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The orbit-determination solutions that model Cassini's motion have been exceptionally stable lately, thanks to the professionals on the Navigation team, the consistently high-quality radiometric data from the Deep Space
Network (DSN), and JPL's world-class Navigation software suite. Given this rock-solid stability, the flight team was able to create commands early for Orbit Trim Maneuver (OTM)-468A. The commands were approved on Thursday Feb. 16 and then sent up to Cassini.
This maneuver will not execute until the following Wednesday, Feb. 22. It was good, though, to have multiple opportunities to assure that this important OTM got safely aboard the flight system, because this is not just any OTM. This is the final Titan-targeting
propulsive maneuver in the Cassini Mission. The targeted Titan flyby T-126 takes place on April 22, and the gravity-assist boost from the encounter will bump Cassini into its series of 20 Proximal orbits. There may be final-adjustment OTMs a few days before
and after T-126, but never will there be another Cassini "targeting" maneuver for any encounter.
Wednesday, Feb. 15 (DOY 046)
For just about three hours today, Cassini watched the bright Sun pass behind Saturn's rings. The Visible and Infrared Mapping Spectrometer (VIMS) and the Ultraviolet Imaging Spectrograph (UVIS) used their solar ports to observe while
the Sun egressed, passing behind the C ring, and on outward through the F ring. The instruments were able to measure the size and spatial distributions of many of the smallest ring particles. This illustration shows Cassini's viewing geometry: http://go.nasa.gov/2mm9JjW
.
Following the solar occultation, UVIS and VIMS took advantage of darkness in the south polar region, and observed Saturn's auroral oval there for 5.7 hours. Finally today, the Imaging Science Subsystem (ISS) examined the planet's
thin bright crescent for two hours, to study the composition of the high atmosphere; UVIS and VIMS rode along.
Saturn is very much like a miniature solar system of its own in many surprising ways, as described in this feature article released today: https://saturn.jpl.nasa.gov/news/2998/like-a-mini-solar-system .
Thursday, Feb. 16 (DOY 047)
The Composite Infrared Spectrometer (CIRS) spent 11.3 hours today mapping Saturn at mid-infrared wavelengths to measure temperatures in the upper troposphere and tropopause; VIMS rode along.
An image featured today highlights rays of ejecta seen on the surface of Saturn's moon Dione: https://saturn.jpl.nasa.gov/resources/7605 .
In today's news is an article and a video with the compelling story of one of the most amazing places in our solar system: Saturn's moon Enceladus: https://saturn.jpl.nasa.gov/news/3001/an-ice-worldwith-an-ocean .
Friday, Feb. 17 (DOY 048)
Cassini made a non-targeted flyby (no dedicated propulsive maneuvers) of Saturn's largest moon Titan today. The cloudy, planet-like object came as close as 186,799 kilometers from the spacecraft. Titan’s high-latitude regions were
visible, enabling comparisons from observations in late 2013 and early 2014, when Cassini was previously orbiting Saturn at high inclinations. Cassini kept its "eyes" trained on Titan for today's observing segment of about 23 hours. Each of the optical Remote-Sensing
(ORS) instruments -- ISS, VIMS, UVIS, and CIRS -- participated, as did the Magnetospheric and Plasma Science (MAPS) instruments.
Shortly after closest approach to Titan, Cassini coasted through apoapsis, marking the start of Saturn Orbit #262.
At about the time Cassini was wrapping up its previous orbit, the 71st Cassini Project Science Group wrapped up its week-long meeting today near Pasadena. About 165 scientists, engineers, and administrators were in attendance.
Saturday, Feb. 18 (DOY 049)
Cassini trained its High-Gain Antenna dish on Earth for 15 hours today for a routine session of two-way digital communications and tracking, using Deep Space Network (DSN) stations in Spain and California. In addition to the complete
playback of science and engineering telemetry, the Navigation team obtained routine Doppler and Range signals, the radiometric data needed to maintain the precise model of Cassini's orbit.
The flight team also used today's DSN period to uplink two sets of commands that were freshly prepared, based on the latest navigation solutions. One set of 36 individual commands updated the vectors Cassini refers to when it points
its instruments towards Saturn, and towards Saturn's small moon Epimetheus. Another set of 125 commands will wait on board until next Wednesday, when it will carry out Cassini's final Titan-targeting Orbit Trim Maneuver #468A.
