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SUG: Added information on adding orbital elements from JPL Horizon
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- old wiki info
- updated formatting and discussion
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@gzotti
gzotti committed Dec 3, 2023
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Expand Up @@ -589,6 +589,98 @@ \subsubsection{Comet section}
(\texttt{orbit\_good=1000} days) from the epoch (defaults to pericenter time).


\subsection{JPL Horizons}
\label{sec:ssystem.ini:JPLHorizons}


The \indexterm{JPL Horizons}\footnote{\url{https://ssd.jpl.nasa.gov/horizons/}} on-line solar system data and ephemeris computation service provides access to key solar system data and
flexible production of highly accurate ephemerides for solar system objects (Late 2023: 1,334,683 asteroids, 3,904 comets, 290 planetary satellites {includes satellites of Earth and dwarf planet Pluto},
8 planets, the Sun, L1, L2, select spacecraft, and system barycenters). Horizons is provided by the Solar System Dynamics Group of the Jet Propulsion Laboratory.

\subsubsection{How to manually look up a body}

A comet discovered in early 2023, C/2023 A3 (Tsuchinshan-ATLAS), may become bright in October 2024.
Let's find out its orbital elements for the most interesting time!

Use the Horizons web interface. You're going to look up the body you are interested in and copy down its orbital elements.
The Horizons web interface will display something like this.

\fbox{\begin{tabular}{rcl}
1& & Ephemeris Type: \button{Osculating Orbital Elements}\\
2& \button{Edit} & Target Body: C/2023 A3 (Tsuchinshan-ATLAS)\\
3& \button{Edit} & Coordinate Center: Sun (body center) [500@10]\\
4& \button{Edit} & Time Specification: Start=2024-07-03 TDB, Stop=2024-12-02, Step=5 (days)\\
5& \button{Edit} & Table Settings: \emph{custom}\\
\multicolumn{3}{l}{\button{Generate Ephemeris}}
\end{tabular}}

\begin{enumerate}
\item Ephemeris Type \emph{must} be set to \button{Osculating Orbital Elements}.
\item Target Body is the body you want to add to Stellarium. Click \button{Edit} to look it up.
\item Center should be set to Sun for all heliocentric trajectories.
\item Time Specification: Use a short interval or discrete dates near the time you are most interested. The returned JDE date (Julian Day number) will be used as epoch in Stellarium.
\item Table Settings: We must \button{Edit} and select Output units: \button{au and days} to create data compatible with the appropriate fields in \file{ssystem\_minor.ini}
\end{enumerate}

The result begins with listing osculating elements for a certain epoch in the past and its own initial conditions and settings.
Horizons then applies numerical integration and simulates the movement and gravitational interaction between all its solar system objects, icluding our comet.
Then, for each requested time, orbital elements are listed. Here you could see the slight changes these elements undergo over weeks.
However, in Stellarium, we can only use one particular element set and show the comet's movement under the assumption that these elements do not change.
When we have read it will be brightest in October, let's take this section. The first number is the date where thjese elements are valid,
i.\,e., the element epoch.

\begin{configfile}[\scriptsize]
2460584.500000000 = A.D. 2024-Oct-01 00:00:00.0000 TDB
EC= 1.000086779013436E+00 QR= 3.914011809348247E-01 IN= 1.391089312570287E+02
OM= 2.156049287759364E+01 W = 3.084955943429219E+02 Tp= 2460581.248532149941
N = 3.253815615607083E-06 MA= 1.057967686457091E-05 TA= 1.819722055160837E+01
A =-4.510320703532996E+03 AD= 9.999999999999998E+99 PR= 9.999999999999998E+99
\end{configfile}
Below you also find the keys:

\begin{configfile}
Symbol meaning:

JDTDB Julian Day Number, Barycentric Dynamical Time
EC Eccentricity, e
QR Periapsis distance, q (km)
IN Inclination w.r.t X-Y 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 Sidereal orbit period (sec)
\end{configfile}

\noindent Now we can add an entry into our \file{ssystems\_minor.ini}.
You can leave the exponential writing style (trailing ``E+02'' etc.)
or convert to the more human-readable as shown here.
Symbols \texttt{M1} and \texttt{k1} are given only in the top list of initial elements.
The symbols in brackets refer to the above table, do not enter those!

