Galileo travelling through Jupiter and Callisto...

[Evil Dr Ganymede]

I'm just running through the Galileo orbiter tour of the Jovian system, starting in 1995. Right from the start, there's a problem - the orbiter seems to travel through Jupiter when it first arrives at the system! Also, it goes through the body of Callisto around 17:30 hours (PDT) on 1997 09 16 (the C10 flyby). And again at about 02:00 hours on 1999 08 14 (PDT) on the C22 flyby.

So the orbit of something - either the satellites or Galileo (more likely the latter, I think) seems to be in error here.

[Evil Dr Ganymede]

Also, my Voyager 1 and 2 don't seem to actually fly past any planets.

Though now I'm thinking of it, I've forgotten whether these and Galileo actually come with Celestia, or if they were someone's add-ons that I incorporated later...

[JackHiggins]

xyz files don't take any account of where objects are at the time of encounters, they only show the craft moving between point xyz1 and xyz2 between the julian dates on those lines.

I have a really accurate voyager 1 & 2 xyz on my site, but because galileo has so many flybys, an accurate xyz of the entire jovian orbital tour would be a really big download... I don't think anyone has made a full one yet!

[chris]

There is also error in the positions of the planets, which while not significant to an Earth based observer makes a big difference if you're designing an orbiter mission! I could add more terms to the VSOP-87 series, but there are already over 1000 per major planet . . . I've been saying for some time that Celestia needs to be able to use JPL's DE-405 (or later) ephemerides for serious accuracy. The drawback is that the DE-405 data files are large enough that supplying a set for all the major planets over a thousand year timespan . . . well, it'd make a big download Using DE405 ephemerides from 1950-2050 and VSOP-87 elsewhere might be a reasonable compromise.

--Chris

[HankR]

Another possibility might be to allow XYZ files to specify planetocentric or satellite-ocentric coordinates (rather than heliocentric coordinates) for close approaches. Then inaccuracies in positioning the target body would not affect the position of the flyby spacecraft relative to the target.

- Hank