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Cassiniís journey at Saturn continues with Rev 44, its 45th orbit of the ringed planet, as Cassini observes Titanís trailing hemisphere, Saturnís ring system, and several of Saturnís icy satellites.
Cassini begins Rev44, on May 2 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 2.2 million km (1.36 million mi) from Saturn. The first week of Rev44 is filled with observations of Saturnís small satellites (to refine their orbits), the F ring, and the rest of the ring system. F ring observations, planned for May 5, are designed to look at gores in the F ring created by close approaches of the ring by the shepherd moon Prometheus. These gores and streamers represent places where Prometheus has pulled particles from the F ring.
As mentioned above, a number of observations are dedicated to observing several of Saturnís small moons to refine our estimates of their orbital paths. These moons include a few of the outer satellites of Saturn. Of these moons, only Phoebe has been photographed so far by Cassini. But during the first week of Rev44, the tiny moon Skathi will be observed. These images will only show the moon as a couple of bright pixels, and no surface features will be visible: the resolution of this observation will be 63 km/pixel and the moon is only 6.4 km across.
Cassini reaches periapse, the closest point in its orbit around Saturn, on May 10 at distance of 253,000 km (157,000 mi). On May 11, Cassiniís Visual and Infrared Spectrometer and Imaging Science Subsystem will observe Saturnís rings. During this observation, the spacecraft will be directly between the Sun and the ring system. This allows for observations of the ďopposition surge.Ē Visually, on the rings, the opposition surge looks like a bright spot on the ring (see PIA08247 for earlier views of this phenomenon).
Cassini encounters Titan for the 31st time on May 12, with a closest approach distance of only 960 km (600 mi). Like the last few encounters with Titan, this flyby (known as T30) will allow for imaging of the northern portion of Titanís trailing hemisphere following closest approach. With each encounter in this sequence of the mission (starting in February with T25), Cassini observes terrain farther and farther south within the trailing hemisphere; ie, the side of Titan that always faces away from its direction of motion along its orbit. The imaging cameras will observe the surface starting around 5 hours after closest approach to acquire the main ISS mosaic from this encounter. At the center of the planned mosaic is the 1,700-km (1,050-mi) wide bright region known as Adiri. This region contains several dark patches known to consist of dune fields 100-200 km (60-120 mi) across. In addition, the Huygens probe landed just off the eastern tip of Adiri in January 2005. This observation will also yield additional images of several intriguing features seen during the last few flybys, such as a 90-km wide dark spot, thought to be an impact crater, to the north of Adiri.
The Cassini RADAR instrument will be in the driverís seat for much of the close approach period. In the 15 minutes before closest approach, RADAR will take the longest altimetry swath obtained to date. The altimetry mode allows the RADAR team to determine the altitude of various features on Titanís surface. The purpose of this long swath is to validate a new technique developed by the RADAR team to use the Synthetic Aperture Radar modeís central beam as an altimeter. The new swath will cross several earlier SAR swaths, correlating the altitudes determined by the ďtopography from SARĒ technique. If this technique can be validated, the RADAR team will no longer have to choose between obtaining SAR or altimetry data, and it will greatly increase altimetry coverage, since SAR has covered much more of the moon than altimetry to this point.
In the 15 minutes after closest approach, a Synthetic Aperture Radar (SAR) swath will be obtained. This swath will cover the central portion of the Caspian Sea-sized dark region first observed by ISS and RADAR in late February. This observation will help determine if all of this dark region is covered with liquid methane and/or ethane, or just the northernmost part observed several times so far by RADAR. The end of this SAR swath will also cover some of the dark lineaments observed by ISS and RADAR north of Belet.
The last 5 days of Rev44 continue with the ring and small satellite observations that characterized the first week of this orbit. Several observations are planned at this time for imaging Saturnís small satellites and refining the orbits of these little worlds. Like the observation before periapse on this orbit, these observations include several of Saturnís tiny outer satellites, such as Kiviuq, Mundilfari, Ijiraq, and Bestla.
On May 15, Cassini will observe Titan from a distance of 1.4 million kilometers (880,000 miles), allowing for images of Titanís north polar region (on the trailing side) at image scales better than 7.4 kilometers (4.6 miles) per pixel. This observation was designed to study the light scattering behavior of Titanís atmosphere and surface, as well as observe cloud motions in the northern hemisphere. Clouds were first observed in Titanís northern hemisphere, by ISS, in February, but only solitary images have been obtained to this point. This 12-hour long observation will allow for cloud tracking. By measuring cloud motions during this time period, scientists hope to measure wind speeds in Titanís troposphere (the lowest layer of Titanís atmosphere).
Cassini begins the next orbit, number 46 (Rev45) on May 19, during which it will encounter Titan for the 32nd time. Those who like Saturnís mid-sized icy satellites will love Rev45. Observations are planned for all of Saturnís mid-sized icy satellites (except Iapetus). Look for numerous observations of Tethys and its giant canyon, Ithaca Chasma, Mimas and its giant impact crater Herschel, Enceladus at very low phase angles, Dione, and Rheaís leading hemisphere. There will also be several observations of Saturnís atmosphere and ring system.