Central Body Components

Gravity | Parents & Children | Shape | Attitude | Ephemeris

The Component Browser allows you to examine the gravitational, atmospheric, ephemeris, and other properties of the planets, the Sun, the Moon, and other central bodies. In addition, you can create a new central body, such as a comet, asteroid, or moon, by duplicating Ceres or another editable central body and redefining one or more of its parameters.

To view and/or edit the properties of a central body, double-click it in the right pane of the Component Browser. This brings up the parameters window for the selected central body.

Gravity

The Central Body parameters window allows you to specify the gravitational parameter () and to define and select among gravity models. The value entered in the Gravitational Parameter field is used in point mass force models and propagators and in element set conversions. Enter a value in the selected distance unit cubed per the selected time unit squared (e.g. km^3/sec^2).

You can add new gravity models by clicking the ellipsis button to the right of the Gravity Models field, bringing up the Gravity Model Selection window, which is a multi component select window. To edit the features of an existing or newly created model, select it from the list and click Analytic Details.... The Gravity Model parameters window appears, offering the following choices:

Field	Description
Gravitational Parameter	Enter the gravitational parameter to be used for purposes of this gravity model (e.g. for inclusion in a full force model) in the selected distance unit cubed per selected time unit squared (e.g. km^3/sec^2).
Reference Distance	The distance from the center of mass of the central body to its surface, i.e., approximately the radius of the central body. Typically defaults to the Maximum Radius entered in the Shape frame of the Central Body parameters window.
Zonal - J2	Taking into account first order Earth oblateness effects.
Zonal - J3	Taking into account first order longitudinal variations of the Earth's shape.
Zonal - J4	Taking into account first and second order Earth oblateness effects.

Registered gravity models provided by Astrogator for non-editable central bodies include the following:

