A Novel Approach to Asteroid Impact Monitoring

Orbit-determination programs find the orbit solution that best fits a set of observations by minimizing the root mean square of the residuals of the fit. For near-Earth asteroids, the uncertainty of the orbit solution may be compatible with trajectories that impact Earth. This paper shows how incorporating the impact condition as an observation in the orbit-determination process results in a robust technique for finding the so-called virtual impactors, i.e., the regions in parameter space leading to impacts. The impact pseudo-observation residuals are the b-plane coordinates at the time of close approach and the uncertainty is set to a fraction of the Earth radius. The extended orbit-determination filter converges naturally to an impacting solution if allowed by the observations. The uncertainty of the resulting orbit provides an excellent geometric representation of the virtual impactor. As a result, the impact probability can be efficiently estimated by exploring this region in parameter space using importance sampling. The proposed technique can systematically handle a large number of estimated parameters, account for nongravitational forces, deal with nonlinearities, and correct for non-Gaussian initial uncertainty distributions. The algorithm has been implemented into a new impact-monitoring system at JPL called Sentry-II, after undergoing extensive testing. The main advantages of Sentry-II over the previous Sentry system are that Sentry-II can systematically process orbits perturbed by nongravitational forces and that it is generally more robust when dealing with pathological cases. The run times and completeness of both systems are comparable, with the impact probability of Sentry-II for 99% completeness being 3 × 10−7.

From Jupiter-family comets to objects in Encke-like orbits

Numerical integrations of the orbital evolution of objects in orbits resembling the peculiar cometary orbit of 2P/Encke have allowed the identification of the dynamical paths connecting these orbits to the possible source regions and to the possible end states. A certain degree of concentration of the longitudes of perihelion of objects in Encke-like orbits has led to the idea that 2P/Encke and the asteroidal objects in nearby orbits all come from the recent fragmentation of a giant progenitor. However, the fact that the majority of Earth-crossing asteroids are discovered when passing near the Earth induces a correlation between the date of discovery and a range of possible longitudes of perihelion. By combining this correlation with the variable rate of asteroid discoveries over the year, it is possible to show that the discovery of asteroids on orbits of given semimajor axis and eccentricity is favoured if their longitudes of perihelion fall within certain intervals. A question still open is the efficiency of the dynamical path linking the Jupiter family to Encke-like orbits; nongravitational forces seem to play an important rôle in this respect.