( NEODyS )
The differences between the two systems are generally not substantial, and in some sense they are reassuring. Independent systems using different software and theoretical approaches are not expected to produce the same results from statistical searches. Experience has shown that there is excellent agreement between the two systems for the more serious potential collision detections.
One of the differences between the two systems stems from different approaches to computing the impact probability. This computation is rough by its very nature, and different techniques may be used; impact probabilities different by a factor of ten or so are not extraordinary.
Another important variation is that Sentry uses a different sampling strategy, one that should detect nearly all potential impacts with probability greater than 10-8 (1 in 100 million), and does not expend much effort pursuing less likely cases, although it may find some anyway. In any case, nothing with impact probability below 10-10 (1 in 10 billion) is published by Sentry. In contrast, NEODyS may not detect as many potential impacts at probabilities below 10-6 (1 in 1 million), but in certain cases it can detect very low probability events that Sentry does not.