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Cassini continues its extended tour of the Saturn system with the 20-day-long Rev130, the spacecraft's 131st orbit around the Ringed Planet. Cassini begins Rev130 on April 17 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.6 million kilometers (1.62 million miles) from Saturn's cloud tops. With Cassini still in an equatorial orbit, the spacecraft will perform the first of two encounters of Enceladus over the span of three weeks. The cameras will acquire several observations of Titan's and Saturn's atmospheres to look for seasonal changes nine months after the start of northern spring.
Cassini's ISS camera starts its observations for Rev130 the day after apoapse by imaging the outer moon Bebhionn. This 6-kilometer-wide (4-mile-wide) moon will be 9.84 million kilometers (6.11 million miles) away from Cassini, so it will appear as a faint point of light in the images acquired. However, variations in the brightness of the moon can reveal the rotational period of the satellite, particularly when combined with a longer observation later this orbit. Also on April 18, Cassini will acquire calibration images for its narrow-angle camera. These images will be taken with the shutter closed so the team's dark current files can be updated. Dark current is the background response of the camera's CCD detectors, visible even when no actual light is hitting it.
On April 20, ISS will image Titan as Dione appears to pass behind the upper haze layers of Titan's south polar region. Titan will be 1.29 million kilometers (801,000 miles) away at the time, while Dione will be 2.28 million kilometers (1.42 million miles) away. Cassini will take images of Titan each day between April 21 and 24 in order to monitor clouds across Titan's trailing hemisphere and to monitor changes in its upper haze layers due to rapid seasonal changes. During these four observations, Titan will range in distance from Cassini from 1.34 million to 1.85 million kilometers (0.83 million to 1.15 million miles). On April 26, ISS will observe Enceladus from a distance of 975,000 kilometers (605,000 miles) in order to observe the moon's south polar jets to assess their level of activity prior to the encounter that will occur a little more than a day later.
On April 27 at 21:09 UTC, Cassini will reach the periapse of Rev130, its closest point to Saturn in the orbit. At periapse, the spacecraft will be 153,900 kilometers (94,690 miles) above Saturn's cloud tops. A few hours after periapse, Cassini will fly by the icy moon Enceladus, passing at an altitude of 99 kilometers (62 miles) on April 28 at 00:10 UTC. This is Cassini's 10th targeted encounter with Enceladus. Another encounter is planned for May 18 and three more are planned for the second half of 2010. No images are planned for this flyby. Instead, Cassini's high gain antenna will be pointed at Earth for the 24 hours before and after the encounter. This orientation will allow for a radio occultation of Saturn's atmosphere on April 27 and a continuous Doppler tracking observation as Cassini passes over Enceladus' south pole. From the latter measurement, the radio science team will look for mass concentrations, such as a sub-surface diapir, or geologic intrusion, that might be associated with the hotspots over the pole.
On April 29 and 30, Cassini will acquire three observations of Saturn's atmosphere. The first two observations are sequences by the Visual and Infrared Mapping Spectrometer (VIMS) designed to monitor cloud dynamics in the giant planet's atmosphere over the course of 1.5 Saturn days. ISS will ride-along. The third observation is an ISS wide-angle-camera, two-frame mosaic of Saturn using multiple filters for each image footprint, including RED, GRN, and BL1. On May 1 and 2, ISS and VIMS will observe first the dayside ansa and then the night side ansa of Saturn's E ring, a diffuse torus of small water ice particles generated from Enceladus' south polar jets. On May 4 and 7, Cassini will observe a crescent of the sub-Saturn face of Iapetus from distances of 1.44 million (892,000 miles) and 1.27 million kilometers (786,000 miles), respectively. These distant images should allow Cassini to see the large impact basin Naimon near the boundary between Cassini Regio and Roncevaux Terra. On May 6, Cassini will image Titan's trailing hemisphere from a distance of 1.46 million kilometers (907,000 miles), again looking for clouds. Also, ISS will acquire an 11-hour observation of Bebhionn, a small, outer satellite of Saturn more than 10.5 million kilometers (6.5 million miles) from the spacecraft. The observation, the second of Bebhionn in this orbit, is designed to help pin down the spin state of the satellite and better understand the structure of its surface by observing its brightness at different phase angles.
On May 8, Cassini will reach apoapse, bringing this orbit to a close and starting Rev131. During Rev131, Cassini will fly by Saturn's moon Enceladus once again, this time acquiring numerous images of that dynamic satellite.
Image products created in Celestia. Enceladus basemap by Steve Albers. All dates in Coordinated Universal Time (UTC).