Central Body	Model	Description
Earth	WGS84 EGM96	This model uses the coefficients from EGM96 with the shape model of WGS84, of which the defining parameters are GM = 3.986004418E+14, and reference distance (equatorial radius of the Earth) = 6,378,137.0 m. This model is the recommended gravity model of the WGS84 definition document: NIMA TR8350.2, Third Edition, 4 July 1997.
	EGM2008	Earth Gravity Model 2008, an updated version of the EGM96 model.
	EGM96	Earth Gravity Model 1996, a geopotential model of the Earth consisting of spherical harmonic coefficients complete to degree and order 360. Developed jointly by NGA (formerly known as NIMA), NASA Goddard and Ohio State University.
	GEMT1	Goddard Earth Model T1.
	GGM01C	Grace Gravity Model 01C. Derived from Grace data combined with TEG4 information.
	GGM02C	Grace Gravity Model 02C. Derived from Grace data combined with TEG4 information.
	JGM2	Joint Gravity Model version 2, a model that describes the Earth gravity field up to degree and order 70, developed by NASA/GSFC Space Geodesy Branch, the University of Texas Center for Space Research and CNES.
	JGM3	Joint Gravity Model version 3, a model that describes the Earth gravity field up to degree and order 70, developed by the University of Texas and NASA/GSFC.
	WGS84	World Geodetic System 1984, an Earth-fixed reference frame, including an earth model, with primary parameters defining the shape of an Earth ellipsoid, its angular velocity and the earth mass that is included in the ellipsoid reference, and secondary parameters defining a detailed gravity model of the Earth. WGS 84 was created by the Defense Mapping Agency (DMA).
	WGS84 old	Old version of WGS84.
Moon	LP150Q	Lunar Prospector Lunar gravity model. GM = 4.902801076e+012, reference distance = 1,738,000.0 m. The Lunar Prospector Lunar gravity model originates from the Spherical Harmonics Gravity ASCII Data Record (SHGADR) and is produced by the Lunar Prospector Gravity Science Team at JPL under the direction of A.S. Konopliv.
	GLGM2	GM = 4.9028029535968e+12, reference distance = 1,738,000 m.
	LP100J	Lunar Prospector Lunar gravity model. GM = 4.902800476e+012, reference distance = 1,738,000.0 m.
	LP100K	Lunar Prospector Lunar gravity model. GM = 4.902800238e+012, reference distance = 1,738,000.0 m.
	LP165P	Lunar Prospector Lunar gravity model. GM = 4.902801056E+12, reference distance = 1,738,000.0 m.
	LP75D	Lunar Prospector Lunar gravity model. GM = 4.902801374e+012, reference distance = 1,738,000.0 m.
	LP75G	Lunar Prospector Lunar gravity model. GM = 4.902800269e+012, reference distance = 1,738,000.0 m.
Mercury	Icarus1987	GM = 2.203209e+013, reference distance = 2,439,000 m. Anderson, J. J., Colombo, G., Esposito, P. B., Lau E. L., and Trager, G. B. "The Mass, Gravity Field, and Ephemeris of Mercury", Icarus 71, 337-349, 1987.
Venus	MGNP180U	GM = 3.248585920790000E+14, reference distance = 6,051,000.0 m.
Mars	GMM1	GM = 4.28283579647735e+13, reference distance = 3,394,200.0 m.
	GMM2B	GM = 4.28283719012840e+13, reference distance = 3,397,000 m. These numbers came from the GMM-2B model published at http://bowie.gsfc.nasa.gov/926/MARS/GMM2B.html and submitted to Journal of Geophysics Research, November 2000.
	Mars50c	GM = 4.2828370371000e+13, reference distance = 3,394,200 m.
Jupiter	JUP230	GM = 1.26686535e+017, reference distance = 71,492,000 m. Jacobson, R. A. The JUP230 orbit solution, 2003. There is no ephemeris prior to 2000 for Jupiter and its moons.
	jup230Spice	SPICE gravity coefficients.
Saturn	Astron2004	GM = 3.7931284e+016, reference distance = 60,330,000 m. Jacobson, R. A., "The orbits of the major Saturnian satellites and the gravity field of Saturn from spacecraft and Earthbased observations", submitted to Astronomical Journal, 2004.
Uranus	ura083Spice	SPICE gravity coefficients.
Neptune	AstronAstro1991	GM = 6.835107e+015, reference distance = 25,225,000 m. Jacobson, R. A., Riedel, J. E. and Taylor, A. H. "The orbits of Triton and Nereid from spacecraft and Earthbased observations," Astronomy and Astrophysics 247, 565-575, 1991.
	nep016Spice	SPICE gravity coefficients.
Pluto	plu017Spice	SPICE gravity coefficients.
Callisto	Icarus2001	GM = 7.179292e+12, reference distance = 2,410,300 m. Anderson, J. D., Jacobson, R. A., McElrath T. P., Moore, W. B., Schubert, G., and Thomas, P. C., "Shape, Mean Radius, Gravity Field, and Interior Structure of Callisto," Icarus 153, 157-161, 2001.
Europa	Science1998	GM =3.20272e+012, reference distance = 1,565,000 m. Anderson, J. D., Schubert, G., Jacobson, R. A., Lau, E. L., Moore, W. B., and Sjogren, W. L., "Europa's Differentiated Internal Structure: Inferences from Four Galileo Encounters," Science 281, 2019-2022, 1998.
Ganymede	Nature1996	GM = 9.8866e+12, reference distance = 2,634,000 m.
Io	JGeoRes2001	GM = 5.96e+12, reference distance = 1,821,600 m. Anderson, J. D., Jacobson, R. A., Lau, E. L., Moore, W. B., and Schubert, G., "Io's gravity field and interior structure," J. Geophysical Research 106, No. E12, 32963-32969, 2001.
Various	ZonalsToJ4 (Spherical Harmonics Gravity Field)	Gravity model for all central bodies except Sun, Earth, and Moon.

Parents & Children

The Parent field of the Central Body parameters window identifies the parent body of the central body under consideration. To substitute a different parent body, click the ellipsis (...) button to the right of the Parent field and select a new body. For example, if you were using a clone of the asteroid Ceres to model the Saturnian moon Titan, this is where you would assign Saturn the role of parent.

The Children field lists any celestial bodies for which this central body is the principal source of gravitational influence. Thus, for example, if the central body is a planet, this is where any of its moons for which a file exists will be listed.

You can, of course, create a new moon by duplicating an editable Moon central body, entering an appropriate name and user comment, and redefining one or more of its parameters.

The Children field cannot be edited; parent-child assignments are handled from the Parent field of the child body. However, you can bring up a list of the central body's children with more detailed information by clicking the ellipsis (...) button to the right of the Children field.

