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Cassini continues its extended tour of Saturn on March 27 with Rev106, the spacecraftís 107th orbit around the Ringed Planet. Cassini begins Rev106 late on March 15 at its farthest distance from Saturn, called apoapse. At this point, Cassini is more than 1.1 million kilometers (684,000 miles) from Saturn. The spacecraft remains in a high-inclination orbit, providing an opportunity to study the rings and the polar regions of Saturn and its satellites. During Rev106, Cassini will encounter Titan for the 52nd time since its first flyby in late 2004.
For the first five days of this revolution, ISS and the other optical remote sensing (ORS) instruments will focus on Saturnís ring system and satellites from high above Saturnís ring plane. Following a downlink session near apoapse, Cassini will spend much of March 16 using the Composite Infrared Spectrometer (CIRS) to acquire radial temperature scans of the ring system. ISS will be used to observe the trailing hemisphere of Saturnís largest moon Titan from a distance of 1.9 million kilometers (1.2 million miles). ISS also will acquire astrometric observations of several of Saturnís small satellites, including: Prometheus, Pan, Anthe, Atlas and the newly discovered G ring moonlet S/2008 S 1. Astrometric observations are designed to improve our knowledge of the orbits of Saturnís small satellites.
On March 18, ISS will repeat some of its observations from two days earlier. First, ISS will observe the northern part of Titanís sub-Saturn hemisphere from a distance of 1.9 million kilometers (1.2 million miles). These images are taken to look for clouds on Titan as well as take a look at the dark surface region of Kraken Mare to monitor potential changes there as spring approaches. ISS will also take another astrometric observation of Methone, Atlas, Janus, Prometheus, Epimetheus, and Pandora. In addition to these ISS observations, the Visual and Infrared Mapping Spectrometer (VIMS) will observe a stellar occultation of the F and outer A rings by Gamma Crucis. Stellar occultations -- situations in which a star is positioned on the opposite side of the rings from spacecraft -- are used to measure the opacity of the rings system. CIRS will also take a few more radial temperature scans of the unlit face of the rings, using this and similar observations to measure grain sizes and thermal inertia, or heat capacity, across the rings.
On March 20, ISS will take a wide-angle camera mosaic of the unlit face of the rings. Several similar mosaics acquired over the next few months will allow scientists to monitor how the brightness of the rings changes as the system approaches equinox in August. ISS also will observe the shadow of Tethys on the outer A ring. Finally, ISS will image Titanís sub-Saturn hemisphere from a distance of 1.1 million kilometers (682,000 miles), looking for clouds west of Kraken Mare. CIRS also will observe the unlit face of the rings from 20 degrees above the ring plane.
Cassini reaches periapse, its closest point to Saturn on Rev106, late on March 21. At this point, Cassini will be 759,000 kilometers (472,000 miles) from Saturnís cloud tops, between the orbits of Rhea and Titan. Shortly before periapse, ISS will observe Titanís sub-Saturn hemisphere from a distance of 992,000 kilometers (616,000 miles). Comparing these images to those acquired the day before should allow scientists to measure cloud speeds if clouds are visible. ISS also will take a look at a crescent Enceladus, looking for changes in the jets that erupt from that moonís south polar region. Finally, CIRS will observe the lit face of the rings from 20 degrees below the ring plane.
In the days following periapse, Cassini will focus on the rings and Titan. ISS will observe Titan twice, on March 22 and 24, looking at the southern sub-Saturn hemisphere. The March 24 occasion provides another opportunity to look at a playa -- or dry lake -- that filled with rainfall in late 2004. This should help to confirm recent results revealing new changes at this feature. On March 23, ISS will take a 13-hour movie of the F ring, staring at the left ansa of the narrow ring. During the movie, Prometheus and Pandora pass through the narrow-angle cameraís field of view, appearing very close together after a conjunction between the two moons the day before. On March 24, ISS will observe the small, inner satellite Methone to help better determine that moonís orbit. On March 25, ISS will acquire a 12-hour observation of the small, outer moon Siarnaq. Although the moon is too far away and too small for Cassini to resolve surface features, scientists hope to measure how light reflected from Siarnaq changes over time in order to calculate the length of the moonís day.
Cassini encounters Titan on March 27 at 04:44 UTC for the 52nd time and the second of 2009. This flyby is the first of a pair of Titan flybys that will occur only eight days apart. The second will take place on April 4. The close approach distance is only 960 kilometers (597 miles), close to the lowest safe altitude for a Titan flyby. This flyby, known as T51, will allow for imaging of the southern leading hemisphere of Titan inbound to the encounter. For the 17 hours before close approach, Cassini will observe Titan at moderate phase angles over the southwestern Xanadu and western Tsegihi regions. During that time, the ISS, VIMS, and CIRS teams will control spacecraft pointing -- or be ďprimeĒ. When ISS is not prime during the approach, ISS will ride along with the CIRS and VIMS observations, continuing to capture images. ISS will acquire three mosaics during this approach period. The first, MONITORNA001, will cover the visible surface of Titan with 10 frames and will be taken from distances ranging from 258,000 to 284,000 kilometers (160,000 to 176,000 miles). The second, GLOBMAP001, will first cover the terrain north of Hotei Arcus, then will zig-zag across the terrain south of Hotei and around the south pole, with 28 frames of three images apiece. GLOBMAP001 will be taken from distances ranging from 108,000 to 186,000 kilometers (67,000 to 116,000 miles). The final mosaic, REGMAP001, will cover the south polar playa with four frames. Changes on this seasonal lake feature were observed between July 2004 and June 2005 as a result of a lowland region filling with methane rainfall during a storm in October 2004. Since then, the lake may have dried up and reverted back to its pre-storm appearance. REGMAP001 will be taken from distances ranging from 29,000 to 39,000 kilometers (18,000 to 24,000 miles). In addition to these ISS observations, CIRS will acquire a mid-infrared atmospheric composition observation and a far-infrared temperature map. VIMS will look for clouds near Titanís equator.
During closest approach on T51, VIMS will be prime. The near-infrared spectrometer will observe several features in the south polar region, looking for changes since RADAR observed the region in late 2007. VIMS also will observe Ontario Lacus. This lake was observed by VIMS during the T38 flyby in December 2007. VIMS revealed the presence of liquid ethane within the lake, but found the lakeís depth to be shallow compared to expectations based on RADAR observations of other Titan lakes. VIMS will follow-up on these observations, looking for potential changes in both the extent and the depth of Ontario.
Following close approach, the Radio Science team will obtain a bistatic radar observation of the surface of Titan. The team will use the high-gain antenna on Cassini to bounce a radio signal off Titanís surface that will then be received on Earth, almost 1.3 billion kilometers (784 million miles) away. The characteristics of the returned signal can tell scientists quite a bit about the roughness of a planetary surface. CIRS also will obtain a number of observations, including mid-infrared temperature maps, limb composition integrations, and nadir composition measurements. ISS will observe Titanís nightside, looking at changes in the haze layers over Titanís north polar region as winter nears an end at that pole.
Cassini reaches apoapse on March 27, bringing Rev106 to an end.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).