Sentry: Earth Impact Monitoring

Frequently Asked Questions

• How is an orbit calculated?

  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.

• How far into the future does Sentry search for impacts?

  100 years.

• Why do your first calculations of an orbit often look more threatening than later ones?

  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. Most often, the threat associated with a specific object will decrease as additional observations become available, and so objects will be posted to, and later removed from, our Impact Risk Page. The Palermo Scale values will typically start out at less negative values when the object's orbit is most uncertain and evolve to more negative values (and eventually off the list) as more and more observations allow the object's orbit to be continually improved.

  On the other hand, in the unlikely case where a particular potential impact event persists until the orbit is relatively well constrained, the impact probability and associated risk will tend to increase as observations are added. This is not too paradoxical: If an asteroid is indeed going to come very near the Earth then a collision cannot be ruled out early on. The impact probability will tend to grow as the orbit is refined and alternative and safer trajectories are eliminated. 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%.

• How soon after the discovery is a search for potential collisions initiated?

  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. If the prioritization analysis indicates that the asteroid cannot pass near the Earth or that its orbit is very well determined then the computationally intensive nonlinear search for potential impacts is not pursued. If, on the other hand, a search is deemed necessary then the object is added to a queue of objects awaiting analysis. Its position in the queue is determined by the estimated likelihood that potential impacts may be found.

• How often do these results change?

  NEA orbits and close approach tables are continuously and automatically updated whenever new observations are made available, generally within a couple of hours of the release of the information. Whenever an NEA orbit is updated the object is re-prioritized and, if appropriate, it is re-queued for a new potential impact search. This process is ongoing - taking place anytime, day and night, seven days a week.

• Why are the results published by NEODyS not the same as those published by Sentry?

  ( 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.