An asteroid's orbit is computed by finding the elliptical path about the sun that best fits the available observations of the object. That is, the object's computed path about the sun is adjusted until the predictions of where the asteroid should have appeared in the sky at several observed times match the positions where the object was actually observed to be at those same times. As more and more observations are used to further improve an object's orbit, we become more and more confident in our knowledge of where the object will be in the future.
Because orbits stemming from very limited observation sets are more uncertain it is more likely that such orbits will "permit" future impacts. However, such early predictions can often be ruled out as we incorporate more observations and reduce the uncertainties in the object's orbit...
...Eventually, the impact probability will drop (usually quite abruptly) to zero or, if the asteroid is really on a collision trajectory, it will continue to grow until it reaches 100%.
When the discovery of a new NEA is announced by the Minor Planet Center (MPC), Sentry automatically (usually within an hour or two) prioritizes the object for an impact risk analysis...
NEA orbits and close approach tables are continuously and automatically updated whenever new observations are made available...
( NEODyS )
The differences between the two systems are generally not substantial...
One of the differences between the two systems stems from different approaches to computing the impact probability...
Another important variation is that Sentry uses a different sampling strategy...
