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Sentry (monitoring system)

Sentry (monitoring system)

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Sentry is a highly automated collision monitoring system that continually scans the most current asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

 catalog for possibilities of future impact with Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 over the next 100 years. Whenever a potential impact is detected it will be analyzed and the results immediately published on the Near Earth Object Program.

Two or three weeks of optical data is not enough to conclusively identify an impact years in the future. By contrast, eliminating an entry on the risk page is a negative prediction; a prediction of where it will NOT be. As far as the public is concerned, it just isn't worth getting worried about an object with a couple weeks of optical data showing a possible Earth encounter years from now.

The Impact Risk page lists a number of lost objects such as that are, for all practical purposes, permanent residents of the risk page; their removal may depend upon a serendipitous rediscovery. Some objects on the Sentry Risk Table, such as , might even be man-made.

Notable objects on the risk page include (numbered asteroids listed first): 99942 Apophis
99942 Apophis
99942 Apophis is a near-Earth asteroid that caused a brief period of concern in December 2004 because initial observations indicated a small probability that it would strike the Earth in 2029. Additional observations provided improved predictions that eliminated the possibility of an impact on...

, , , , and .

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