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Cassini continues its extended tour of the Saturn system with the 19-day-long Rev129, the spacecraft's 130th orbit around the Ringed Planet. Cassini begins Rev129 on March 29 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.38 million kilometers (1.48 million miles) from Saturn's cloud tops. With Cassini still in an equatorial orbit during Rev129, the spacecraft will encounter two of Saturn's satellites, Titan and Dione, and will conduct a non-targeted flyby with Janus. As it did during the previous orbit, Cassini will continue its observations of satellite mutual events. Mutual event observations such as these, in which one moon passes close to or in front of another as seen by Cassini, help scientists refine their understanding of the orbits of Saturn's moons.
Cassini's ISS camera starts its observations for Rev129 a few hours after apoapse by imaging the star Alpha Virginis (Spica) for instrument calibration. The team will take a number of images using this wide-angle camera to update the camera's absolute photometry and shutter offset. This observation will make use of a standard stellar target - Spica -- to assess the changing properties of the camera's charge-coupled device and computer systems by comparing the calibrated results with earth-based measurements of the star. On April 2, Cassini will image Epimetheus as the moon transits the south polar region of the larger co-orbital satellite, Janus. At the time of the occultation, Epimetheus will be 2.14 million kilometers (1.33 million miles) from the spacecraft, while Janus will be 2.19 million kilometers (1.36 million miles) away.
Cassini encounters Titan on April 5 at 15:55 UTC for the 68th time. This is the first Titan flyby since January and one of the last encounters of the current extended mission, which ends on July 1. The close approach distance for the encounter (known as T67) is 7,462 kilometers (4,636 miles). This flyby will allow for imaging of the anti-Saturn hemisphere of Titan outbound to the encounter. For much of the inbound segment of the encounter, when only a thin crescent will be visible, the Composite Infrared Spectrometer (CIRS) team will control spacecraft pointing, or be "prime." CIRS will focus on measuring the composition of Titan's atmosphere. One focus for their observations will be the atmosphere over Titan's north pole. Titan recently started its 7.5-year-long northern spring, and sunlight directly reached the north pole for the first time since 1995. CIRS will examine how Titan's atmosphere has responded to the change in seasons. CIRS will also perform a scan of Titan southern hemisphere using the instrument's far-infrared channel. These observations continue until just before closest approach, when ISS takes over to acquire one of its two closest observations of Titan during this current mission. This observation (VHIGHRES001) will focus on the center of the dark equatorial terrain region, Belet. RADAR has previously shown Belet to be a large sand sea, also known as an erg. ISS will try to image some of these dunes, as well examine the emission-angle dependence of the brightness of the area. During the observation, Cassini will be moving toward Titan's sunlit side, and the emission angle of the imaged region will change from looking almost straight down to imaging the area as it approaches Titan's limb.
After this closest approach observation, which runs for 2 hours and 15 minutes, ISS will observe Titan again in an observation named REGMAP001. This 12-frame mosaic will cover the central and western portions of the dark equatorial region Senkyo. Like VHIGHRES001 before it, one goal of the observation will be direct observation of the dunes that fill Senkyo. Doing so will require that the dunes be darker than the substrate on which they lie. REGMAP001 will be taken from a distance of 43,000 to 72,000 kilometers (27,000 to 45,000 miles) from Titan. The next observation, GLOBMAP001, is a 25-frame mosaic that will cover much of the visible terrain on Titan within 40 degrees of the equator. It will be taken from distances ranging from 103,000 to 172,000 kilometers (64,000 to 107,000 miles) from Titan. The Visible and Infrared Mapping Spectrometer (VIMS) and CIRS will also acquire observations of Titan outbound from the moon, mapping the composition of Titan's sub-Saturn hemisphere.
On April 7 at 12:53 UTC, Cassini will reach the periapse of Rev129, its closest point to Saturn in the orbit. At periapse, the spacecraft will be 152,390 kilometers (94,690 miles) above Saturn's cloud tops.