\begin{configfile}[\scriptsize]
[c2023A3(tsuchinshanatlas)]
iau_designation = C/2023 A3
name = C/2023 A3 (Tsuchinshan-ATLAS)
type = comet
orbit_Epoch = 2460584.5
orbit_ArgOfPericenter = (W) 308.4955943429219
orbit_AscendingNode = (OM) 21.56049287759364
orbit_Eccentricity = (EC) 1.000086779013436
orbit_Inclination = (IN) 139.1089312570287
orbit_PericenterDistance = (QR) 0.3914011809348247
orbit_TimeAtPericenter = (Tp) 2460581.248532149941
absolute_magnitude = (M1) 6.1
slope_parameter = (k1/2) 4.25
orbit_good = 1000
discovery = 2023-01-09 [Not given by JPL Horizons]
ref = JPL Horizons: JPL#25
\end{configfile}



%%% Local Variables:
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2 comments on commit 47ba336

@JonSeamans
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Very nice!
First, I've not communicated with Stellarium since it's moved to github. It's great to read Georg's and Alexander's comments again regarding the spectacular, and still evolving, Stellarium program!
Second, a few questions:

  1. Do the above definition and configuration comments apply to existing Stellarium versions, or do I have to wait for the next version?
    I'm currently using version 20.4
  2. I'm trying to recreate the Artemis / Orion trajectory and put the observer at Orion's location (i.e. the astronaut's view). I pick the appropriate epoch amongst the concatenation of osculating element sets JPL Horizons list. Depending on trajectory location / time, elements in the sets change based on the time increment. Now the details:
  • I assume orbit "type" will still be comet, but less clear on what other parameters required?
  • Specifically, because both Earth and Moon gravitation is involved, I don't know to pick the coordinate center in Horizons, Earth/Moon barycenter or Geocentric? The elements do change. (I'll also ask JPL this question. but I think it's the barycenter.)
    However, Horizons lists 12 elements but Stellarium uses only six.

Is this because there are redundant parameters in Horizons list? 
I also don't understand the "orbit_good" input parameter, and why it = 1000?
Needless to say, I was not successful with my initial attempt (prior to this discussion). BTW, I did define and construct the trajectory in ssystem_minor.ini. I've tried again.

Thanks again for your responses!
Jon

@gzotti
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@gzotti gzotti commented on 47ba336 Dec 3, 2023

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Very nice! First, I've not communicated with Stellarium since it's moved to github. It's great to read Georg's and Alexander's comments again regarding the spectacular, and still evolving, Stellarium program! Second, a few questions:

1. Do the above definition and configuration comments apply to existing Stellarium versions, or do I have to wait for the next version?
   I'm currently using version 20.4

This is 0.20.4 then, and massively outdated, so old that I don't remember what has changed when. We have finally solved our accuracy goals in 2021. After the long-expected version 1.0 (October 2022) we are now at 23.3.

2. I'm trying to recreate the Artemis / Orion trajectory and put the observer at Orion's location (i.e. the astronaut's view).  I pick the appropriate epoch amongst the concatenation of osculating element sets JPL Horizons list. Depending on trajectory location / time, elements in the sets change based on the time increment.  Now the details: 

* I assume orbit "type" will still be comet, but less clear on what other parameters required?

I have meanwhile updated the text further with an example for Voyager 2 and assign "interstellar object" to make the trajectories stand out better when observing from e.g. Solar System Observer. However, all non-planets are coded more or less the same.

* Specifically, because both Earth and Moon gravitation is involved, I don't know to pick the coordinate center in Horizons, Earth/Moon barycenter or Geocentric?  The elements do change. (I'll also ask JPL this question. but I think it's the barycenter.)

Our elements for non-moons are sun-centered only. When centering on a planet (type=moon), distances are in km. You can try that, but this is beyond specs. Follow the D2.1 Moons section, we don't know more. If in doubt, check the implementation in Planet.cpp. I would have to do the same now to check again.

  However, Horizons lists 12 elements but Stellarium uses only six.

Is this because there are redundant parameters in Horizons list?

Stellarium uses the classical 7 elements (including epoch date) needed to compute an undisturbed Kepler orbit. The other data given by Horizon are redundant.

I also don't understand the "orbit_good" input parameter, and why it = 1000?

Quote from the Stellarium User Guide, the essential reference for beginning and advanced and expert users:

orbit_good can be given in days to limit computation of the object to the time range epoch ±orbit_good. If specified as 0 there is no check for out-of-range dates. If specified as -1, half
the orbital period is used. The main purpose of this parameter is to avoid an element clash or the inadvertent use of outdated comet orbit elements when a periodic comet reappears.

The examples given in subsequent pages should help understanding the number. 1000 is an arbitrarily simple number. 3*365 would mean 3 years, but is harder to read.

Needless to say, I was not successful with my initial attempt (prior to this discussion). BTW, I did define and construct the trajectory in ssystem_minor.ini. I've tried again.

Thanks again for your responses! Jon

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