Sentry (monitoring system)
Sentry is 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.[1] Whenever a potential impact is detected it will be analyzed and the results immediately published on the Near Earth Object Program.[1] Two or three weeks of optical data is 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] From the point of view of the general public, it isn't worth getting worried about an object with a couple of weeks of optical data showing a possible Earth encounter years from now.[2]
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.[3] 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.[1] 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).[1] 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.[4]
In August 2013, the Sentry Risk Table started using planetary ephemeris (DE431) for all NEO orbit determinations.[5] 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.[5]
References
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- SENTRY - An Automatic Near-Earth Asteroid Collision Monitoring System@ neo.jpl.nasa.gov
- IMPACT RISK ASSESSMENT: AN INTRODUCTION
- Sentry Risk Table
- NEODyS CLOMON2
- List of objects for which all previously detected potential impacts have been eliminated
- Asteroid Hazards, Part 3: Finding the Path – Minor Planet Center on YouTube
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