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Cassini Significant Events 04/29/09 – 05/05/09

The most recent spacecraft telemetry was acquired on May 7 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, April 29 (DOY 119):

Following the completion of the prime reaction wheel (RWA) friction test on Apr. 25, the Project decided to re-examine the existing practice regarding how often these tests are performed. As a result, the frequency of the prime RWA friction test was reduced from every three months to every six months. The back up wheel friction test will be performed yearly, rather than every six months. This decision was based on the determination that the daily trends of friction torque spikes are a better measure of RWA health than are the quarterly friction tests, as well as the fact that this reduction will give a small decrease in the usage of the RCS thrusters.

Uplink Operations continued sending Instrument Expanded Block files (IEB) to the spacecraft today in support of the S50 sequence. Seven files were sent. The background sequence itself will go up on Friday after the final Sequence Approval meeting and the Command Approval meeting. Execution will begin on May 5.

An encounter strategy meeting was held today to cover the period between May 5 and May 21, Titan flybys T54 and T55, and maneuvers 194-196 in S50.

Friday, May 1 (DOY 121):

Cleaning up the final delivery to T54 after Orbit Trim Maneuver (OTM)192 was going to require a very small maneuver - less than 10 mm/sec - implying that time of closet approach biasing would be required if the OTM were to be executed. In addition, a maneuver executed over either the prime or backup pass was going to require a time bias to place the RWAs in an acceptable configuration during the OTM. Finally, canceling this OTM would reduce the down stream delta V estimates by about 0.1 m/s. Science planning reviewed both the “with” and “without” trajectories from the preliminary Navigation review. The pointing error without the OTM was acceptable. As a result, OTM-193, the T54 approach maneuver planned for May 2, has been cancelled.

SCO uplinked a real time command today to increase the command loss timer (CLT) on board the spacecraft from the nominal 90 hours to 6 days. This decision came as a result of uncertainty in the availability of DSS-14. Between Sunday evening and Thursday evening of next week, the only station allocated to Cassini is DSS-14. If DSS-14 were declared “red” and became unavailable again, Cassini might go as long as 96 hours until the next possible uplink. If the 90-hour limit was exceeded, the CLT would call System Fault Protection and the spacecraft would “safe”. Extending the CLT value to 144 hours eliminates this possibility. The timer will be set back to 90 hours sometime after the flyby.

DSS-14 was turned over to operations today. The station supported successful passes with Voyager at 05:25 UTC and with MER at 11:25 UTC. The next support for Cassini will be Monday, May 4.

Saturday, May 2 (DOY 122):

It turns out that even though the azimuth bearing at DSS-14 was fixed in time and was declared green for a while, the sub-reflector controller is now non-operational. DSN personnel are on top of the antenna trying to fix the issue by manually positioning the sub-reflector to select the X-band horn.

The JPL annual open house began today and will continue through Sunday. UPDATE: Approximately 12,500 of the total 32,000 visitors to the Laboratory came to the Cassini exhibit.

Monday, May 4 (DOY 124):

Even with the manual operation of the sub-reflector, the antenna was unable to acquire the signal and the project did lose ~2.8Gb of science data. DSS-25 was brought up in parallel to the scheduled pass to enable the acquisition of Navigation data, and to send up a CLT command, but was unable to acquire the telemetry data at the data rate that had been sequenced for the 70m antenna. The next DSS-14 track for Cassini is Wednesday after the T54 flyby.

Tuesday, May 5 (DOY 125):

The S55 Engineering Activities Review was held today.

An uplink readiness review was held today for the update of AACS flight software version A8.7.7. The update will be sent to the spacecraft at the end of May/early June 2009. The update will be in effect from June 2009 to July 2010, and will modify the default safing attitude and the default thruster magnitudes.

A gorgeous picture of Titan and Epimetheus with Saturn’s rings was Astronomy Picture of the Day today. Check it out at: http://antwrp.gsfc.nasa.gov/apod/ap090505.html.

