Probability of impact - target plane - virtual impactor

Once the region of uncertainty of a NEO has been determined
and its shape and position have been calculated over a long
enough period of time, it is possible to evaluate the probability
of impact with a planet -for example Earth-. If one of the
virtual asteroids belonging to the region of uncertainty has a
chance to hit Earth, it will be called** virtual impactor.
**To evaluate the probability of impact, we first have to
define the concept of "

If we consider the intersection of the region of uncertainty with this plane, we will have a plane figure (par example an ellipse). In this case, if the planet lies inside this figure, the impact is possible and its probability can be estimated as the ratio of the areas of the ellipse and the planet.

What normally happens when a