What is a chaotic orbit? |
version |

When talking about the orbits of asteroids and comets, a very
important concept must be introduced: the concept of chaotic
orbits. In fact, there are many physical mechanisms (click here to know more about these mechanisms)
that act on these bodies, perturbing their initial orbits and
making them become NEOs
(dict.).

Stable and unstable positions

It is much easier to understand the nature of chaotic orbits making a parallelism with stable and unstable positions in the motion of a pendulum. For a pendulum, this concept is very intuitive: a position of stable equilibrium corresponds to a position the body will come back to when slightly perturbed. For unstable equilibrium, a little deviation from this initial position is sufficient to make the body leave its initial condition. |

Forcing this parallelism we can say that steady orbits will be highly predictable orbits (just like the position of stable equilibrium is for the pendulum) while unsteady orbits will depend abruptly from the initial conditions (as for the pendulum, starting from a position of unstable equilibrium, the final position depends strongly on the perturbation).

Chaotic orbits and the Liapounov time

It is important to understand that a chaotic orbit is
unpredictable for its nature and therefore, the position of the
object can't be perfectly determined in the future. In other
words, a chaotic orbit is so sensible to very small changes in
the initial conditions (or in the forces that act upon the body)
that the prevision of the trajectory over long periods of time is
impossible.

This because different orbits are divergent: a very little
difference in the initial conditions leads to totally different
orbits in the future. Even if this explanation is very intuitive,
it is important to understand that chaos is a mathematical
concept, that can be formalized and measured by a parameter
called the Lyapounov time
(dict.).

NEOs' chaotic orbits: an example

The most classical example of chaotic orbits is the
trajectory of a light body such as an asteroid or a
comet. In fact, there are many physical mechanisms (such
as the flyby of a planet, or a resonance phenomena) that
act on asteroids and comets, perturbing their initial
orbits and making them become NEOs (click
here to know more about these mechanisms). An example of these mechanisms, is the close approach of a planet. As shown in the picture, when the asteroid passes near a planet, even for very similar initial conditions, the orbits diverge. |