View source for Sentry (monitoring system)

{{Use dmy dates|date=May 2019}}
{{short description|JPL program to monitor the Minor Planet Center's catalog for Earth impacts}}
[[File:2020VV risk corridor.png|thumb|right|Asteroid [[2020  VV]] risk corridor for the obsolete virtual impactor of 12 October 2033.]]
'''Sentry''' is an automated [[Asteroid impact prediction|impact prediction]] system started in 2002 and operated by the [[Center for Near-Earth Object Studies|Center for Near Earth Object Studies]] (CNEOS) at [[NASA]]'s [[Jet Propulsion Laboratory]]. It continually monitors the most up-to-date [[asteroid]] catalog for possibilities of future impact with [[Earth]] over the next 100+ years.<ref name="risk-table"/> Whenever a potential impact is detected, it will be analyzed and the results immediately published by CNEOS.<ref name="risk-table"/> However, alerts do not imply certainty about future impacts, as the small amounts of optical data that can trigger an alert are not enough to conclusively identify an impact years in the future.<ref name="Giorgini"/> In contrast, eliminating an entry on the risk page is a negative prediction (a prediction of where it will ''not'' be).<ref name="Giorgini"/>

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.<ref name="Giorgini"/> 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.<ref name="Giorgini"/> For example, even though {{mpl|2005 ED|224}} had a 1-in-500,000 chance of impacting Earth on 11 March 2023, its most likely position at that date was farther away than the Sun.<ref name="NEODyS2023"/> Most objects in the Sentry Risk Table have an [[observation arc]] of less than 14 days, making their positions highly uncertain, and have not been observed for years.

There are 1888 near-Earth asteroids listed in the risk table and 41,848 virtual impact dates, so for each asteroid in the risk table, there is an average of about 22 virtual impact dates. Only about 21 objects in the table are large enough, with a diameter greater than about 140 meters, to be considered [[potentially hazardous object]]s. The average size of an object on the default page of Sentry is 120 meters, with an average impact probability of about 1 in 500. More [[Orbital eccentricity|eccentric]] orbits (such as {{mp|2015 RD|36}}) that extend to nearly the orbit of [[Jupiter]] can make [[atmospheric entry]] at velocities of ~{{Convert|40|km/s|mi/s|abbr=unit}}.<ref name=velocity/>

==Sentry Risk Table==
{|class="wikitable sortable floatright" style="text-align:center; font-size:0.9em;"
|+Objects with better than a 1/500 (0.2%) cumulative chance of impact
! Object
! Cumulative<br/>impact<br/>probability
! Date of<br/>greatest risk
! Estimated<br/>diameter<br/>(meters)
! [[Observation arc|Observation<br/>arc]]<br/>(days)
|-
| '''{{mpl|2010 RF|12}}''' || 10% || 2095-09-05 || 7 || 4374
|-
| '''{{mpl|2020 CD|3}}''' || 2.5% || 2082-09-09 || 2 || 742
|-
| '''{{mpl|2006 RH|120}}''' || 1.3% || 2044-02-08 || 4 || 281
|-
| {{mp|2017 WT|28}} || 1.2% || 2104-11-24 || 8 || 19
|-
| {{mp|2024 BY|15}} || 0.94% || 2095-03-19 || 15 || 49
|-
| {{mp|2020 VW}} || 0.70% || 2074-11-02 || 7 || 14
|-
| '''{{mpl|2006 JY|26}}''' || 0.50% || 2074-05-03 || 7 || 3
|-
| {{mp|2020 CQ|1}} || 0.46% || 2070-02-03 || 6 || 29
|-
| {{mp|2022 SX|55}} || 0.40% || 2035-09-17 || 3 || 1
|-
| '''{{mpl|2022 NX|1}}''' || 0.32% || 2075-12-03 || 8 || 142
|-
| '''{{mpl|2000 SG|344}}''' || 0.27% || 2071-09-16 || 37 || 507
|-
| '''{{mpl|2020 VV}}''' || 0.23% || 2056-10-11 || 12 || 61
|-
| {{mp|2017 LD}} || 0.22% || 2079-06-10 || 11 || 45
|-
| {{mp|2000 LG|6}} || 0.21% || 2094-05-27 || 5 || 2
|}

