Celestia Forums

Hi,

I think I heard recently that Cassini's trajectory was modified so that it would spend more time at Saturn.
Firstly does anyone know if this is true?
If so, has anyone generated updated XYZ's for the changes in Cassini's path?

I'd find it quite interesting if I could goto Cassini at a given date and time corresponding to a published photo, and then reproduce the view for comparison in Celestia. For example: http://celestiaproject.net/forum/viewtopic.php ... ght=#74730

Cheers
CC

Hi Chuft,I don't know of any recent changes in the orbit of Cassini.I have just looked on the JPL Horizons computer and the latest ephemeris they have are from March 23rd 2005,I updated the xyz orbit for Cassini using these ephemeris in May 2005,and this is available on the Motherlode in this addon
http://celestiamotherlode.net/creators/jestr/cassini_huygens_1_1.zip
However the other Cassini addon of mine on the Motherlode still uses an older version of Cassini's orbit-I should delete this one as it doesnt have any other differences from the updated one.I havent checked lately how the xyz compares with the recent photos from Cassini,but I will have a look,cheers,Jestr

Hi Jestr,
Thanks, I think I've already installed that download of yours, but I'll check.

As a test I've tried to replicate this view in Celestia
(click on the image to see details about it):


but it seems to be not quite right: if you notice the phase of Mimas, it doesn't match the photo.
(click to open in Celestia):

Hello again Chuft.The position of Cassini looks to be about right though,dont forget that the position of the moons themselves may be slightly out,Mimas has a customorbit definition,but I remember when I was compiling the Cassini xyz orbits and checking it against the planned flybys,several times I had to alter the xyz orbit by hand because it flew straight through Titan,I cant remember now if it happened with any other moons,but it only meant changing a few values in the xyz data.Cheers,Jestr

Oh,

I had noticed that it flies thru Titan for me.
Does that mean I haven't got the latest XYZ?

The xyz file I made for the addon linked to above is called
jestr_cassinigrandtour0505.xyz
If I remember correctly,Runar updated this file for me on the Motherlode.
If you have this file already and Cassini still flies through Titan,let me know when this happens and I will correct it,I'm pretty sure I checked all of the flybys though to make sure it didnt happen.Jestr

jestr wrote:The position of Cassini looks to be about right though,dont forget that the position of the moons themselves may be slightly out,Mimas has a customorbit definition,but I remember when I was compiling the Cassini xyz orbits and checking it against the planned flybys,several times I had to alter the xyz orbit by hand because it flew straight through Titan,I cant remember now if it happened with any other moons,but it only meant changing a few values in the xyz data.

I've just searched my Celestia directories and It appears I don't have jestr_cassinigrandtour0505.xyz. This would make sense, because I don't believe that such a big difference in the phase angle of Mimas relative to Cassini (looks like 10-20 degrees difference between the photo and my simulation) could be explained by minor customorbit differences (CustomOrbits are usually VSOP87 aren't they?-- so should be very accurate).

Anyway, downloading your new version as we speak. Will let you know how I get on.


Thanks for the help.

Hi Chuft,only eight of the nine planets use VSOP87 (not Pluto),most of the moons have their own hard coded Customorbits,but I dont know how these are worked out.Jestr

OK Jestr,
Assuming you have the new track for Cassini, if you click on my original URL (copied from my earlier post), and then select Cassini, you'll notice it's new position is now 1147.7 km away from that position.

Here is the new position using your new XYZ.

Chuft-Captain wrote:Hi Jestr,
Thanks, I think I've already installed that download of yours, but I'll check.

As a test I've tried to replicate this view in Celestia
(click on the image to see details about it):


but it seems to be not quite right: if you notice the phase of Mimas, it doesn't match the photo.
(click to open in Celestia):

For one possible reason why, see this thread.

bdm wrote:For one possible reason why, see this thread.

