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Cassini begins the month-long Rev149 on May 30 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 4.03 million kilometers (2.50 million miles) from Saturn's cloud tops. The spacecraft is in the middle of the first equatorial phase of the Cassini Solstice Mission, a phase which lasts until May 2012. During this phase, the spacecraft's orbits lie within the equatorial plane of the planet as well as the orbital plane of most of Saturn's major moons and within the ring plane. This orbit allows Cassini to image Titan's equatorial region and Saturn's atmosphere unobstructed by its rings. Twenty-five ISS observations are planned for Rev149, including six associated with a Titan flyby planned for June 20. A little more than half of the 25 observations this orbit target Saturn's largest moon. The majority of the remainder will focus on Saturn's icy satellites.
ISS begins its observations for Rev149 on June 4 with a Titan monitoring observation. Titan will be 3.81 million kilometers (2.37 million miles) away at the time. A half-phase Titan will be visible from Cassini, allowing surface features and clouds across Titan's Xanadu region to be observed. Immediately afterward, ISS will acquire an astrometric observation of Saturn's small, inner moons. During this observation, the camera system will image Helene, Anthe, Janus, Calypso, and Prometheus. In addition, the wide-angle camera (WAC) will image Saturn at the end of the Titan and at the beginning of the astrometric observations. These will allow Cassini to continue to monitor the massive storm raging across the planet's northern hemisphere. Starting on June 6, Cassini will take a series of three lengthy observations of Iapetus as the spacecraft performs one of its closest encounters with the distant icy satellite of the second extended mission. These observations run until early on June 10. The closest approach to Iapetus occurs on June 7 at 10:30 UTC at a distance of 862,490 kilometers (535,930 miles), although the closest ISS images will be taken early the next day. These images will allow for imaging of the moon's south polar region. Also on June 10, ISS will perform photometric calibration using the second brightest star in the night sky, Canopus. The observations are designed to check for changes in the narrow-angle camera's point-spread function, or PSF. The PSF is a measure of how the camera system responds to sources that are far enough from the camera to appear as a single point of light, like a distant moon in an astrometric observation. Understanding the PSF ensures the integrity of the data acquired during those types of observations.
On June 11, Cassini will repeat its observations from a week earlier by acquiring a Titan cloud monitoring observation as well as a small satellite astrometric one, each with Saturn storm monitoring images added in just as before. The Titan observation will be acquired from a distance of 2.78 million kilometers (1.73 million miles) and will cover the trailing hemisphere of the moon. The small satellites imaged in the astrometric observation include Helene, Pallene, Atlas, Janus, and Telesto. Another Titan monitoring observation (with Saturn WACs added on to the end) will be taken on June 13 from a distance of 3.06 million kilometers (1.90 million miles); this time Cassini will image the Senkyo region of Titan. Following this Titan observation, two dedicated Saturn monitoring observations are planned. During both, the WAC will acquire images every two hours in order to track the evolution of the northern hemisphere storm system. On June 14, another astrometric observation, with added Saturn imagery, is planned. This time ISS will take a look at Telesto, Pallene, Epimetheus, Pandora, Methone, and Anthe. Another dedicated Saturn monitoring observation is also planned immediately afterward. Finally, on June 16, ISS will track Rhea as the heavily cratered moon passes in front of Titan. Rhea will be 1.83 million kilometers (1.14 million miles) away, while behind it, Titan will be 2.54 million kilometers (1.58 million miles) from Cassini.
On June 18 at 23:46 UTC, Cassini will reach the periapse of Rev149. At a distance of 294,710 kilometers (183,120 miles), this will be the closest point to Saturn in this orbit. A single ISS observation is planned for the periapse period thanks to a close encounter with the small moon, Helene. At 19:31:37 UTC, Cassini will make it closest approach to the small moon with a distance of 6,968 kilometers (4,329 miles) from its center. During the encounter, Cassini will approach Helene from the moon's night side, acquiring first 20 clear-filter images every minute. Then the cameras will switch to 14 sequences of red, green, blue, IR3, IR1, and UV3 filters. Each of these color sequences includes one clear filter image. Seven clear filter images taken after those sequences will finish off the flyby. As Cassini passes toward Helene's sunlit side, the spacecraft will image the moon's north polar region as well as the Saturn-facing side of Helene, a region seen only in Saturn-shine during the last encounter in March 2010. This should help to finish off a global map of the satellite, which can be useful to assess the impact history of Helene as well as provide a global survey of the "gullies" seen during the last encounter.
