Many small objects orbit the Sun coming very near Earth. These objects are named NEOs (Near Earth Objects), and are comets and asteroids that come near our planet. Scientists would say that NEOs have orbits characterized by a perihelion distance less than 1.3 AU.
A first classification of NEOs divides NEC (Near Earth Comets) from NEAs (Near Earth Asteroids) that constitute the vast majority of NEOs and are further divided into three main families, depending on the features of their orbits. In particular they are classified in three groups (Amor, Apollo and Atens) according to their perihelion and aphelion distances and their semi major axes.
IEOs (Inner Earth Objects) or asteroids with trajectories that lie entirely in the Earth orbit, represent a last class of objects. These asteroids can occasionally come very close to our planet, but are very difficult to detect from the ground, since the observing geometry places them at small angular distances from the Sun for most of the time, making them invisible against the bright sky background.
How can we study them?
NEOs are studied in many different ways. First of all, scientists have to discover new NEOs. The task isn’t an easy one, since these objects are small, fast-moving and star-like points in the night sky. Usually it is accomplished with imaging telescopes, identifying NEOs by their motion against the slower background stars.
As a second task, the orbits of these objects have to be tracked, in order not to loose them in the future and to know if they could be of some danger for Earth. To do this, many observations need to be combined in what is called a follow-up procedure and an archive of dangerous objects that need to be monitored in time is kept up-to-date.
Once the object has been tracked, its properties (such as dimension, shape, rotation and chemical composition) will be studied by photometric techniques, examining the spectrum reflected by the object. Instruments such as radars are also very useful to determine NEOs' orbits and to create detailed models of their tridimensional shapes.
Of course, going to space to study NEOs is also a possible, valuable solution.
Last update: 9 May 2012