Predicting Close Earth Approaches of Asteroids and Comets

The motions of all known Earth approaching asteroids and comets with reasonably secure orbits have been numerically integrated forward in time to A.D. 2200. Special care was taken to use the best available initial conditions including orbits based upon radar data. Each object was integrated forward with Earth and moon perturbations treated separately, with general relativistic equations of motion and with perturbations by all planets at each integration step. For the active short-period comets whose motions are affected by the rocket-like effects of vaporizing ices, a nongravitational force model was employed. When a close approach to the Earth was sensed by the numerical integration software, an interpolation procedure was used to determine the time of the object’s closest approach and the minimum separation distance at that time. For those objects making the closest Earth approaches in the next two centuries, an error analysis was conducted to determine whether or not the object’s error ellipsoid at the time of closest approach included the Earth’s position (i.e., an Earth collision could not be ruled out). Although there are no obvious cases where a known near-Earth asteroid or comet will threaten the Earth in the next two centuries, there area few objects that warrant special attention, The Aten type asteroid 2340 Hathor makes repeated close Earth approaches and because most of its orbit lies within that of the Earth, it is often a difficult object to observe in a dark sky. For both asteroids and comets, there are generally dramatic increases in their position uncertainties following close planetary encounters. Because of their short observational data intervals, their unmodeled nongravitational effects, and the possibility of escaping early detection by approaching the Earth from the sun’s direction, long-period comets may present the largest unknown in assessing the long-term risk of Earth approaching objects. Fortunately the frequency with which these objects approach the Earth is very small compared with the numerous approaches by the population of near-Earth objects with short periodic orbits. For the short-period comets, rocket-like outgassing effects and offsets between the observed center-of-light and the comet’s true center-of-mass can introduce large uncertainties in their long-term orbital extrapolations. The uncertainty in the future motion of an active short-period comet is substantially larger than the motion of an asteroid with a comparable observational history. While asteroids dominate the list of close Earth approaches in the next two centuries, their motions are relatively predictable when compared to the active comets. For the rapidly growing population of known near-Earth asteroids and comets, efficient procedures are suggested for monitoring their long-term motions thus allowing early predictions of future close Earth approaches.