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Sentry: Keeping an Eye on Near-Earth Objects
Sentry: Keeping an Eye on Near-Earth Objects
12 Mar 2002
(Source: Jet Propulsion Laboratory)

SENTRY - An Automatic Near-Earth Asteroid Collision Monitoring System

NASA's Near-Earth Object Program Office ( announces the arrival of the Sentry automatic impact monitoring system. In development for nearly two years, Sentry is a highly automated, accurate, and robust system for continually updating the orbits, future close Earth approaches, and Earth impact probabilities for all Near-Earth Asteroids (NEAs).

When interpreting the Sentry Impact Risks Page (, where information on known potential NEA impacts is posted, one must bear in mind that an Earth collision by a sizable NEA is a very low probability event. Objects normally appear on the Risks Page because their orbits can bring them close to the Earth's orbit and the limited number of available observations do not yet allow their trajectories to be well-enough defined. In such cases, there may be a wide range of possible future paths that can be fit to the existing observations, sometimes including a few that can intersect the Earth.

Whenever a newly discovered NEA is posted on the Sentry Impact Risks Page, by far the most likely outcome is that the object will eventually be removed as new observations become available, the object's orbit is improved, and its future motion is more tightly constrained. As a result, several new NEAs each month may be listed on the Sentry Impact Risks page, only to be removed shortly afterwards. This is a normal process, completely expected. The removal of an object from the Impact Risks page does not indicate that the object's risk was evaluated mistakenly: the risk was real until additional observations showed that it was not.

While completely independent, the Sentry system is meant to be complementary to the NEODyS CLOMON impact monitoring system operated in Pisa, Italy. Personnel from both the Sentry and NEODyS systems are in constant communication, cross checking each other's results and providing constructive feedback to continuously improve the efficiency, accuracy, and robustness of both systems.

The Sentry system was developed largely by Drs. Steve Chesley and Alan Chamberlin with significant technical help from Dr. Paul Chodas. Ron Baalke provided our web site updates.

Donald K. Yeomans
Manager, NASA's Near-Earth Object Program Office

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Last Updated: 13 Mar 2002