[X] My Assistant

[Toma B]

26 Nov. Close Flyby (500 km.) of Rhea...(Size: 1538 km.)
27 Nov. Distant Flyby (108,000 km.) of Enceladus...(Size: 512 x 494 x 490 km.)
27 Nov. Distant Flyby (86,000 km.) of Helene...(Size: 36 x 32 x 30 km.)

This should be interesting...

Can anybody give a bit more information on the upcoming flybys???

Is this the best image of Helene that we have???
Vojager 2 - distance :341000 km)

[jmknapp]

Is Cassini set to do hi-res ISS imaging of Rhea? I was looking at the pointing info using the CK kernel and it seems that the ISS pallet is pointed away from Rhea during the closest approach (from ~50,000 km inbound to ~50,000 km outbound). The RADAR doesn't seem to be pointed that way either during that interval. So what is the plan for imaging at Rhea?

Could be a problem with my software, but here's an animation (7MB MPG) showing (via a yellow dot) when and where during the flyby the NAC is pointed:

Rhea/NAC movie

On edit: I spoke too soon--it went by so fast I missed it: the NAC sweeps across Rhea's latitude 10 degrees south (approx.) in a straight line right at the time of closest approach, lasting only 4-5 minutes. Here's an animation showing the camera boresight as a yellow dot:

Rhea NAC -- closest approach

[Guest_AlexBlackwell_*]

Is Cassini set to do hi-res ISS imaging of Rhea?

I don't know if this has been mentioned elsewhere or not but the Rhea-1 Mission Description Document (for the upcoming November 26, 2005, flyby) is now online.

[volcanopele]

I don't know if this has been mentioned elsewhere or not but the Rhea-1 Mission Description Document (for the upcoming November 26, 2005, flyby) is now online.

Thanks! To follow up on jmknapp's comments, here is what that document has to say about ISS observations:

"For ISS, this Rhea flyby provides an opportunity to capture the best images of the wispy terrain and to obtain high-resolution images of a relatively fresh, bright crater on the leading hemisphere. ISS will perform mosaics on the inbound leg (178,000-54,000 km) for regional mapping. At closest-approach, ISS will capture the highest-ever resolution images of Rhea as the moon passes through the field-of-view of the camera."

Note the comment about the ray crater being imaged at very high resolution.

[jmknapp]

ISS will perform mosaics on the inbound leg (178,000-54,000 km) for regional mapping. At closest-approach, ISS will capture the highest-ever resolution images of Rhea as the moon passes through the field-of-view of the camera."

Yes, thanks, Alex. It says much of the approach time is taken up by pointing towards Earth at T-2h04m 54,000 km out and then at T-1h39m:

"Radio Science Rhea Mass Determination -- 3h downlink -- Canberra and Goldstone 34M; Rhea passes through ORS boresight near C/A for highest-resolution images"

Pretty wild if I'm reading that right. So the NAC boresight just happens to pass through Rhea at closest approach while the HGA is being continuously pointed at Earth all the while for the radio/mass science? Funny how that worked out.

[elakdawalla]

Pretty wild if I'm reading that right. So the NAC boresight just happens to pass through Rhea at closest approach while the HGA is being continuously pointed at Earth all the while for the radio/mass science? Funny how that worked out.

It's not really that much of a coincidence. Pointing the HGA to Earth fixes one orientation of the spacecraft, but the spacecraft can still be rotated around that axis to point the camera in any other direction perpendicular to that (the camera boresight is perpendicular to the HGA). So you want the triangle formed by Earth, Cassini, and some point on Dione's surface to be a right triangle, with Cassini at the right angle. Since this is a close pass by Rhea, the Earth-Cassini and Earth-Dione lines are very nearly collinear, and the angle becomes a right angle when Cassini is very very close to its closest approach distance. All that's required for this to work is for the planners to be tricky enough to have Cassini oriented properly so that its Z axis (and the HGA) are pointed at Earth and the Y axis (camera direction) is pointed in the direction where Rhea will be at the closest point of the flyby. And the Cassini planners are very very tricky!

--Emily