[For trouble viewing the images/movies on this page, go here]
Cassini continues its exploration of the Saturn system with the 6.5-day Rev 271, which begins on April 9 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 271 is the 1st of 22 proximal orbits that will take place between April 2017 and the end of the mission in September, where Cassini will approach Saturn between the ring system and Saturn’s atmosphere. Eleven ISS observations are planned for Rev 271 with the majority focused on Saturn’s atmosphere.
On April 24, ISS will ride along with the Composite Infrared Spectrometer (CIRS) to observe a crescent Titan from a distance of 760,000 kilometers (472,000 miles). ISS will be acquiring a timelapse movie over a 3-hour period, useful for tracking clouds in Titan’s lower atmosphere. ISS will acquire a follow-up cloud monitoring observation later on April 24.
On April 26 at 09:03 UTC, Cassini will reach periapse for Rev 271 at an altitude of 2,941 kilometers (1,827 miles) above Saturn’s cloud tops, the first of 22 periapses where Cassini will pass between the rings and Saturn to provide unique observations of Saturn’s internal structure, for example. On this pass, Cassini will focus on optical-remote sensing observations of Saturn’s atmosphere. Inbound on April 25, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) as it maps Saturn’s northern hemisphere during a set of three observations. During the two-and-a-half hours before periapse, ISS will acquire high-resolution, Wide-Angle Camera images (WACs) of Saturn’s north polar vortex. During periapse, the spacecraft will be rotated so that the High-Gain Antenna (HGA) will be pointed in the direction Cassini is moving to protect the spacecraft from D-ring particles. ISS will ride along to acquire very-high-resolution WACs during periapse as Saturn passes through the WAC’s field-of-view, providing the highest resolution images of Saturn to-date.
Outbound, CIRS will acquire a regional thermal map of Saturn’s south polar region. ISS will ride along with VIMS to image Saturn’s rings while Cassini is in the planet’s shadow. Afterwards, VIMS will watch the Sun as it emerges from behind Saturn. ISS will ride along as the Ultraviolet Imaging Spectrometer (UVIS) observes Saturn’s south polar aurorae. Early on April 27, VIMS will observe a stellar occultation of the red supergiant star Betelgeuse by Saturn. Finally, UVIS will acquire several ultraviolet imaging slews of Saturn’s south polar aurorae.
Late on April 27, ISS will acquire a 12-hour time-lapse movie of Enceladus’ south polar plumes from 1.20 million kilometers (0.74 million miles) away, watching how the plume’s intensity changes over a third of an orbit. The plumes are thought to vary in intensity due to tidal stresses opening and closing the plume’s source fractures. On April 29, ISS will image the bright limb of Saturn to monitor its upper haze layers. A few hours later, Cassini will reach apoapse, bringing Rev 271 to a close and starting up the next orbit, Rev 272. Nine ISS observations are planned for this orbit, most will be covering Saturn’s rings. Later on April 29, ISS will take WAC calibration images in order to track changes in the dark current of the camera’s CCD sensor. On April 30, ISS will ride along with a VIMS observation of a stellar occultation of the A-type star Sirius by Saturn’s ring system. Afterward, ISS will acquire a seven-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 May 2 at 19:42 UTC, Cassini will reach periapse for Rev 272 at an altitude of 2,861 kilometers (1,778 miles) above Saturn’s cloud tops. Late on May 1, ISS will observe a crescent Titan from 1.95 million kilometers (1.21 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 Rev275. Next, ISS will ride along with CIRS to observe Saturn’s moon Rhea from 368,000 kilometers (229,000 miles) away, allowing for two sets of five-color imaging of the moon’s northern hemisphere. This is the last Rhea observation of the Cassini mission. Next, ISS will image the outer A ring and the F ring. At closest approach, the Magnetometer will be performing a series of rolling calibration observations while the Ion and Neutral Mass Spectrometer (INMS) directly measures the composition of Saturn’s ionosphere.
Early on May 3, ISS will acquire a scan across Saturn’s ring system while the spacecraft is in Saturn’s shadow. This observation will also include a long stare at the C and D rings in the hopes of catching impacts between ring particles and small meteoroids. Afterward, ISS will acquire a four-hour observation of the small, distant satellite Bebhionn. On May 4, ISS will observe several of Saturn’s faint rings including the F ring, G ring, and the Methone ringlet.
On May 6, Cassini will reach apoapse, bringing Rev 272 to a close and starting up the next orbit, Rev 273, which will include a non-targeted encounter with Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC). Rhea basemap by Steve Albers.