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Cassini continues its new tour of the Saturn system with the 20-day-long Rev138, the spacecraft's 139th orbit around the Ringed Planet and the fourth full orbit of the Cassini Solstice Mission. Cassini begins Rev138 on September 13 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.57 million kilometers (1.59 million miles) from Saturn's cloud tops. Cassini is in a slightly inclined orbit, 4.7 degrees out of the ring plane. This orbit is still close enough to provide Cassini an opportunity to examine some of Saturn's moons, though the camera and other remote sensing instrument teams will focus on Saturn during the orbit. Cassini also will perform a targeted flyby of Saturn's largest moon, Titan. Later in the orbit, Cassini and Saturn will pass behind the sun from the perspective of the Earth, precluding remote-sensing observations and communications between September 28 and October 3.
Cassini's ISS camera system starts its observations for Rev138 on September 13 with an astrometric observation of several of Saturn's small, inner satellites in order to improve our knowledge of the motions of these moons. This observation will include images of Janus, Epimetheus, Pallene, Methone, and Anthe. Another astrometric observation will be acquired on September 17, covering Polydeuces, Calypso, Anthe, Methone, Epimetheus, Prometheus, and Janus. Also on September 13, Cassini will image Titan while the satellite appears half-illuminated. This will permit Cassini to search for clouds across Titan's trailing hemisphere. This observation will be acquired from a distance of 2.46 million kilometers (1.53 million miles). ISS will repeat this observation later in the orbit on September 21 when the spacecraft is 2.11 million kilometers (1.31 million miles) away from Titan and on September 27 when it is 1.29 million kilometers (0.8 million miles) away from the haze-shrouded moon. Titan may be at too high a phase angle during the September 21 opportunity to allow cloud monitoring. The September 27 observation covers Titan's anti-Saturn hemisphere a few days after the Sept. 24 Titan flyby. On September 17, ISS will take a seven-hour observation of the small irregular satellite, Kiviuq. During this sequence, Kiviuq will be the brightest it will ever be during the Cassini Solstice Mission, making this a great opportunity to acquire a light curve of the moon. This observation is part of a campaign of observations of this and other small satellites that are taken determine their rotational periods, study their surface properties, and to determine if they are binary objects. This 16-kilometer-diameter (10-mile-diameter), outer satellite will be 9.8 million kilometers (6.09 million miles) away.
In the 10-day run up to periapse during Rev138, Cassini's remote sensing instruments will be focused primarily on studying Saturn's atmosphere. For example, between September 14 and 23, ISS will ride along with six Ultraviolet Imaging Spectrometer (UVIS) observations of Saturn. ISS will acquire wide-angle-camera images of Saturn during these observations to study the photometry and polarimetry of Saturn's clouds and hazes. Twice on September 15 and again on September 18 and 19, ISS will acquire a set of wide-angle-camera frames designed to study the photometry of Saturn's clouds and narrow-angle-camera movie frames to monitor cloud motions for over two hours. The WAC images will include a red-green-blue image set for true-color composites. Finally, on September 21 ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) observation of Saturn. ISS will take a two-by-two mosaic of Saturn's day side and several frames that cover its night side to monitor for lightning.
On September 22 at 23:59 UTC, Cassini will reach the periapse of Rev137, its closest point to Saturn in the orbit. At periapse, the spacecraft will be 148,210 kilometers (92,093 miles) above Saturn's cloud tops. Inbound, ISS will image a crescent Enceladus from a distance of 246,000 kilometers (153,000 miles) to monitor the activity of its south polar jets. Afterward, during periapse, the camera system will image the bright B-type star Rigel (beta Orionis) as the ring system appears to move in front of the star. Measurements of how the star brightens and fades as ringlets pass in front of it allow researchers to calculate the rings' opacity and density. The next day, ISS will take a look at Enceladus from a distance of 570,000 kilometers (354,000 miles). Researchers hope to study the satellite's surface at very low phase angles and to study how the brightness of various regions change as the phase angle increases from around 3.5 degrees to 7.6 degrees near the end of the sequence. Unlike in the images the day before, the plumes are not expected to be visible at such low phase angles.
Cassini encounters Titan on September 24 at 18:38 UTC for the 73rd time. This is the second of 56 Titan flybys planned for the new extended mission with the next encounter scheduled for November 11. The close approach distance for the encounter (known as T72) is 8,175 kilometers (5,079 miles). This flyby will allow for imaging of the anti-Saturn hemisphere of Titan outbound to the encounter. For much of the inbound leg of T72, when Titan will appear as a thin crescent from Cassini, the Composite Infrared Spectrometer (CIRS) team will be controlling pointing in order to measure the composition of Titan's atmosphere and haze layer aerosols. These observations include two polar limb integrations to measure how the atmosphere's composition changes with altitude at 87 degrees south and 60 degrees north. Outbound, UVIS and VIMS will control pointing, again measuring atmospheric composition and mapping Titan's surface and cloud features. ISS will take images during this flyby by riding along with other instruments' observations, so no large mosaics are planned. These images should be useful for cloud monitoring, and if present, the clouds motions and development can be tracked.
At closest approach, VIMS will be prime so that it can acquire high resolution images of Titan's equatorial region, achieving a resolution of 5 kilometers (3 miles) per pixel. Their highest resolution coverage will be focused near the boundary between two of Titan's great sand dune seas: Belet and Senkyo.
Starting on September 27 and running through October 3, Cassini and Saturn will pass behind the sun from the perspective of the Earth during a period known as solar conjunction. The spacecraft's proximity to the sun in the Earth's sky restricts communications between Cassini and its controllers on the ground. As a result, remote-sensing observations during this period are prohibited. When Cassini emerges from solar conjunction on October 3, the camera system will focus its attention in what remains of Rev138 on the irregular moons in the outer reaches of the Saturn system. Although Cassini is far away from these small, icy bodies, it can still acquire useful data on their size, shape, and rotational period based on how their brightness changes with time as the little moons rotate. Three four- to five-hour light curve observations are planned for October 3 and 4 of three different satellites: Kiviuq, Siarnaq, and Hyrrokkin.
On October 4, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev139. Rev138 includes non-targeted encounters with Titan, Mimas, Dione, and Rhea.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).