From Newsgroup: sci.space.news


Cassini Significant Events
for 02/17/16 - 02/23/16

Cassini is currently 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 Feb 24 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

The Cassini flight team's work becomes less compressed when Cassini is
flying a longer orbit about Saturn. Typically there are three opportunities
to fire the rocket engine or thrusters to adjust the trajectory during
each orbit, and each opportunity requires the team to accomplish multiple tasks and work several meetings. Since the T-117 Titan encounter last
week set the spacecraft onto an orbit that takes about eight days longer
to go around Saturn, there is more time in between pressing realtime operations,
and therefore a bit less pressure for some team members.

Still, work continues at a steady pace to prepare the next command sequences that will orchestrate the spacecraft's activities, in concert with Deep
Space Network (DSN) resources that need to be scheduled. The S93 sequence continues running on Cassini until April 18, when the S94 sequence will
have been completed and installed on the spacecraft. By then, Cassini's
orbit will have expanded another eight days to a 32-day period, thanks
to the Titan flyby T-118 on April 4.

Wednesday, Feb. 17 (DOY 048)

Today marks 6,700 days since Cassini was launched from Earth, on Oct.
15, 1997. The DSN has kept the flight team's engineers and scientists
in close regular contact with the spacecraft ever since that day, despite
the enormous physical distances.

Cassini's T-117 close encounter with Saturn's mysterious, haze-enshrouded, moon Titan finished up today, after having come within 1,018 kilometers
of its surface. The Radio Science atmospheric occultation went perfectly,
and the bistatic experiment succeeded in "spotlighting" Titan's surface;
the DSN captured specular reflections of Cassini's radio beam coming from
some of Titan's surface wetlands, ponds, or small lakes. From this, the
Radio Science team will be able to extract information to characterize
the surface reflectivity, dielectric constant, and roughness.

Data from Cassini's remote-sensing observations during the encounter were stored onboard and then played back successfully via telemetry. Among
them, the Visible and Infrared Mapping Spectrometer (VIMS) had looked
for a specular reflection of sunlight from some small northern lakes.
The Composite Infrared Spectrometer (CIRS) made measurements that add
to a three-dimensional picture of Titan's atmospheric circulation. The
Imaging Science Subsystem (ISS) imaged Titan's surface and atmosphere
at high-phase solar illumination over Titan's anti-Saturnian hemisphere. Additional information about the flyby may be found at

http://saturn.jpl.nasa.gov/mission/flybys/titan20160216

Thursday, Feb. 18 (DOY 049)

ISS monitored Titan for nine more hours today, with CIRS, VIMS, and the Ultraviolet Imaging Spectrograph (UVIS) participating at times, to track clouds and their evolution. The huge planet-like object had receded to
over one million km from the speeding spacecraft by the end of the day.

Data from CIRS appears in an image featured today, showing changing temperatures
on the surface of Titan as its seasons progress:

http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=5309

Friday, Feb. 19 (DOY 050)

The spacecraft turned its telescopes to Saturn today while UVIS created
radial profiles of the atmosphere for 80 minutes. Next, UVIS observed
Saturn's thermosphere, up above the visible atmosphere, for nine hours.
ISS then looked for small objects near Saturn for an hour as part of the satellite orbit campaign.

Saturday, Feb. 20 (DOY 051)

Cassini's Magnetospheric and Plasma Science (MAPS) instruments collected
data for most of today, as they often do, even during the many different optical remote-sensing observations. When the spacecraft turned to face
its high-gain antenna to Earth, commands arrived that had been prepared
based on the Navigation team's analysis of the past few days of radiometric tracking data from the DSN. In response, Cassini turned and fired its
small, hydrazine-fed rocket thrusters for 69 seconds. This gave the spacecraft the change in velocity of 69 millimeters per second that was necessary
to clean up its trajectory following T-117. This maneuver was performed
on its backup opportunity, in order to incorporate one more day's tracking data, and to allow for slightly better orbital-mechanics performance.

In addition to trajectory cleanup, the maneuver also targeted the spacecraft for a line-up on March 11 during which UVIS will lead an observation of
a stellar occultation: the bright star Epsilon Orionis (the middle star
in Orion's belt) will pass behind Enceladus and its south-polar plume
of ejecta.

Sunday, Feb. 21 (DOY 052)

ISS performed another 60-minute satellite orbit observation. Following
this, ISS, CIRS and VIMS jointly observed the Encke Gap in Saturn's A
ring for 19 hours; the data will be combined into a movie. This was more
than enough time for particles in that gap to make a complete orbit of
Saturn. Next, CIRS stared at the sunlit side of Saturn's broad A ring
for 10 hours today, obtaining high-quality spectra at thermal-infrared wavelengths. This allows CIRS to infer information about ring-particle composition.

Monday, Feb. 22 (DOY 053)

ISS began a 30-hour observation of Saturn's irregular moon Skoll. Named
for a giant wolf in Norse mythology, this object is about 6 km in diameter; its inclined retrograde orbit takes it more than 25 million km from Saturn.

Three of Saturn's icy moons are captured straddling the rings in an image featured today:

http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=5303

Tuesday, Feb. 23 (DOY 054)

On five occasions during the week, when the optical instruments were pointing at or near Saturn, ISS made two-minute Saturn storm-watch observations;
VIMS rode along with two of them. Also this week, the Deep Space Network communicated with and tracked Cassini on six occasions, using its stations
in Spain and Australia. A total of 135 individual commands were uplinked,
and about 1,233 megabytes of telemetry data were downlinked and captured
at rates as high as 124,426 bits per second.

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This illustration shows Cassini's position on Feb. 23: http://go.nasa.gov/1OdNmE9.
It's clear that the kick from Cassini's T-117 encounter has put the spacecraft on a longer orbit than the previous one. 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. ------------------------
Milestones spanning the whole orbital tour are listed here: http://saturn.jpl.nasa.gov/mission/saturntourdates .
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For a glossary of technical terms relating to these events, click the
"full story" link on this page: http://saturn.jpl.nasa.gov/news/significantevents/ .
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Information on the present position and speed of the Cassini spacecraft
may be found on the "Present Position" page at: http://saturn.jpl.nasa.gov/mission/presentposition/ .
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Visit the JPL Cassini home page for more information about the Cassini Project: http://saturn.jpl.nasa.gov/
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