How many NEOs are out there?

Two methods are used to calculate a
first statistic of the population of NEOs surrounding Earth:

- direct observations of NEOs;
- observations of craters on our planet and on the planets around us.

From these two methods it is possible to
obtain the following curve representing the population of NEOs:

Image courtesy of Andrea Carusi. |

This image represents today's best estimation of the number of NEOs surrounding Earth. The number of NEOs is represented as a function of the diameters of the objects (the two axes are in logarithmic scale). This estimation is far from being precise (the dashed lines represent the error of the estimation). The error is much bigger for small asteroids, and this is due to a bias observation. |

It is clear form this figure that the number of NEOs grows inversely from diameters: there are much more small NEOs than big ones. Furthermore, the dependence between the diameter and the number of NEOs is not linear: three different regions can be identified where the dependence is linear (in logarithmic scale). This could be explained saying that there are different production laws for objects of different dimensions: when they break up, the very big objects produce a much larger number of fragments than the smaller ones.

A first statistics can be deduced from the number of observed NEOs, which changes very fast with the progress of observational techniques. From direct observations, it is possible to measure the brightness and consequently the dimension of these objects. These observations are obviously easier for large NEOs, and so there is what is called an observational bias.

This is a representation of how the number of known
NEAs (Near Earth Asteroids) grows each year. Two
important things are to notice: first, the big growth of
this number in the last years is due to new technologies.
Second thing, the observational bias that leads to detect
more easily big objects, seems to have every year minor
effects. In fact, in the last year, large NEAs have
become a very smaller fraction of all discovered NEAs (in
2000 , 4/10 of all discovered NEAs while in 1980 almost
all known NEAs were large ones). Image courtesy NASA |

Observations of the surfaces of surrounding planets

A Moon's crater (NASA) |
Analyzing the craters' distribution on the planets' without atmosphere surfaces, it is possible to deduce the number of NEOs that have impacted the planets and therefore, the number of NEOs that are orbiting around them.. In fact the dimension of craters are proportional to the diameters of the impactors. The study and cataloging of craters is also a good method to date the surface, since the number of observable craters will be proportional to the time the surface has been exposed to bombardment. |

Normally this estimation is done on the
Moon's surface, being quite sure that the distribution of objects
falling on the Moon is similar to the one of Earth, being the two
objects near.

This estimation is done taking in
consideration some hypothesis:

- The Earth and the Moon have different masses and dimensions: the number of objects that have impacted the Moon is surely smaller than the number that have impacted Earth;
- the surface of moon has not been impacted uniformly, since some parts have been exposed to bombardment for longer periods;
- the number of impacts must practically be constant in
time. This hypothesis cannot be considered for sure, but
it seems very likely, over the last 3 billions of years.