Cassini is orbiting Saturn with a 23.9-day
period in a plane inclined 53.4 degrees from the planet's equatorial
plane. The most recent spacecraft tracking and telemetry data were
obtained on Aug. 7 using one of the 34-meter diameter Deep Space
Network stations at Canberra, Australia. Except for some science
instrument issues described in previous reports (for more information
search the Cassini website for CAPS and USO), the spacecraft continues
to be in an excellent state of health with all of its subsystems
operating normally. 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/.
http://saturn.jpl.nasa.gov/mission/presentposition/.
There are three opportunities during each
Saturn orbit for the flight team to design, test, approve, uplink, and
execute the propulsive events known as Orbit Trim Maneuvers (OTMs).
Each maneuver's design is based on iterations of orbit estimation
using the most recent radiometric tracking data (Doppler shift for
velocity, Range measurements for distance) and other minor inputs.
Since Cassini's orbit period has just recently been increased from
twelve days to sixteen days, and now to twenty-four days, pressure on
the operations teams seems a bit more relaxed. Nonetheless, work
proceeds on the next command sequences: S80 will go active on the
flight system for ten weeks beginning on Aug. 14; S81 will go active
October 22, and S82 on December 28. Work also continues for the 2016
start of the F-ring and Proximal Orbits phase, which will bring an end
to Cassini's mission late in 2017. Meanwhile, timed commands from the
on-board S79 sequence continued to control most of the spacecraft's
activities in flight this week.
Wednesday, July 31 (DOY
212)
The Imaging Science Subsystem (ISS) team had the spacecraft turn to train its narrow-angle camera (NAC) on Saturn's small irregular moon Paaliaq, which has a diameter of around 20 kilometers and orbits 15.2 million kilometers from Saturn. For 37.25 hours, the spacecraft's attitude and articulation control subsystem constantly adjusted the three reaction-wheels' speeds, trading their angular momentum with that of the whole spacecraft. This way, the ISS telescope tracked the rock precisely while it and the spacecraft were in constant motion, orbiting Saturn in different planes. The NAC's 19-centimeter aperture reflector created 624 images of Paaliaq's very dark surface; the Ultraviolet Imaging Spectrograph (UVIS) rode along taking data as well. The wide-angle camera (WAC), with its smaller-aperture refracting telescope, was not used. The results of the observations may lead to discovery of the body's rotation rate, and other general characteristics of this moon.
Five views of Saturn by the Earth-orbiting Hubble Space Telescope, each image taken a year apart, were featured as NASA's Astronomy Picture of the Day today. As any halfway-good amateur telescope will show, the rings' opening angle changes as Saturn's seasons progress. The planet's shadow on the rings is barely discernible since the Sun was behind the Earth when each image was taken. The collage may be seen here:
http://apod.nasa.gov/apod/ap130721.html
A news release released today pins down the time-varying nature of Enceladus's enigmatic, continuously erupting geysers. It includes an image with the orbit of that small icy moon artificially portrayed as a white ring-like band:
http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20130731/
Thursday, Aug. 1 (DOY
213)
The flight team developed plans and designs
for the next firing of Cassini's primary main engine, one of a pair of
400 newton bipropellent-fed rockets on board (the backup main engine
has never been used in flight). OTM 357 will execute next Tuesday
night, after the spacecraft has reached apoapsis, and will cause the
spacecraft to change its velocity by 3.6 meters per second. This
relatively large maneuver will set up for the next Titan flyby, the
1400-kilometer T94 encounter on September 12.
Friday, Aug. 2 (DOY
214)
The Visible and Infrared Mapping Spectrometer (VIMS) began a 22-hour observation of Saturn, making a global movie mosaic.
The design for OTM 357 was finalized,
commands approved, and an Autorad session was enabled. Autorad is a
software convenience for the flight team that will send the OTM
command to the spacecraft during a DSN tracking period next Monday
night.
Saturday, Aug. 3 (DOY
215)
The Composite Infrared Spectrometer (CIRS)
watched Saturn for nearly eight hours to discern atmospheric
composition. Next, the spacecraft turned on its Ka-band (32 GHz) radio
transmitter so that back on Earth, the Radio Science team was able to
conduct an operations readiness test with a 34-meter diameter DSN
station in Australia. This was the last such test before carrying out
a Radio Science ring occultation experiment on Aug.
8.
Sunday, Aug. 4 (DOY
216)
ISS led an observation of Titan to monitor
its clouds for an hour and a half, with VIMS also taking data.
Following this, UVIS began an observation lasting more than sixteen
and a half hours to capture Saturn in the extreme-ultraviolet and the
far-ultraviolet parts of the spectrum, with CIRS riding along to take
data in the infrared, and ISS making visible-light images.
Monday, Aug. 5 (DOY
217)
Cassini passed through apoapsis going 12,793 kilometers per hour, having coasted "up" about 2.1 million kilometers from Saturn, almost twice as high as Titan's orbit. CIRS then observed the planet for twelve hours and ten minutes to map its atmosphere with the instrument's mid-infrared sensor. It repeated the observation with a slightly longer duration on the following day.
The flight team's Autorad tool connected to the 34-meter diameter DSN station in Australia that had been scheduled to track Cassini, and uplinked the pre-approved OTM-357 commands. Autorad then disconnected itself as planned, permitting the Realtime Operations Team to take over and send additional routine commands. All were confirmed via telemetry as received on the spacecraft after a round-trip light time of two hours 46 minutes.
An image featured today provides a stunning view of the strange atmospheric features centered on Saturn's north pole: the hurricane and the hexagon:
http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=4875
Tuesday, Aug. 6 (DOY
218)
A member of the ISS and VIMS teams contributed an interesting blog entry today discussing his work with results of the backlit observation of Saturn and ring system that was taken on July 19 (while many of us were outside smiling for the NAC). It may be seen here:
http://saturn.jpl.nasa.gov/news/waveatsaturn/blogs/blog20130806/ .
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