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Cassini Significant Events 09/30/15 – 10/06/15

Cassini is currently orbiting Saturn with a period of 13.9 days in a plane inclined 0.6 degree from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on Oct. 7 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/news/significantevents/anomalies .

Typically, Cassini has three opportunities to fire its rocket thrusters or main engine during each of its orbits about Saturn. Last week's T-113 encounter with Saturn's planet-like moon Titan changed the spacecraft's orbit period to just under 14 days, so the time between Orbit Trim Maneuvers (OTMs) is compressed. This week, Cassini fired its 400-newton bipropellent-fed main engine on two occasions to make the small adjustments needed to remain on the planned orbital path.

Wednesday, Sept. 30 (DOY 273)

Cassini's Imaging Science Subsystem (ISS) tracked features on Saturn, at varying emission angles, for two hours while the planet rotated. The Composite Infrared Spectrometer (CIRS), the Ultraviolet Imaging Spectrograph (UVIS), and the Visible and Infrared Mapping Spectrometer (VIMS) took advantage of the pointing and rode along acquiring data as well.

Next, Cassini turned to point its telescopes toward Saturn's moon Dione, as it passed within about 48,800 kilometers of the spacecraft. This icy moon is 1,123 km in diameter, one-third the diameter of Earth's rocky moon. All of the optical remote-sensing (ORS) instruments, ISS, CIRS, UVIS and VIMS, made observations at various times during the period. CIRS, in particular, observed how areas on the surface cool at different rates as they move from sunlight to darkness, including a thermally anomalous region of interest. ISS had a good equatorial view on Dione's leading hemisphere, where there are ridges, grooves, and tectonically deformed craters, as well as large plains that are relatively crater-poor and thus younger.

Turning from Dione, VIMS watched a distant red star named Delta Ophiuchi for 1.3 hours. Due to the spacecraft's motion, the bright star passed slowly behind Saturn in its southern hemisphere. Stellar occultations such as this are valuable for learning about the structure and makeup of the planet's atmosphere; in this case revealing the vertical temperature profile and aerosol abundance in the stratosphere. Next, still pointing to Saturn, VIMS mapped the planet's sunlit side, including the area around 35 degrees north latitude where storms have been seen. The planet rotated for three hours during the observation, while the other ORS instruments rode along. This illustration shows the viewing geometry: https://space.jpl.nasa.gov/cgi-bin/wspace?tbody=699&vbody=-82&month=9&day=30&year=2015&hour=11&minute=50&fovmul=1&rfov=40&bfov=30&showsc=1&showac=1 . (The red star Delta Ophiuchi can be seen to the lower left, just about to go behind Saturn near the B-ring shadow.)

Upon completion of these observations, VIMS turned to another distant red star, this one named 30 Piscium. VIMS captured data about the rings' structure while the star made a slow ingress occultation over the course of 2.75 hours.

During the ring-occultation experiment, Cassini coasted through periapsis of its orbit #222. It came within 114,566 km of the planet's visible limb, skirting just outside the faint G ring, going 72,010 km per hour relative to Saturn.

CIRS took the reins away from VIMS following the occultation, and studied the thermal structure in the stratosphere at Saturn's limb, north and south of the equator, in the mid-infrared part of the spectrum. The observation lasted 3.2 hours while VIMS rode along.

Thursday, Oct. 1 (DOY 274)

The bright blue-white stars Epsilon Orionis and Zeta Orionis are interesting objects in themselves, and they form part of Orion's belt in our sky. Today, though, they both aided UVIS in probing Saturn's atmosphere. UVIS spent one hour watching them being occulted by Saturn's high atmosphere in the west, from less than one microbar down to the millibar level. The lowest altitudes sampled by UVIS overlap with the highest that CIRS can sample. Today's UVIS experiment measured temperatures and vertical profiles of hydrocarbons in the thin Saturnian air. Photochemical reactions are known to occur in this region, and some still-unidentified process generates substantial heat there as well. Adding to the interest, this is the region where Cassini will pass during its final orbits in 2017.

As soon as the stars were fully hidden by the planet, UVIS began making one slow scan across Saturn's illuminated hemisphere to form spectral images in the far- and extreme-ultraviolet. The other ORS instruments rode along for this four-hour observation. Next, it was time for Epsilon and Zeta Orionis to rise through Saturn's eastern limb, and UVIS observed the occultation-egress for just over two hours.

Cassini quickly passed around to Saturn's night side, where VIMS finished up the day by mapping the whole night-time hemisphere four times, covering 50 degrees south to 50 degrees north latitude, and spanning about 180 degrees of longitude. CIRS rode along.

Friday, Oct. 2 (DOY 275)

Commands stored onboard Cassini in the S91 sequence came to a planned pause just as it received commands built and uplinked near real time to perform OTM-422, the post T-113 trajectory clean up maneuver. The spacecraft turned and fired its main rocket engine for 1.4 seconds, providing the required change in velocity of 0.25 meter per second.

ISS turned to watch the faint G and E rings for 6.4 hours from low elevation angles while they were sunlit at high phase angles, meaning basically from behind. VIMS rode along.

Cassini scientists were represented during the week at the European Planetary Science Congress in Nantes, France. The meeting came to a close today.

Saturday, Oct. 3 (DOY 276)

CIRS observed Saturn's atmosphere for 13 hours, gathering high-quality infrared spectra to determine upper troposphere and tropopause temperatures. VIMS rode along.

Sunday, Oct. 4 (DOY 277)

The Cosmic Dust Analyzer (CDA) observed interstellar dust for 13.5 hours today.

Monday, Oct. 5 (DOY 278)

UVIS began a 60-hour series of mosaic scans of Saturn’s magnetosphere, focusing on the vicinity of the Saturn disk to image oxygen. The first scan was 15 hours in duration. When this was done, Cassini executed commands sent from Earth in real time to turn and fire its main rocket engine for 15 seconds. This OTM-423 provided the spacecraft a change in velocity of 2.6 meters per second, targeting for the Enceladus flyby E-20 coming up on Oct. 14.

An unusual view of Saturn's tiny moon Pandora joins the more distant, hazy Titan in an image featured today: /resources/16247 .

Tuesday, Oct. 6 (DOY 279)

UVIS made a 12.5-hour magnetosphere mosaic scan. After this was completed, the navigation team used ISS to take images of Saturn's moon Rhea against the background stars for optical navigation purposes.

This week, ISS conducted a total of four Saturn storm-watch observations; VIMS rode along with two of them; all were two minutes long, squeezed in while the optical instruments were already pointing on or near Saturn. Throughout the week, and largely independent of which direction the spacecraft was turning to point its other instruments, the Magnetospheric and Plasma Science (MAPS) instruments continued collecting data in an ongoing survey of Saturn's magnetosphere.

During the week, the Deep Space Network communicated with and tracked Cassini on nine occasions, using DSN stations in California and Australia. A total of 6,271 individual commands were uplinked, and about 2,168 megabytes of telemetry data were downlinked and captured at rates as high as 110,601 bits per second.

This illustration shows Cassini's position on Oct. 6: http://go.nasa.gov/1jOtsaB . The format shows Cassini's path over most of its current orbit up to today; looking down from the north, all depicted objects revolve counter-clockwise, including Saturn along its orange-colored orbit of the Sun.