  Short-arc orbit determination is crucial when an asteroid is first
discovered. In these cases usually the observations are so few that the
differential correction procedure may not converge. We have developed an
initial orbit computation method, based on the systematic ranging, an orbit
determination techniques which systematically explores a raster in the
topocentric range and range-rate space region inside the admissible region. We
obtain a fully rigorous computation of the probability for the asteroid that
could impact the Earth within few days from the discovery, without any a priori
assumption. We test our method on the two past impactors 2008 TC3 and 2014 AA,
on some very well known cases, and on two particular objects observed by the
ESA Gaia mission.
