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Cassini begins the 26-day Rev147 on April 3 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 3.21 million kilometers (1.99 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. For this orbit, this equatorial positioning allows Cassini to image Titan's equatorial region and Saturn's atmosphere unobstructed by its rings. Twelve ISS observations are planned for Rev147, including two associated with a Titan flyby planned for April 19 and five other non-flyby observations that will target this largest Saturnian moon.
On April 17 at 09:46 UTC, Cassini will reach the periapse of Rev147. At a distance of 223,930 kilometers (139,140 miles), this will be the closest point to Saturn in this orbit. Only two observations are planned prior to periapse in Rev147. Both are Titan observations with the first planned for April 6 and the second for April 12. The April 6 observation will be taken from a distance of 2.04 million kilometers (1.27 million miles) and will provide another opportunity to monitor surface changes across the southern boundary of Belet and within the bright region Adiri. These observations and others like them this orbit will tell scientists how methane is drained across the surface after a major rain storm as well as provide a measure for how quickly stratospheric debris is deposited on the surface. The April 12 observation will be taken from a distance of 3.19 million kilometers (1.98 million miles). Centered over Titan's sub-Saturn hemisphere, the observation will allow ISS researchers to monitor clouds on the moon in the days leading up to the T75 flyby.
Two days after periapse, Cassini encounters Titan on April 19 at 05:00 UTC for the 76th time. This is the second of six Titan flybys planned for 2011 with the next encounter scheduled for May 8. T75 is a high altitude flyby with a close-approach distance of 10,053 kilometers (6,246 miles). This flyby will allow for imaging of the anti-Saturn and trailing hemispheres of Titan outbound to the encounter. Inbound to this flyby, the Cassini Plasma Spectrometer (CAPS) will be prime. CAPS will be measuring magnetospheric interactions with the moon near the dusk terminator of Titan.
ISS has no prime observations during this flyby, but will be used after closest approach during a pair of observations by the Ultraviolet Imaging Spectrometer (UVIS). The first observation has UVIS slowly scanning across the moon's equator and ISS will snap wide-angle-camera images (WACs) during parts of the scan. This observation is primarily a photometry observation of Titan, but it does include true-color images of the satellite (using the RED, GRN, and BL1 filters). During the second observation, UVIS will stare at Adiri located at the center of Titan's disk. ISS will capture 80 images during this observation, most of them CB3 surface frames, though a few will be taken in other filters to determine cloud heights, if any are visible. On the surface, researchers will be interested in taking a closer look at a group of bright features within Adiri that were first seen after last year's arrow storm. This observation will be researchers' first close-up look at them. These features are thought to be bright areas that had been covered with dust-sized particles that precipitated out of the stratosphere. Last year's storm may have washed this dust coating away in select regions.
Cassini will continue to monitor clouds on Titan during a pair of observations on April 20 and a single observation on April 25. The two observations on April 20 are also known as "caboose" observations because they are attached to the end of a Titan flyby. These two, five-hour observations will allow researchers to monitor clouds if they are present and to track their motions. Even without clouds, the two observations will provide the best images to date of the southern boundary of Belet where extensive changes were observed during the final quarter of 2010. As with the more distant images taken on April 6, these images will tell us more about how methane rain is drained in this region of Titan. The bright spots within Adiri will also be clearly visible.
On April 22 and 25, ISS will perform a pair of astrometric observations of Saturn's small, inner moons. During these two observations, the camera system will image Polydeuces, Pallene, Prometheus, Helene, Telesto, Methone, and Pandora. In addition, the wide-angle camera (WAC) will image Saturn. These images will allow Cassini to continue to monitor the massive storm raging across the planet's northern hemisphere.
ISS will finish up Rev147 with a pair of Saturn monitoring observations and one of a Rhea-Dione-Enceladus mutual event. The mutual event observation will be taken on April 25 and involves Enceladus and Dione passing first behind Rhea and then behind the dark limb of Saturn. Rhea will be 2.23 million kilometers (1.38 million miles) away from Cassini, while Dione and Enceladus will be 3.12 million kilometers (1.94 million miles) and 2.98 million kilometers (1.85 million miles) away, respectively.
On April 29, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev148. A Titan flyby (T76) is planned for May 8.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).