[Kstars-devel] Pluto

[Jason Harris]

Hello,

You may recall that our model of Pluto's orbit fails rather spectacularly if 
you wander more than a century or so from the present epoch.  One idea I have 
is to rewrite KSPluto as a derivative of KSAsteroid, and rely on the 
asteroid-like orbital elements of Pluto published here:
http://ssd.jpl.nasa.gov/elem_planets.html

After doing some transformations (Mean Longitude -> Mean Anomaly; Longitude of 
Perihelion -> Argument of Perihelion), I added this line to my asteroids.dat:
    0 Pluto2            51545 39.48168677 0.24880766  17.14175 113.76329 
110.30347  14.86205  1.00 0.15 JPL 0

Pluto2 is always within 30 arcmin of Pluto, and if you add Trails to both and 
set a timestep of 1 week, you can see that they track each other very well in 
ecliptic longitude.  However, for some reason Pluto2's retrograde loops are 
significantly "flatter" than Pluto's.

Now the question is, which one is correct?  For 8 June 2005, I am getting the 
following coords:

Pluto:  ( 17h 32m 14s,  -14d 28' 46" )
Pluto2: (  17h 32m 14s, -14d 58' 18" )

I tried looking up Pluto's coordinates for today (suprisingly difficult to 
find!).  Here are some sites and their coords:

http://www.fourmilab.ch/yoursky/ : ( 17h 32m 15s,  -14d 58' 36" )
http://aa.usno.navy.mil/cgi-bin/aa_micaform2?calc=8&ZZZ=END : 
( 17h 32m 42s ,  -14d 58' 52" )

The second one is the US Naval Observatory's web interface for the 
Astronomical Almanac, so it's pretty authoritative.  So, it looks like there 
may be something wrong with our current Pluto model, even for modern dates.  

I would like to go ahead with my plan to make KSPluto a derivative of 
KSAsteroid, and use the JPL orbital elements.  Note that the "Pluto2" 
coordinates reported above are not quite correct.  This may be due to the 
fact that I have not taken into account the fact that the elements change 
with time ("Pluto2" used the elements from J2000).  The JPL site presents 
rates of change for each value, and I will include these in the new KSPluto 
class.

Jason
-- 
KStars: KDE Planetarium
http://edu.kde.org/kstars