Following the DSN pass today, CIRS took the reins for about 8.5 hours, and made thermal-infrared observations of Saturn's optically dense B ring; UVIS and VIMS rode along. The data will help constrain the ring's composition and structure.
After this was done, VIMS led CIRS and ISS in an observation of the red star VY Canis Majoris for 10.5 hours while it was occulted by Saturn's rings. The chord occultation covered the narrow F ring, the broad A ring, the dusty Cassini Division, and almost
all of the dense B ring. The line of sight to the star then turned around just inside the B ring's inner edge, and proceeded back outward.
Next, ISS spent one hour observing the smaller bodies in the inner part of the Saturn system, to refine knowledge of their orbits. ISS then made images of the ever-changing F ring, at medium resolution for about 9.5 hours, with CIRS
and VIMS riding along.
Monday, Feb. 20 (DOY 051)
The Deadline for U.S. students to enter the 2016-2017 Scientist-for-a-Day contest will be Friday, Feb. 24. Other participating countries may have different deadlines. More information is available here: https://solarsystem.nasa.gov/educ/scientist-for-a-day
.
Tuesday, Feb. 21 (DOY 052)
With the spacecraft rapidly approaching periapsis, UVIS tracked the ultraviolet-bright star Beta Crucis for four hours to watch it being occulted by Saturn's rings, with CIRS and ISS riding along. This ingress occultation covered
the main ring system from the F ring inward to the D ring before the star passed behind Saturn from Cassini's perspective. UVIS then targeted the A ring for an observation, recording high-quality spectra for about 2.5 hours to investigate its composition and,
to study how the ring responds locally to strong resonant gravitational perturbations; CIRS rode along.
During the current F-ring grazing orbits, as Cassini approaches periapsis it flies directly over the sunlit side of the rings, and then rapidly plunges to their unlit side. Today, VIMS used the spacecraft's motion to scan the main
rings for three hours as Cassini approached them; CIRS rode along. ISS then began a series of three high-priority observations.
First, ISS spent one hour examining "propellers" (http://go.nasa.gov/17oqTWF ) at close range, with CIRS and UVIS riding. Next, ISS and the other ORS instruments made use of the recently updated vectors and observed Epimetheus for
an hour, squarely centered in the telescopes' fields of view, while the distance shrank to a mere 8,300 km. This was Cassini's final observation of Epimetheus, the little moon that strangely trades orbits with Janus. After passing through the ring plane and
whipping past periapsis, ISS turned back to look at some propellers from the dark side for nearly an hour.
Continuing the day's flurry of ORS activity, VIMS and CIRS made a 2.5-hour observation of the rings' dark side, complementing the lit-side view from earlier today. Next, ISS targeted the C ring for another 2.5 hours with CIRS and
VIMS riding, to view narrow features known as "plateaus." ISS then studied more of the A ring's propellers for about two hours, with CIRS riding along.
Finally, CIRS rounded out the week's science activities with a seven-hour far-infrared observation intended to reveal azimuthal variation in the Cassini Division between the A and B rings.
The DSN communicated with and tracked Cassini on six occasions this week, using stations at all three of the DSN's locations: Australia, California, and Spain. A total of 174 individual commands were uplinked, and about 2,125 megabytes
of telemetry data were downlinked and captured at rates as high as 110,601 bits per second.
Wrap up:
Cassini is executing its set of F-ring-grazing orbits of Saturn, with a period of 7.2 days in a plane inclined 63.5 degrees from the planet's equatorial plane. The 20 orbits are nearly identical, with Cassini's nearest point at about
150,000 km, and furthest point at about 1.28 million km from Saturn. Speeds relative to Saturn at those points (periapsis and apoapsis), are close to 76,150 km per hour and 9,000 km/h respectively.
The most recent spacecraft tracking and telemetry data were obtained on Feb. 22 using one of the 34-meter diameter DSN 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/anomalies .
Cassini's orbit looks the same again this week, but the spacecraft's position is different on this illustration of Cassini's path up to mid-day Feb. 21: http://go.nasa.gov/2j9xZHV .
The countdown clock in Mission Control shows 206 days until the end of the mission.
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Milestones spanning the whole orbital tour are listed here:
<https://saturn.jpl.nasa.gov/mission/saturn-tour/tour-dates/>
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Information on the present position and speed of the Cassini spacecraft may be found on the "Present Position" page at:
<https://saturn.jpl.nasa.gov/mission/saturn-tour/where-is-cassini-now/>
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