Shape

For an editable central body, the Shape frame of the Central Body parameters window allows you to select among three different basic shape types:

Shape	Parameters
Sphere	Enter the radius of the spherical central body in the selected distance unit.
Triaxial Ellipsoid	Enter the Semi-Major Axis, the Semi-Mid Axis, and the Semi-Minor Axis of the ellipsoidal central body in the selected distance unit.
Oblate Spheroid	Enter the Minimum and Maximum radii of the oblate spheroidal central body in the selected distance unit. When you click OK or Apply, the Flattening Coefficient appears in a read-only field. This coefficient is calculated by dividing the minor radius by the major radius and subtracting the quotient from 1:

To add a new shape profile, click the ellipsis (...) button to the right of the Shapes field and make the desired selection in the Shape Selection window that appears. After dismissing the Shape Selection window and returning to the Central Body parameters window, select the new profile in the Shape list and enter its parameters in the appropriate fields.

For non-editable central bodies, the shape information in the Central Body parameters window and in the Shape Selection window is read-only.

Attitude

To define attitude for an editable central body, click the ellipsis (...) button and select either an IAU 1994 attitude definition or a Rotation Coefficient file type.

Click Details to select a specific Rotation Coefficient file or to view the properties of an IAU 1994 definition, depending on which option you have selected.

You cannot edit the properties of an IAU 1994 definition using the Component Browser user interface. You must edit IAU 1994 definition files manually, outside of STK.

Ephemeris

Astrogator provides the following sources of ephemerides for a central body:

Source	Description
Analytic Orbit	Enter the values and rates of change for the classical orbital elements (see below).
Ephemeris File	Enter the path and filename of the external ephemeris (*.e) file in the Ephemeris File field that appears when you select this option, or click the ellipsis (...) button to the right of the Ephemeris File field to browse for a file. You can use a satellite to model a comet with Astrogator. Generate the satellite's ephemeris, create a data file and then use it as the ephemeris source for the central body.
JPL DE	Ephemerides from the Jet Propulsion Laboratory's JPL DE set are used.
JPL SPICE	The SPICE propagator reads ephemeris from binary files that are in a standard format produced by the Jet Propulsion Laboratory for ephemeris for celestial bodies but can be used for spacecraft.
Planetary Ephemeris	Enter the path and filename of the planetary ephemeris (*.pe) file in the Planetary File field that appears when you select this option, or click the ellipsis (...) button to the right of the Planetary File field to browse for a file.

To create one or more ephemeris profiles, click the ellipsis (...) button to the right of the Ephemeris field, and make appropriate selections in the Ephemeris Selection window that appears. Then, to edit the orbital elements of an analytical ephemeris profile, select it in the Ephemeris list and click Analytic Details.... An Ephemeris window appears, allowing you to specify an Epoch and the value and rate of change for the classical (Keplerian) orbital elements listed below. These data must be entered with respect to the body's parent, in the parent-inertial coordinate system. The following fields appear in the Value column of the Ephemeris window:

Orbital Element	Definition
Semimajor Axis	One-half the distance along the long axis of the elliptical orbit. Enter a value in the selected distance unit.
Eccentricity	The ratio of the distance between the two foci of the ellipse and its major axis. Dimensionless.
Inclination	The angle from the Z axis of the inertial coordinate system to the orbit angular velocity vector. Enter a value in the selected angle unit.
Right Ascension of Ascending Node	The angle from the X axis of the inertial coordinate system to the point where the orbit crosses the X-Y plane in the +Z direction. Enter a value in the selected angle unit.
Argument of Periapsis	The angle measured in direction of the body's orbital motion, and in the orbit plane, from the ascending node to the periapsis of the orbit. Enter a value in the selected angle unit.
Mean Longitude	Sum of the Right Ascension of the Ascending Node, the Argument of Periapsis, and the Mean Anomaly. Enter a value in the selected angle unit. (See note below concerning Mean Longitude Rate.)

Mean Anomaly is the angle measured from periapsis of a hypothetical body moving with a uniform speed that is equal to the Mean Motion, i.e. the uniform rate of a body in a circular orbit of the same period.

In the Rate column of the Ephemeris window, enter rate of change information in the appropriate unit for each orbital element per the selected time unit.

It is essential to set Mean Longitude Rate correctly, since it is used by the propagator rather than the gravitational parameter of the user-defined central body.

In addition to editing orbital elements, you can use the Ephemeris window to review the ephemeris of non-editable central bodies.

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