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Cassini Significant Events 01/07/2015 – 01/13/2015

Cassini is currently orbiting Saturn with a 31.9-day period in a plane inclined 19.1 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on Jan. 14 using one of the 34-meter diameter Deep Space Network 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/

Cassini's main attraction this week was a close encounter with Saturn's 5152-kilometer-wide moon Titan. Generally, each of Cassini's orbits of Saturn includes a Titan flyby, not only for the value of its scientific observations, but also to shape Cassini's orbital tour. This week's T-108 gave Cassini a gravity-assist deflection to further reduce the inclination of its orbit from 28.6 degrees with respect to Saturn's equatorial plane -- the ring plane -- down to 19.1 degrees. This time, Cassini's orbital period was not changed; the vehicle remains in a 31.9-day orbit of the gas giant.

There were other highlights as well: stellar occultations, in which a star passes behind Saturn's atmosphere, or its rings, are enormously productive scientific observations, so Cassini takes advantage of as many of these opportunities as it can. Often they are among the first activities to be scheduled during command sequence development. Four such observations took advantage of three fortuitously placed stars this week.

Meanwhile on Earth, Cassini's managers and scientists made final arrangements for the next Project Science Group meeting; the weeklong PSG #65 will begin Monday of next week at the Italian Space Agency (ASI) headquarters near Rome.

Wednesday, Jan. 7 (DOY 007)

The Imaging Science Subsystem (ISS) controlled spacecraft pointing for 11 hours while its telescopes tracked and observed Saturn's dark-surfaced, spongy-looking moon Phoebe, which orbits nearly 13 million kilometers from the planet. The Ultraviolet Imaging Spectrograph (UVIS) and the Visible and Infrared Mapping Spectrometer (VIMS) took advantage of this pointing, and made observations in ride-along mode. These observations will help scientists compare Phoebe to Saturn's more distant irregular satellites. Next, UVIS took control for 1.75 hours to measure the ultraviolet albedo of the anti-Saturnian hemisphere of Saturn's slightly larger moon Mimas.

Thursday, Jan. 8 (DOY 008)

UVIS watched the bright blue star Alpha Virginis, also known as Spica, for nearly five hours while Saturn's A ring, then the B ring, and then Saturn itself occulted the star; ISS and the Composite Infrared Spectrometer (CIRS) rode along. Next, UVIS spent two hours observing the rings while its field of view drifted out towards the ansa; ISS rode along. By the time the observation was completed, Cassini's orbital motion had caused Spica to rise from behind Saturn, down near the high southern latitudes, so UVIS observed this occultation egress for two hours with CIRS riding along. Next, ISS performed a high-resolution color scan of the sunlit faces of the A and B rings for 3.5 hours. Finally, ISS and VIMS carried out an observation in the Titan monitoring campaign, while the huge hazy moon was about 1.9 million kilometers from the spacecraft.

A news feature released today describes how Cassini has helped the astronomical community pin down the position and orbit of Saturn in the solar system. Highly precise knowledge of solar system objects aids basic understanding of the solar system, and enables accurate navigation through interplanetary space. The feature may be seen here:

Friday, Jan. 9 (DOY 009)

ISS spent four hours making a close-up observation of the ripples in Saturn's D ring -- the innermost ring, which is difficult to see from Earth. Following this, VIMS turned and tracked the red star Alpha Hercules while it was occulted by Saturn's rings for 2.75 hours. Finally, CIRS studied ring thermal properties for seven hours while CIRS and ISS rode along.

Cassini passed through periapsis in its orbit about Saturn late in the day, coming in as close as 431,000 kilometers from the visible top of the atmosphere. The spacecraft had sped up to 41,780 kilometers per hour relative to the planet -- nearly seven times its speed at apoapsis on Dec. 24.

While very close to the periapsis point, Cassini turned and fired its rocket thrusters for 51 seconds, based on commands uplinked near real time for Orbit Trim Maneuver (OTM)-400. This provided a change in velocity of 55 millimeters per second to fine-tune the approach to Titan. The flight team had postponed this maneuver from its normally scheduled execution on the previous day, to make use of an orbital-mechanics advantage that the navigation team realized would save some precious propellant.

Saturday, Jan. 10 (DOY 010)

ISS reacquired and tracked the orbits of known "propeller" features ( http://go.usa.gov/YyGR ) in Saturn’s rings for 1.3 hours. Next, ISS led CIRS and VIMS to make high-resolution observations of Saturn's narrow, bright, "twisted" F ring. When this was finished, CIRS continued observing the rings for four hours to further its study of ring-particle thermal properties. To close out the day's activities, UVIS turned to the blue-white star Gamma Pegasus when it appeared behind the Cassini Division in Saturn's rings and made a grazing occultation behind the dense B ring; ISS and CIRS rode along.

Sunday, Jan. 11 (DOY 011)

Cassini turned to begin conducting telescopic observations as it came within 400,000 kilometers of Titan -- as close to Cassini as our Moon is to Earth. As the distance rapidly decreased from there, the spacecraft switched its attitude control from the fine-pointing reaction wheels to the hydrazine-fed thrusters, which have enough control authority to deal with passage through Titan's upper atmosphere. Cassini's Radar instrument operated in its various modes, including synthetic-aperture imaging near closest approach, providing the best resolution of Titan's haze-hidden, complex surface. As usual, Cassini's direct-sensing Magnetospheric and Plasma Science instruments acquired data amid all the remote-sensing observations. Today's flyby is further described on the T-108 encounter page:

Monday, Jan. 12 (DOY 012)

Outbound from the close encounter, VIMS, ISS and UVIS rode along with CIRS to obtain low-resolution images of Titan's north polar region. CIRS led the other instruments while observing temperatures and aerosols in Titan's dense atmosphere, and then making a mid-infrared thermal map to obtain information on the thermal structure of Titan's stratosphere.

An image featured today shows off a relatively young impact crater on Saturn's icy moon Rhea:

Tuesday, Jan. 13 (DOY 013)

For an extra day after the encounter, ISS monitored Titan's atmosphere to track springtime clouds and their evolution, with VIMS riding along.

During the past week, the Deep Space Network communicated with and tracked Cassini on nine occasions, using stations in Spain, Australia and California. A total of 164 individual commands were uplinked, and about 1920 megabytes of telemetry data were downlinked and captured at rates as high as 124,426 bits per second.

If your outside before dawn on Friday morning Jan. 16, look for Saturn almost "touching" the crescent Moon in the East.