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Cassini continues its exploration of the Saturn system with the 6.5-day Rev 287, which begins on August 4 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.21 million kilometers (0.75 million miles) from Saturn’s cloud tops. Rev 287 is the 17th of 22 proximal orbits that will take place between April 2017 and the end of the mission in September. During these orbits, Cassini’s closest approach to Saturn occurs between the ring system and Saturn’s atmosphere. Eight ISS observations are planned for Rev 287, with most occurring during a non-targeted encounter with Saturn’s largest moon, Titan.
For its first observation of Rev 287, on August 4, ISS will observe the outer A ring at high-phase angles, which can emphasize the appearance of dust in the Encke, Keeler, and Roche Gaps. On August 6, ISS will ride along with the Composite Infrared Spectrometer (CIRS) as it observes the lit face of the C and D rings, the innermost of Saturn’s main rings. On August 7, ISS will conduct a survey of the propellers in the outer A ring. Propellers are voids in the ring created by the gravity of large, 100 – 1000-meter (328 – 3280 foot) ring particles. Due to the influence of the rings on their motion, these observations are needed to keep track of previously discovered propellers, like Earhart and Bleriot. The Ultraviolet Imaging Spectrometer (UVIS) will acquire several scans of Saturn’s north polar region, observing the entire auroral oval.
On August 7 at 17:23 UTC, Cassini will reach periapse for Rev 287 at an altitude of 2,897 kilometers (1,800 miles) above Saturn’s cloud tops. On approach to Saturn, ISS will ride along as the Visual and Infrared Mapping Spectrometer (VIMS) acquires a scan across Saturn’s main rings. Through the ring plane crossing and closest approach, the Ion and Neutral Mass Spectrometer (INMS) will directly measure the composition of gas between Saturn and the D ring. Much of this gas originates in Saturn’s rings. As Cassini travels away from Saturn, UVIS will observe Saturn’s south polar auroral oval.
On August 8 and 9, ISS will observe the distant moon Kiviuq from 10.9 million kilometers (6.80 million miles) away. While Kiviuq won’t be resolved in these images, measurements of its brightness can provide information on its rotation speed and the orientation of its spin axis, as well as provide a rough estimate of its overall shape and determine whether it is a binary system, like many similar-sized objects orbiting the Sun. Afterward, early on August 10, ISS will observe the planet Neptune and its large moon Triton from 4.38 billion kilometers (2.72 billion miles) away. This observation will provide a measurement of Neptune and Triton’s brightness at a higher phase angle than is achievable from Earth. Triton will appear about 15 pixels away to Neptune’s upper left.
On August 11 at 05:04 UTC, Cassini will perform a non-targeted encounter with Titan at an altitude of 194,990 kilometers (121,160 miles). CIRS and ISS will acquire several observations over the course of a segment lasting nearly two days. ISS will acquire six mosaics during this period to improve mapping of Titan’s north polar region and to track clouds across its northern hemisphere. The first mosaic, with three frames, will be taken from 300,000 kilometers (186,000 miles) away and will be centered northeast of the Menrva impact basin. The highest resolution mosaic, using four frames, will be taken 30 minutes after closest approach and will cover Titan’s north polar region. The next mosaic, taken from 235,000 kilometers (146,000 miles) away, will include a frame that will be ISS’s best view of Kraken Mare and Ligeia Mare. CIRS will acquire several mid-infrared compositional maps of Titan’s north polar atmosphere.
On August 10, after the first ISS mosaic and during the first CIRS observation of Titan, Cassini will reach apoapse, bringing Rev 287 to a close and starting up the next orbit, Rev 288. At this point, Cassini is 1.21 million kilometers (0.75 million miles) from Saturn’s cloud tops. Seventeen ISS observations are planned for this orbit, with most focused on Titan and Saturn’s atmosphere. On August 12, ISS will ride along with VIMS to image a crescent Saturn. Afterwards, ISS will ride along with UVIS as it observes Saturn’s north polar aurorae. Next, ISS will acquire an observation of the limb of a crescent Saturn to monitor its upper haze layers and to observe cloud shadows.
On August 13, ISS will ride along with VIMS as it maps Saturn’s north polar region. After a CIRS observation of Saturn’s limb hazes, ISS will look at streaks in the C ring plateau. On August 14 at 04:23 UTC, Cassini will reach periapse for Rev 288 at an altitude of 1,691 kilometers (1,051 miles) above Saturn’s cloud tops. This is the first of five final passes where Cassini dips into Saturn’s upper atmosphere. INMS will conduct the first direct sampling of Saturn’s atmosphere and the first of a gas planet since the Galileo probe plunged into Jupiter in December 1995. RADAR will ride along to acquire radiometry data of Saturn’s atmosphere, useful for understanding the ammonia concentration in Saturn’s atmosphere. On the outbound leg, ISS will ride along with UVIS as it observes Saturn’s south polar aurorae.
On August 16, ISS will image a crescent Titan from 1.16 million kilometers (0.72 million miles) away to continue monitoring seasonal changes to its upper haze layers. Later, ISS will again observe Kiviuq, from 10.7 million kilometers (6.68 million miles) away, to measure its lightcurve. ISS will also acquire another photometric observation of Neptune and Triton. On August 17, Cassini will reach apoapse, bringing Rev 288 to a close and starting up the next orbit, Rev 289, when Cassini will perform a non-targeted encounter with Titan and image Triton.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).