I was hoping that perhaps a umsf member could shed some light on our knowledge regarding perceived risk to Cassini from material in the Saturn system, and whether this has changed since orbit insertion.

This pertains specifically to the 'protective attitude' that Cassini is commanded to adopt during ring-plane crossings (with the high-gain antenna pointing in the direction of travel to act as a shield to the rest of the spacecraft)

This orientation obviously has costs in terms of hydrazine, data loss (ring-plane crossings are often at or near periapse, when multiple targets are within useful range) and must therefore affect command sequence-planning quite heavily

I was surprised at the low-level flybys at Enceladus, with the spacecraft in effect flying through the south-polar plumes, and also Cassini being directed through the G-ring arc (surely a largely unknown environment in terms of particles at the time) for the Aegaeon encounter.

The cosmic dust analyzer on board is perhaps less glamorous than the remote-sensing instruments, but I wonder if its data continue to indicate danger in crossing the ring plane just outside of the F ring?

Any answers much appreciated...

Jase

While the turns to the Dust RAM orientation do take away from time that the science teams could use for observations, these turns very rarely consume any hydrazine. Cassini uses reaction wheels for virtually all slews and the hydrazine used to bias the wheels isn't really effected by the dust mitigation turns. Also, Cassini performs slews for virtually every science observation, so the turns tot he Dust RAM direction are relatively benign to the science teams.

To answer your larger question, the mission planners for Cassini have already analyzed the trajectory the spacecraft will travel during the coming 6 years and identified all instances where the expected particle flux is larger than some threshold that they've predetermined (unfortunately I don't know what that threshold is. Sorry!). Since all of these events are known in advance based on a mission planning analysis, there's very little feedback from the CDA team or the other in-situ instruments required to quantify the risk to the spacecraft. Also, besides turning to use the high gain antenna as an umbrella, the operations team can also simply point the instrument boresights or radiators away from the expected particle flux direction or close the main engine cover to protect the fragile engine bells.

Flying through Enceladus plume is not as cavalier as it might at first seem. Early flybys were approached cautiously and post flyby analysis was used to quantify the risk for Cassini on each subsequent pass through the plumes.

Thanks for the detailed response Todd - much appreciated.

No doubt the 'expected particle flux threshold' has increased significantly since orbit insertion with the discovery of the dusty rings and ring arcs outside of the F ring

I'd be cautious too with $3+ billion worth of hardware to look after.

I think that's what Mark Showalter said when they based the original hazard assessment on his Voyager-based E ring model...

Just for reference, there are only six more dust hazard periods which will require pointing the high gain antenna to ram. Those are all Janus-Epimetheus ring crossings in 2015-2017, and all about 10-15 minutes long, plus turn time. (This doesn't include the final orbits inside the D ring; analysis and plans for that are still pending.) All the other hazard periods just require closing the main engine cover. Also, although there was some early talk about it, Cassini has never turned to keep instrument optics out of the ram direction. There was lots of analysis and testing on mirror samples before they decided this would be ok. (Part of the reason for all the testing and analysis was to impact those turns would have had on science: If "turn optics out of the ram direction when the spacecraft is in the E ring" were a rule, imaging during the inbound leg of an Enceladus encounter would have been forbidden.)

I see I see!
When 'dust hazard' is flagged in Cassini's routine schedules, I imagined the whole bus having to be flipped over, HGA pointing forward, with the resultant loss of targeting potential for the remote sensing instruments. If it's just a question in most cases, of closing the main engine cover, then this has much less of an impact on imaging and so on than I thought. Some of the dust hazard durations in the XXM are listed at 416 minutes and similar, so around periapse this would obviously have impacted heavily on the best observation opportunities.

Thanks for the insights Frank.