Question Pluto System

[Nighthawke]

Does anyone know if Tony Dunn's cfgData program still works correctly with JPL Horizons ephimeris system? I am trying to add Nix and Hydra to my Pluto add-on and I seem to be getting some bogus ephimerides from JPL via cfgData. Here is what I'm getting from cfgData after pasting e-mail contents from JPL and generating data:

Epoch = 2009.44922075823
SemiMajorAxis = 0
Eccentricity = .542887868836594
Inclination = 1.97055075472226
LongAscNode = 3.96619405789097
LongPerihelion = 5.1691425491519
MeanLongitude = 11.4081776751846

Everything seems OK except for eccentricity and SMa. Why would cfgData give a value of zero for SMa??

Here are the contents of the return e-mail from JPL for reference:

Automated mail xmit by MAIL_REQUEST, PID= 13018 Sat Jun 13 19:59:47 2009
++++++++++++++++++++++++++++++++ (part 1 of 1) +++++++++++++++++++++++++++++++

*******************************************************************************
Revised: Jun 20, 2007 Nix / (Pluto) 902

Fit to all available observations.
*******************************************************************************


*******************************************************************************
Ephemeris / MAIL_REQUEST Sat Jun 13 19:59:46 2009 Pasadena, USA / Horizons
*******************************************************************************
Target body name: Nix (902) {source: PLU017}
Center body name: Pluto (999) {source: PLU017}
Center-site name: BODY CENTER
*******************************************************************************
Start time : A.D. 2009-Jun-13 19:52:09.0000 CT
Stop time : A.D. 2009-Jun-13 19:52:10.0000 CT
Step-size : 60 minutes
*******************************************************************************
Center geodetic : .000000000,.000000000,.00000000 {E-lon(deg),Lat(deg),Alt(km)}
Center cylindric: .000000000,.000000000,.00000000 {E-lon(deg),Dxy(km),Dz(km)}
Center radii : 1195.0 x 1195.0 x 1195.0 km {Equator, meridian, pole}
System GM : 8.7317411466673764E+02 km^3/s^2
Output units : KM-S, deg, Julian day number (Tp)
Output format : 10
Reference frame : ICRF/J2000.0
Output type : GEOMETRIC osculating elements
Coordinate systm: Ecliptic and Mean Equinox of Reference Epoch
*******************************************************************************
JDCT
EC QR IN
OM W Tp
N MA TA
A AD PR
*******************************************************************************
$$SOE
2454996.327881944 = A.D. 2009-Jun-13 19:52:09.0000 (CT)
EC= 5.428878688365937E-01 QR= 5.039629420511769E+04 IN= 1.129042415619046E+02
OM= 2.272461802470180E+02 W = 6.892387152088178E+01 Tp= 2454996.960923331790
N = 4.624973783964202E-05 MA= 3.574703809555429E+02 TA= 3.498703702516962E+02
A = 1.102493037689745E+05 AD= 1.701023133328313E+05 PR= 7.783827905105082E+06
$$EOE
*******************************************************************************
Coordinate system description:

Ecliptic and Mean Equinox of Reference Epoch

Reference epoch: J2000.0
xy-plane: plane of the Earth's orbit at the reference epoch
x-axis : out along ascending node of instantaneous plane of the Earth's
orbit and the Earth's mean equator at the reference epoch
z-axis : perpendicular to the xy-plane in the directional (+ or -) sense
of Earth's north pole at the reference epoch.

Symbol meaning

JDCT Epoch Julian Date, Coordinate Time
EC Eccentricity, e
QR Periapsis distance, q (km)
IN Inclination w.r.t xy-plane, i (degrees)
OM Longitude of Ascending Node, OMEGA, (degrees)
W Argument of Perifocus, w (degrees)
Tp Time of periapsis (Julian day number)
N Mean motion, n (degrees/sec)
MA Mean anomaly, M (degrees)
TA True anomaly, nu (degrees)
A Semi-major axis, a (km)
AD Apoapsis distance (km)
PR Orbital period (sec)

Geometric states/elements have no aberration corrections applied.

Computations by ...
Solar System Dynamics Group, Horizons On-Line Ephemeris System
4800 Oak Grove Drive, Jet Propulsion Laboratory
Pasadena, CA 91109 USA
Information: http://ssd.jpl.nasa.gov/
Connect : telnet://ssd.jpl.nasa.gov:6775 (via browser)
telnet ssd.jpl.nasa.gov 6775 (via command-line)
Author : [email protected]
*******************************************************************************


Any help certainly appreciated. The quicker I can get the details sorted out the quicker I can update the add-on.

Nighthawke

 

[Chode]

Does anyone know if Tony Dunn's cfgData program still works correctly with JPL Horizons ephimeris system?

Yes, it still works fine for me. It can be a bit finicky when it comes to parsing the data you give it. In your posted example, I can get it to work correctly if I insert a single space before "A =". It is best to copy and paste the data directly from the e-mail response, which should have that space in it (at least it does in my e-mail program).

Hope that helps.

Regards

 

[2552]

I usually use the Horizons web interface and copy the text that gives me, that always works for me. cfgData gives me this:

Epoch = 2009.4
SemiMajorAxis = 122551204.258821
Eccentricity = .585518998253685
Inclination = 1.9702743196618
LongAscNode = 3.96614514298737
LongPerihelion = 6.69020200075955
MeanLongitude = 6.69631012461578

Although for the Pluto system I just use the Pluto system spice kernel.

 

[Nighthawke]

Chode, 2552,

Thanks for your replies. The problem with configData was the missing space. I downloaded the spice kernel and tried that also. It seems to give much better performance than my cobbled together config files. However I did notice that the orbits of Nix and Hydra fluctuate greatly with either config files or spice kernel. I was fairly proficient at trigonometry, physics and orbital mechanics in high school, but I'm not an astrophysics wiz by any stretch of the imagination. So, I guess the question I'm feeling a need to ask is if this is normal behavior for Nix and Hydra??

Thanks again fellas,
Nighthawke

 

[Chode]

If you look at the orbital elements of Nix and Hydra with repect to the Pluto body center from Horizons and Spice, there are large fluctuations, mostly because of the presence of Charon, which causes Pluto to move around alot. You get much smaller fluctuations if you look at elements with respect to the system barycenter. In Orbiter, the elements are assumed to be parent body centered, and currently you cannot specify elements with repect to the barycenter.

I've tested out the spice module a bit, and it should give good results, however, there is a bug in Orbiter 2006-P1 that incorrectly calculates the position of a system barycenter, which causes the spice module to give incorrect results. This bug was corrected in Orbiter betas going back about a year or so. Also, the kernals only cover from 1900-2050, and it's accuracy degrades outside that time period.

EDIT: CORRECTION: I re-checked the Spice module and it is NOT affected by the barycenter bug in Orbiter 2006-P1.

Regards