Dione Orbit ?

[jestr]

Have you seen this nice image from Cassini ?

I have been trying to replicate it in Celestia but it seems the orbit for Dione (or Cassini I guess) is a bit out.Looking in my ssc it says it is a custom orbit but not VSOP I guess ?Is there any way to improve the accuracy?
All the best Jestr

[t00fri]

Javier,

Yes, I also checked:

The distance of Cassini is far too small (230 000 Km
instead of the correct 603 000 Km) and either Cassini is
too high up or Dione too low down (see purple arrow)
relative to Saturn's southern hemisphere.


Bye Fridger

[jestr]

Hi Fridger,Could it be there have been corrections made to Cassinis orbit since our xyz file was put together,I guess they want everything to be right for the upcoming release of Huygens?It seems pretty accurate (as you have pointed out)with respect to Titan though.Is there a VSOP version of Dione's orbit?Jestr

[t00fri]

Hi Fridger,Could it be there have been corrections made to Cassinis orbit since our xyz file was put together,I guess they want everything to be right for the upcoming release of Huygens?It seems pretty accurate (as you have pointed out)with respect to Titan though.Is there a VSOP version of Dione's orbit?Jestr

Javier,

yes, that was also my 'rationale'. But, unfortunately, I have no quantitative knowledge of what has been really done in terms of orbit corrections.

Anybody knows?

Bye Fridger

[Matt McIrvin]

The thread on the Titan-B flyby mentioned that Celestia had it occurring at about 1800 km distance, whereas the real distance was more like 1200 km. That makes me think that the relevant change might have been the correction mentioned here, designed to improve the margin for error in Iapetus's effect on the Huygens probe:

http://planetary.org/news/2004/cassini_titan00b_plan_1211.html

The upcoming Titan encounter, which is referred to as "Titan-B" or simply "Tb" by mission planners, was originally planned to take place at a higher altitude of 2200 kilometers (1370 miles). However, new observations of Iapetus have indicated that its mass is less well known than was previously thought, leading to uncertainty about how the force of Iapetus' gravitational pull on the Huygens probe might affect the precision of the delivery of the probe to Titan in January. This uncertainty made mission planners leery of allowing the probe and the orbiter to pass too closely by Iapetus on December 31 -- which is after the orbiter and probe will separate, but before Huygens descends to Titan.

So the navigators changed the orbital trajectory of Cassini and Huygens slightly to make them pass farther from Iapetus on December 31 than originally planned. These trajectory changes lowered the planned Titan-B flyby altitude, did not change the altitude of the TItan-C flyby on January 14 (during which Huygens will descend), and raised the planned altitude of the Titan-3 flyby, which will take place on February 15.

[Matt McIrvin]

...On the other hand, come to think of it, I've noticed that in the Celestia orbit Cassini passes much further from Iapetus around Dec. 31/Jan. 1 than mission descriptions say it will. So there may be more to it than that.

[Guest]

Can you remind me when they will release the Huygens?

IIRC, my Celestia showed a very close flyby sometime in January.

[Evil Dr Ganymede]

Can you remind me when they will release the Huygens?

IIRC, my Celestia showed a very close flyby sometime in January.

Huygens is released on Christmas Day. The January flyby is when it'll be landing and we'll be (hopefully) getting data.