Titan Flyby - 25 March 2008
25 March 2008Today, Tuesday 25 March, Cassini returns to Titan for the mission's forty-third targeted encounter: Titan-42. The closest approach occurs at 14:27:48 UT, at an altitude of 1000 kilometres above the surface and at a speed of 6.3 kilometres per second. The latitude at closest approach is 27° S and the encounter occurs on orbit number 62.
This encounter is set up with two manoeuvres: an apoapsis manoeuvre on 17 March and a Titan approach manoeuvre on 22 March. T42 is the seventh in a series of outbound encounters that will last until the end of the prime mission, and it occurs less than two days after Saturn closest approach. This is the last in a series of seven Titan southern hemisphere encounters.
- Ion and Neutral Mass Spectrometer (INMS)
This will be the second time in the entire mission when INMS has used its open source neutral beaming mode to see reactive neutrals and understand wall effects, where detected particles end up sticking to the source. The instrument is unable to measure both ions and neutrals at the same time. Part of the Magnetosphere and Plasma Science Titan campaign, this observation will help sort out compositional changes of minor species during ingress vs. egress.
- Visible and Infrared Mapping Spectrometer (VIMS)
VIMS rides along on the INMS closest approach observations using the noodle mode, where VIMS captures a long noodle-shaped swatch of Titan's surface. The noodle mode was first tried in the T20 flyby: the noodle observation, using a RADAR swath for context, showed correlation between interdune terrain and types of ices. If this particular observation is successful, VIMS scientists expect to exploit it more during the extended mission. Pointing is especially tricky for this mode. VIMS is not expected to capture the Huygens landing site, but will get medium resolution imaging of adjacent unknown territory. VIMS non-noodle prime observations during T42 include a mosaic at 5 kilometres per pixel of unknown territory at the northern rim of the dark units and cloud tracking.
- Imaging Science Subsystem (ISS)
Similar to the T41 encounter, ISS captures regional mapping and full-disk mosaics covering Adiri and East Belet, giving one of the best views of the high northern latitudes and filling in coverage. The team is looking forward to seeing a high resolution image of a complex boundary in that northern region. The instrument may be able to see all the way up to the pole due to scattering by the atmosphere. In addition, the Narrow Angle Camera monitors for surface/atmosphere changes, attempts to see surface colour variations, and monitors limb hazes. As the geometries of the T41 through T44 flybys are very similar, it will be possible to monitor how the clouds change in this region every few weeks.
- Composite Infrared Spectrometer (CIRS)
CIRS continues to extend spatial and temporal coverage of Titan, from low-spectral resolution disk maps to high spectral resolution nadir and limb integrations. Getting good time resolution is very important in the search for seasonal changes in the stratosphere, especially the expected break-up of the northern polar vortex in northern spring.
- Radio and Plasma Wave Science (RPWS)
As Titan is out "in front" of Saturn, the T41 through T44 flybys put the spacecraft in an ideal location to have another opportunity to see Titan outside of Saturn's magnetosphere, in shocked solar wind ahead of the magnetosheath as happened on T32. It is interesting to duplicating the flyby geometry to look for shorter time-scale phenomena in Titan's plasma environment, so this series of four flybys, especially T41 through T43, will offer that opportunity.
Table of Events
22 March 2008
|OTM #151 prime. Titan-42 targeting manoeuvre
23 March 2008
|Start of sequence S39 which contains Titan-42
|Descending ring plane crossing
|Pallene non-targeted flyby.
Altitude = 32 565 km
|Epimetheus non-targeted flyby.
Altitude = 84 132 km
|Methone non-targeted flyby.
Altitude = 102 556 km
|Saturn periapse, R = 3.7 RS, lat = -23°, phase = 149°
24 March 2008
|OTM #151 backup
25 March 2008
|Start of the TOST segment
|Turn cameras to Titan
|Deadtime, 7 minutes and 48 seconds long. Used to accommodate changes in flyby time
|Titan atmospheric observations-VIMS. Cloud map
|Titan atmospheric observations-CIRS. Obtain information on trace constituents in Titan's stratosphere. Integrate on limb at two positions
|Titan atmospheric observations-CIRS. Obtain information on surface & tropopause temperatures, and on tropospheric CH4. Scan or contiguous steps across disk
|Transition to thruster control
|Titan atmospheric observations-CIRS. Vertical sounding of stratospheric compounds on Titan, including H2O. Integrations at 2 locations on the limb displaced vertically
|Titan atmospheric observations-CIRS. Vertical aerosol sounding of Titan's stratosphere
|Titan atmospheric observations-CIRS. Vertical temperature sounding of Titan's tropopause & stratosphere
|Turn cameras to new waypoint
|Titan magnetosphere and plasma science campaign-INMS. Determine atmospheric and ionospheric composition and thermal structure
|Titan-42 Flyby Closest Approach Time. Altitude = 1000 km, speed = 6.3 kms-1, 21° phase at closest approach
|Turn cameras to new waypoint
|Titan surface observations-VIMS. High Resolution Mapping of landing site
|Ascending ring plane crossing
|Transition off of thruster control
|Titan surface observations-VIMS. High-resolution cubes of Titan's surface
|Titan surface observations-ISS. High resolution regional map
|Titan atmospheric observations-CIRS. Obtain information on surface & tropopause temperatures, and on tropospheric CH4
|Titan atmospheric observations-CIRS. Obtain vertical profiles of temperatures in Titan's stratosphere
|Titan atmospheric observations-CIRS. Obtain information on CO, HCN, CH4. Integrate on disk at airmass 1.5-2.0
26 March 2008
|Titan atmospheric observations-ISS. Monitoring for surface/atmosphere changes; attempt to see surface colour variations; monitor limb hazes, 1-3 km/px
|Titan surface observations-VIMS. Regional map
|Titan surface observations-VIMS. Global map
|Deadtime, 15 minutes and 12 seconds long. Used to accommodate changes in flyby time
|Turn to Earth-line
|Playback of T42 Data. Madrid 70 arrayed
Titan Ground Trace
The above image shows the ground trace of the Titan-42 flyby on a mosaic of Titan's surface, for a period of 32 hours around closest approach (going from ~30° to ~243° longitude). The colour of the trace indicates Cassini's altitude above the surface. Blue: > 100 000 km, green: < 100 000 km, light blue: < 50 000, yellow: < 10 000 km, orange: < 5000 km, red: < 2000 km. The point of closest approach is marked T42 (Cassini at 1000 km altitude).