Celestia/Tutorials/Comets

This tutorial assumes you have read the previous page: Simple Asteroids. Once you do, you should have a good idea on how to create comet SSCs for Celestia, as they are similar to asteroids.

List of parameters

In Celestia, comets are treated very similarly to asteroids. An SSC should look very similar in its format to that of an asteroid, but with several key changes:

Name

"Name" "Sol"

Here, Name refers to the name(s) of the comet. However, comet naming works differently from asteroid naming. A full name might look something like this: "81P Wild", where 81P refers to the number of the comet plus the letter "P" indicating that it is a periodic comet. "Wild" refers to the comet's discoverer, Paul Wild. The name of the comet should technically be "81P/Wild" with a slash, but slashes inside names do not work in Celestia for technical reasons. So in Celestia, it might be given a name like this:

"81P Wild:Wild 2" "Sol"

The "2" after "Wild" is necessary because it is the second comet discovered by Wild.

For non-periodic comets (comets that do not return), the nomenclature may look something like this:

"C1995 O1 (Hale-Bopp):Hale-Bopp" "Sol"

A few objects have been catalogued as asteroids and were then discovered to show cometary activity. For these, it's common to include the asteroid names and the comet designation, like below:

"2060 Chiron:Chiron:1977 UB:95P Chiron" "Sol"

Class

	Class "comet"

This line simply tells Celestia that the object is a comet. Objects defined as comets in Celestia show a tail when they are sufficiently close to the Sun. It is not possible to define an object with more than one type of object, so dual objects like 2060 Chiron tend to be defined as asteroids.

Radius

	Radius <number>

As with asteroids, this parameter simply refers to the radius of the asteroid, in kilometers. If you're lucky, JPL's Small Body Database will have a diameter that you can divide by two to get the radius. However, many will not. In that case for comets, the next best thing to use is the total nuclear magnitude (M2), for which a published value should exist for many (but not all) comets.

The total nuclear magnitude is essentially a value of how intrinsically bright the nucleus of the comet is, with a lower value meaning a brighter asteroid. Using this and the albedo value (explained in detail later), it is possible to estimate the size of the comet's nucleus.

Unfortunately, most comets do not have known albedos either. So, it is customary to assume an albedo of about 0.04 for comet nuclei, as they are known to be quite dark. To calculate the radius in kilometers, use this formula:

$R={\frac {664.5}{\sqrt {p}}}10^{-0.2M_{2}}$

where $p$ stands for the albedo and $M_{2}$ stands for the absolute magnitude.

Albedo

	Albedo <number>

The albedo of an asteroid is a value from 0 to 1 that refers to what fraction of light gets reflected off the asteroid, with 0 being a complete dark object and 1 reflecting all of the light that hits it.

If you cannot find a published albedo value, it is customary to assume an albedo for 0.04 for comets.

Example code

For the sake of reference, here is what some .ssc code might look like for 81P/Wild:

"81P Wild:Wild 2" "Sol"
{
	Class "comet"
	Mesh "asteroid.cms"
	Texture	"asteroid.jpg"
	Radius 2.0

	EllipticalOrbit
	{
	Epoch	        2455809.5
	Period          6.41340443513
	SemiMajorAxis	3.45186352633
	Eccentricity	0.53707073006
	Inclination	3.23724754968
	AscendingNode	136.097685541
	ArgOfPericenter	41.7562320856
	MeanAnomaly	85.9757611643
	}

	Albedo 0.04 # guess
	RotationPeriod 8 # guess
}