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The first part of this week found Cassini speeding inbound toward Saturn. On the way, it flew by the 5,152-kilometer-wide planet-like moon Titan on Thursday, coming as close as 2,271.5 kilometers above its complex surface. Relative to Titan, the spacecraft
was going at a meteoric speed of more than 20,000 kilometers per hour. Relative to Saturn, though, its speed was over 23,000 kilometers per hour and increasing every minute. By Saturday, when Cassini arrived at its nearest point to Saturn, it had reached 60,000
kilometers per hour relative to the planet. From this periapsis, it began gradually slowing while climbing along its orbital path toward apoapsis on May 19. Thursday's Titan flyby was designated the T-111 encounter. This page lists some of the important events
of that day:
Wednesday, May 6 (DOY 126)
At the start of the day, the Visible and Infrared Mapping Spectrometer (VIMS) was in control of spacecraft pointing, carrying out the remainder of an 18-hour observation of Saturn's misty E ring and faint G ring. The rings were sunlit at a phase angle
of 105 degrees -- almost side-on. Participating in this observation were two of Cassini's other telescopic, optical remote-sensing (ORS) instruments, the Ultraviolet Imaging Spectrograph (UVIS), and the Composite Infrared Spectrometer (CIRS).
Upon completion of the ring observation, the Imaging Science Subsystem (ISS, the fourth ORS instrument) slewed over to Saturn for a two-minute storm-watch observation. Next, Cassini's Navigation team took their turn on the telescope, and pointed ISS to
Saturn's moons Tethys and Dione to take images for 90 minutes for optical navigation purposes. Finally, ISS took one hour for an observation in the satellite orbit campaign, looking for small objects near Saturn.
Thursday, May 7(DOY 127)
Three and one-half hours into the day, Cassini had come closer to Titan than our own moon is to Earth. Since Titan is nearly 1.5 times the diameter of our moon, though, it would have seemed huge to an observer. As Cassini observed throughout the day, Titan
and its thick atmosphere grew ever larger. CIRS commanded the pointing for the first 8.5 hours, and then VIMS took control for its observations during the next nine hours on approach, while the other ORS instruments made ride-along observations at selected
times. Closest approach to the surface of Titan came late in the day, at which time CIRS took the reins again for observations lasting another eighteen hours. The other ORS instruments again made selected observations as riders.
The T-111 encounter changed Cassini's own orbit by gravity assist. As planned, the spacecraft's orbit period decreased from 28.1 days to 18.9 days. Its inclination was unchanged.
Friday, May 8(DOY 128)
Cassini continued its T-111 encounter activities on Titan's dayside. The ORS observations poured data into the on-board solid-state recorders (SSRs) at rates that had been carefully negotiated months in advance. When the SSRs were full, it was time for
Cassini to make use of time on two of the Deep Space Network's (DSN) largest antennas, each 70 meters in diameter, which had also been negotiated long before. The spacecraft turned to point its 4-meter-diameter dish antenna to Earth. First the DSN antenna
in Madrid, Spain, and later the one at Goldstone, California, captured every bit of the telemetry data that Cassini sent home. As always, these DSN stations also provided uplink to the spacecraft and two-way coherent tracking -- Doppler-shift and ranging data
-- for navigation.
Saturday, May 9(DOY 129)
As Cassini continued to coast inbound to Saturn, it entered the E ring, the ring created and maintained by watery geysers from Saturn's small moon Enceladus. Since Cassini is orbiting Saturn in its equatorial plane, it was a prolonged ring encounter. The
Cosmic Dust Analyzer (CDA) took control of spacecraft pointing so it could collect and report on the ring's ice grains for nearly eight hours, investigating in particular the variations in their orbital inclinations, as well as differences in their chemical
composition. As part of this direct-sensing observation, CDA also looked for the icy dust trails of several moons, large and small, that orbit Saturn within the E ring.
When CDA completed its survey, CIRS turned to Saturn's 1,123-kilometer-diameter moon Dione for 90 minutes. The other ORS instruments rode along while the observation gathered data on Dione's thermally anomalous regions and their boundaries and backgrounds.
CIRS's data will also allow scientists to continue the search for any activity on Dione, particularly over its extensive, step-edged ice cliffs. The spacecraft glided through periapsis in its Saturn orbit during this observation.
CIRS next turned to Saturn's 1,060-kilometer moon Tethys for four hours, investigating this moon's thermally anomalous region. The data should help provide a better constraint on the region's anomalously high thermal inertia. Again the other ORS instruments
took data as ride-along observers.
Polydeuces, which Cassini discovered in 2004, is only a few kilometers in diameter. It orbits Saturn at the same distance as Dione, trailing it by 60 degrees at Dione's L5 Lagrange point. Once CIRS completed its Tethys observation, it began a two-hour
study of Polydeuces, which was well placed for observing. (Dione has another small orbital companion; Helene occupies the L4 Lagrange point 60 degrees ahead of Dione.)
Sunday, May 10(DOY 130)
ISS spent 6.5 hours observing the plumes of Saturn's 504-kilometer-diameter moon Enceladus while they were backlit by the Sun; UVIS and VIMS rode along. CDA then spent four hours measuring dust in the outer part of the E ring, where the ice particles are
more varied than those in the inner E ring. CDA then handed control back to ISS, which spent 10.5 hours studying Saturn's distant, irregular moon Kiviuq. A very dark-surfaced object only about 14 kilometers in diameter, Kiviuq follows an inclined, eccentric
orbit as far as 11.11 million kilometers out from the planet. VIMS rode along. The goals are to determine the object's pole orientation, rotation direction, and shape, as well as to address the question whether Kiviuq might be a contact-binary or even a binary
object.
Monday, May 11(DOY 131)
The flight team generated commands near real time and uplinked them to perform Orbit Trim Maneuver (OTM)-410, the post T-111 trajectory cleanup maneuver. In response, the spacecraft turned and burned its small hydrazine-fed rocket thrusters for 60 seconds,
obtaining a change in velocity of 61 millimeters per second.
Last February, while Cassini was still in an orbit inclined about 19 degrees from Saturn's equatorial plane, it obtained a beautiful side-on view of Saturn and its rings that was featured today:
Tuesday, May 12(DOY 132)
UVIS led observations of Saturn today in two sets. The first one studied Saturn's aurorae in the ultraviolet part of the spectrum for six hours with ISS and VIMS riding along. The second one studied Saturn's atmosphere in the extreme- and far-ultraviolet,
with ISS and CIRS riding along.
During the past week, the DSN communicated with and tracked Cassini on six occasions, using stations in Spain, California and Australia. A total of 141 individual commands were uplinked, and about 1,881 megabytes of telemetry data were downlinked and captured
at rates as high as 142,201 bits per second.
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This illustration shows Cassini's position on May 12: http://go.nasa.gov/1HfT4GE .
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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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