Sentry (monitoring system)

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Asteroid 2020 VV risk corridor for the virtual impactor of 12 October 2033.

Sentry is a highly automated impact prediction system operated by the JPL Center for NEO Studies (CNEOS) since 2002. It continually monitors 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 by the Near Earth Object Program.[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] 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[edit]

Plot of orbits of known potentially hazardous asteroids

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.[3] Lost asteroids 2007 FT3 and 2014 MV67 with their very short 1-day observation arcs have missed virtual impactor dates. 1997 XR2 was serendipitously rediscovered in 2006 after being lost for more than 8 years. 2004 BX159 was determined to be a harmless main belt asteroid in 2014. Some objects on the Sentry Risk Table, such as 2000 SG344, might even be artificial.

2010 RF12 is the asteroid with greatest probability (5%) of impacting Earth, but is only ~7 meters in diameter. Numbered notable objects with observation arcs of several years include: (29075) 1950 DA, 99942 Apophis, 101955 Bennu, and (410777) 2009 FD.[1] Notable asteroids removed from Sentry in the last few years include (most recently removed listed first): 2006 QV89, 2017 XO2, 1994 WR12, 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.

In August 2013, the Sentry Risk Table started using planetary ephemeris (DE431) for all NEO orbit determinations.[4] 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.[4]

JPL launched major changes to the website in February 2017 and re-directed the classic page on 10 April 2017.


As of June 2020 there are over 23,000 near-Earth objects of which roughly 1023 near-Earth asteroids are listed on the risk table.[1] 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. As of June 2020, only around 38 objects on the risk table are large enough to qualify as a PHO.[1] Roughly 700 of these risk-listed near-Earth asteroids are estimated to be about the size of the Chelyabinsk meteor (H>26), 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.[5]

The only two comets that briefly appeared on the Sentry Risk Table are 197P/LINEAR (2003 KV2) and 300P/Catalina (2005 JQ5).[5]


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.[6] The impact risk assessment is rated on a scale of 0–4 (negligible, small, modest, moderate, or elevated). Scout is used to help identify imminent impactors.

See also[edit]


  1. ^ a b c d e f "Sentry Risk Table". NASA/JPL Near-Earth Object Program Office. Retrieved 13 April 2017. (Click "Use Unconstrained Settings" to see how many objects are on the list; H<=22 for list of PHAs)
  2. ^ a b c d Jon Giorgini (30 July 2002). "Understanding Risk Pages". Columbine, Inc. (hohmanntransfer). Retrieved 21 November 2011.
  3. ^ "IMPACT RISK ASSESSMENT: AN INTRODUCTION". NASA/JPL Near-Earth Object Program Office. 31 August 2005. Retrieved 14 October 2011.
  4. ^ a b "Sentry Notes". NASA/JPL Near-Earth Object Program Office. 12 August 2013. Retrieved 13 April 2017.
  5. ^ a b "Removed Objects". NASA/JPL Near-Earth Object Program Office. Archived from the original on 25 February 2017. Retrieved 16 February 2017.(Search for "P/" to list comets removed.)
  6. ^ "Scout: NEOCP Hazard Assessment". NASA/JPL Near-Earth Object Program Office. Retrieved 26 June 2019.

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