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Cassini continues its extended tour of Saturn on May 2 with Rev110, the spacecraft's 111th orbit around the Ringed Planet. Cassini begins Rev110 early on May 2 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 1.36 million kilometers (846,000 miles) from Saturn. The spacecraft remains in a high-inclination orbit, providing an opportunity to study the rings and the polar regions of Saturn and its satellites.
Cassini starts its observations for Rev110 shortly after apoapse. First, Cassini will turn its cameras to M48, an open star cluster in the constellation Hydra and to M45, another open star cluster otherwise known as the Pleiades. These observations will be used to update the calibration of the two ISS cameras. Later, ISS will take an astrometric observation of several of Saturn's small satellites, including Prometheus, Pan, Daphnis, Atlas, and Janus. Astrometric observations are used to help provide better orbital calculations for some of these small rocks, which can be effected by gravitational interactions with the larger icy moons. On May 3 and 4, ISS will acquire two movies of spokes on the unlit side of Saturn's B ring. Also on May 4, Cassini will observe Enceladus's north polar region from a distance of 1.22 million kilometers (760,000 miles) as well as take a movie of the Encke Gap in the outer A ring.
Cassini encounters Titan on May 5 at 22:54 UTC for the 55th time and the first encounter of the month. This flyby is also the third in a series of 10 flybys between April and August 2009 that will be spaced 16 Earth days, or one Titan day, apart. This encounter is one of the more distant of Cassini's targeted Titan flybys, with a close approach distance of 3,244 kilometers (2,016 miles). This flyby, known as T54, will allow for imaging of the southern trailing hemisphere of Titan outbound to the encounter, similar to the area observed during the previous two encounters in April. On approach to Titan, the Composite Infrared Spectrometer (CIRS) team will be controlling spacecraft pointing, or be considered "prime," with a short ISS observation 9 to 10 hours prior to closest approach. The ISS observation is designed to study the photometry of Titan's north polar haze layers. CIRS will focus on understanding the composition of Titan's stratosphere by performing nadir and limb integrations of the satellite's atmosphere in the far- and mid-infrared. The CIRS instrument will point straight down at Titan to perform the nadir integrations, staring at particular spots. During the limb integrations, Cassini will observe the atmosphere at the edges of the visible disk of Titan. These measurements include full-disk, mid-infrared temperature scans as well as several limb scans designed to look at different layers of Titan's atmosphere in the far-infrared.
During closest approach on T54, CIRS will be prime. CIRS will perform several scans of the limb of Titan during the close approach period. These scans will allow CIRS to measure the composition of Titan's atmosphere at different altitudes, particularly looking at aerosols in Titan's stratospheric haze layers. CIRS also will measure the temperature of Titan's atmosphere at different altitudes, acquiring a temperature profile of Titan's nightside atmosphere. These measurements are complimentary to those acquired in the previous encounter, when CIRS primarily focused on the atmosphere over Titan's southern hemisphere.
Following close approach, ISS, CIRS, the Ultraviolet Imaging Spectrometer (UVIS), and the Visual and Infrared Mapping Spectrometer (VIMS) will trade off being prime. ISS will take a nine-frame mosaic shortly after closest approach, from a distance of 8,600 to 32,000 kilometers (5,300 to 20,000 miles). This mosaic, HIRESNA001, will focus on a region near 65 degrees south latitude, 260 degrees west longitude. Several dark features have been previously observed in this region, though these features do not have a low enough albedo to be considered lake feature candidates. Their current origin is unknown. Ride-along VIMS observations could be used to constrain their composition. UVIS will take a far- and extreme-ultraviolet scan of Titan's atmosphere. CIRS then will take another look at Titan to measure the composition of the stratosphere by observing the satellite's atmosphere in the mid- and far-infrared. Finally, VIMS and UVIS will search for clouds.
Following the T54 flyby, Cassini imaging will be primarily focused on Saturn's ring and satellite systems. Cassini will turn its cameras to Titan for three observations between May 7 and 9. The first of these will be taken while the satellite is in Saturn's shadow. Although Cassini has acquired similar observations in the last few orbits, this time ISS will acquire numerous long-exposure color filter observations designed to study the photometry of Titan while it is in eclipse. The other two observations are designed to look for clouds across Titan's southern sub-Saturn hemisphere from 994,000 and 1.202 million kilometers (618,000 and 747,000 miles) away. On May 9, Cassini reaches periapse, its closest point to Saturn on Rev110. At this point, Cassini will be 850,000 kilometers (528,000 miles) from Saturn's cloud tops, between the orbits of Rhea and Titan. Also on May 9, ISS will acquire an observation of the shadows of Pandora and Mimas on Saturn's ring system as well as an astrometric observation of several of Saturn's small satellites, including Pandora, Epimetheus, Prometheus, Janus, Pan, and Daphnis.
On May 10, ISS will observe Titan's sub-Saturn hemisphere from a distance of 1.224 million kilometers (748,000 miles). The bright region Tsegihi should be near the center of the frame. Next, ISS will take an 11-hour movie of the F ring. The observation is designed to study the evolution of channels and streamers created by Prometheus when it is at apoapse in the narrow ring. These effects are predicted to become more pronounced as Prometheus dives deeper into the F ring as the apoapse of Prometheus's orbit and periapse of the F ring become aligned in December. ISS also will take two more astrometric observations of several of Saturn's small satellites on May 10 and 11, including Atlas, Daphnis, Janus, Telesto, S/ 2008 S 1, Methone, and Epimetheus.
On May 12, ISS will observe the shadows of Tethys and Mimas on Saturn's ring system. On May 13, Cassini will take a look at Titan's northern leading hemisphere from a distance of 1.116 million kilometers (693,000 miles). Afterward, ISS will take another astrometric observation, this time taking a look at Janus, Anthe, Prometheus, and Pandora. Next, ISS will observe its first eclipse of one moon by another. In this case, parts of Mimas will experience a total solar eclipse by Enceladus. ISS will observe the shadow of Enceladus move across the surface of Mimas. On May 14, ISS will observe several of Saturn's small satellites, including Atlas, S/ 2008 S 1, Anthe, Pallene, and Pan. ISS will also perform more instrument calibrations using the star Vega.
Cassini reaches apoapse on May 17, bringing Rev110 to an end and starting Rev111. Rev111 will include Cassini's 56th flyby of Titan, T55.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).