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 "target plane" which is the plane orthogonal to the trajectory of the object, containing both the point of maximum approach of the asteroid and the position of Earth.
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 virtual impactor is found, is a "follow-up" meaning that the position of the asteroid is measured again after some time has passed. In this way, the position is better determined and the region of uncertainty associated to the asteroid becomes smaller. After a second observation, the ellipse on the target plane is surely smaller. For this reason if the impact is still possible (the planet is inside the ellipse) the probability of the event will have grown up. If after a third observation the planet falls out of the region, the probability will fall to 0.