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Cassini continues its extended tour of Saturn on January 28 with Rev102, the spacecraft's 103rd orbit around the Ringed Planet. The spacecraft and its many instruments will be particularly busy in this orbit.
Rev102 includes a targeted flyby of Saturn's largest moon, Titan, and a non-targeted encounter of its second largest moon, Rhea. Cassini begins Rev102 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 1.18 million kilometers (738,000 miles) from Saturn. The spacecraft is in a high-inclination orbit, providing an opportunity to study the rings and the polar regions of Saturn and its satellites.
For the first five days of this revolution, ISS and the other optical remote sensing (ORS) instruments will focus on Saturn's ring system and satellites. Only a few hours into the new orbit, ISS will observe an arc of material in the G ring as Cassini crosses above the ring plane. This arc is a dense concentration of material that is confined by Mimas's gravity within the faint G ring. The Composite Infrared Spectrometer (CIRS) will then perform a radial scan of the left side of the ring system, measuring the system's spectra at mid-infrared wavelengths. Cassini will spend much of the next day, January 30, communicating with Earth or staring at Saturn with its ORS instruments.
On January 30, after a downlink session, ISS will observe Titan's sub-Saturn hemisphere from a distance of 2.27 million kilometers (1.41 million miles). Scientists will use this and similar observations during this orbit to look for methane clouds and to track them if any are observed. Clouds on Titan during the present season are intermittent. Most of the cloud observations that have been acquired over the last few months revealed cloud-free skies (ignoring, of course, the ever-present haze). Following the Titan observation, Cassini will turn its cameras to the small moon Prometheus, which orbits just inside the narrow F ring. ISS will observe Prometheus for more than 11 hours, nearly an entire orbit for the moon. Prometheus's gravity has a strong effect on the particles within the F ring and scientists hope to watch the effects in action during this sequence. Next, ISS will acquire an astrometric observation, designed to improve our knowledge of the orbits of Saturn's small satellites. The moons to be imaged include Anthe, Atlas, and Pan. Pan, however, will be in Saturn's shadow when it's imaged. Finally, ISS will acquire optical navigation images of Enceladus and Mimas before downlinking its day's worth of data to Earth.
On January 31, ISS will again observe Saturn's small satellites, in this case Pandora, Janus, Epimetheus, and Daphnis, in order to better understand their orbital motions. Next, ISS will image again Titan's sub-Saturn hemisphere, this time from a distance of 1.94 million kilometers (1.21 million miles). In addition to looking for clouds, this observation provides an opportunity to image Titan's largest methane sea, Kraken Mare, albeit at a moderate emission angle. Next, the Visual and Infrared Mapping Spectrometer (VIMS) will observe a stellar occultation of Saturn's ring system and dayside atmosphere by Gamma Crucis, a 1.59-magnitude, red giant star in the constellation Crux. ISS will then observe Daphnis, taking 21 images over a span of two hours. The lengthy observation will improve our knowledge of Daphnis's orbit which is within the Keeler Gap in the outer A ring. The Ultraviolet Imaging Spectrometer (UVIS) will then observe another stellar occultation of the rings and Saturn's atmosphere by Beta Centauri, a 0.61-magnitude, blue-white giant star in the constellation Centaurus. Finally, ISS will take a 24-footprint, eight-color, wide-angle camera mosaic of the unlit side of Saturn's ring system.
Following the large, ISS-mosaic observation, the Cassini will turn its ORS instruments to Alpha Trianguli Australis, a bright K-type star in the constellation Triangulum Australe. The star will then be occulted by the A and B rings. Next, CIRS will perform radial scans of the F ring and Cassini Division before Cassini points its high-gain antenna at Earth to downlink its two days worth of observations.
Cassini reaches periapse, its closest point to Saturn on Rev102, on February 2. At this point, Cassini will be 548,000 km (341,000 mi) from Saturn's center, or just outside the orbit of Rhea. During this periapse passage, Cassini will also conduct a non-targeted encounter of Rhea. At its closest approach, Cassini will be 97,427 km (60,538 mi) from the satellite. ISS will acquire five mosaics of Rhea, approaching the moon from almost directly over its north pole (observed near the sub-spacecraft point in the first mosaic) to progressively further south until Cassini is just below Rhea's equator over the moon's trailing hemisphere. ISS's fourth mosaic, acquired shortly after closest approach, will cover a half-phase Rhea with six, narrow-angle camera frames using three color filters and a broadband, clear filter. After ISS has completed taking its five mosaics, ISS will observe from a distance of 1.16 million kilometers (718,000 miles) a gibbous Titan featuring the two equatorial sand seas of eastern Aztlan and western Senkyo. Afterward, CIRS will scan across Rhea as the satellite recedes from Cassini, measuring the moon's thermal emission before and during an eclipse when Saturn passes between the Sun and the satellite. ISS then will make a time-lapse movie of the left ansa of the narrow F ring. During at least a few of the frames, the small satellite Prometheus should pass through the field of view of the Narrow-angle camera. Finally, the Wide-angle camera will observe dust spokes in the B ring. Following optical navigation imaging of Enceladus and Mimas, Cassini will downlink its data back to Earth.
