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Cassini begins the month-long Rev148 on April 29 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.85 million kilometers (1.77 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 and within the ring plane as well as the orbital plane of most of Saturn's major moons. For this orbit, an equatorial orbit allows Cassini to image Titan's equatorial region and Saturn's atmosphere unobstructed by its rings. Thirty ISS observations are planned for Rev148, including 11 associated with a Titan flyby planned for May 8. A little more than half of the 30 observations this orbit target Saturn's largest moon. The majority of the remainder will focus on Saturn or its rings.
ISS begins its observations for Rev148 on May 2 with a ride-along sequence with the Visual and Infrared Spectrometer (VIMS), imaging the sunward ansa of the E ring. The E ring is a diffuse torus of icy material orbiting Saturn that is sourced in large part by material from Enceladus' south polar jets. This observation will last for 15 hours and is designed by the VIMS team to help capture phase curve and color data of faint rings like the E ring. On May 3 and again on May 7, ISS will perform a pair of astrometric observations of Saturn's small, inner moons. During these two observations, the camera system will image Methone, Telesto, Janus, Polydeuces, Calypso, Prometheus, and Pandora. In addition, the wide-angle camera (WAC) will image Saturn, allowing Cassini to continue to monitor the massive storm raging across the planet's northern hemisphere. On May 4, ISS will ride along with the Composite Infrared Spectrometer (CIRS) to acquire a set of WAC images of Saturn. On May 6, ISS perform photometric calibration using the bright star Vega. Since Vega is visible and its near-infrared spectrum is well known, the results of the ISS calibrated intensity values can be compared to Vega's reference spectrum to fix any issues that remain with the CISSCAL calibration program or allow for recent changes that have occurred on the camera.
Two days before periapse, Cassini encounters Titan on May 8 at 22:54 UTC for the 77th time. This is the third of six Titan flybys planned for 2011 with the next encounter scheduled for June 20. T76 is a moderate-altitude flyby with a close-approach distance of 1,873 kilometers (1,163 miles). This flyby will allow for imaging of the sub-Saturn side and trailing hemisphere of Titan outbound to the encounter. CIRS 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, including temperature and aerosol density profiles along the limb of the moon, mostly over its north pole. ISS will acquire images during the CIRS 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.
During periapse, VIMS will control primary pointing, acquiring high resolution images as the spacecraft passes over northern Adiri and the margins of that bright terrain region with the surrounding dune fields. Afterwards, VIMS (with ISS riding along) will acquire a mosaic across northern Senkyo, a region of dark dunes on Titan's trailing hemisphere. Following closest approach, VIMS will have a few more observations, including a mosaic across the trailing hemisphere of Titan and a stare observation from a greater distance (200,000 kilometers or 124,000 miles). Between those two observations the Ultraviolet Imaging Spectrometer (UVIS) will slowly scan north to south along the morning terminator (the boundary line between night and day) of Titan. Finally, after the second VIMS observation, CIRS will perform a series of scans across Titan. During each of these observations, ISS will ride along, taking images of Titan mostly using the CB3 near-infrared filter. This will allow researchers to track any clouds that might be visible, though no equatorial clouds have been seen since the cloud outbreak in September and October 2010.
On May 10 at 20:44 UTC, Cassini will reach the periapse of Rev148. At a distance of 295,010 kilometers (183,310 miles), this will be the closest point to Saturn in this orbit. Three observations are planned near periapse. The first is an aurora movie that will cover the south pole while Cassini sees the sunlit side of Saturn as a very thin crescent. The second observation also requires high phase angles to pull off. ISS will image Enceladus from a distance of 245,000 kilometers (150,000 miles) in order to image its south polar plumes. About 45 minutes into the observation, Enceladus will pass into Saturn's shadow. During this period, the moon will still be visible in Saturn-shine. Helene will pass through the background during the observation, appearing below Enceladus' south pole after Enceladus eclipse ingress.
On May 11, as part of its periapse sequence, ISS will acquire its first of four Titan monitoring observations planned for Rev148. This sequence will cover Titan's Saturn-facing hemisphere from a distance of 1.75 million kilometers (1.09 million miles). The other three observations are planned for May 12, May 16, and May 18. The May 12 observation will cover Titan's Saturn-facing hemisphere from a distance of 2.31 million kilometers (1.44 million miles). Saturn's rings will appear in front of a portion of Titan's northern hemisphere. On May 16, ISS will image Titan's leading hemisphere from a distance of 3.13 million kilometers (1.94 million miles). The final observation on May 18 will cover Titan's Xanadu region from a distance of 2.74 million kilometers (1.71 million miles). Each of these observations will also include a short WAC image sequence covering Saturn. Like the Saturn images tacked onto the SATEORB observations, these are intended to improve the frequency with which Saturn is imaged. This allows for better monitoring of Saturn's storm systems, like the one currently active on the planet's northern hemisphere.
On May 12, ISS will ride along with VIMS to image the Saturn ring system as the spacecraft crosses the ring plane while the planet is at a low phase angle of 10 degrees. Also on May 12, ISS will ride along with UVIS to image Saturn's south polar region while the spectrometer scans the planet's aurorae. On May 13 and May 14, ISS will acquire rotational light curve observations of two of Saturn's small, outer moons, Suttungr and S/ 2004 S 12. Suttungr is 5.6 kilometers (3.4 miles) across, while S/ 2004 S 12 is 5 kilometers (3.1 miles) across. A successful detection of S/ 2004 S 12 could provide enough of an orbital determination that it could be given a name by the International Astronomical Union to replace its current provisional designation.
On May 16 and 18, ISS will perform a pair of astrometric observations of Saturn's small, inner moons. During these two observations, the camera system will image Calypso, Epimetheus, Prometheus, Atlas, Telesto, Methone, Helene, Anthe, Janus, and Pallene. In addition, the WAC will image Saturn.
ISS will finish up Rev148 with an E-ring observation with VIMS and a spectacular Titan-Dione-Tethys-Saturn mutual event. Both observations will be taken on May 21. The mutual event observation consists of watching Titan as it passes in front of Saturn. Next, while Titan is still transiting Saturn, it will pass in front of Dione, then Tethys. Cassini will be 2.31 million kilometers (1.44 million miles) away from Titan at the time while Dione will be 3.15 million kilometers (1.96 million miles) away.
On May 30, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev149. A Titan flyby (T77) is planned for June 20. A close, non-targeted encounter with Helene is also planned.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).