What is an orbit?

To understand what
exactly is an* orbit*, a basic physical concept is needed:
any two bodies in the universe are attracted by each other by a
force called the *Newton force of attraction* which
depends on the distance between the two bodies and on their
masses (click
here to know more about this force).
The existence of this force is the only reason why planets,
moons, asteroids and comets do not run away from the solar system
and instead follow precise paths around the Sun.

In fact, the
role of this force can be easily understood if we consider
a simple system made of two bodies, such for example as
the Earth and a celestial body - an asteroid- that, at
first, approaches the Earth with a simple, straight
trajectory. When the smaller body comes near the Earth,
it is attracted by the gravitational force of the planet.
It begins then to follow a different trajectory from the
initial straight one. This trajectory, called orbit, has
a shape that depends on the physical conditions, such as
the masses, the initial speed, the initial trajectory,
etc (in the animation on the right a parabolic orbit has
been represented). All orbits can be described mathematically by some simple physical characteristics, called orbital
elements. |

The determination of the orbit in the
precise conditions described in the above example is a simple
problem of dynamics, called the 2-body
problem. If more than two bodies are involved, the problem,
named n-body problem is much, much
more complicated.