Cassini Significant Events 04/15/09 – 04/21/09

The most recent spacecraft telemetry was acquired on Apr. 21 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 15 (DOY 105):

The orientation of Orbit Trim Maneuver (OTM) #190 was such that the reaction wheels would have experienced unacceptable speed ranges. However, due to a combination of excellent delivery from OTM-189 and the relatively high altitude for the upcoming Titan flyby, cancellation of OTM-190 had essentially no impact on either science pointing or downstream delta-V. As a result, OTM-190, due to execute on Apr. 16, has been cancelled.

A beautiful image of the jagged shadows on Saturn's rings was Astronomy Picture of the Day today. Check it out at: http://antwrp.gsfc.nasa.gov/apod/ap090415.html

Friday, April 17 (DOY 107):

An encounter strategy meeting was held today to cover the period between Apr. 20 and May 6, Titan flybys T53 and T54, and maneuvers 191-193.

For more than three hours on Saturday, the Cassini Plasma Spectrometer and the other Magnetosphere and Plasma Science instruments studied the dusk side of the magnetosphere. This was followed by a Magnetometer subsystem (MAG) 6-hour calibration roll. The day ended with an Ultraviolet Imaging Spectrograph (UVIS) star calibration observation using the bright star "Alpha Vir," also known as "Spica."

Sunday, April 19 (DOY 109):

Closest approach for the Titan 53 flyby occurred today at 6:32 pm PDT. Cassini passed by at an altitude of 3,600 kilometers above the surface, a speed of 5.8 kilometers per second, and a latitude of 7.7 degrees south. T53 was the second flyby in a series of eleven inbound encounters and the ninth Titan encounter in Cassini’s Equinox Mission.

Shortly before the flyby, the Cassini Project learned that the primary playback pass for the T53 data would be affected by unexpected downtime on the Goldstone 70-meter antenna. This pass was subsequently allocated a 34-meter station, which was utilized at lower data transmission rates to preserve the highest scientific priority data. Much of the science described below was able to be preserved.

Solar and stellar occultations by Titan are the most valuable Titan observations for UVIS because they provide detailed vertical profiles of nitrogen and hydrocarbons, HCN, and aerosols in Titan’s atmosphere. For T53, UVIS observed a long stellar occultation and a solar occultation. The two observations probed different parts of the atmosphere of Titan.

The solar occultation, using the extreme ultraviolet channel, sampled the neutral nitrogen from about 900 kilometers altitude up to about 2300 kilometers. This range overlapped the atmospheric region sampled by the Ion and Neutral Mass Spectrometer (INMS) and by the attitude control system, or AACS. The stellar occultation for T53 was a very slow one. Slow occultations allow scientists to acquire more signal at a given altitude range, and are therefore more valuable. This occultation used the UVIS far ultraviolet channel. The FUV wavelengths sample a number of hydrocarbons and haze from about 300 to 1600 kilometers altitude. The upper end of this altitude range overlaps with data acquired by INMS and the lower end overlaps with the Composite Infrared Spectrometer (CIRS). Both the solar and stellar occultations show a complex picture of the upper atmosphere. Density profiles and mixing ratios cannot be described as a simple function of latitude and longitude. There is more going on, perhaps gravity wave activity, or perhaps some other phenomena, which make the upper atmosphere more variable than simple models would predict. Continued observations like the ones in T53 will help scientists sort out these issues. Visit the Cassini website for a full account of science activities for T53: http://saturn.jpl.nasa.gov/mission/flybys/titan20090420/.

Monday, April 20 (DOY 110):

Science Planning kicked off the Science Operations Plan process for S54 today. The sequence development website is now up and has been populated with initial products.

Tuesday, April 21 (DOY 111):

The sequence leads for S49 hosted a kickoff meeting today for the DOY-116 Rhea Live Inertial Vector Propagator update. The schedule is very tight with a go/no go meeting planned for tomorrow, and if approved, uplink on Friday.

Uplink Operations sent commands to the spacecraft today for modifications to data policing during the orbit 109 rings segment. The file will update five data policing tables between DOY-111 and DOY-117 to account for the loss of DSS-14 on DOY-115 and the addition of three-hour DSS-63 tracks on DOY-115 and 117. An additional file to be uplinked later in the week will take care of any necessary telemetry mode changes. According to the most recent information released by the DSN, DSS-14 is currently scheduled to be up and operational some time on DOY-120.