Thanks for that bdm. It's actually that thread which inspired these experiments of mine. Unless Mimas has suddenly acquired an atmosphere, I don't think this apparent phase difference can be explained by refraction or light scattering, however you may be onto something if it's due to limb darkening as a result of Celestia's use of the Lambert model. (It would be interesting to see this same scene with Chris's new photometric code once he's completed it.)

Now that I have the latest XYZ from Jestr, if it's not due to limb darkening as discussed above, then I can only think of 3 other possible reasons...

1. CustomOrbit declaration for Mimas is inaccurate (as Jestr suggested earlier).
One of Celestia's main design goals is accuracy, so I would be surprised if the CustomOrbit was in error by enough to explain an apparent difference in phase of 10-20 degrees.
2. Date publicised for the photo is incorrect (or not UTC perhaps).
Note that the image in question comes from the RAW image section of the website and is not calibrated or validated yet
3. Cassini's track has actually changed since Jestr updated it, and the new ephemeris data hasn't been published.

I'm no expert in these areas, so I'd be interested in anyone else's opinion.
(Where are you Henry Harris or RocketMan?...I Think It's Going To Be A Long Long Time)

PS. thought of a 4th explanation...Mimas really is the DeathStar and has moved it's orbit!

Actually,I tend to agree with bdm,the problem is a rendering one,the positions of Cassini,Mimas and Saturn all look reasonably accurate to me (though of course,I'm no expert).I just think there isn't enough light reflected off Mimas,but we've had this discussion many times on the forum,and our monitors are not capable of showing the full range of stars luminosities,and we'd all be blinded when looking at stars if they could.Jestr

I've pretty much come to the same conclusion, except I also think the jury is out on whether the photo or the Celestia version is more accurate. It's possible the Lambert issue is affecting the rendering, but given that the reference photo I used is a RAW un-calibrated one, it's also likely that it's not an accurate representation either. (We don't know the exposure time etc. for this photo...perhaps it's over-exposed and, if and when it's calibrated it may look more like the Celestia version). Probably not the best choice of reference photo by me. Given that Celestia is designed to display visible light/naked eye visuals, then it's best to use similarly calibrated images for comparison.

I don't think this is a monitor issue at all because the photo has more contrast than the Celestia version, so Celestia's obviously not exceeding the monitor capabilities in this circumstance.

As for the accuracy of the position, that's hard to tell, given the scale of the distances involved and the FOV setting. For example I'd be surprised if anyone could actually distinguish visually which of THE OLD TRACK POSITION and THE NEW TRACK POSITION is correct. They're virtually identical even though they're 1147.7 km apart.
Here they are together:

Thanks jestr and bdm for your comments,

It will be interesting to see how this is affected by Chris' new photometric code when it arrives.

Cheers
CC

PS. Jestr, I just followed Huygens trajectory down to Titan in your new addon, and I really enjoyed it. Excellent attention to detail with all the transitions, deployment of parachutes etc!!
You should consider writing a script to best show it off to newbies who may not know how to track it, etc.

jestr wrote:If I remember correctly,Runar updated this file for me on the Motherlode.

Yes, it is here:
http://www.celestiamotherlode.net/creat ... ns_1_1.zip
... It seems not to be listed in the catalog, i don??t know why, but it is archived in your creators folder.
It is the only version that includes your XYZ for the course correction in may 2005: i uploaded it in july last year, so it is the most up-to-date one.

- rthorvald

PS. Jestr, I just followed Huygens trajectory down to Titan in your new addon, and I really enjoyed it. Excellent attention to detail with all the transitions, deployment of parachutes etc!!
You should consider writing a script to best show it off to newbies who may not know how to track it, etc.

Check the Motherlode Chuft,there are a few to choose from,Jestr

rthorvald wrote:It is the only version that includes your XYZ for the course correction in may 2005: i uploaded it in july last year, so it is the most up-to-date one.

- rthorvald

I just now received a message from one of the people who maintains Horizons. The Horizons Cassini trajectory was updated on March 31, 2006, so it might be reasonable to update Celestia xyz files again.

Will do,Jestr