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Concept# Parabolic trajectory

Summary

In astrodynamics or celestial mechanics a parabolic trajectory is a Kepler orbit with the eccentricity equal to 1 and is an unbound orbit that is exactly on the border between elliptical and hyperbolic. When moving away from the source it is called an escape orbit, otherwise a capture orbit. It is also sometimes referred to as a C3 = 0 orbit (see Characteristic energy).
Under standard assumptions a body traveling along an escape orbit will coast along a parabolic trajectory to infinity, with velocity relative to the central body tending to zero, and therefore will never return. Parabolic trajectories are minimum-energy escape trajectories, separating positive-energy hyperbolic trajectories from negative-energy elliptic orbits.
The orbital velocity () of a body travelling along parabolic trajectory can be computed as:
where:
is the radial distance of orbiting body from central body,
is the standard gravitational parameter.
At any position the orbiting body has the escape velocity for that position.
If a body has an escape velocity with respect to the Earth, this is not enough to escape the Solar System, so near the Earth the orbit resembles a parabola, but further away it bends into an elliptical orbit around the Sun.
This velocity () is closely related to the orbital velocity of a body in a circular orbit of the radius equal to the radial position of orbiting body on the parabolic trajectory:
where:
is orbital velocity of a body in circular orbit.
For a body moving along this kind of trajectory an orbital equation becomes:
where:
is radial distance of orbiting body from central body,
is specific angular momentum of the orbiting body,
is a true anomaly of the orbiting body,
is the standard gravitational parameter.
Under standard assumptions, the specific orbital energy () of a parabolic trajectory is zero, so the orbital energy conservation equation for this trajectory takes the form:
where:
is orbital velocity of orbiting body,
is radial distance of orbiting body from central body,
is the standard gravitational parameter.

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Hyperbolic trajectory

In astrodynamics or celestial mechanics, a hyperbolic trajectory or hyperbolic orbit is the trajectory of any object around a central body with more than enough speed to escape the central object's gravitational pull. The name derives from the fact that according to Newtonian theory such an orbit has the shape of a hyperbola. In more technical terms this can be expressed by the condition that the orbital eccentricity is greater than one.

Elliptic orbit

In astrodynamics or celestial mechanics, an elliptic orbit or elliptical orbit is a Kepler orbit with an eccentricity of less than 1; this includes the special case of a circular orbit, with eccentricity equal to 0. In a stricter sense, it is a Kepler orbit with the eccentricity greater than 0 and less than 1 (thus excluding the circular orbit). In a wider sense, it is a Kepler orbit with negative energy. This includes the radial elliptic orbit, with eccentricity equal to 1.

Specific orbital energy

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