Two days after periapse, Cassini encounters Titan on June 20 at 18:32 UTC for the 78th time. This is the fourth of six Titan flybys planned for 2011 with the next encounter scheduled for September 12. This flyby, T77, is a low-altitude flyby with a close-approach distance of 1,358 kilometers (844 miles). This flyby will allow for imaging of the anti-Saturn hemisphere of Titan outbound from the encounter. The Huygens landing site will be near the center of the visible disk during the outbound leg. The Ultraviolet Imaging Spectrometer (UVIS) will be the primary pointing instrument during the inbound leg of this flyby while Titan is visible as a narrow crescent. The instrument will perform a number of spectral measurements of Titan's atmosphere. ISS will acquire images during the UVIS observations. These are designed to study the moon's high altitude haze layers. Currently, the moon is undergoing a shift as the north polar hood disappears and a south polar hood (visible as complex layers of high altitude haze) develops.
The RADAR team will control pointing during the two hours before and after closest approach. This is the RADAR group's only observations of Titan for 2011. First, the RADAR team will acquire a scatterometry observation covering the equatorial region of Titan's sub-Saturn hemisphere. Next, when Cassini is 26,000 kilometers (16,000 miles) from Titan, it will take an oblique high-altitude synthetic aperture radar (SAR) observation that will cover far northwestern Fensal, an area that will bridge the SAR coverage gap between the large impact basin Menrva and the smaller, central peak crater, Ksa. Starting at a distance of 4,800 kilometers (3,000 miles) and continuing until Cassini is 1,654 kilometers (1,027 miles) above Titan, the RADAR team will acquire a short SAR swath across northwestern Fensal, including over Ksa. When combined with the T17 data from September 2006, the team will be able to obtain stereo coverage of the crater. Armed with improved topographic information of the site, RADAR scientists hope to learn more about the structure of Titan's upper crust when the crater formed. Cassini will then turn its high-gain antenna so that it is pointed straight down at Titan so that it can acquire an altimetry swath across Xanadu and the Xanadu/Shangri La boundary. The swath will help in determining Titan's global shape as well as provide more information on the Xanadu's high radar backscatter. As Cassini departs from Titan, the RADAR team will acquire another high-altitude SAR swath, this time covering southwestern Xanadu and western Tui Regio, an area that is unusually bright in the near-infrared. Tui Regio could be a dry sea bed or it could have a cryovolcanic origin. Finally, RADAR will acquire more scatterometry data, this time covering the equatorial region on the anti-Saturn hemisphere.
Afterward RADAR's observation period, the Composite Infrared Spectrometer (CIRS) and the Visual and Infrared Mapping Spectrometer (VIMS), will have control of pointing with ISS riding along. The ISS observations are designed to track clouds across Titan's anti-Saturn hemisphere, if any are visible, and to monitor changes within Adiri.
ISS will finish up Rev149 with a series of Titan observations -- the first in a series of "Titan Exploration at Apoapse," or TEA campaigns. These observations are designed to allow ISS and VIMS to acquire low phase angle images in order to monitor cloud systems on Titan. This first campaign includes four separate observations that run as long as 21 hours. There will be one observation each day between June 22 and 25. Sets of images will be taken every hour in order to monitor the evolution of cloud systems if they are visible. The first observation on June 22 will cover the region south of Belet that saw a massive storm dump heavy rains across a region 2,000 kilometers (1,200 miles) long and 130 kilometers (80 miles) wide. When last seen in April, much of the area had dried up, though a pair of playas does remain. This observation will also be useful for monitoring these temporary lakes.
On June 29, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev150. Rev150 is primarily a Saturn-focused orbit, with a number of observations planned specifically to study the great northern hemisphere storm.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).