A few hours before periapse, ISS will perform a targeted encounter with one of Saturn's mid-sized icy satellites, Dione. The altitude for this encounter is 504 kilometers (313 miles); this is the closest encounter with Dione since October 2005. Only one encounter during the Solstice Mission, on December 12, 2011, will allow Cassini to fly closer to the satellite. ISS' observations for the flyby start four hours before closest approach. The camera system will search for cryovolcanic plumes along Dione's dark limb as the moon enters Saturn's shadow. At the end of this 10-minute observation, Dione will be in darkness, illuminated from behind Cassini by light reflected off Titan. At closest approach, the Cassini Plasma Spectrometer (CAPS) will be prime. However, ISS will take images across Dione's leading hemisphere as the satellite crosses the fields-of-view of the wide-angle camera (WAC) and narrow-angle camera (NAC). Included in these images will be a WAC of southern Fidena Fossae near the terminator, a NAC and a WAC of Petilia Fossae, a NAC and WAC over cratered terrain southwest of the crater Camilla, a NAC and WAC of the crater Remus, a WAC over cratered terrain east of the crater Dido, and a NAC and WAC over southern Eurotas Chasmata. After this observation, the Ultraviolet Imaging Spectrometer (UVIS) will map Dione's ultraviolet reflectivity, scanning north-south along 340 degrees west longitude. ISS will take six observations during this scan. Next, ISS will acquire a 20-frame mosaic across Dione's sub-Saturn and leading hemispheres, including the area shown above right. This mosaic will be taken from distances ranging from 38,800 to 91,000 kilometers (24,100 to 56,500 miles) from Dione. This mosaic will cover the western portions of Dione's tectonic "wispy" terrain and crater terrain that includes three of Dione's largest craters, Aeneas, Dido, and Turnus. Following this mosaic, CIRS and VIMS will acquire data over the same region. CIRS will measure the temperature of Dione's leading hemisphere while VIMS will make their own mosaic of the region. ISS will ride along with both, taking a multi-spectral data set during the VIMS observation.
After periapse, Cassini will observe the leading and anti-Saturn hemispheres of Janus during a non-targeted encounter with that satellite. Cassini will fly within 74,597 kilometers (46,352 miles) of the larger of Saturn's two co-orbital satellites. ISS will image the moon using a number of narrow-angle-camera filters. ISS will then observe a half-phase Enceladus in order to image that moon's south polar plume at high-resolution. Cassini will be 198,000 kilometers (123,000 miles) from Enceladus.
Between April 8 and 17, Cassini will take 12 astrometric observations of Saturn's small satellites. Astrometric observations add further data points for the calculations of the complex orbits of many of these bodies. This complexity results from these moons' proximity to much larger moons. During these nearly daily observations, known as SATELLORBs, ISS will image Methone, Anthe, Calypso, Telesto, Epimetheus, Prometheus, Janus, Polydeuces, Mimas, Pallene, Tethys, Atlas, Pandora, Helene, and Hyperion. Between April 8 and 12, Cassini will image Titan four times in order to monitor changes in the distribution of clouds in its atmosphere. These observations of its sub-Saturn hemisphere will be taken from a distance of 2.06 million to 3.27 million kilometers (1.28 million to 2.03 million miles) from Titan.
With Cassini still in an equatorial orbit, the cameras will be able to image several mutual events between two or more of Saturn's moons. On April 8, ISS will image Prometheus transit across the more distant Rhea, which will also be partially obscured by Saturn's rings. On April 10, ISS will take a look at a transit of Titan by Dione, with Dione 1.8 million kilometers (1.12 million miles) away from the spacecraft. On April 11, Cassini will image another transit of Janus by Epimetheus.
Finishing up Rev129, on April 16 Cassini will acquire a 3-hour observation of Albiorix, a small, outer satellite of Saturn more than 14.9 million kilometers (9.3 million miles) from the spacecraft. The observation 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 April 17, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev130. Shortly before that on April 17, ISS will acquire a seven-frame, wide-angle-camera mosaic of Saturn's faint E ring. This mosaic will cover most of the inner Saturn system out to the orbit of Tethys.
Image products created in Celestia. Dione basemap by Steve Albers. All dates in Coordinated Universal Time (UTC).