A change of plans for Cassini: higher altitude for the "T7" Titan flyby
The June 15 Cassini Project Update includes a note about a difficult decision -- they are raising the altitude of an upcoming Titan flyby, "T7," which is scheduled for September 7. The flyby was originally planned for a 950 kilometer altitude, and then it was raised to 1025. Now it's up to 1075. Raising the altitude has direct consequences on the science plans, in particular for any mosaic observations, so people really don't like to make these changes unless they absolutely have to. But spacecraft safety trumps all other considerations, and apparently they are worried that Titan's atmosphere is too thick in the warmer southern hemisphere for the 1025-kilometer altitude (which is what they used on the "T5" April 16 flyby) to be a safe one. Here's a quote from the update:
Last week at the Titan Atmosphere Model Working Group (TAMWG) telecon, the key topic of discussion was the appropriate altitude for the upcoming T7 flyby in September. Because of variations in atmospheric density observed to date, there is no consensus model to predict the atmospheric density to be encountered at T7. Use of worst-case density values indicate that the T5 altitude of 1025 km may not be safe for T7 with its far southern hemisphere latitude of closest approach, and might lead to spacecraft loss of attitude control and safing. The closer the spacecraft (S/C) gets to Titan, the greater the atmospheric torque applied to the spacecraft. At some point the atmosphere becomes dense enough that the S/C cannot maintain its orientation while flying through it. Then it executes safing and has to find the sun in order to regain its proper orientation so that it can communicate with Earth again. Safing terminates the executing sequence, so all science observations planned between the safing activity and when the sequence is resumed are lost. By raising the flyby altitude we essentially eliminate any chance of this happening because the atmosphere will be thinner.
Following the TAMWG meeting, a follow-on meeting was held to discuss the operational issues related to making this change. There were concerns expressed about trying to accommodate changes....However, a schedule for product deliveries was developed that would allow the changes to be accommodated, and the decision was made to proceed with the change. As of today the target altitude for the T7 flyby has been officially raised from 1025 to 1075 km.
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