On Tuesday of this week, Cassini was poised to begin what can be called a whole new mission. A gravity assist on that day from the T-125 targeted flyby of Titan flung the spacecraft into a daring set of 20 highly-inclined orbits of Saturn. On each of these "ring-grazing" orbits, the spacecraft will come near the outer edge of the main ring system, closer to the gas giant than ever before (except for the day of arrival in 2004). The direct-sensing, in-situ instruments will be sampling particles and gasses in the ring plane, and the telescopic instruments will be making observations from many new perspectives. This annotated image of Saturn's rings makes it clear where Cassini will pass just outside of the F ring, but well inside the G ring: https://saturn.jpl.nasa.gov/internal_resources/589 .
A table showing the times for Cassini's apoapses and ring-plane crossings (which are all near periapsis) during this new part of the mission is available here: https://saturn.jpl.nasa.gov/news/2967/ring-grazing-orbits-quick-reference .
Wednesday, Nov. 23 (DOY 328)
The command sequence S97 began its 10 weeks of controlling Cassini's activities today. First off, the Composite Infrared Spectrometer (CIRS) mapped Saturn's northern hemisphere at near-infrared wavelengths, to measure temperatures in the upper troposphere and tropopause. The 22-hour observation covered two full rotations of the planet. The Visible and Infrared Mapping Spectrometer (VIMS) rode along.
While the CIRS observation was underway, Cassini coasted through apoapsis, marking the start of its orbit #250 of Saturn. It had reached an altitude of 1.32 million kilometers from Saturn, and slowed to 10,282 km per hour relative to the planet.
Thursday, Nov. 24 (DOY 329)
For the first time in five years, Cassini turned to point the Imaging Science System (ISS)’s telescope to Saturn's irregular moon Suttungr, which it tracked for four hours. This little moon takes almost a year to go around Saturn, in a retrograde orbit whose inclination is close to the ring plane; it reaches as far as 21.7 million km from the planet. The approximately 6-km diameter object was named after a giant in Norse mythology.
Next, the Ultraviolet Imaging Spectrograph (UVIS) took over spacecraft pointing for 8.3 hours to map Saturn's northern hemisphere at extreme- and far-ultraviolet wavelengths, to study the distribution of hazes and organic compounds high in Saturn’s atmosphere; CIRS and ISS rode along.
An annotated slice across Saturn's main ring system was featured today as NASA's Astronomy Picture of the Day today: http://apod.nasa.gov/apod/ap161124.html .
Friday, Nov. 25 (DOY 330)
VIMS took three hours to map Saturn’s high northern latitudes. Included was a single image centered on the planet's north pole, where a huge hurricane persists; CIRS, ISS, and UVIS rode along.
Saturday, Nov. 26 (DOY 331)
ISS viewed Suttungr again today for 2.3 hours, from a distance of about 17 million km. The observation would repeat for 8.3 hours on Monday. These observations are intended to determine how the object rotates.
The exploding star system Eta Carinae is fascinating in its own right, but today it served as a bright point of infrared light for CIRS to watch as it passed behind Saturn's main rings. Many such occultation experiments have been performed with UVIS, VIMS and Radio Science, but few with CIRS. Occultation observations by CIRS fill in gaps in wavelength coverage (9 to 16.6 microns) that are not sampled by the other instruments. This particular chord occultation, which had a turnaround point in the mid-C ring, was particularly valuable for providing information about the rings' optical depth.
Today's observations by UVIS of Saturn's icy moons Tethys and Dione, 2.5 hours each, should reveal information about the nature and sunlight-scattering properties of their icy surfaces. CIRS, ISS and VIMS rode along with both.
Sunday, Nov. 27 (DOY 332)
Today, Cassini came within about 21,000 km of Saturn's small but active moon Enceladus. It was a "non-targeted" encounter, meaning Cassini did not have to use any rocket-thruster firings to achieve the encounter, it just happened along the way to the next targeted Titan flyby. The Optical Remote-Sensing (ORS) instruments took full advantage of the opportunity, though.
First, CIRS, VIMS and UVIS spent two hours examining Enceladus's north-polar region, in a search for signs of previously undetected endogenic activity. Next, Cassini's RADAR instrument spent two hours examining Enceladus. It used its passive radiometry mode, which is sensitive to natural radio emissions from short distances below Enceladus's surface, and it also used its active scatterometry mode. Next, ISS spent 1.7 hours on an observation with the primary goal of obtaining broadband multispectral images of Enceladus, with all the other ORS instruments riding along. The body's active southern hemisphere, now in its local winter season, came under scrutiny of CIRS and the other ORS instruments as Cassini completed its flyby.
Cassini coasted through periapsis during the Enceladus observations, going 62,149 km per hour relative to the planet. It came within 158,000 km of Saturn's visible edge. This was Cassini's final periapsis at such a relatively large distance; next week's periapsis will be only 84,000 km above the clouds.
Monday, Nov. 28 (DOY 333)
The Radio Science team conducted an experiment today, using several stations of the Deep Space Network and one European Space Agency (ESA) ground station. They observed Cassini's three radio-frequency emissions while the spacecraft passed behind Saturn's rings, collecting information about physical properties of the ring system structure, including gravitational wakes and the host of density waves that populate the A ring.
Saturn's moon Mimas is less than 1/700 the diameter of the main ring system. An image featured today illustrates the size comparison; to compare their mass, though, is a surprisingly different story: https://saturn.jpl.nasa.gov/resources/7562 .
Tuesday, Nov. 29 (DOY 334)
Cassini will have only one more targeted flyby of Titan in its mission after today's T-125 encounter. Today, the spacecraft's ORS instruments were prime for control of spacecraft pointing. Cassini's RADAR instrument will be prime for the final targeted encounter, T-126, this coming April. Today's event, as mentioned, provided the nudge Cassini needed to begin its series of so-called "F-ring orbits." In doing so, the spacecraft's orbit period was shortened to 7.2 days, and its inclination was raised to 63.7 degrees.
Today's encounter is described here: https://saturn.jpl.nasa.gov/news/2965/titan-flyby-t-125-gateway-to-the-ring-grazing-orbits .
The Cassini Scientist for a Day essay contest opened recently. The U.S. deadline for participating is Feb. 24, 2017; other countries may have different deadlines. The contest challenges students in grades 5 through 12 worldwide to participate with NASA scientists in studying Saturn for a day. More information may be found here: https://solarsystem.nasa.gov/kids/scientist-for-a-day/ .
The Deep Space Network communicated with and tracked Cassini 10 times this week, using stations in Spain, Australia and California; an ESA station participated one time from Australia. A total of 17 individual commands were uplinked, and about 940 megabytes of telemetry data were downlinked and captured at rates as high as 110,601 bits per second.
Cassini is orbiting Saturn with a period of 7.2 days in a plane inclined 63.7 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on Nov. 28, using one of the 34-meter diameter DSN stations in Australia. 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 .
Milestones spanning the whole orbital tour are listed here:
Information on the present position and speed of the Cassini spacecraft may be found on the "Present Position" page at: