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Cassini continues its exploration of the Saturn system with the 32-day Rev 204, which begins on April 27 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 3.38 million kilometers (2.10 million miles) from Saturn's cloud tops. Rev 204 occurs during the first inclined phase, which lasts until March 2015, of the Cassini Solstice Mission. The inclined phase will allow for polar views of Saturn and Titan as well as better vistas of Saturn's rings than those Cassini had while in the earlier, equatorial phase of the Solstice Mission. Thirty-nine ISS observations are planned for Rev 204 with the majority focused on Saturn's atmosphere and on Titan during the T101 flyby.
Five hours into the orbit on April 27, ISS will image Titan, looking for clouds across its northern hemisphere, at a distance of 4.46 million kilometers (2.77 million miles). Afterward, ISS will acquire an astrometric observation of Saturn's small, inner moons. Astrometric observations are used to improve our understanding of the orbits of these small satellites, which can be influenced by Saturn's larger icy moons. Two more astrometric observations will be taken on May 3 and 7. Finally on April 27, ISS will acquire a quick observation of Saturn using the Wide-Angle Camera (WAC). This observation is part of a series of "Storm Watch" observation sequences designed to take advantage of short, two-minute segments when the spacecraft turns the optical remote sensing (ORS) instruments back to Saturn as a waypoint between other experiments' observations. These sequences include blue, clear, two methane band, and one full-frame, continuum band filter images. Ten more Storm Watch observations will be taken between April 28 and May 11. On April 29, and again on May 3, 4, and 7, ISS will take a look at Titan, searching for clouds across the northern hemisphere. This series of observations will be taken from distances ranging from 4.25 million kilometers (2.64 million miles) down to 1.78 million kilometers (1.11 million miles). On May 3, ISS will acquire a set of calibration image for the Narrow-Angle Camera (NAC). These are designed to look for changes in the sensitivity of the camera's CCD sensor.
On May 7 into May 8, ISS will observe the Anthe ring arc, created by micrometeorites blasting material off the small moon Anthe, and the faint ringlets of Saturn's inner D ring. On May 10, ISS will acquire a scan of NAC images across Saturn's main ring system using various color filters. Prometheus will pass through the field of view when the NAC looks at the A and F rings. On May 11, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to acquire distant observations of Tethys and Enceladus at low phase angles. These are designed to look at color and brightness variations across their surfaces (as opposed to higher phase angle images which highlight surface texture variations, topography, or dust near the moons). Tethys will be 1.6 million kilometers (1.0 million miles) away during the observation, while Enceladus will be 1.65 million kilometers (1.03 million miles) away. On May 15 at 11:30 UTC, Cassini will reach periapse for Rev 204 at an altitude of 686,290 kilometers (426,440 miles) from Saturn's cloud tops. During periapse, ISS will ride along with UVIS to observe Saturn's south polar aurorae.
Cassini encounters Titan on May 17 at 16:12 UTC for the 102nd time. This is the fifth of eleven Titan flybys planned for 2014, with the next encounter scheduled for June 18. T101 has a relatively high close-approach altitude of 2,994 kilometers (1860 miles). This is the first in a series of flybys where Cassini approaches over Titan's autumnal south pole and departs over the sun-lit north polar region. Cassini will also be encountering Titan while the moon is over Saturn's nightside, deep in its magnetotail. Inbound, ISS will observe Titan's southern, sub-Saturn hemisphere. Outbound, ISS will be able to observe a crescent Titan over its northern, sub-Saturn hemisphere. Observations for this encounter will start with ISS riding along with the Composite Infrared Spectrometer (CIRS) as it measures Titan's atmospheric and surface temperatures in the mid-infrared. The observation will include stares at the beginning and ending of the observation for ISS to acquire NAC images of Titan. Next, ISS will acquire a global mapping mosaic of Titan. This four-frame mosaic will cover much of the visible face of Titan and will be centered just south of eastern Aztlan near Nath. The images from this mosaic will have a resolution around 2.15 kilometers (1.34 miles) per pixel. Afterward, ISS will ride along with two more CIRS observations, one focused on Titan's surface and the other on the limb of the moon, as well as two VIMS observations of Titan's surface.
Afterward, ISS will ride along with UVIS as that instrument observes a stellar occultation when Titan passes in front of the star Eta Ursa Majoris. For UVIS, stellar occultations are used to provide high-resolution profiles of the hydrocarbons and haze in Titan's atmosphere as well as a temperature and pressure profile. While the Ion and Neutral Mass Spectrometer (INMS) can directly provide such information during close flybys, UVIS can probe deeper into Titan atmosphere, down to an altitude of 200 kilometers (124 miles), well below the minimum altitude Cassini can safely approach Titan. Ingress will be over Titan's northern mid-latitudes while egress will be over its southern mid-latitudes.
The main focus of the flyby is the Radio Science Subsystem (RSS) observation at closest approach. In the two hours before and after closest approach, RSS will be used to acquire bistatic and radio occultation observations. Both before and after closest approach, RSS will acquire bistatic observations, bouncing radio signals off Titan's surface that will then be received back on Earth. This experiment is used to study the physical properties of Titan surface, whether the area studied is liquid or solid, reflectivity at microwave wavelengths, and its dielectric constant from which its composition and roughness can be estimated. Before closest approach RSS will examine a portion of eastern Tsegihi, just east and south of Shiwanni Virgae. As Cassini departs from Titan, RSS will acquire a bistatic observation of Ligeia Mare, a Lake Superior-sized sea of liquid hydrocarbons in Titan's north polar region. That swath will extend to the northern tip of neighboring Kraken Mare. At closest approach, RSS will acquire a radio occultation of Titan's atmosphere, sending a signal through it that will then be acquired back on Earth. By studying variations in the strength of the signal, information about Titan's atmosphere like density, tropospheric wind and temperature profiles, and variations due to seasonal change can be gleaned. After closest approach, CIRS and UVIS will acquire more data about Titan's upper haze layers - its structure, temperature, and composition. ISS will ride along.
On May 18 and 19, ISS will also image a crescent Titan from a distance of 600,000 kilometers (370,000 miles) to 720,000 kilometers (447,000 miles). While the sequence is designed to observe Titan's upper haze layers, the phase angle is not so high that the surface around the moon's north polar region won't be visible. A similar, but shorter, observation will be acquired on May 20. Finishing up the orbit, on May 30, ISS will again observe Titan, this time from a distance of 4 million kilometers (2.49 million miles). Titan's northern sub-Saturn hemisphere will be visible.
On May 31, Cassini will reach apoapse, bringing Rev 204 to a close and starting up the next orbit, Rev 205, which will include another targeted flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).