Sentry is a highly automated impact prediction system operated by the JPL Center for NEO Studies since 2002. It continually monitors the most current asteroid catalog for possibilities of future impact with Earth over the next 100+ years. Whenever a potential impact is detected it will be analyzed and the results immediately published by the Near Earth Object Program. However, several weeks of optical data are 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. 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. Sometimes, it can't even be said for certain what side of the Sun such an object will be at the time of the listed virtual impactor date. Most objects on the Sentry Risk Table have an observation arc of less than 14 days and have not been observed for years.
Sentry Risk Table
The Impact Risk page lists a number of lostminor planets that are, for all practical purposes, permanent residents of the risk page; their removal may depend upon a serendipitous rediscovery. Lost asteroids and with their very short 1-day observation arcs have missed virtual impactor dates. was serendipitously rediscovered in 2006 after being lost for more than 8 years. was determined to be a harmless main belt asteroid in 2014. Some objects on the Sentry Risk Table, such as, might even be man-made. is the asteroid with greatest probability of impacting Earth, but is only ~7 meters in diameter. Numbered notable objects with observation arcs of several years include: 1950 DA, 99942 Apophis, 101955 Bennu, and 2009 FD. Notable asteroids removed from Sentry in the last few years include :,,,,,,,, 367943 Duende, and. 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. 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 for all NEO orbit determinations. DE431 better models the gravitational perturbations of the planets and includes the 16 most massive main-belt asteroids. JPL launched major changes to the website in February 2017 and re-directed the classic page on 10 April 2017.
Numbers
there are over 23,000 near-Earth objects of which roughly 1023 near-Earth asteroids are listed on the risk table. About 95% of the objects on the risk table are too small to qualify as a potentially hazardous object because the objects are less than roughly 140 meters in diameter, or have an absolute magnitude fainter than 22., only around 38 objects on the risk table are large enough to qualify as a PHO. Roughly 700 of these risk-listed near-Earth asteroids are estimated to be about the size of the Chelyabinsk meteor, which killed no one but had 1,491 non-direct injuries; or smaller. More than 2,490 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 and 300P/Catalina.
Scout
Sentry's little brother Scout scans recently detected objects on the Minor Planet Center's NEO Confirmation Page with designations that are user-assigned and unofficial as they have not been confirmed by additional observations. The impact risk assessment is rated on a scale of 0–4. Scout is used to help identify imminent impactors.
