At the end of next month, Cassini will yet again be on course to go where no spacecraft has gone before. This time it will be Saturn's narrow, bright F ring. From its November-30 orbital high-point 1.28 million kilometers from Saturn, which is a little farther from the planet than the orbit of its moon Titan, Cassini will begin descending toward its December-4 crossing through the plane of Saturn's rings. This time, which will be the first of twenty such passages, it will come a mere 10,000 kilometers from the F ring -- a distance that is only twice the body-diameter of Titan itself. The most actively changing of any of Saturn's majestic rings, the dusty F ring is constantly being perturbed by the small moon Prometheus and other factors, some of which are unknown. For this and other reasons it is a target of great scientific interest. This week, though, the nineteen-year-old spacecraft had more routine work to do.
Wednesday, October 5 (DOY 279)
Cassini's Imaging Science Subsystem (ISS) had the spacecraft rotate and point its eight-inch reflecting telescope to track one of Saturn's small, irregular moons, Kiviuq, for 12.5 hours. This observation repeated beginning late on Thursday and lasting 24 hours. The very dark-surfaced object is only about 14 kilometers in diameter, and it follows an inclined orbit that reaches as far as 14.8 million kilometers from the planet. The goals are to determine the object's pole orientation, rotation direction, and shape, as well as to address whether Kiviuq might be a contact-binary or even a binary object.
Next, ISS undertook a 90-minute observation in the Titan monitoring campaign at a range of two million kilometers. The Visible and Infrared Mapping Spectrometer (VIMS) and the Composite Infrared Spectrometer (CIRS) rode along. This observation was repeated on Monday from a similar distance, though without CIRS's participation.
Thursday, October 6 (DOY 280)
ISS tracked a known propeller in Saturn's rings (http://go.nasa.gov/17oqTWF) for two hours, with CIRS, VIMS, and the Ultraviolet Imaging Spectrograph (UVIS) participating. When this was finished, ISS led CIRS and UVIS in observing Saturn's faint G ring and E ring for 9.5 hours while they were sunlit from behind at high phase angles.
Friday, October 7 (DOY 281)
Saturn appears dim and low in the western sky as viewed from Earth, and it's setting early these evenings. Mostly because of Earth's motion about the Sun, this means that soon we and Saturn will be on opposite sides of our central star. The distance is increasing, as today's diagram of our solar system shows: http://go.nasa.gov/2edoFkW.
The growing distance means Cassini's radio communications signals are a little weaker now, compared to the naturally occurring radio noise near the same wavelengths. As a result, Cassini has to speak more slowly to be reliably heard across the distance: the highest downlink telemetry communication rate these days is close to 110,000 bits per second. By comparison, when Earth and Saturn are nearer to each other, on the same side of the Sun, the bit rate can reliably be in excess of 142,000 bits per second. The signals still propagate at the speed of light; that will never change. But the amount of embedded information has to be adjusted to remain viable.
Saturday, October 8 (DOY 282)
ISS devoted 17 hours today to observing Saturn’s shadow on the faint, distant Phoebe ring, whose particles orbit Saturn nearly 13 million kilometers out, going in the retrograde direction. UVIS rode along. During the observation, Cassini coasted through apoapsis, marking the start of its orbit #245 of Saturn. The spacecraft had climbed to 1.399 million km and slowed to 11,664 km per hour relative to the planet.
Sunday, October 9 (DOY 283)
Watching the 325-km wide Encke Gap in the outer part of Saturn's A ring was ISS's task for 14 hours today. CIRS and UVIS rode along.
Monday, October 10 (DOY 284)
ISS and VIMS made a one-hour observation in the satellite orbit campaign, looking for small objects near the gas giant. Next, CIRS measured temperature differentials across the northern face of Saturn's rings, during an eight-hour activity while UVIS rode along.
VIMS took the reins next, and with ISS and CIRS riding along, watched a distant orange star for nearly nine hours; Lambda Velorum passed behind Saturn's ring system thanks to Cassini's orbital motion. While its light was being occulted by material in the rings, and alternately shining through their gaps, the optical instruments obtained data about the rings' structure.
Back on the home planet today, Cassini's 70th Project Science Group (PSG) meeting began its week of work and discussions near JPL, with about 180 Cassini scientists in attendance.
In conjunction with the PSG this week are two science workshops of note. The Titan Cold Case workshop is revisiting some of the big questions from earlier in the mission or pre-mission such as the fate of Titan’s methane, the strange behavior of Titan’s clouds, and reconciling Arecibo results with Cassini’s radar observations. The goal is to determine whether scientists are now able to ‘close’ the case files, or whether some topics will require new dedicated missions to address them. The PSG will close with the Geologically Young Rings and Moons workshop that will examine the dynamical and geological evidence for and against a geologically recent origin of Saturn's icy moons and rings.
An image featured today captures a unique view of Saturn's rings: https://saturn.jpl.nasa.gov/resources/7531.
Tuesday, October 11 (DOY 285)
CIRS, UVIS and VIMS performed a joint eight-hour study of Saturn's large icy moon Rhea to better understand the composition of its surface. Finally, ISS spent five hours making a high resolution, color scan of the main rings. UVIS, CIRS, and VIMS participated.
On three occasions during the week when the optical instruments were already pointing on or near Saturn, ISS made two-minute storm watch observations with VIMS riding along.
The Deep Space Network communicated with and tracked Cassini five times this week, using stations in Australia and California. A total of ten individual commands were uplinked, and about 1,260 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 9.6 days in a plane inclined 57.9 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on October 11, using the 70-meter diameter DSN station at Goldstone, California. 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.
This week's feature article is "Care and Feeding of an Aging Spacecraft": https://saturn.jpl.nasa.gov/news/2947/care-and-feeding-of-an-aging-spacecraft.
Cassini's analysis of Saturn's magnetosphere is summarized here: https://saturn.jpl.nasa.gov/science/magnetosphere.
Cassini's path up to mid-day October 11 is illustrated here: http://go.nasa.gov/2cErR5C. At that time, the countdown clock in Mission Control was showing 338 days until the end of the mission.
Milestones spanning the whole orbital tour are listed here: https://saturn.jpl.nasa.gov/mission/saturn-tour/tour-dates.
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.