Saturn and Mimas

From high above Saturn's northern hemisphere, NASA's Cassini spacecraft gazes over the planet's north pole, with its intriguing hexagon and bullseye-like central vortex. › Full image and caption

Cassini is one of the best-instrumented scientific machines ever to fly in interplanetary space. Unique among all its other instruments, Cassini's Radar was designed to actively examine the surface of Saturn's planet-like moon Titan, which hides under a thick, hazy nitrogen atmosphere. For such Titan observations, Cassini's microwave-remote-sensing Radar instrument would transmit five individual radar beams to image the hidden surface. Now during Cassini's proximal orbits, with no more Titan Radar observations remaining to be made, this instrument is getting new assignments. This week, on the spacecraft's seventh passage between the rings and atmosphere, one of the Radar instrument's five beams was selectively used alone, in order to obtain Synthetic-Aperture Radar (SAR) imaging of Saturn's rings. In between its quick, active imaging pulses, the instrument also served as a microwave radiometer; the sophisticated instrument can passively discern some of the natural radio emissions that come from the ring particles.

Wednesday, May 31 (DOY 151)

Cassini passed apoapsis today. This marked the beginning of Orbit #277.

Thursday, June 1 (DOY 152)

Looking "down" to the gas giant planet from on high, Cassini's Ultraviolet Imaging Spectrograph (UVIS) began making slews across Saturn's vast northern auroral region today; the Composite Infrared Spectrometer (CIRS) rode along. This remote-sensing observation lasted over 15 hours, while Saturn rotated more than once. Meanwhile, the Magnetospheric and Plasma Science (MAPS) instruments were directly measuring the concurrent plasma and field conditions in-situ.

Friday, June 2 (DOY 153)

This week's featured image shows giant Saturn and mere Mimas: https://saturn.jpl.nasa.gov/resources/7686.

Saturday, June 3 (DOY 154)

For six hours today, CIRS stared at the sunlit side of Saturn's broad A ring to determine the thermal-infrared emissivity of its particles; The Ultraviolet Imaging Spectrograph (UVIS) rode along. The observation was centered near the 325-kilometer wide Encke gap, to help scientists learn about the structure of this region, in which Saturn's little moon Pan orbits the planet once every 13.9 hours.

Next, ISS led two consecutive observations with CIRS and UVIS riding along. The first target was the area between the broad A ring's outer edge and the narrow F ring. For its second activity, ISS reacquired and tracked propellers, objects which raise waves in the rings like the ones pictured here: https://go.nasa.gov/2rVC9ab.

When this was done, CIRS produced a high-resolution thermal scan across the main rings at an elevation angle close to 90 degrees, which minimized the foreshortening that typically reduces effective resolution. UVIS rode along with the three-hour observation.

ISS then led the rest of the Optical Remote-Sensing (ORS) instruments for nearly three hours, scanning the rings' sunlit side. This complements the high-resolution scan of their unlit side from Cassini's previous orbit. These images will be combined with other systematic ring scans to generate a full, high-resolution radial mosaic of Saturn's main rings.

Sunday, June 4 (DOY 155)

Diving in for its seventh plunge between rings and planet, Cassini came some 300 km closer to Saturn than during the previous plunge. Again, as the spacecraft used the High Gain Antenna as a shield, the Radio and Plasma Wave Science (RPWS) instrument recorded hits by ring particles, which are detectable to RPWS by the plasma clouds they generate on impact. For two hours starting at periapsis passage, Cassini pointed its high-gain antenna dish to Saturn's rings, and conducted its single-beam SAR imagery. Resolutions are expected to make features visible that are from 0.5 up to 4 km in size.

Now with the spacecraft flying directly under the unlit side of the rings, CIRS produced another high-resolution thermal scan across them. This one lasted four hours, and had UVIS riding along again. Comparing scans of the sunlit and unlit sides, scientists can examine the energy exchange across the thickness of the rings, to reveal more about the ring particles and structures.

