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The number of days remaining in Cassini's mission went below 850 this week. Accordingly, the flight team has begun preparing for some final encounters with objects in the Saturnian system before the historic tour comes to an end. The "first of the lasts" will happen on May 31, with one final scrutiny of the "spongy" moon Hyperion, whose mean density is just over half that of water. The next "lasts" will include two flybys of Saturn’s moon Dione, with its enigmatic ice-cliffs, in June and then in August. Then come Cassini's three final targeted encounters with Enceladus before the end of this year, examining its geysers and snowy surface. This week the Navigation team observed Enceladus to update knowledge of its positioning, in preparation for those last close visits.
Wednesday, May 20 (DOY 140)
The Visible and Infrared Mapping Spectrometer (VIMS) completed a six-hour observation of Saturn's aurorae, with the other Optical Remote Sensing (ORS) instruments' telescopes riding along to
make their own observations. VIMS then handed control of spacecraft pointing over to the Ultraviolet Imaging Spectrograph (UVIS) for another six hours. During this allotment of observing time, UVIS slewed its field of view repeatedly across Saturn’s auroral
ovals to complement the previous VIMS observation. VIMS, in fact, rode along with this one, as did the Composite Infrared Spectrometer (CIRS).
Next, Cassini spent nine hours linked with Earth while the Deep Space Network (DSN) tracked and communicated with the spacecraft. Today, radio communications from Cassini took 74 minutes and
38 seconds to propagate across the 1,343,000,000 kilometers of interplanetary space.
After the DSN employed its cutting-edge technology to capture every bit of telemetry data that Cassini played back from its most recent observations, the Imaging Science Subsystem (ISS, the
fourth ORS instrument) had Cassini turn towards Titan to monitor changes in its dynamic atmosphere. CIRS and VIMS rode along. Finally, UVIS turned back to Saturn to observe the planet in the extreme- and far-ultraviolet parts of the spectrum, while the other
ORS instruments rode along.
Thursday, May 21 (DOY 141)
CIRS led the three other ORS instruments while it stared at a determined location on Saturn for 17 hours, to derive signatures of its chemical composition in the infrared.
Friday, May 22 (DOY 142)
The Navigation team used the ISS for 90 minutes to observe Enceladus against a background of stars for optical navigation purposes. As mentioned above, this was in preparation for Cassini's
final targeted encounters with that perplexing moon late this year. When this was done, CIRS took control for 22 hours to map Saturn in the mid-infrared part of the spectrum.
During a routine DSN communications session, the flight team uplinked commands to update some of the spacecraft's stored pointing vectors, increasing their accuracy in anticipation of Cassini's
last encounter with Hyperion at the end of the month when it will be "only" 34,287 kilometers away from the spacecraft.
Tonight, Earth reached the point in its orbit of the Sun where Saturn is directly opposite our star. From our planet's surface, Saturn rose in the east at sunset, and reached its zenith at midnight.
The ringed planet is a beautiful sight in a telescope for much of the year, but especially when opposition minimizes its distance.
Saturday, May 23 (DOY 143)
UVIS and then VIMS took turns, seven hours each, observing Saturn’s southern aurorae. The other ORS instruments rode along at times.
Sunday, May 24 (DOY 144)
UVIS imaged Saturn in the ultraviolet for 14 hours, while CIRS, ISS, and VIMS rode along. These repeated ultraviolet mappings are meant to determine how the planet’s appearance changes with
observing geometry, so that scientists can use it to study Saturn’s atmospheric make-up.
In two years from today, less than seven percent of Saturn's own year, it will be solstice on the ringed planet; the Sun will be at its highest elevation in the gas giant's northern sky, marking
the beginning of Saturn summer in the north, and winter in the south.
Monday, May 25 (DOY 145)
ISS controlled pointing for 16 hours today to image a variety of latitudes on Saturn at a range of emission angles. All the other ORS instruments made ride-along observations.
In a not-so-routine DSN pass today, Cassini served as a deep-space calibration target for a new 80,000-watt transmitter at the DSN’s Goldstone facility in California. Received signal strength
measured by Cassini was used to assess the performance of the transmitter. Spacecraft and transmitter performance were nominal.
Perhaps Saturn's 1,528-kilometer wide moon Rhea looks most like our own Moon, although is more than twice Rhea's size. Its ancient surface is full of impact craters dating back to an early history.
An image of this moon that Cassini took earlier this year is featured today: http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=5192 .
Tuesday, May 26 (DOY 146)
VIMS began an observation of 19 hours, creating repeated mosaics of Saturn's globe while it rotated one and a half times while in view. ISS and CIRS rode along.
During the past week, the DSN communicated with and tracked Cassini on nine occasions, using stations in California and Australia. A total of 27 individual commands were uplinked, and about 1,238
megabytes of telemetry data were downlinked and captured at rates as high as 142,201 bits per second.
Wrap-up
Cassini is currently orbiting Saturn with a period of 18.9-days in a plane inclined 0.3 degree from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were
obtained on May 27 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.
For a glossary of technical terms relating to Cassini's events, click the "full story" link on this page: http://saturn.jpl.nasa.gov/news/significantevents.
Milestones spanning the whole orbital tour are listed here: http://saturn.jpl.nasa.gov/mission/saturntourdates.
This illustration shows Cassini's position on May 26: http://go.nasa.gov/1J3ubNs.
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