[For trouble viewing the images/movies on this page, go here]
Cassiniís journey at Saturn continues with Rev 59, its 60th orbit of the ringed planet. Cassiniís slate of observations this orbit includes a Titan flyby, a distant encounter with Janus, and numerous observations of Saturnís rings, atmosphere, and small moons as the spacecraftís orbit increases in inclination. Cassini begins Rev59 on February 14 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 1.82 million km (1.13 million mi) from Saturn. On February 15, Cassini performs several observations of Saturnís small satellites. The observations are designed to study the orbits of these objects and how they might evolve over short time periods due to perturbations from the other satellites in the system. On February 16, Cassini captures several distant images of Iapetus. During this observation, Cassini will be 2.2 million km (1.4 million mi) from the two-tone satellite. Only a thin crescent will be visible from Cassini, so while this observation will not be best for mapping, these images should help with characterizing the shape of Iapetus as well as looking at the fine-scale surface properties using photometry. (In addition, the area to be imaged was observed at much higher resolution, albeit in Saturn-shine, during the earlier close flybys.)
Two long sequences covering Saturnís atmosphere are planned for February 18 and 19. The observations include filters designed to look at the dynamics of clouds at different levels of Saturnís upper atmosphere. Cassiniís position high above the ring plane will provide a largely unobstructed view of Saturnís northern hemisphere. The only other imaging sequence during the pre-periapse period consists of an F-ring movie sequence scheduled for February 17. This sequence is designed to observe the current state of clumps, gaps, channels, and fans within the F-ring, caused by its interaction with a retinue of moons and temporary clumps of material orbiting in the region. Cassini reaches periapse, the closest point in its orbit, on February 20 when the spacecraft is 199,000 km (124,000 mi) above Saturnís cloud tops. Two imaging sequences are planned for the periapse period. The first occurs just as the Sun passes behind Saturn from Cassiniís vantage point. ISS and the Visual and Infrared Mapping Spectrometer (VIMS) will view the ring system at high phase angles, perfect for observing dust in the system. Cassini then performs a distant encounter with the co-orbital satellite Janus at 144,000 km (89,000 mi). Images planned for the encounter will show the southern sub-Saturn hemisphere of the irregularly shaped moon. For the first hour of the encounter, the Composite Infrared Spectrometer (CIRS) will stare at the moon to determine the properties and depth of Janusí surface regolith. Following this ďsit Ďní stare,Ē ISS will observe the satellite using a multitude of spectral filters. Cassini encounters Titan for the 42nd time on February 22, with a close approach distance of only 1,000 km (602 mi). This flyby (known as T41) will allow for imaging of the trailing hemisphere of Titan, centered northwest of the bright region named Adiri. Inbound to the encounter, when only a thin, sunlit crescent is visible from Cassini, the VIMS, ISS, and CIRS teams trade control of spacecraft pointing (taking turns being "prime"). CIRS will have control of spacecraft pointing for the first three hours of the encounter. During that time, CIRS will stare at an area to the east of Hotei Arcus using its far-infrared detector. This observation is designed to study the composition of the atmosphere in this area and to measure the surface temperature. VIMS and ISS have prime coverage for the next four hours, examining the visible crescent of Titan. For ISS, this will allow for study of Titanís upper haze layers.
The RADAR instrument will be in control of spacecraft pointing for the five hours before and two hours after closest approach. During this flyby, RADAR will use all of its observation modes: Synthetic Aperture Radar (SAR) at closest approach, altimetry before and after the SAR swath, scatterometry, and radiometry. The altimetry swaths, designed to study regional topography, will cover central Tsegihi inbound and the terrain north of Adiri outbound. The SAR swath, which will provide RADAR imaging at up to 300 m/pixel resolution, is a little more complicated than most swaths. When the RADAR instrument is building up a SAR image, Cassiniís dish-like antenna (which transmits and receives the radio pulses) is pointed at an angle away from straight down at the surface. The RADAR team can decide (in advance) whether looking left or right will best suit their science goals. For all prior encounters, the antenna was pointed in the same orientation for the entire swath. However, for the T41 SAR swath, the antenna will be pointed to the right for the first half of the swath and to the left for the second. This arrangement will allow the RADAR team to image Hotei Arcus and the region just south of Xanadu during the first part of the SAR coverage, and the Huygens landing site during the second part. Hotei Arcus is a bright arc to the southeast of Xanadu. This feature is a suspected cryovolcanic feature based on its morphology and its bright appearance at five microns in VIMS data. The RADAR team hopes to confirm this interpretation with its higher resolution dataset. RADAR coverage of the Huygens landing site during T41 will provide regional stereo coverage when combined with the lower resolution T8 data from October 2005. The second half of the RADAR swath will also cover a part of far southeastern Adiri, seen by ISS as an interesting patchwork of bright and dark material.
Following RADARís closest approach coverage, UVIS, ISS, VIMS, and CIRS will trade control of spacecraft pointing. Between three and five hours after closest approach, the UVIS team is in the driverís seat. UVIS will examine Titanís atmosphere using a stellar occultation of Eta Canis Majoris. This observation is designed to study the haze layers over the dawn terminator of Titan. UVIS will also perform two scans of Titan using its extreme and far ultraviolet detectors. Two ISS mosaics are planned for the period after closest approach: the first, a 26-frame, full-disk mosaic taken from distances ranging from of 110,000-184,000 km (68,000-114,000 mi); the second, a 14-frame, full-disk mosaic taken from distances ranging from 235,000-275,000 km (146,000-171,000 mi). VIMS will acquire distant observations of Titan designed to study cloud evolution (assuming clouds are visible at the time), and CIRS will perform several scans to examine temperature and compositional differences in Titanís atmosphere. Near the end of the encounter UVIS will watch another stellar occultation, this time using Epsilon Canis Majoris, which is the brightest star in the sky at the wavelengths UVIS is measuring, making it an excellent star to use for stellar occultations. These occultations will allow the UVIS team to examine differences in Titanís high-altitude haze layers between the night- and day-sides. Cassini will focus on Titan again on February 25, in a distant observation of the moonís trailing hemisphere.
Cassini finishes Rev59 with a small satellite orbital determination sequence and wide-angle camera mosaic of Saturnís atmosphere.
The spacecraft begins the following orbit, number 61 (ďRev60Ē), on February 25.