Earth impact threat of newly discovered asteroids: an a posteriori analysis

In the past decade both scientists and laymen have probably heard at least once through the newspapers, TV and Internet that a new aster-oid has been discovered with non-zero (sometimes “high”) probability of collision with the Earth in the near future. Since early 2000’s, such prob-abilities are routinely calculated by two impact monitoring systems (one in the US, the other in Europe) on preliminary orbits of newly discovered Near Earth Asteroids (NEAs), and are regularly updated whenever addi-tional new astrometric observations for each threatening object become available. A typical pattern is that as the orbit becomes more precisely determined, impact probability often increases initially, but then turns around and decreases until it falls to zero, or some very low number. In the present study we define a probability measure which provides a simple tool to evaluate from the very beginning the chance that the im-pact probability calculated by monitoring systems for a threatening object will reach unity at the end of the overall process of orbit refinement. We stress that this chance is independent of the specific (fluctuating) value of the probability calculated by the monitoring systems. In a precise sense, which should be clear throughout the paper, the concrete impact proba-bility of a newly discovered asteroid is not that given by the monitoring systems