Editing Orbital speed (section)

==Radial trajectories==
In the following, it is assumed that the system is a two-body system and the orbiting object has a negligible mass compared to the larger (central) object. In real-world orbital mechanics, it is the system's barycenter, not the larger object, which is at the focus.

[[Specific orbital energy]], or total energy, is equal to ''E''<sub>k</sub>&nbsp;−&nbsp;''E''<sub>p</sub> (the difference between kinetic energy and potential energy). The sign of the result may be positive, zero, or negative and the sign tells us something about the type of orbit:<ref name="lissauer2019">{{Cite book |title=Fundamental Planetary Sciences: physics, chemistry, and habitability |last1=Lissauer |first1=Jack J. |last2=de Pater |first2=Imke |year=2019 |publisher=Cambridge University Press |isbn=9781108411981 |location=New York, NY, US |pages=29–31 }}</ref>
* If the [[specific orbital energy]] is positive the orbit is unbound, or open, and will follow a [[hyperbola]] with the larger body the [[focus (geometry)|focus]] of the hyperbola. Objects in open orbits do not return; once past periapsis their distance from the focus increases without bound. See [[radial hyperbolic trajectory]]
* If the total energy is zero, (''E''<sub>k</sub>&nbsp;=&nbsp;''E''<sub>p</sub>): the orbit is a [[parabola]] with [[focus (geometry)|focus]] at the other body. See [[radial parabolic trajectory]]. Parabolic orbits are also open.
* If the total energy is negative, {{nowrap|''E''<sub>k</sub> − ''E''<sub>p</sub> < 0}}: The orbit is bound, or closed. The motion will be on an [[ellipse]] with one [[focus (geometry)|focus]] at the other body. See [[radial elliptic trajectory]], [[free-fall time]]. Planets have bound orbits around the Sun.