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.