# How to transform orbital elements into Celestia's SSC format

## 0.0: introduction

To display a comet or asteroid in Celestia, use your favorite text editor to create a file with the filetype ".ssc" in Celestia's "extras" folder. In it you need to specify the object's name and its orbital parameters.

The International Astronomical Union publishes circulars which have orbital elements for comets and for asteroids. This is a brief, incomplete, description of how to transform the IAU orbital elements of a comet or asteroid into Celestia's SSC format.

## 1.0: Cometary Orbital Elements

### 1.1:  Translation between orbital element names and Celestia SSC keywords

• http://www.lns.cornell.edu/~seb/celestia/orbital-parameters.html
• http://cfa-www.harvard.edu/iau/info/OrbElsExplanation.html

Cometary Orbital Elements
Element name or symbol Celestia SSC keyword Description
EpochEpoch (1,2) Julian date when elements are valid
e or Ecc Eccentricity Elongation of ellipse: 0.0 = circle, 1.0 = parabola, .gt. 1.0 = hyperbola (pure number)
q PericenterDistanceClosest approach to Sun (in AU)
T or Tp Epoch (1,2) Time of PeriCenter passage (Julian date)
Node or W or ΩAscendingNode Where the Orbit crosses the Ecliptic (longitude, in degrees)
Peri or w or ω ArgOfPericenterangle from "Node" to Pericenter (in degrees)
Incl or i InclinationTilt of orbit from Ecliptic (in degrees)
(not provided) Period calculated orbital period (in Earth years), using either
P = (q/(1-e))^1.5 for a closed, elliptical orbit
P = (q/(e-1))^1.5 for an open, hyperbolic orbit
Unfortunately, P is undefined for a parabolic orbit.
____
Notes
1. By convention, cometary elements don't include a mean anomaly value, so the Epoch you put into the SSC's EllipticalOrbit must be the time of the pericenter passage, when the mean anomaly is, by definition, 0.0.
2. You can translate conventional Gregorian dates into astronomical Julian dates with any of a number of conversion utilities. For example, use the calculator provided by Bill Jeffery of the University of Texas at http://quasar.as.utexas.edu/BillInfo/JulianDateCalc.html

### 1.2: IAU Comet Circular information

As an example, here are orbital elements for Comet Hale-Bopp. See
```C/1995 O1 (Hale-Bopp)
Epoch 1997 Mar. 13.0 TT = JDT 2450520.5
T 1997 Apr. 1.1373 TT                                   Nakano
q   0.914142             (2000.0)            P               Q
z  +0.005395       Peri.  130.5887      -0.1331204      -0.1703043
Node   282.4707      +0.2822928      +0.9378055
e   0.995068       Incl.   89.4300      +0.9500472      -0.3025183

From 3000 observations 1993 Apr. 27-2001 Feb. 17, mean residual 0".8.
Nongravitational parameters A1 = +1.27, A2 = +0.1144.
```

### 1.3: Celestia SSC Format

http://members.fortunecity.com/guilpain/Fichiers%20ssc_uk.htm

Note that Celestia does not properly handle object names which include a "/". That character is used by Celestia as a separator between object names. As a result, a name which contains a "/" cannot be defined to have a satellite, nor can it be selected by typing a `[carriage-return]` followed by the name of the object. Otherwise such a name is OK: the name can be selected using Celestia's "Solar System Browser", for example.

Celestia ignores anything starting with a #, so everything on a line after a # is a comment meant only for your information.

```
#C/1995 01 Hale-Bopp (CfA)
# "name of object"  "name of primary"
"Hale-Bopp"       "Sol"
{
Class   "comet"
Texture "asteroid.jpg"
Mesh    "asteroid.cms"
RotationPeriod 11.34  # arbitrary

# taken from
# http://cfa-www.harvard.edu/iau/Ephemerides/Comets/1995O1.html
EllipticalOrbit
{
Period             2523.363 #calculated: (q/(1-e))**1.5
PericenterDistance    0.914142  # q
Eccentricity          0.995068  # e
Inclination          89.4300    # Incl.
AscendingNode       282.4707    # Node
ArgOfPericenter     130.5887    # Peri.
MeanAnomaly           0.0       # position at T
Epoch           2450539.6373    # T
}

InfoURL "http://www.jpl.nasa.gov/comet/"
}
```

## 2.0: Asteroidal Orbital Elements

### Contents

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