Cassini continues its exploration of the Saturn system with the 32-day Rev 202, which begins on February 20 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.84 million kilometers (1.76 million miles) from Saturn's cloud tops. Rev 202 occurs during the first inclined phase, which lasts until March 2015, of the Cassini Solstice Mission. The inclined phase will allow for polar views of Saturn and Titan as well as better vistas of Saturn's rings than those Cassini had while in the earlier, equatorial phase of the Solstice Mission. Thirty-two ISS observations are planned for Rev 202 with the majority focused on Saturn's atmosphere and rings and on Titan during the T99 flyby.
Two days into the orbit on February 22, ISS will image Titan, looking for clouds across its northern hemisphere, at a distance of 3.77 million kilometers (2.34 million miles). Cassini will take another look at Titan on February 24 from a distance of 3.55 million kilometers (2.20 million miles). Afterward on February 24, ISS will observe the outer satellite, Kiviuq, from a distance of 8.06 million kilometers (5.01 million miles). This is one of the closest observations of this small satellite. On February 26, ISS will acquire an astrometric observation of Saturn's small, inner moons. Astrometric observations are used to improve our understanding of the orbits of these small satellites, which can be influenced by Saturn's larger icy moons. Another astrometric observation will be acquired on February 28. Immediately after the February 28 observation, ISS will acquire a quick observation of Saturn using the WAC. This observation is part of a series of "Storm Watch" observation sequences designed to take advantage of short, two-minute segments when the spacecraft turns the optical remote sensing (ORS) instruments back to Saturn as a waypoint between other experiments' observations. These sequences include blue, clear, two methane band, and one full-frame, continuum band filter images. Three more storm watch observations will be taken on February 27 and 28. On March 1, ISS will acquire a movie of the F ring, observing its various channels and streamers created by the interaction between the ring material and the nearby moon, Prometheus.
Cassini encounters Titan on March 6 at 16:27 UTC for the 100th time. This is the third of eleven Titan flybys planned for 2014, with the next encounter scheduled for April 7. T99 has a close-approach altitude of 1,500 kilometers (932 miles). This flyby will allow for imaging of the north polar region and northern anti-Saturn hemisphere inbound to the encounter and the southern anti-Saturn hemisphere of Titan on the outbound leg. ISS will start the encounter off by acquiring a global mapping mosaic of Titan. This ten-frame mosaic will cover much of the visible face of Titan at a resolution of 1.89 kilometers (1.17 miles) per pixel. The observation includes a long stare at the north pole for both cloud monitoring and for looking at the lakes and seas in the region.
The main focus of the flyby is the Radio Science Subsystem (RSS) gravity pass during the 24 hours surrounding closest approach. During a gravity pass, the spacecraft points its high-gain antenna at Earth so that researchers can observe changes in Cassini's signal caused by Titan's gravity. The RSS team will use this measurement to determine the density of Titan's outer layers, like the global sub-surface ocean (by looking for short-term changes in the gravity field caused by Saturn's gravitational pull on Titan), to measure the geoid (global shape) of Titan and search for the presence of large gravity anomalies, and to determine the deformability (also known as its rheology) of the outer ice shell. After RSS finishes its gravity pass, the Composite Infrared Spectrometer (CIRS) will acquire more data about Titan's upper haze layers - its structure, temperature, and composition - as well as the now-cooling south polar vortex. ISS will ride along with the observation.
