Sentry (monitoring system)

Sentry is a highly automated impact prediction system operated by the JPL Center for NEO Studies (CNEOS) since 2002. It continually monitors the most up-to-date asteroid catalog for possibilities of future impact with Earth over the next 100+ years.^[1] Whenever a potential impact is detected it will be analyzed and the results immediately published by the Center for Near-Earth Object Studies.^[1] However, several weeks of optical data are not enough to conclusively identify an impact years in the future.^[2] By contrast, eliminating an entry on the risk page is a negative prediction (a prediction of where it will not be).^[2]

Scientists warn against worrying about the possibility of impact with an object based on only a few weeks of optical data that show a possible Earth encounter years from now.^[2] Sometimes, it cannot even be said for certain what side of the Sun such an object will be at the time of the listed virtual impactor date.^[2] For example, even though 2005 ED224 had a 1-in-500,000 chance of impacting Earth on 11 March 2023, it was expected to be farther than the Sun at the time.^[3] Most objects on the Sentry Risk Table have an observation arc of less than 14 days and have not been observed for years.

There are 1620 near-Earth asteroids listed on the risk table with 37,736 virtual impactor dates. For each asteroid listed on the risk table there are on average about 23 virtual impactors. Only about 19 objects on the risk list are large enough to be considered potentially hazardous objects with a diameter greater than about 140 meters. The average size of an object on the default page of Sentry is 120 meters with an average impact probability of 1:500. More eccentric orbits (such as 2015 RD₃₆) that extend to nearly the orbit of Jupiter can make atmospheric entry at velocities of ~40 km/s (25 mi/s).^[4]