Package org.orekit.bodies

This package provides interface to represent the position and geometry of space objects such as stars, planets or asteroids.

The position of celestial bodies is represented by the CelestialBody interface. This interface provides the methods needed to either consider the body as an external one for its gravity or lighting influence on spacecraft (typically in perturbing force computation) or as an internal one with its own frame.

The CelestialBodyFactory class is a factory providing several predefined instances implementing the CelestialBody interface for the main solar system bodies. The Sun, the Moon, the eight planets and the Pluto dwarf planet are the supported bodies. In addition to these real bodies, two points are supported for convenience as if they were real bodies: the solar system barycenter and the Earth-Moon barycenter. The CelestialBodyFactory factory relies on the JPL DE 405, 406 or similar binary ephemerides files to compute all positions and velocities. Note that the binary files are used, not the ASCII ones, regardless of the processor endianness.

As an example, computing the position of the Sun and the Moon in the EME2000 frame, this done as follows: 

  CelestialBody sun      = CelestialBodyFactory.getSun();
  CelestialBody moon     = CelestialBodyFactory.getMoon();
  Vector3D sunInEME2000  = sun.getPosition(date, Frame.getEME2000());
  Vector3D moonInEME2000 = moon.getPosition(date, Frame.getEME2000());

Since the supported bodies implement the CelestialBody interface, they all provide their own body-centered inertial frame, hence adding a few more frames to the ones provided by the org.orekit.frames package. Since the frames tree is rooted at an Earth-centered frame, the solar system bodies frames tree does not seems in canonical shape. This of course is only a side effect of the arbitrary choice of GCRF as the root frame and has no effect at all on computations.

The shape of celestial bodies is represented by the BodyShape interface.

Only one implementation is provided by OREKIT for now: the OneAxisEllipsoid class which represents the natural flattened shape of big space rotating bodies like planets or the Sun.

For asteroids, it is expected that users provide their own shape models, for example based on triangulation. They should implement the BodyShape interface in order to be used by Orekit.

When using OneAxisEllipsoid body representation, points are generally described in associated body frame, by so-called geodetic coordinates (longitude, latitude, altitude). The GeodeticPoint class allows handling of such coordinates. It is a simple container that does not provide processing methods.

Author:
L. Maisonobe