The Cassini Mission Website is the winner of the 2009 Webby Award in the Science category. With nearly 10,000 entries from more than 60 countries, the Webby Award is the leading international award honoring excellence on the Internet, and is selected by judges from the International Academy of Digital Arts and Sciences. In addition to the Cassini site, the main NASA site --www.nasa.gov-- received the Webby People's Voice Award in the Government category. More information is available at: http://www.webbyawards.com/. Cassini Outreach points out that this award recognizes the Web site, but also honors the hard-working people who run the mission, including the Raytheon team that helped produce the site, and the millions of people who visit the site regularly. The site is for them.

The S49 sequence concluded and S50 began execution today at 2009-125T07:16 SCET. The sequence will run for 39 days and conclude on June 13, 2009. During that time there will be three targeted encounters of Titan. Seven OTMs are scheduled, numbered 194 through 200.

Just under 16 days since its previous visit, Cassini returned to Titan for the T54 flyby. Closest approach to Titan occurred today at 5:08 PM PDT, at an altitude of 3,244 kilometers, a speed of 5.8 kilometers per second, and latitude at 14.1 degrees S.

As Cassini approached Titan, the Composite Infrared Spectrometer (CIRS) focused on the moon's atmosphere, analyzing its temperature and aerosol profiles. Inbound, the phase angle was much larger than 90 degrees and the Visual and Infrared Mapping Spectrometer (VIMS) observations provided information on Titan's atmosphere composition. After closest approach, VIMS observed the south polar region and looked for variations at the surface and in the atmosphere, then monitored clouds in the southern hemisphere.

Imaging Science (ISS) acquired high-resolution data of Titan’s trailing hemisphere at high southern latitudes and rode along with VIMS cloud monitoring. The Ultraviolet Imaging Spectrograph (UVIS) obtained an image cube of Titan's atmosphere at Extreme Ultraviolet and Far Ultraviolet wavelengths. These cubes provide spectral and spatial information on nitrogen emissions, H emission and absorption, absorption by simple hydrocarbons, and the scattering properties of haze aerosols. This was one of many such cubes gathered over the course of the mission to provide latitudinal and seasonal coverage of Titan's middle atmosphere and stratosphere.

For the fields and particles instruments, the Magnetospheric Imaging instrument measured energetic ion and electron energy input to Titan’s atmosphere. The Radio and Plasma Wave Science instrument measured thermal plasmas in Titan's ionosphere and surrounding environment, searched for lightning in Titan's atmosphere, and investigated the interaction of Titan with Saturn's magnetosphere. The Magnetometer obtained data that will complement the data acquired at T52 and T53 in assisting to characterize the background field for a similar local time with respect to Saturn and different SKR longitudes. For more information on this flyby, go to: http://saturn.jpl.nasa.gov/mission/flybys/titan20090505/.

Status as of Tuesday morning indicated that DSS-14 had been declared “orange”, or partially operational. Maintenance personnel were able to get the sub reflector axis to a good 45-degree focus. Downlink would be possible but some degradation is expected for upcoming supports. For Cassini, this means that the data from the Titan 54 flyby is at risk. DSN schedulers have begun looking into acquiring another station for Cassini in order to meet Navigation requirements for the next OTM.

Cassini and the DSN had a Radio Science test planned over DSS-26 at the same time as our DSS-14 pass. The project now has a station in hand to meet Navigation needs. RSS has been notified and will reschedule the test later in S50.

Tuesday afternoon the flight team met to review options for salvaging the T54 data. After much discussion, a plan evolved to lower the highest telemetry rate during the playback to increase chances of getting back good data. The file is to be uplinked 10 minutes after the transmitter is turned on at DSS-14. Science Planning has impacted the plan and determined that lowering the data rate from 165900 kbps to 142200 kbps for DSS-14 still leaves margin, so data volume will be OK. Getting back the T54 data is beginning to look more hopeful than it did not too long ago.

UPDATE: Tuesday Night 22:22 PDT

The maintenance team has repaired the Sub Reflector Controller (SRC) and is tracking Voyager at this time for validation of operation. The Cassini support starting tomorrow afternoon, May 6, should be nominal. DSS-14 will have supported three passes before the Cassini track, elevating confidence. Given the latest information, Cassini will NOT be sending a command to lower the data rate from 165K to 142K during today’s pass.

UPDATE: Thursday Morning

DSN came through again; we got all the data from the T54 flyby.