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NASA Cassini Significant Events 08/05/09 - 08/11/09

Status Report From: Jet Propulsion Laboratory
Posted: Saturday, August 22, 2009

image The most recent spacecraft telemetry was acquired on Aug. 11 from the Deep Space Network tracking complex at Goldstone, California. The Cassini spacecraft is in an excellent state of health and all subsystems are 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/operations/present-position.cfm.

Wednesday, Aug. 5 (DOY 217)

The Spacecraft Operations Office (SCO) held an engineering activities review for S58 today. At the review the team discussed all engineering events to execute from March 1 to April 5, 2010.

According to the Navigation Team, the target miss distance at T60 resulting from cancelling Orbit Trim Maneuver (OTM) #211 would be less than 1 km, and the downstream consequences are not significant. In addition, when comparing pointing performance with and without the maneuver, Science Planning determined that there was very little change. Because of this, OTM-211, due to execute on Aug. 6, has been cancelled.

Thursday, Aug. 6 (DOY 218):

All participating teams have submitted files for the first input port of the S56 Science Operations Plan process. On Monday, Aug. 10, Science Planning will host an S56 Science Highlights meeting. The presentation will focus on what observations will be unique and highest priority during that sequence, and will include presentations from the Target Working Teams and Orbiter Science Team leads, with comments from Investigation Scientists and other instrument team representatives.

Friday, Aug. 7 (DOY 219):

A new moonlet, situated about 500 kilometers inward from the outer edge of the B ring, was found by detection of its shadow which stretches 41 kilometers across the rings. The shadow length implies the moonlet is protruding about 200 meters above the ring plane. If the moonlet is orbiting in the same plane as the ring material surrounding it, it must be about 400 meters across. This type of observation is only possible around the time of the Saturn equinox crossing. The illumination geometry that accompanies equinox lowers the sun's angle to the ring plane and causes out-of-plane structures to cast long shadows across the rings. For more details on this and to view the image link to: http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20090807/ and http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=3617.

Sunday, Aug. 9 (DOY 221):

Cassini completed its 61st targeted flyby of Titan on Aug. 9, passing the moon at a distance of 970 kilometers above the surface and at a speed of 6.0 kilometers per second. The onboard sequence clocked out as expected and all data was waiting to be transmitted to the DSN. As unique, high-priority data from Saturn's equinox crossing was to be acquired soon after the flyby, there was no redundant playback scheduled. In addition, a redundant playback of AACS data over a DSS-15 pass tomorrow had been intentionally removed within the last few weeks in order to implement a Y-thruster RWA bias demonstration on Monday. The Titan and AACS data were lost during transmission due to a problem at the Goldstone Deep Space Network facility. The Antenna Logic Controller (ALC) at DSS14 went down just prior to the start of the Cassini track. Goldstone was able to provide limited support at DSS-25 in order to supply radiometric data for Navigation, and to support some real-time commanding. However, the downlink data rates sequenced on the spacecraft in anticipation of 70m support could not be acquired by the 34m antenna.

Monday, Aug. 10 (DOY 222):

As a result of the very accurate flyby at T60 and an upcoming 13 m/s deterministic maneuver, there is a delta V savings to cancel OTM-212 and accomplish all of the correction at OTM-213. Since the orbit determination solution had converged, there was no reason to wait until the planned final Navigation review tomorrow. Therefore OTM-212 has been cancelled.

An RWA momentum bias demonstration using only the B-branch Y facing thrusters, conducted today, was completely successful. The use of Y-thruster only biases is expected to reduce hydrazine consumption as well as more evenly balance hydrazine throughput between the Y and Z thrusters. Y thruster biases are necessarily done off Earth-line, and hence complicate the navigation process due to the lack of visibility, which is the primary reason why they haven't been used to this point in the mission. The benefit of more balanced hydrazine throughput is the lessened use of the Z thrusters and the corresponding presumed mitigation of the problems recently experienced on the A-branch thrusters. The intention is to implement Y thruster biases as a regular part of operations beginning with S60 in May of 2010. Sequence development for S60 begins in November of this year.

Tuesday, Aug. 11 (DOY 223):

A non-targeted flyby of Atlas occurred today.

A real-time command was sent to the spacecraft today to power Ka-band transmission on, then off, in support of a test at the tracking complex at Narrabri, Australia.

Since Saturn's axis is tilted relative to its orbit about the sun, Saturn has seasons -- seasons that last for over seven Earth years. One result of this is a varying exposure angle of the rings to the sun. When this angle goes to zero, that is, an equinox crossing, sunlight will hit Saturn's thin rings edge-on. The light reflecting off this extremely narrow band is so small that for all practical purposes the rings simply vanish. The solar ring plane crossing which occurred today provides a unique opportunity for Cassini scientists. The sunlight hitting the rings edge-on can illuminate, or throw shadows, revealing vertical ring structures and oddities previously unseen. This phase of Cassini's Equinox Mission was designed with just this opportunity in mind.

Because Saturn's position in the sky as seen from Earth is so close to the sun now, it is extremely difficult to see from Earth. But Cassini is there, ready and waiting. Around equinox, Cassini's thermal instrument is tasked with measuring the temperature of both sides of the rings to see how the rings cool as they go through this seasonal change. The spacecraft's cameras are looking for topographic features in the rings, like tiny moons and possible ring warps, which are only visible at equinox, while the near-infrared and ultraviolet instruments will be on the hunt for signs of seasonal change on the planet. For more details, go to: http://saturn.jpl.nasa.gov/news/cassinifeatures/feature2009087/

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