Next, VIMS and UVIS used their solar ports to observe an occultation of the Sun for 5.6 hours as it passed behind Saturn's rings. By comparing the transmission of sunlight through the rings, VIMS and UVIS can constrain knowledge of the smallest particles there. Finally today, VIMS observed the 2.5-hour occultation of the bright red star Alpha Orionis, Betelgeuse, as it passed behind the rings; ISS and CIRS rode along.

Monday, June 5 (DOY 156)

Betelgeuse next passed behind Saturn's atmosphere from Cassini's view, and VIMS probed the atmosphere by tracking the star for about an hour until it disappeared, deep in the upper atmosphere. CIRS then turned back towards the rings, and stared at the C ring to measure its particles' emissivity at long, thermal-infrared wavelengths; ISS and UVIS rode along for this 7.5-hour observation.

UVIS watched the blue star Epsilon Orionis, the middle star in Orion's belt, for 2.8 hours as it passed behind the rings, with ISS and CIRS riding along. Next, UVIS turned and tracked the star Zeta Orionis, another belt star, for two hours as it took a similar path behind the rings. The low opening angle of this "double occultation" allowed UVIS to study the distribution of material in the more diffuse ring regions.

The week-long Cassini Project Science Group (PSG) meeting began at the European Space Research and Technology Centre (ESTEC) in Noordwijk, the Netherlands today.

Tuesday, June 6 (DOY 157)

For yet another stellar occultation, UVIS pointed to blue Epsilon Orionis, which today passed behind Saturn's icy moon Tethys. The 33-minute observation was looking for possible telltale absorption in the star's spectrum by any intervening atmospheric material at Tethys. Finally, ISS took advantage of the low ring elevation-angle and high-phase lighting angle, and began an 18-hour observation, staring at the ring ansa with ISS and UVIS riding along, to map out the fainter components in the rings. By the time this observation finished, well into the next day, Cassini would be at apoapsis again, beginning another new orbit.

JPL's "What's Up" video this week includes a description of the best Saturn viewing in a telescope, as the ringed jewel closes in on opposition on June 15. From now until mid-September, but then never again, Earthlings can view Saturn at the same time Cassini is viewing it (given light-travel time, of course, which at opposition is 75 minutes): https://www.jpl.nasa.gov/video/details.php?id=1474.

The DSN communicated with and tracked Cassini on 10 occasions this week, using stations in California, Spain, and Australia. A total of 48 individual commands were uplinked, and about 1,300 megabytes of science and engineering telemetry data were downlinked and captured at rates as high as 142,201 bits per second.

Wrap up:

Cassini is executing its set of 22 Grand Finale Proximal orbits, which have a period of 6.4 days, in a plane inclined 62.0 degrees from the planet's equatorial plane. Each orbit stretches out to an apoapsis altitude of about 1,272,000 km from Saturn, where the spacecraft's planet-relative speed is around 6,000 km/hr. At periapsis, the distance shrinks to about 2,500 km above Saturn's visible atmosphere (by comparison, Saturn is about 120,660 km in diameter), and the speed is around 123,000 km/hr.

The most recent spacecraft tracking and telemetry data were obtained on June 7 using the 70-meter diameter DSN station in Spain. The spacecraft continues to be in an excellent state of health with all of its subsystems operating normally except for the instrument issues described at http://saturn.jpl.nasa.gov/anomalies.

The countdown clock in Mission Control shows 100 days until the end of the Mission.

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This page offers all the details of the Mission's ending: <https://saturn.jpl.nasa.gov/mission/grand-finale/overview/>
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Milestones spanning the whole orbital tour are listed here:
<https://saturn.jpl.nasa.gov/mission/saturn-tour/tour-dates/>
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Information on the present position and speed of the Cassini spacecraft may be found on the "Present Position" page at:
<https://saturn.jpl.nasa.gov/mission/saturn-tour/where-is-cassini-now/>
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