In case a lost asteroid is Earth-crossing, there might be one (or more) of the virtual asteroids associated with it for which an actual collision is possible. Then there is a small nearby region filled with collision orbits still compatible with the observations. We call a virtual asteroid with an orbit in this region a Virtual Impactor (VI).
In practice, let us suppose that some time after the discovery apparition the orbit undergoes a close approach to the Earth; the Modified Target Plane (MTP) for that encounter is the plane perpendicular to the geocentric velocity at closest approach [Milani and Valsecchi 1999]. For some initial condition in the confidence region in element space there is a point on the MTP representing the close approach of the corresponding orbit. If some of the points on the MTP are inside the cross section of the Earth, these correspond to VIs.
For this paper, we have modified the procedure to find potential collisions described recently in [Milani et al. 1999, Sec. 3]. Here we find VIs in the following way:
In the neighborhood of a VI there is a small region of the element space containing other collision solutions; this we call a Virtual Impactor Region (VIR). To exclude the possibility of an impact (at a given encounter) we need not only to perform negative observations of the VI, but of all the orbits belonging to the VIR.