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Cassini Significant Events 03/09/16 – 03/15/16

Cassini is orbiting Saturn with a period of 23.9 days in a plane inclined 21.9 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on March 15, using the 70-meter diameter Deep Space Network station 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 crowning achievement for this week was an ICY PIE. This high-priority observation of an icy moon (ICY) was a pre-integrated event (PIE). This means the observation was scheduled (integrated) into Cassini's timeline of planned science observations before any other activities were scheduled near that time. It was also a "targeted" observation for Cassini, meaning that the spacecraft's trajectory had been adjusted, using propulsive maneuvering. This allowed the spacecraft to be in position to align with a star that would pass behind the plume of ejecta continuously issuing from Saturn's small but active icy moon Enceladus. The remarkable observation is described on its own page, which includes a beautifully animated simulation of the event: http://saturn.jpl.nasa.gov/mission/flybys/enceladus20160311/ .

Wednesday, March 9 (DOY 069)

Saturn's broad A ring is the outermost of its main rings. The outer part of that ring is home to most of the larger "propeller" features that Cassini has discovered. A "propeller" is a visible pair of wakes in the ring-material created by the gravitation from a small, orbiting lump (http://go.nasa.gov/17oqTWF). Today, Cassini's Imaging Science Subsystem (ISS), along with the Composite Infrared Spectrometer (CIRS), finished up a 13.25-hour survey of that region, in an attempt to reacquire many of the largest propellers. Lumps and particles in that region complete an orbit of Saturn about once every 14 hours.

For the next science activity, the Visible and Infrared Mapping Spectrometer (VIMS) trained its telescope on the distant, red star R Aquilae for 2.25 hours while it was being occulted by the A ring, moving behind it by virtue of Cassini's orbital motion. During this 2.25-hour occultation, the line of sight to the star cut all the way into the inner B ring; CIRS rode along. VIMS then observed the occultation of another bright red star, Epsilon Pegasi, by the rings. This 2.25-hour occultation spanned from outside the F ring, across the A, B, and C rings, to the innermost portions of the D ring; CIRS and ISS rode along. Finally, the day closed with a 3.5-hour CIRS observation of the southern hemisphere of Saturn's small icy moon Mimas, with ISS, VIMS, and the Ultraviolet Imaging Spectrograph (UVIS) riding along.

During the Mimas observation, Cassini sped through periapsis of its Saturn orbit #233. It came within 153,700 kilometers of Saturn's visible limb, going 49,300 km per hour relative to the planet.

Thursday, March 10 (DOY 070)

ISS spent one hour generating images of Saturn's narrow, bright F ring and the edge of the A ring; CIRS and VIMS rode along. CIRS then watched the unlit side of the A ring, across its 325-km wide Encke gap, with VIMS riding along. The infrared spectra, acquired at moderate wavelength-resolution, will provide for insight into the composition of Saturn's rings and the structure of their particles. Next, ISS spent another 4.75 hours on its propeller survey, with CIRS riding along. Cassini's remarkable viewing geometry is illustrated here. (Recall that the latest raw images from Cassini are always posted here: http://saturn.jpl.nasa.gov/photos/raw .)

Late in the day, ISS took further advantage of the unique viewing geometry -- high-phase sunlight angles and low elevation angles -- by starting a 2.5-hour observation of Saturn's faint D ring. These phase and elevation angles highlight the very small particles that make up that ring, which is the closest ring to the planet. CIRS and VIMS rode along.

Friday, March 11 (DOY 071)

Still taking advantage of the viewing angles today, CIRS observed the Cassini Division, which is the area between the A ring and the dense B ring, to investigate the composition and structure of the particles there; VIMS rode along. Following this four-hour observation, ISS turned back to Saturn's rings for nearly five hours to acquire movies of their fainter, more tenuous components, while they were lit up at high-phase sunlight; CIRS and VIMS rode along.

Next came the highly anticipated ICY PIE. During the observation, UVIS controlled spacecraft pointing, while the other optical instruments (ISS, VIMS, and CIRS) rode along. Having achieved the right alignment in space and time, bright blue Epsilon Orionis, the middle star in Orion's belt, passed behind the Enceladus plume. This occultation experiment was particularly valuable, because it represented Cassini's final opportunity to fill in a key gap in UVIS's coverage of Enceladus's orbital phase. It will allow the UVIS team to determine whether or not the gas abundance in Enceladus's plume is modulated with the moon's orbital phase, as is true with the plume's dust abundance.

Immediately following the ICY PIE, the spacecraft turned to point its high-gain antenna towards Earth. After Cassini's radio signal propagated for 82.4 minutes, the 70-meter diameter Deep Space Station in Australia locked onto it, and captured all the telemetry from this observation and others, over the course of nine hours. Several of ISS's raw images bear witness to the remarkable stellar occultation as it occurred; here is one of them: http://go.nasa.gov/1STsbgO .

Saturday, March 12 (DOY 072)

VIMS observed the disk of Saturn for 11 hours to create movies of the planet as Cassini receded from the planet towards a March-21 apoapsis; CIRS rode along. Following this observation, UVIS spent 14 hours mapping Saturn at ultraviolet wavelengths, to study the distribution of hazes and organic compounds high in the gas giant's atmosphere. The other optical instruments also observed.

Sunday, March 13 (DOY 073)

Today's science observation was led by CIRS, which repeatedly scanned north and south across Saturn’s disk for 11 hours, creating a global map of the planet in the infrared part of the spectrum. VIMS rode along.

Cassini's multi-spectral view of Saturn from 2007 was selected as NASA's Astronomy Picture of the Day. Back then it was summer in the planet's southern hemisphere, and thermal-infrared light showed bands of low-altitude storm clouds in the dark northern latitudes: http://apod.nasa.gov/apod/ap160313.html .

Monday, March 14 (DOY 074)

VIMS took the opportunity to create another global movie of Saturn at near-infrared wavelengths today; CIRS rode along. Following this 14-hour activity, CIRS focused on one particular latitude of Saturn, observing for 22 hours while the planet rotated twice; CIRS and ISS rode along.

New views of northern terrain are possible now that it's springtime there in the cold Saturnian system. Enceladus is the subject of today's featured image.

Tuesday, March 15 (DOY 075)

On five occasions during the week, the DSN communicated with and tracked Cassini, using stations in Australia. A total of 20 individual commands were uplinked, and about 1,398 megabytes of telemetry data were downlinked and captured at rates as high as 124,426 bits per second.

This illustration shows Cassini's position mid-day on March 15:

. The format shows Cassini's path over most of its current orbit up to today; looking down from the north, all depicted objects (except the background stars of course) revolve counter-clockwise, including Saturn along its orange-colored orbit of the Sun.