Since 2002, Sentry has been a highly automated collision monitoring system that continually scans 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 on the Near Earth Object Program. However, several 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. 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.
Sentry Risk Table
The Impact Risk page lists a number of lost objects that are, for all practical purposes, permanent residents of the risk page; their removal may depend upon a serendipitous rediscovery. 1997 XR2 was serendipitously rediscovered in 2006 after being lost for more than 8 years. Some objects on the Sentry Risk Table, such as 2000 SG344, might even be man-made.
Notable objects currently on the risk page include (numbered asteroids listed first): (29075) 1950 DA, 99942 Apophis, 101955 Bennu, 2009 FD, 1994 WR12, and 2010 RF12. Notable asteroids removed from Sentry in the last few years include (most recently removed listed first): 2007 VK184, 2013 BP73, 2008 CK70, 2013 TV135, 2011 BT15, 367943 Duende, and 2011 AG5.
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. More than two dozen known asteroids have higher than a one in a million chance of impacting into Earth within the next 100 years.
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.
JPL launched major changes to the website in February 2017.
As of February 2017 there are roughly 680 near-Earth asteroids listed on the risk table and roughly 2000 asteroids have been removed from the risk table since it launched in 2002.