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Cassini continues its extended tour of Saturn with Rev120, the spacecraft's 121st orbit around the Ringed Planet. Cassini begins Rev120 on October 23 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.49 million kilometers (1.55 million miles) from Saturn's cloud tops. During Rev120, Cassini orbits in the ring plane and orbital plane of Saturn's main satellites, providing an opportunity for Cassini to encounter some of Saturn's moons as well as to look at mutual events between the various satellites.
Cassini ISS starts its observations for Rev120 the day after apoapse with the start of two observation campaigns. Between October 24 and 30, the ISS camera will acquire 2-frame, wide-angle-camera mosaics of a crescent Saturn each day except on Oct. 27, when it will acquire two mosaics, and on Oct. 29 when it will acquire none. The mosaics are designed to study how the photometry and polarimetry (which focuses on the high altitude hazes over the poles) of Saturn's atmosphere has changed since earlier in the Cassini mission. Starting on Oct. 24 and running through the 28th, Cassini ISS will acquire 15-image, north-south strips across Saturn's nightside, using the narrow-angle camera. These mosaics are designed to search for lightning within storms in the planet's atmosphere. In addition to these two observation campaigns, on October 24, ISS will acquire a sequence of images of the distant moon Bestla (Saturn XXXIX), a 7-kilometer-wide (4.3-mile-wide) moon 7.7 million kilometers (4.8 million miles) away. On October 26, ISS will observe the crescent of Titan, 1.2 million kilometers (749,000 miles) distant, as part of a monitoring campaign of Titan's upper atmospheric haze layers. On October 26 and 27, ISS will image three satellite mutual events. The first is a Rhea occultation of Tethys, while Rhea is 1.9 million kilometers (1.2 million miles) away from Cassini and Tethys is 2.5 million kilometers (1.5 million miles) away. The second, on October 27, is an occultation of Enceladus's south polar plume across Tethys, when Enceladus is 2.1 million kilometers (1.3 million miles) away and Tethys is 2.6 million kilometers (1.6 million miles) away. Finally, the third mutual event is an occultation of Rhea by Titan when Saturn's largest moon is 1.01 million kilometers (630,000 miles) away and Rhea is 2.3 million kilometers (1.5 million miles) away.
On October 30, as Cassini nears Saturn, the spacecraft will turn and point its cameras to Titan as it nears the giant moon for a non-targeted flyby on November 1. This observation, along with a similar one on October 31, is designed to monitor clouds across the western half of the trailing hemisphere. Both observations will be acquired from a distance of approximately 1.3 million kilometers (808,000 miles). On October 31, ISS will acquire another north-south mosaic of Saturn's night side, this time using the wide-angle camera. Like the earlier narrow-angle mosaics, this observation is designed to help monitor lightning on Saturn. Finally on October 31, ISS will acquire images of several of Saturn's faint rings at high-phase angles and low-elevations angles above the ring plane, including the E ring around Enceladus and Tethys, the Methone and Pallene rings, and the D ring. In its final observation before the Enceladus flyby sequence, the Visual and Infrared Mapping Spectrometer (VIMS) and ISS will image the narrow crescent and nightside of Saturn as part of a global cloud dynamics campaign. While ISS will be search for lightning in its images of the nightside, VIMS can directly observe cloud formations by looking for Saturn's internal heat filtered through its clouds at five microns.
On November 2, at 7:42 UTC, Cassini will perform its seventh targeted encounter of Saturn's moon Enceladus. During this flyby, Cassini will approach from Enceladus's nightside, pass almost directly over its south pole at a distance of 100 kilometers (62 miles), and then depart the satellite over the moon's dayside, anti-Saturn hemisphere. On approach, ISS will acquire several observations of the crescent of Enceladus, including two observations while Enceladus is entering and leaving Saturn's shadow. In addition to providing information on particle grain sizes and macro-scale roughness of the surface of Enceladus, high-phase angle observations such as these are also a great opportunity to image the plumes of Enceladus. Although the plumes sources will remain near the limb, a geometry that would normally provide a great chance to monitor these features at high resolution, for the 6.5 hours prior to closest approach, Enceladus will appear too close to the sun to safely image until a few minutes prior to closest approach. At that point RADAR will take over, acquiring several radiometry scans and stares of Enceladus's nightside. During these RADAR observations, Cassini reaches periapse, its closest point to Saturn on Rev120. At this point, Cassini will be 132,850 kilometers (82,550 miles) from Saturn's cloud tops, between the orbits of Mimas and Enceladus. For the 30 minutes prior to periapse, Cassini's high-gain antenna will be pointed at Saturn's southern mid-latitude for a brief radiometry observation.
