  Summary: In the past decade both scientists and laymen have probably heard at
least once through the newspapers, TV and Internet that a new asteroid has been
discovered with non-zero (sometimes "high") probability of collision with the
Earth in the near future. Since early 2000's, such probabilities 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 additional 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 impact 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 probability of a newly discovered
asteroid is not that given by the monitoring systems.