On March 8 at 19:26 UTC, Cassini will reach periapse for Rev 202 at an altitude of 798,890 kilometers (496,410 miles) from Saturn's cloud tops. Early in the day, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) to observe a pair of stellar occultations by Saturn's ring system from Alpha Lyrae (also known as Vega), an A-type star in the constellation Lyra and one of the brightest stars in the night sky, and R Lyrae, a red giant star also in the constellation Lyra. ISS and VIMS will observe the ingress and egress phases of both occultations. Occultations like these are used to understand the structure of Saturn's ring system and to look for any changes that might occur due to meteor impacts or slight changes in Saturn's gravity field. ISS images will focus on the portions where the stars are occulted by the F ring. Next, ISS will acquire a Wide-Angle Camera (WAC) mosaic of the entire ring system in multiple colors as well as a Narrow-Angle Camera (NAC) scan along the edge of Saturn's shadow on the ring system. Both mosaics will be taken while Cassini is over the unlit side of Saturn's rings. Next, ISS will image the outer A ring, where it will be looking at propellers previously imaged by Cassini. Propellers are small voids in Saturn's rings created by the gravitational interaction between large ring particles and the surrounding ring.
On March 9, ISS will acquire a 10-hour observation of the faint, Phoebe ring, created from micrometeorite impacts on that outer moon. On March 10 and 19, ISS will ride along with VIMS to observe aurorae at Saturn's south pole. On March 11 and 12, ISS will acquire a pair of limb observations at Saturn, which will be helpful for understanding the structure of Saturn's upper haze layers as well as a ride along observation with the Ultraviolet Imaging Spectrometer (UVIS). Similar ride along observations with UVIS will be taken on March 17 and 20. ISS will also observe Saturn during several CIRS observations on March 15, 21, 22, and 24. Starting late on March 13, ISS will acquire a day and a half-long observation of the small outer moon, Fornjot. While Fornjot will be too far away (18.8 million kilometers or 11.7 million miles) for Cassini to observe any surface details, this very long observation will be used to measure its rotational period by observing how its brightness changes as it rotates. Finally, ISS will observe Titan and its upper haze layers over its north polar region on March 17 from a distance of 1.71 million kilometers (1.06 million miles).
On March 24th, Cassini will reach apoapse, bringing Rev 202 to a close and starting up the next orbit, Rev 203, which will include another targeted flyby of Titan.
Cassini continues its exploration of the Saturn system with the 6.5-day Rev 281, which begins on June 26 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.21 million kilometers (0.75 million miles) from Saturn’s cloud tops. Rev 281 is the 11th of 22 proximal orbits that will take place between April 2017 and the end of the mission in September. During these orbits, Cassini’s closest approach to Saturn occurs between the ring system and Saturn’s atmosphere. Eight ISS observations are planned for Rev 281, with the majority focused on Saturn’s atmosphere.
On June 27, Cassini’s Optical-Remote-Sensing instruments will observe a pair of stellar occultations by Saturn and its main rings. First, the Ultraviolet Imaging Spectrometer (UVIS) will watch as Beta Canis Majoris passes behind Saturn. This will allow UVIS to observe the structure of Saturn’s south-polar, upper atmosphere. Afterward, ISS will ride along as the Visual and Infrared Mapping Spectrometer (VIMS) observes Sirius as it passes behind the main rings. On June 28, VIMS will acquire several mosaics of Saturn’s north polar region and northern hemisphere to track cloud motion on Saturn’s nightside.
On June 29 at 22:14 UTC, Cassini will reach periapse for Rev 281 at an altitude of 3,595 kilometers (2,234 miles) above Saturn’s cloud tops. Inbound on this periapse pass, UVIS will observe the limb of Saturn to measure airglow and hydrocarbons in Saturn’s upper atmosphere. The Composite Infrared Spectrometer (CIRS) will map the temperature and composition of the vortex at Saturn’s north pole. As Cassini approaches Saturn, ISS will acquire a noodle of Wide-Angle Camera (WAC) images of Saturn’s atmosphere, running from 80 degrees North Latitude to 17 degrees North Latitude. At periapse, the spacecraft will rotate so that the High-Gain Antenna (HGA) is pointed in the RAM direction, protecting the rest of the spacecraft from inner D ring particles. ISS will continue to acquire WACs in this new orientation through closest approach to Saturn. Early on June 30, VIMS will acquire mosaics of Saturn’s unlit, south polar region.
