A “natural body” catalog file defines trajectory, orientation, appearance, name label, and trajectory plot parameters for a natural body. Natural body catalog files can be created to define new bodies. It is possible but usually not necessary to redefine existing bodies because the majority of Solar system bodies in the SPICE-enhanced version of Cosmographia already use SPICE trajectory and orientation data. This is an example of a SPICE-based natural body catalog file for Saturn, just to illustrate how it is done:
"name": "Cosmographia CASSINI Example",
"mass": "95.152 Mearth",
"duration": "10759.22 d",
The version and name attributes set the catalog format/content version (must be “1.0”) and the internal name of the catalog file.
The class attribute sets the object class to “planet”. Other applicable class values are “satellite”, “asteroid”, “dwarf planet”, “comet”, “reference point”, and “other”.
The name attribute sets the name that will be used to identify the body object within Cosmographia.
The mass and density attributes set the body’s mass and density values that will be shown in the Cosmographia Object Properties panel.
The startTime and endTime attributes, omitted in this example, can be used to set the body’s “lifetime” span within Cosmographia. Usually these times are not needed for natural bodies and can be left out. In some cases though, for bodies for which SPICE data provides limited coverage, they can be used and set the start and stop times of the period for which the body ephemeris data are available in the SPICE kernels listed in the catalog file defining the mission’s SPICE data.
The center attribute contains the Cosmographia name for the center of the body’s trajectory plot. This parameter will usually be set to “Sun” for planets, asteroids, and comets or the Cosmographia name of the planet for satellites.
The trajectory property defines how Cosmographia computes the body’s trajectory. The example above tells the program to compute the trajectory of Saturn relative to the Sun using SPICE. In addition to SPICE-based trajectories, Cosmographia supports a few other types of trajectories described in the Trajectory Types section of this User’s Guide.
The bodyFrame property defines how Cosmographia computes the orientation of the body’s body-fixed frame. This can be done in two ways – in one step, using just the bodyFrame property (more common, as is done using SPICE data to compute orientation of the IAU_SATURN frame in the example above), or in two steps, using a combination of the bodyFrame and rotationModel properties. The one-step way defines the orientation of the body-fixed frame with respect to the ICRF frame using one of the supported bodyFrame types described in the Body Frame/Intermediate Frame Types section of this User’s Guide. The two-step way uses the rotationModel property in addition to the bodyFrame property to define the orientation of the body-fixed frame – first step rotates from the ICRF frame to an intermediate frame as specified by the bodyFrame property, and second step rotates from the intermediate frame to the body-fixed frame as specified by the rotationModel property, using one of the supported rotationModel types described in the Rotation Model Types section of this User’s Guide. When the two-step way is used, the property name bodyFrame should be changed to the property name intermediateFrame in the catalog files, solely for clarifying purposes.
The geometry property defines the shape that will be used to visualize the body. The example above tells the program to model Saturn as an ellipsoid (“type”: “Globe”) with radii set by the radii attribute and a base map image provided in the file specified by the baseMap attribute. More details about Globe and other geometry types are provided in the Geometry Types section of this User’s Guide.
The label property defines the appearance of the body’s name label shown on the screen. The example above tells the program to use a certain RGB color set by the color attribute. The label property also supports a few additional attributes shown in the Spacecraft Catalog section.
The trajectoryPlot property defines the appearance of the spacecraft trajectory line shown on the screen. The example above tells the program to use a certain RGB color set by the color attribute and to plot the trajectory over the time period given by the duration attribute. The trajectoryPlot property also supports a few additional attributes shown in the Spacecraft Catalog section.
A more detailed description of some of the parameters in this catalog file is provided in the “CATALOG FILE DEFINING A NATURAL BODY” section of the Cosmographia-SPICE User’s Guide.