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Cassini continues its exploration of the Saturn system with the 13-day Rev 227, which begins on November 30 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.89 million kilometers (1.17 million miles) from Saturn’s cloud tops. Rev 227 occurs during the second Equatorial phase of Cassini’s extended-extended mission. During this 10-month phase, Cassini will orbit within the orbital plane of Saturn’s rings, allowing for frequent encounters with Saturn’s icy satellites. Twenty-seven ISS observations are planned for Rev 226 with the majority focused on Saturn’s atmosphere and its various small, icy satellites.
Cassini begins Rev 227 in the middle of its solar conjunction period, when Saturn and Cassini are on the other side of the Sun from the Earth. Between November 27 and December 2, Cassini will have very limited communications with Earth due to interference from the Sun’s atmosphere, so very few science observations will be acquired (mostly lower bitrate fields-and-particles observations). No ISS observations are planned for this six-day period.
For its first observation of Rev 227, after solar conjunction, on December 3, 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. ISS will acquire a similar observation on December 11. Another way to improve our understanding of satellite orbits is to observe mutual events, when one moon appears to transit in front of another. After the astrometric observation, ISS will observe a mutual event as Mimas passes in front of Enceladus’s south polar region. Mimas will be 1.61 million kilometers (1.00 million miles) away during this sequence, while Enceladus will be 1.86 million kilometers (1.15 million miles) away. A similar mutual event will be observed later that day as Enceladus passes in front of Mimas’s north polar region. Mimas will be 1.75 million kilometers (1.09 million miles) away during this sequence, while Enceladus will be 1.35 million kilometers (0.84 million miles) away.
Immediately after the mutual event observation, 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 to point the optical remote sensing (ORS) instruments back at Saturn, as a waypoint between other experiments’ observations. These sequences include blue, clear, two methane band, and one full-frame, continuum band filter images. Between December 3 and 5, ISS will acquire seven more Storm Watch observations, and between December 7 and 12, it will take six more.
On December 3 and 4, ISS will observe a crescent Titan in order to monitor changes in its upper haze layers. The observation on December 3 will be acquired from a distance of 1.44 million kilometers (0.90 million miles), while the observation on December 4 will be taken from 1.63 million kilometers (1.01 million miles) away. Early on December 6, ISS will observe a mutual event between the distant moons Ijiraq and Siarnaq, where both moons will fit in the same Narrow-Angle Camera (NAC) field-of-view. Ijriaq will be 7.49 million kilometers (4.65 million miles) away, while Siarnaq will be 12.0 million kilometers (7.45 million miles) away.
On December 6 at 20:31 UTC, Cassini will reach periapse for Rev 227 at an altitude of 92,380 kilometers (57,402 miles) from Saturn's cloud tops, just outside the orbits of Janus and Epimetheus. First up after periapse, the camera system will image the outer A ring, where it will be looking at propellers previously imaged by Cassini. Propellers are small voids in Saturn’s rings created by the gravitational interaction between large ring particles and the surrounding ring. At 20:38 UTC, Cassini will perform a close, non-targeted encounter of Epimetheus at an altitude of 2,616 kilometers (1,626 miles). This is the closest flyby planned of Epimetheus that Cassini will perform, though a similar close pass will occur in January 2017 during Rev 259. ISS will acquire a pair of multi-spectral observations of Epimetheus prior to the encounter. The first set will be acquired from a distance of 36,000 kilometers (22,400 miles) away, while the second set will be taken from 26,600 kilometers (16,500 miles) away, after a mid-infrared scan by the Composite Infrared Spectrometer (CIRS). Cassini will then turn away from Epimetheus so that it can use the high-gain antenna to shield the spacecraft from micrometeoroid impacts during the ring-plane crossing. During the passage, Epimetheus will pass through the field-of-view of the Wide-Angle Camera, allowing for twelve images to be taken by the camera during closest approach.
After the Epimetheus encounter, Cassini will perform a pair of non-targeted encounters in quick succession, first of tiny Atlas at a distance of 20,857 kilometers (12,960 miles) then, 15 minutes later, of Prometheus at a distance of 20,985 kilometers (13,040 miles). Cassini will observe both shortly after closest approach to both inner moons. Atlas will be imaged first. While it will be imaged at a relatively close range, due its small size (only 30 kilometers or 19 miles across), it will only appear 215 pixels across. This would still be the best images acquired to date of this moon which orbits just outside Saturn’s A ring. Cassini will next image Prometheus at a distance of 36,400 kilometers (22,600 miles). Cassini will be roughly looking down the long end of Prometheus, over its anti-Saturn hemisphere.
On December 7, ISS will observe a crescent Titan from a distance of 1.88 million kilometers (1.17 million miles). Later that day, ISS will observe a crescent Enceladus in order to monitor its south polar plume. On December 8, ISS will take a look at the G ring, a narrow, dusty ring produced by micrometeorite impacts on the tiny moon Aegaeon. On December 12, ISS will observe the E ring, a larger, more diffuse, dusty ring generated by jets of water and other material at Enceladus’s south pole.
On December 13, Cassini will reach apoapse, bringing Rev 227 to a close and starting up the next orbit, Rev 228, which will include encounters with Titan, Enceladus, and Aegaeon.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).