Sentry (monitoring system)

The Impact Risk page lists a number of lost minor planets that are, for all practical purposes, permanent residents of the risk page; their removal may depend upon a serendipitous rediscovery. is the asteroid with greatest probability (10%) of impacting Earth, but is only ~7 meters in diameter. The only numbered objects with observation arcs of several years are (29075) 1950 DA and 101955 Bennu. The diameter of most near-Earth asteroids that have not been studied by radar or infrared can generally only be estimated within about a factor of 2 based on the asteroid's absolute magnitude (H). Their mass, consequently, is uncertain by about a factor of 10. For near-Earth asteroids without a well-determined diameter, Sentry assumes a generic albedo of 0.15. In August 2013, the Sentry Risk Table started using planetary ephemeris (DE431) for all NEO orbit determinations. DE431 (JPL small-body perturber ephemeris: SB431-BIG16) better models the gravitational perturbations of the planets and includes the 16 most massive main-belt asteroids. In April 2021, Sentry transitioned to DE441 which removed the very low impact probability of short-arc 2014 MV67 which had been less than 1:1-billion. The switch to DE441 also briefly added in the harmless Jupiter trojan 2014 ES57 with a very low impact probability of about 1:1-billion. JPL launched major changes to the website in February 2017 and re-directed the classic page on 10 April 2017. In 2021 JPL launched Sentry-II which handles the Yarkovsky effect that can significantly change a small asteroid's path over decades and centuries. Sentry-II defaults to an impact pseudo-observation (IOBS) analysis technique that runs an extended orbit-determination filter that tries to converge to an impacting solution compatible with the observational data. == Numbers ==

s , there are over 37,000 near-Earth objects of which roughly 1,900 near-Earth asteroids are listed on the risk table. Only around 21 objects on the risk table are large enough to qualify as potentially hazardous objects with a diameter greater than 140 meters (absolute magnitude brighter than 22). About 99% of the objects on the risk table are less than roughly 140 meters in diameter. Roughly 1400 of these risk-listed near-Earth asteroids are estimated to be about the size of the Chelyabinsk meteor or smaller (H>26), which killed no one but had 1,491 indirect injuries. More than 3,300 asteroids have been removed from the risk table since it launched in 2002. The only two comets that briefly appeared on the Sentry Risk Table are 197P/LINEAR (2003 KV2) and 300P/Catalina (2005 JQ5). == JPL SBDB comparison ==

The JPL Small-Body Database close approach table lists a linearized uncertainty. Sentry computations explore alternate orbit solutions along the line of variations and account for orbit propagation nonlinearities. == Scout ==

Sentry's "little brother" Scout scans recently detected objects on the Minor Planet Center's Near-Earth Object Confirmation Page with designations that are user-assigned and unofficial as they have not been confirmed by additional observations. Scout is used to help identify imminent impactors. ESA's equivalent to Scout is Meerkat Asteroid Guard. ==See also==