On June 30, ISS will image a crescent Titan from 1.94 million kilometers (1.21 million miles) away to continuing monitoring seasonal changes to its upper haze layers. On July 1, UVIS will observe a pair of stellar occultations by Saturn’s atmosphere of Zeta Orionis and Episilon Orionis, two of the three stars that make up Orion’s belt. VIMS will acquire a pole-to-pole mosaic of Saturn’s nightside. Later on July 1, ISS will observe the bright limb of Saturn to monitor changes in the planet’s upper haze layers. On July 2, ISS will observe the distant, outer moon Bebhionn from 9.66 million kilometers (6.00 million miles) away. On July 3, ISS will image the outer A ring at high phase angles.
On July 3, during the A ring observation, Cassini will reach apoapse, bringing Rev 281 to a close and starting up the next orbit, Rev 282. At this point, Cassini is 1.21 million kilometers (0.75 million miles) from Saturn’s cloud tops. Eleven ISS observations are planned for this orbit, with the majority focused on Saturn’s main rings. On July 4, Cassini’s Optical-Remote-Sensing instruments will observe a pair of stellar occultations by Saturn and its main rings. First, ISS will ride along as the Visual and Infrared Mapping Spectrometer (VIMS) observes Sirius as it passes behind the main rings. Afterward, Ultraviolet Imaging Spectrometer (UVIS) will watch as Beta Canis Majoris passes behind Saturn. This will allow UVIS to observe the structure of Saturn’s south-polar, upper atmosphere. CIRS will also acquire temperature measurements of the lit face of Saturn’s rings during a pair of observations. ISS will ride along with the second of these covering the C and D rings. On July 5, ISS will image a crescent Titan from 1.94 million kilometers (1.21 million miles) away to continuing monitoring seasonal changes to its upper haze layers.
On July 6 at 09:36 UTC, Cassini will reach periapse for Rev 282 at an altitude of 3,615 kilometers (2,246 miles) above Saturn’s cloud tops. Inbound on this periapse pass, CIRS will acquire a temperature map of the boundary of Saturn’s shadow on the main rings. Afterwards, UVIS will observe a stellar occultation by the main rings of the B-type star Kappa Canis Majoris. Next, ISS will acquire a short, color radial scan across the lit face of the B and C rings. ISS will also conduct a survey of the propellers in the outer A ring. Propellers are voids in the ring created by the gravity of large, 100 – 1000-meter (328 – 3280 foot) ring particles. Due to the influence of the rings on their motion, these observations are needed to keep track of previously discovered propellers, like Earhart and Bleriot. ISS will then observe the edge of the A ring. Afterward, ISS will ride along as VIMS observes the red supergiant star Gamma Crucis passing behind the main rings. Finally, UVIS and ISS will acquire a scan across the C ring, studying small-scale structures in the rings.
During the ring plane cross a few minutes before closest approach, the spacecraft will rotate so that the High-Gain Antenna (HGA) is pointed in the RAM direction, protecting the rest of the spacecraft from inner D ring particles. During the ring plane crossing right before periapse, Cassini’s high-gain antenna will be pointed in the direction the spacecraft is moving to shield it from ring particles. It will then turn to acquire a RADAR scatterometry scan across the main ring system, measuring cm-scale roughness from the A ring down to the C ring.
On July 6, the Radio Science Subsystem (RSS) will perform a radio occultation of the rings by transmitting a signal to Earth through the rings. Changes in the strength of the received signal can provide information about the fine-scale structure of the rings. Afterwards, ISS will ride along with a CIRS observation of the unlit side of the A ring. On July 8, ISS will observe the distant, outer moon Bebhionn from 9.93 million kilometers (6.17 million miles) away.
On July 7, Cassini will reach apoapse, bringing Rev 282 to a close and starting up the next orbit, Rev 283, when Cassini will perform a non-targeted encounter with Titan.