The Cassini Spacecraft's time in orbit is getting shorter. This refers to the long term, of course, since the Grand Finale is a little over a year away. But it also refers to each orbit Cassini makes of Saturn. Since Aug. 10, each full orbit of the planet has been taking only twelve days to complete. Late next month Cassini will shorten its orbit period further, to 9.6 days, and then to eight days in November. By the end of Cassini's tour of Saturn next year, the orbits will have shortened to an awfully brief 6.5 days.
Add to this Cassini's orbit's high inclination -- more than 50 degrees off the equatorial ring plane -- and it's clear the quicker orbits offer a wide range of astounding views, which change dramatically every day. To illustrate, this week's account is accompanied by links to simulated viewing geometry at several points.
Wednesday, Aug. 17 (DOY 230)
Cassini's Imaging Science Subsystem (ISS) finished up two hours of observing a known "propeller" (http://go.nasa.gov/17oqTWF) feature in Saturn's ring system, with the Composite Infrared Spectrometer (CIRS) also observing. ISS then pointed to Saturn's largest satellite Titan for 90 minutes, monitoring the planet-like moon's atmospheric features, with CIRS and the Visible and Infrared Mapping Spectrometer (VIMS) riding along. Titan was about 1.5 million kilometers from the spacecraft. The observation was repeated on Saturday, and again on Tuesday, from similar distances.
Next, the Ultraviolet Imaging Spectrograph (UVIS) conducted slews across Saturn's northern auroral region, taking data for 10.4 hours. Here's an illustration of the viewing angle.
Thursday, Aug. 18 (DOY 231)
UVIS and VIMS spent another nine hours watching Saturn's northern auroral zone, and then CIRS took the reins to observe Saturn's large icy moon Dione for 4.5 hours, detecting the spectra in reflected sunlight to determine Dione's surface composition. VIMS and UVIS rode along.
The Cassini project scientist made a presentation to Deep Space Network (DSN) personnel today, describing Cassini science objectives, key events and tracking requirements for the upcoming F-ring and Proximal orbit mission phases.
The view to Saturn has changed abruptly from the day before, as Cassini speeds through its short, highly inclined orbit.
Friday, Aug. 19 (DOY 232)
After only two days, Cassini was below Saturn's ring plane. Looking back toward the planet, UVIS led a 15-hour observation of southern auroral region, which was now largely in the shade of night on the planet. ISS and VIMS participated at selected times. This time the auroral observation included looking for the "footprint" where an arc of electrical current meets Saturn's atmosphere. The associated "tube" of electric current is constantly being generated by the motion of the small icy moon Enceladus and its ejecta through Saturn's magnetic field: /resources/15289/ . The activity is being coordinated with observations by the Hubble Space Telescope near Earth.
During the observation, the spacecraft coasted past periapsis at 400,000 km above Saturn's visible limb, going 40,974 km per hour relative to the planet. Here's Cassini's enormously different view of the planet today.
Saturday, Aug. 20 (DOY 233)
Looking back toward the dark side of Saturn's rings, CIRS and VIMS tracked the 325-km-wide Enke gap in outer the A ring to determine particle composition. Back-lighting by the Sun caused the finest particles in and near Saturn's rings to shine brightly in forward-scattered light.
Orbit Trim Maneuver (OTM)-459, the Titan T-123 encounter targeting maneuver near apoapsis, executed on its backup opportunity today. Postponing this maneuver by one day conserved a fair amount of propellent. The 53-second burn of Cassini's small rocket thrusters provided the necessary change in velocity of 56 millimeters per second.
This week's simulated views of Saturn from Cassini are all with the same wide field of view, set arbitrarily to 60 degrees. Today's view: https://space.jpl.nasa.gov/cgi-bin/wspace?tbody=699&vbody=-82&month=8&day=20&year=2016&hour=23&minute=00&fovmul=1&rfov=60&bfov=30&showac=1.
Sunday, Aug. 21 (DOY 234)
VIMS observed the bright red star Alpha Orionis, also known as Betelgeuse, as it became occulted by the Saturn's rings, passing behind them due to Cassini's motion in orbit. CIRS and ISS rode along with this four-hour observation, gaining information about ring-system structure.
Following the stellar occultation, ISS took control of spacecraft pointing for 16 hours to track and observe Saturn's small, distant irregular moon, Skathi (also transliterated from the Norse as Scadi). This very dark-surfaced object orbits Saturn in a retrograde and highly inclined orbit that reaches as far as 19.7 million km from the planet. The name refers to a giantess in Norse mythology. Given the parameters of Skathi's orbit, it might have formed from material knocked off of Saturn's moon Phoebe by an ancient collision. Today's viewing geometry is again very different, and arguably beautiful.
Monday, Aug. 22 (DOY 235)
All the optical remote-sensing instruments, which are CIRS, ISS, UVIS and VIMS, spent nearly nine hours jointly observing Saturn's moon Hyperion, which has a spongey appearance. The spacecraft had come within 350,000 km from the dark-surfaced object, which orbits Saturn at a distance of 1.4 million km.
A close-up of tectonic features on the surface of Saturn's icy moon Dione is today's featured image: /resources/17482 .
Tuesday, Aug. 23 (DOY 236)
ISS, CIRS, and VIMS observed Saturn's narrow, contorted F ring for nearly eight hours today. Later, ISS spent an hour looking for small objects near Saturn, as part of the satellite orbit campaign. VIMS rode along. Near the end of the day, ISS and VIMS began an observation of Saturn’s irregular moon Erriapus that would last nearly 28 hours. Named for a giant in Gaulish mythology, Erriapus is only about eight kilometers in diameter. It has a very dark surface, and its inclined orbit reaches 25.6 million km from Saturn.
One final viewing angle illustration, on the way back "up" towards apoapsis on Aug. 25 (the Sun is in view this time): http://go.nasa.gov/2bgLRIF .
On three days this week, while Cassini's optical instruments were already pointing at or near Saturn, ISS carried out two-minute Saturn storm-watch observations. The Deep Space Network communicated with and tracked Cassini via eight sessions this week, using stations in Spain, California and Australia. A total of 1,078 individual commands were uplinked, and about 1,494 megabytes of telemetry data were downlinked and captured at rates as high as 142,201 bits per second.
Cassini is orbiting Saturn with a period of 12 days in a plane inclined 53.7 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on Aug. 23, using one of the 34-meter diameter DSN stations 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.
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 .