Description of an orbit: the orbital elements
To know the state of
motion of a body, it would be necessary to know its position and
its speed. Furthermore, the values of these six parameters (the three components of the
position and the three components of the speed) should be known
at every instant. But, during a general motion, these values
change constantly and knowing them at every moment is impossible.
In the reality, constant, stable
orbits (as in the 2body problem), can be described by 6 constant
parameters, called orbital elements. In other words, to
know the state of motion of the body, it is not necessary to know
the position and the speed of the object at every instant. The
knowledge of the numerical values of the 6 constant parameters
that characterize the orbit is sufficient.
2 dimensional orbit
Let's first consider the orbit solution of
a 2body problem: the orbit lies
completely on a plane, called orbital plane. If the
orbit is elliptic (click here
to know more about ellipse ) and periodic, only two of the 6
orbital parameters are needed to completely define the orbit:
The orbit is totally defined by two constant parameters:

For what concerns the other conics, these two parameters
have different definitions: in the case of a parabola, M,
the major semiaxis is considered infinite, while for an
hyperbola, it is considered negative. For what concerns the eccentricity,
in the case of an ellipse, it will assume values ranging from 0
(for the circle) to 1. In the case of a parabola, it will assume
value =1 and will be superior to 1 in the case of an hyperbola.
The values of eccentricity and the major
semiaxis, together, define completely the shape and the other
characteristics of the orbit. In fact, the value of the major
axis is inversely proportional and of the opposite sign of the binding energy of the
orbit. This means that a very little semiaxis corresponds to a
strongly negative energy and therefore to a very bound orbit.
On the other hand, eccentricity is a clear indicator sign of the
shape of the orbit giving, for example in the case of the
ellipse, an information on how much the orbit is crushed.
If this planar orbit is considered in a
3dimensioal volume, a third parameter is needed to indicate the
inclination of the orbital plane.
3 dimensional orbit: definition of the orbital parameters
If instead of a planar
orbit, we consider a tridimensional orbit (for example, the orbit
of an asteroid not considered on its plane) 6 parameters will be
needed to define completely the orbit. These parameters are
called the orbital elements.
On the following
drawing, the coordinates axes are chosen to have a reference
plane xy that is different from the orbital plane.
In the
drawing, the orbital plane is the plane where
the orbit lies. The intersection between the orbital
plane and the reference plane is called nodal
line. This nodal line passes through the ascending
and descending node. The first 3 orbital elements needed to define the kind, the shape and the inclination of the orbit are:  a, the major semiaxis. If the orbit is elliptical the definition corresponds to the planar case, while the case of parabola corresponds to an infinite value of a;  e, the eccentricity (as defined in the planar case);  i the inclination of the orbital plane, the angle it forms with the plane xy. Other three parameters are needed to describe the orientation of the orbit on the orbital plane:  argument of the perihelion  nodal longitude  T period of the perihelion 

How to confront the orbit of an asteroid with the orbit of the Earth
When considering the chance of an impact between two bodies, it is very important to compare the two orbits. For example, let's examine the orbit of an asteroid in comparison with the orbit of the Earth. To do so, it is possible to choose the orbital plane of the Earth as the plane xy. In this case we will talk of mutual nodes. 