On February 3, CIRS will spend much of the time until a downlink early on February 4 acquiring several temperature scans of the ring system at different solar longitudes. To determine the thermal inertia of different parts of the rings, this new data can be compared to data taken when the Sun was a higher elevation over the ring plane.
Following a downlink session during the first half of February 4, ISS will observe the southern portion of Titan's leading hemisphere from a distance of 1.1 million kilometers (686,000 miles), once again looking for clouds. Next, CIRS will perform four additional temperature scans of the ring system. ISS will then acquire an astrometry observation of several of Saturn's small satellites, including Atlas, Pan, Pallene (for a long stare), Pandora (though this satellite will be in the shadow of Saturn at the time), and Epimetheus. Following various CIRS stares at different portions of the ring system, ISS will take a partial azimuthal scan of the outer A ring, focusing on the Keeler Gap.
Following another downlink session, on February 5 ISS again will image several of Saturn's small moons, including Prometheus, Epimetheus, Pandora, Pan, and Atlas. Afterward, the high-gain antenna will turn to Saturn as the RADAR instrument acquires a radiometry observation of the planet. Next, ISS will again look for dust spokes on the B ring of Saturn. Early on February 6, Cassini will turn its cameras to Titan, examining the large moon's leading hemisphere and south polar region from a distance of 615,000 kilometers (382,000 miles). Combined with the observation of the previous day, scientists hope to track cloud formations in Titan's atmosphere by comparing their position(s) in the two sequences. This, again, assumes that Titan does not experience yet another cloud-free day. Following optical navigation imaging of Dione and Rhea, Cassini will clear its solid-state recorders by downlinking its data to Earth.
Cassini encounters Titan on February 7 at 08:51 UTC for the 51st time and the first of 2009. The close approach distance is only 960 kilometers (597 miles) -- close to the lowest safe altitude for a Titan flyby. This flyby, known as T50, will allow for imaging of the southern leading hemisphere of Titan inbound to the encounter. For the 19 hours before close approach, when Cassini will observe Titan at moderate phase angles over southwestern Xanadu and western Tsegihi, the VIMS and CIRS teams will control pointing until five hours before the encounter when RADAR will take over as "prime". During this approach period, ISS will ride along with the CIRS and VIMS observations. VIMS will control spacecraft pointing between 13 and 19 hours prior to close approach. During this observation, VIMS will conduct long-range tracking of cloud features on Titan. Between 5 and 13 hours prior to close approach, CIRS will perform two observations of Titan. The first is a far-infrared compositional measurement of Titan while CIRS is pointed at the sub-spacecraft point on the surface. The second is a mid-infrared integration over the limb of Titan. This observation allows CIRS and other instruments to measure the composition of Titan's stratosphere. Starting around five hours before closest approach, Cassini will turn to point the high-gain antenna across Titan, allowing the RADAR instrument to acquire radiometry and scatterometry scans. The radiometry scan measures Titan's temperature and ability to store an electric charge. The scatterometry scan looks at surface roughness on Titan.
During closest approach on T50, the Ion and Neutral Mass Spectrometer (INMS) will be prime, while RADAR will ride along to acquire a short Synthetic Aperture Radar (SAR) swath. INMS will measure the composition of Titan's upper atmosphere. Right around closest approach, RADAR will ride along with the INMS observation by taking a SAR swath over Titan's trailing hemisphere, running roughly from southwest to northwest from 50 degrees south, 350 degrees west to 0 degrees north, 310 degrees west [link to Titan map, ID#3778]. Among the features that RADAR will observe include the far northern portion of Mezzoramia, a possible southern analog to the north polar methane sea, Kraken Mare. Next, RADAR will sweep over an infrared dark spot at 44 degrees south, 332 degrees west. This spot, observed at high-resolution by ISS in October 2006, resembles some of the lake features seen by RADAR in the north polar region and could be an example of a mid-latitude lake or a cryovolcanic caldera RADAR also will observe a set of bright streaks south of Senkyo, previously observed by ISS and VIMS. The VIMS team has hypothesized that these streaks are mountain ranges. RADAR's observations of these features could prove or disprove this possibility. Finally, the SAR swath will cover an area where the instrument had previously seen at lower resolution a complex set of bright terrain features and small dune ergs. Altimetry will also be taken by RADAR before and after closest approach. Regions to be sampled include an area informally called the "baseball diamond" in the southern leading hemisphere and a swath paralleling the earlier, T21 RADAR SAR swath. Finally, RADAR will also take several low-resolution, HiSAR swaths before and after closest approach.
Following close approach, RADAR will spend five hours taking radiometry and scatterometry scans across the northern trailing hemisphere. Afterward, ISS, UVIS, and CIRS will trade off control of spacecraft pointing. UVIS will measure Titan's haze layer composition at extreme and far ultraviolet wavelengths. CIRS will acquire several mid-infrared temperature maps. Finally, ISS will acquire two wide-angle camera monitoring sequences before Cassini plays back the Titan data it acquired early on February 8.
Cassini reaches apoapse on February 8, bringing Rev103 to an end.
Image products created in Celestia. Rhea map by Steve Albers. All dates in Coordinated Universal Time (UTC).