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Cassini begins the three-week Rev150 on June 29 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.69 million kilometers (1.67 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, Cassini will be able to image Titan's equatorial region and Saturn's atmosphere unobstructed by its rings. Thirty-three ISS observations are planned for Rev150, the vast majority designed to monitor the large northern hemisphere storm first seen in December 2010.
ISS begins its observations for Rev150 on July 1 with a pair of dark current calibration observations for both the narrow-angle (NAC) and wide-angle (WAC) cameras. These images will be taken with the shutter closed and are designed to measure the response of the two cameras' CCDs when they are not actively exposed to light. This dark current response can change over the course of the mission so this observation is being taken to update the ISS calibration software, CISSCAL. On July 3, ISS will acquire a Titan monitoring observation. Titan will be 3.39 million kilometers (2.11 million miles) away at the time. A half-phase Titan will be visible from Cassini, allowing surface features and clouds across Titan's Fensal-Aztlan region to be observed. Immediately afterward, ISS will acquire an astrometric observation of Saturn's small, inner moons, including Prometheus, Epimetheus, Calypso, Pallene, Helene, and Polydeuces. After the astrometric observation, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) as that team images the G ring. On July 5, ISS will acquire another observation of Saturn's small, inner satellites, this time taking a look at Helene, Pallene, Anthe, Prometheus, and Janus.
On July 9 and 10, with Cassini speeding to periapse, its closest point to Saturn in its orbit, ISS will ride along with VIMS to image the night side of Saturn. NAC images will be taken to search for lightning while WAC images will be used to monitor Saturn's clouds illuminated by the rings. Three observations are planned: a 12-hour sequence on July 9 and two, two-hour sequences on July 10. Three minutes after the end of the first observation on July 10 at 15:59 UTC, Cassini will reach periapse for Rev150 at a distance of 183,410 kilometers (113,970 miles). The next day, July 11, ISS will image the Senkyo region of Titan from a distance of 1.78 million kilometers (1.11 million miles), again to look for clouds and to monitor surface changes first seen late last year. On July 12, ISS will take a two-by-two mosaic of Saturn using the WAC in a series of filters that include red, green, and blue for true color imaging. This mosaic will be taken while VIMS acquires a movie of Saturn's northern hemisphere.
Beginning on July 13, ISS will acquire "storm watch" WAC observations of Saturn during brief periods when Cassini happens to be pointed at the planet. These two-minute sequences include summed images taken in blue, clear and two methane band filters. Each of those sequences will include one full-frame (not summed), continuum band image taken with the CB2 filter. In addition to the set taken on July 13, thirteen more will be taken between then and July 20. Also on July 13, ISS will take a low-phase, low elevation (within the ring plane) observation of the E ring. The observation is intended to acquire color photometry of the diffuse ring and to study its vertical profile, or its vertical characteristics with respect to the main ring plane and the sun, now that we are past equinox. On July 14, in addition to six storm watch observations, ISS will search for small satellites around Rhea's L4 Lagrange point, image the sub-Saturn hemisphere of Titan to look for clouds, and observe a Tethys transit of Titan's south pole. The Tethys-Titan mutual event observation occurs with Cassini being 1.86 million kilometers (1.54 million miles) from Tethys and 3.21 million kilometers (2.00 million miles) from Titan.
On July 17, two Saturn storm watch observations are planned as well as another Titan cloud monitoring sequence covering the Fensal-Aztlan region and an astrometric sequence. The astrometry observation will target the small moons Epimetheus, Prometheus, Atlas, Pandora, Anthe, Methone, Pallene, Helene, and Janus. On July 18, in addition to two Saturn storm watch observations, ISS will image Dione as it passes in front of the southern half of Rhea. Dione will be 2.19 million kilometers (1.36 million miles) away, while behind it Rhea will be 3.09 million kilometers (1.92 million miles) from Cassini. During this transit, portions of Rhea's northern hemisphere will be obscured by Saturn's main rings. Finally on July 20, ISS will take one more astrometric observation, this time imaging Calypso, Polydeuces, Helene, Pandora, and Epimetheus. The WAC will also take the final two storm watch observations for this orbit.
On July 21, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev151. Rev151, like this one, 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).