CICLOPS: Cassini Imaging Central Laboratory for OPerationS
Rev 267-268: Mar 25 - Apr 9 '17

Cassini continues its exploration of the Saturn system with the 7.2-day Rev 267, which begins on March 25 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.22 million kilometers (0.76 million miles) from Saturn’s cloud tops. Rev 267 is the 17th of 20 F-ring orbits that will take place between November 2016 and April 2017 where Cassini will approach Saturn just outside the main ring system. Eleven ISS observations are planned for Rev 267 with most focused on Saturn’s atmosphere.

On March 25, ISS will perform its first observation of Rev 265 by observing a crescent Titan from 1.93 million kilometers (1.20 million miles) away. This observation will be used to monitor changes to the moon’s outer atmospheric haze layers resulting from seasonal changes. Northern summer begins on Titan (and the rest of the Saturn system) on May 24 during Rev 275. On March 27, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) as it observes a stellar occultation by Enceladus’s south polar plume of the bright B-type star Epsilon Canis Majoris. Stellar occultations such as this are used to measure the density of gases within the plume, like water and nitrogen.

Between March 26 and March 31, many of Cassini various instruments will focus their attention on Saturn’s atmosphere. These include compositional measurements of its atmosphere by the Composite Infrared Spectrometer (CIRS) on March 26 and 27, limb mapping by ISS on March 27 and 31, polar auroral mapping by UVIS on March 27 and 28, and near-infrared mapping of Saturn’s north polar region on March 27 and 28 and south hemisphere on March 29. UVIS will also observe a stellar occultation of Beta Crucis on March 28 that will provide an improved estimate of the density of gas in Saturn’s upper atmosphere through to the innermost D ring, a region where Saturn will be passing through starting on Rev 271.

On March 29 at 06:09 UTC, Cassini will reach periapse for Rev 267 at an altitude of 86,484 kilometers (53,739 miles), near the orbits of Janus and Epimetheus. During closest approach, the Ion and Neutral Mass Spectrometer (INMS) will be “prime,” performing in-situ measurements of the faint, primarily-oxygen atmosphere that surrounds the rings. Afterwards, ISS will ride along with CIRS as it observes the south polar region of Enceladus. CIRS will look for changes in the amount of heat the region is giving off while ISS will observe the south polar plume from 176,000 kilometers (109,000 miles) away. The VIMS southern hemisphere observations of Saturn, after periapse, will cover the polar regions, “storm alley” around 35 degrees South Latitude, and the equatorial region. Taken over Saturn’s night side, these will be used to estimate cloud coverage across the three different regions as they experience very little sunlight due to the shadow of the rings.

On March 30, ISS will ride along with a CIRS compositional observation of the unlit side of Saturn’s A ring. On March 31, the camera system will observe Enceladus’s once again, this time from 1.01 million kilometers (0.63 million miles) away. On April 1, Cassini will reach apoapse, bringing Rev 267 to a close and starting up the next orbit, Rev 268. Seventeen ISS observations are planned for this orbit, with most covering Titan and Saturn’s rings. Late on April 1, ISS will observe a crescent Titan from 1.97 million kilometers (1.22 million miles) away. On April 3, ISS will observe a half-phase Titan from 1.41 million kilometers (0.88 million miles) away. This observation will be used to track clouds across Titan’s northern mid-latitudes on the moon’s sub-Saturn hemisphere. Later on April 3, ISS will acquire an 8-hour, time-lapse movie of the narrow F ring to monitor features in the ring created by the gravity of nearby moons and ring clumps.

On April 5 at 10:01 UTC, Cassini will reach periapse for Rev 268 at an altitude of 86,605 kilometers (53,814 miles), near the orbits of Janus and Epimetheus. Most of the observations during the periapse period focus on Saturn’s rings and measuring the composition of gas near the F ring. First, VIMS will observe a stellar occultation of the star Gamma Crucis by Saturn’s atmosphere. Next, ISS will ride along with a UVIS stellar occultation observation, where Dione will pass in front of the star Alpha Eridani. UVIS will use the observation to look for volatiles like water vapor near Dione. Afterward, CIRS then ISS will perform high-resolution radial scans of Saturn’s A and B rings. ISS will perform a similar radial scan of the unlit side after periapse. During closest approach, INMS will be “prime” once again, performing in-situ measurements of the faint, primarily-oxygen atmosphere that surrounds the rings.

After a radial scan of the unlit side, ISS will perform very high resolution imaging of the A ring edge and the F ring. Afterward, ISS will conduct a survey of the propellers in the outer A ring. Propellers are disturbances in the ring created by the gravity of large, 100 – 1000-meter (328 – 3280-foot) diameter ring particles. Due to the back influence of the rings on their motions, these observations are needed to keep track of previously discovered propellers, like Earhart and Bleriot. Finally, CIRS will do a compositional measurement of the unlit side of the A ring.

On April 5, the Radio Science Subsystem (RSS) will perform a radio occultation of the main ring system by transmitting a radio signal through the rings to Earth. By measuring variations in the strength of the received signal, the radial structure of the rings can be observed. Differences between this and prior occultations can be used to measure spiral density structures within the rings. At 22:08 UTC, Cassini will perform a non-targeted encounter with Titan at a distance of 477,490 kilometers (296,700 miles). ISS will acquire a series of four mosaics and several stares of Titan, while CIRS will build-up a pair of compositional maps. These observations will cover Titan’s sub-Saturn and southern hemispheres to look for stray clouds in these regions. On April 7 and 8, ISS will observe the D ring and the outer A ring at high phase angles.

On April 9, Cassini will reach apoapse, bringing Rev 268 to a close and starting up the next orbit, Rev 269.

Image products created in Celestia. All dates in Coordinated Universal Time (UTC).