With an hour to go until closest approach, Cassini will turn so that the Ion and Neutral Mass Spectrometer (INMS) can acquire samples of the inner E ring as well as Enceladus's plume. Along with an encounter in late April 2010, this will be Cassini's deepest passage into Enceladus's plume, passing 100 kilometers (62 miles) nearly directly over the south pole. This will allow INMS to measure minor chemical species, or different chemical compounds, within the plume, as well as look for variations between the different plume jets, thanks to the potentially strong signal to noise ratio provided by Cassini's low altitude. The INMS team also plans to measure ammonia and nitrogen with this flyby. With such a low altitude, the INMS team hopes to have a stronger ammonia signature as the ammonia will have had little time in the sun to have been broken down into nitrogen and hydrogen by photolysis. Finally, INMS hopes to obtain stronger signals from larger hydrocarbon compounds. The earlier discovery of water vapor with a possible liquid water source, hydrocarbon compounds, and an energy source centered on the south pole has made Enceladus a target of significant astrobiological interest. ISS will be riding along with this observation, acquiring two pairs of WAC and NAC images as Enceladus passes through those cameras' fields of view. The two targets include an area near the terminator in the south polar region and within an unnamed crater located near 38 degrees South latitude, 155 degrees West longitude.
Following closest approach, Cassini will turn its optical remote sensing suite back to Enceladus, this time for a Composite Infrared Spectrometer (CIRS) observation of Enceladus's south polar region. ISS will be riding along with this observation, acquiring NAC and WAC images of this area. From this point, ISS will acquire no more images for this encounter. However, the Ultraviolet Imaging Spectrometer (UVIS), CIRS, and VIMS each have an observation during this outbound leg. UVIS will acquire a full ultraviolet wavelength map of Enceladus's anti-Saturn hemisphere with Saturn in the background, along with the south polar plume, which will appear to block light from Saturn. CIRS will acquire a mid-infrared (10-16 microns) map of Enceladus's dayside. Finally, VIMS will map Enceladus's composition at near-infrared wavelengths. With its observations of Enceladus complete, Cassini ISS and UVIS will observe Tethys's trailing hemisphere from a distance of 314,000 kilometers (195,000 miles) before the spacecraft turns its antenna to Earth to playback the flyby data.
Following the playback, on November 3, Cassini will turn its cameras back to Titan to monitor clouds, this time of Titan's western sub-Saturn hemisphere. Titan will be 2.15 million kilometers (1.33 million miles) away during this observation. Just as it did during the first few days of this orbit, the Cassini ISS camera will acquire 2-frame, wide-angle-camera mosaics of a crescent Saturn each day between November 4 and 10, except for November 6. Also on November 4, ISS will acquire an astrometric observation of several of Saturn's small satellites including Janus, Atlas, Methone, Pandora, and Helene. Astrometric observations are used to help provide better orbital calculations for some of these small rocks, which can be affected by gravitational interactions with the larger icy moons. Over the next four days, ISS will acquire five more such observations. These "SATELLORB" sequences will provide distant imaging of the following moons: Atlas, Pandora, Kari (one of Saturn's small, outer satellites), Calypso, Janus, Methone, Prometheus, Pan, Polydeuces, Epimetheus, Helene, Hyperion, Pallene, and Telesto. On November 5, ISS will begin another imaging campaign, with observations each day until November 8 (with two mosaics that day), this time looking for lightning on Saturn's nightside. Just like the similar campaign earlier in the orbit, this search will use 15-image, north-south NAC strips across the nightside. ISS will also acquire a strip of images of Saturn's dayside southern hemisphere with multiple NAC filters on November 6.
Closing out the orbit, ISS will observe two transits of Janus by Rhea on November 8. In the second of these two observations, the limb of Saturn will be in the background providing a nice photo-op. ISS will observe the small outer satellite, Bestla (formerly known as S/ 2004 S 18) on November 9. The observation is designed to study the photometry of this distant satellite as the moon is too far away and too small to see it as anything more than a point of light in ISS images. On November 9, ISS will take an eight-frame, wide-angle-camera mosaic across much of the Saturn system out to the orbit of Rhea, focusing primarily on mapping the distribution of material in the E ring. A similar observation will be acquired on November 11, this time focusing on the area around the orbit of Enceladus with two, two-frame, WAC mosaics on either side of Saturn. On November 11, after an observation of Titan's crescent, ISS will observe a transit of Janus across Rhea, with both satellites on the far side of the rings. Janus will be 2.7 million kilometers (1.7 million miles) away, while Rhea will be 3.1 million kilometers (1.9 million miles) away. Finally, ISS will take a calibration observation of most of the filters on its wide-angle camera by acquiring numerous images of the B-type star Spica.
Cassini reaches apoapse on November 11, bringing Rev120 to an end and starting Rev121. Rev120 includes another flyby of the active moon Enceladus (E7) and a close, non-targeted encounter of Rhea.
Image products created in Celestia. Rhea and Enceladus basemaps by Steve Albers. All dates in Coordinated Universal Time (UTC).