Rev49, Cassini will also perform its second of two encounters with the satellite Rhea as well as a relatively close encounter with Tethys. As always, a close encounter with Titan is also slated for Rev49.