Emily LakdawallaDec 15, 2006

Breaking a tie on the Cassini science teams

This week's Cassini Project Update was particularly interesting, because it contained a story about how a difficult decision was made regarding the prioritizing of different science teams' desires for an upcoming Titan flyby.

Two competing candidate plans were developed for the scientific observations to be carried out on the T32 Titan flyby in June of 2007. One plan features the collection of Ion and Neutral Mass Spectrometer (INMS) data, and the other emphasizes Optical Remote Sensing (ORS) observations, in particular a solar occultation egress measurement by the Ultraviolet Imaging Spectrograph (UVIS) and the Visual and Infrared Mapping Spectrometer (VIMS). Members of the Titan Orbiter Science Team (TOST) were unable to form a consensus as to which plan to choose, and attempts to "merge" the plans in some way have not been successful.

Cassini has no fewer than 12 different science instruments, and they cannot all point at the same place at the same time. The Optical Remote Sensing instruments -- the cameras and imaging spectrometers, ISS, UVIS, CIRS, and VIMS -- all point in the same direction and can capture data simultaneously, although each one has a different resolution and requires different exposure times, so in practice it's not easy to use them all at once. Most of the rest of Cassini's instruments are so-called "fields and particles" instruments, which measure the magnetic field and snap up energetic particles and ions from the space environment through which Cassini flies. These include CDA, INMS, CAPS, RPWS, MIMI, and MAG. (Too many acronyms, I know; go to that Cassini instruments page to learn what these are.) Even when instruments can be used simultaneously, there's only so much onboard memory available, which must be carefully apportioned to each instrument. Add to that the fact that each flyby affords a unique and possibly never duplicated viewpoint on Titan, and you can see how challenging it can be to identify which instruments get to control the spacecraft pointing and data volume for a given flyby. So, what are they going to do for T32? Read on.

The decision was handed over to the Project Scientist [Dennis Matson]. What made the decision difficult was the fact that both plans offer excellent science at Titan. Also, with respect to Titan's surface and sensible atmosphere, it is difficult to find a scientific criterion for choosing one over the other. As a result, the Project Scientist fell back to the basics of considering the project and the established science goals as a whole. The following is extracted from the scientific objectives of the mission in the NASA Announcement of Opportunity for Cassini:


-Determine abundances of atmospheric constituents (including any noble gases; establish isotope ratios for abundant elements; constrain scenarios of formation and evolution of Titan and its atmosphere

-Observe vertical and horizontal distributions of trace gases; search for more complex organic molecules; investigate energy sources for atmospheric chemistry; model the photochemistry of the stratosphere; study formation and composition of aerosols; - Measure winds and global temperatures; investigate cloud physics, general circulation and seasonal effects in Titan's atmosphere; search for lightning discharges;

-Determine the physical state, topography and the composition of the surface; infer the internal structure of the satellite;

-Investigate the upper atmosphere, its ionization, and its role as a source of neutral and ionized material for the magnetosphere of Saturn.


-Determine the configuration of the nearly axially symmetric magnetic field and its relation to the modulation of Saturn Kilometric Radiation (SKR).

-Determine current systems, composition, sources, and sinks of magnetosphere charged particles;

-Investigate wave-particle interactions and dynamics of the dayside magnetosphere, and the magnetotail of Saturn and their interactions with the solar wind, the satellites, and the rings;

-Study the effect of Titan's interaction with the solar wind and magnetospheric plasm

-Investigate interactions of Titan's atmosphere and exosphere with the surrounding plasma

OK, so here's the decision:

Review of these objectives provided a clear way to resolve the "tie". Both plans address, to some degree, the 1st, 2nd, and 5th Titan objectives. With respect to the magnetospheric objectives, the INMS plan addresses the 4th and 5th objectives. The ORS plan does not significantly address any of the these objectives. The INMS plan was selected as it addresses more of the mission's objectives.

That was a tough call, and seems to give every objective equal weight, which might or might not be fair; I couldn't say. The discussion just makes me glad that I'm not Dennis Matson. Cassini's science team is enormous in size, international in scope, and varied in their interests; they don't always get along. As a casual viewer of the mission I always want to see more optical remote sensing data because, well, I can see it; the ORS stuff is more accessible to the layperson. But that doesn't make the fields and particles stuff any less important.

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