The Impact Risk page lists a number of [[lost minor planet]]s that are, for all practical purposes, permanent residents of the risk page; their removal may depend upon a [[serendipity|serendipitous]] rediscovery.<ref name="intro"/> Lost asteroid [[1979 XB]] has been on the list since the list's inception.<ref name="Sentry2002"/> {{mpl|2007 FT|3}} and {{mpl|2014 MV|67}} with their very short 1-day observation arcs have missed virtual impactor dates as they were likely quite distant from the Earth at the time. {{mpl|1997 XR|2}} was serendipitously rediscovered in 2006 after being [[Lost minor planet|lost]] for more than 8 years. {{mpl|2004 BX|159}} was determined to be a harmless [[main belt]] asteroid in 2014. Some objects on the Sentry Risk Table, such as {{mpl|2000 SG|344}}, might even be artificial.<ref>{{cite web |title=Much Ado about 2000 SG344 |url=https://science.nasa.gov/science-news/science-at-nasa/2000/ast06nov_2 |website=NASA Science |publisher=NASA |access-date=27 September 2023}}</ref>

{{mpl|2010 RF|12}} is the asteroid with greatest probability (10%) of impacting Earth, but is only ~7 meters in diameter. The only [[Minor planet designation|numbered]] objects with [[observation arc]]s of several years are [[(29075) 1950 DA]] and [[101955 Bennu]].<ref name="risk-table"/> Notable asteroids removed from Sentry include (most recently removed listed first): [[99942 Apophis]], [[(410777) 2009 FD]], {{mpl|2006 QV|89}}, {{mpl|2017 XO|2}}, {{mpl|1994 WR|12}}, {{mpl|2007 VK|184}}, {{mpl|2013 BP|73}}, {{mpl|2008 CK|70}}, {{mpl|2013 TV|135}}, {{mpl|2011 BT|15}}, [[367943 Duende]], and {{mpl|2011 AG|5}}.

{{As of|2025|02}}, of the 191 asteroids with better than a 1-in-10,000 chance of impacting Earth only [[(29075) 1950 DA]] and [[101955 Bennu]] are larger than 50 meters in diameter.

{{As of|2025|03}}, the soonest virtual impactor of an asteroid larger than 50 meters in diameter with a better than 1:1-million chance of impact is {{mpl|2022 PX|1}} on 11 August 2040 with a 1:{{val|310000}} chance of impact.<ref name="VIs"/> It is estimated to be 120-meters in diameter, has a short observation arc of 7-days, and is expected to be approximately {{convert|1.75|AU|e6km|abbr=unit|lk=on}} from Earth on 11 August 2040.<ref name=Horizons2022PX1/> The impact scenario is outside the [[3-sigma]] uncertainty region of {{Nowrap|± 242 million km}}.<ref name=Horizons2022PX1/>

{{anchor|2025}}
The asteroid with the greatest chance of impacting Earth in 2025 is {{mpl|2009 VA}} (6-meters in diameter) with less than a 1-day observation arc.<ref name="VIs"/> It has a 1:48,000 chance of impact on 06 November 2023, but is expected to be around {{Convert|0.3|AU|e6km|abbr=unit|lk=on}} from Earth on that date with uncertainty region of ± 900 million km.<ref name="Horizons2025"/>

With a 24-day observation arc, {{mp|2017 SA|20}} has the most virtual impactors with 1244 virtual impactor dates.<ref name="risk-table"/><ref>[https://cneos.jpl.nasa.gov/sentry/details.html#?des=2017%20SA20 Sentry: 2017 SA20] (1244 VIs) using 2022-Aug-30 solution</ref>

The diameter of most near-Earth asteroids that have not been studied by [[Radar astronomy|radar]] or [[Infrared astronomy|infrared]] can generally only be estimated within about a factor of 2 based on the asteroid's [[Absolute magnitude#Solar System bodies (H)|absolute magnitude]] (H).<ref name="risk-table"/> 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#Astronomical albedo|albedo]] of 0.15.

In August 2013, the Sentry Risk Table started using planetary [[ephemeris]] ([[Jet Propulsion Laboratory Development Ephemeris#Ephemerides in the series|DE431]]) for all NEO orbit determinations.<ref name="notes"/> DE431 ([[JPL Small-Body Database|JPL small-body]] perturber ephemeris: SB431-BIG16) better models the gravitational [[Perturbation (astronomy)|perturbations]] of the planets and includes the 16 most massive [[main-belt asteroid]]s.<ref name="notes"/> In April 2021, Sentry transitioned to [[Jet Propulsion Laboratory Development Ephemeris#Ephemerides in the series|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.<ref name="Sentry-II"/> 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 ==
[[File:Potentially Hazardous Asteroids 2013.png|thumb|Plot of orbits of known [[potentially hazardous asteroid]]s ]]
{{As of|2025|02}}, there are over 37,000 [[near-Earth object]]s of which roughly 1,900 near-Earth asteroids are listed on the risk table.<ref name="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#Solar System bodies (H)|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.<ref name="removed"/>

The only two comets that briefly appeared on the Sentry Risk Table are 197P/LINEAR (2003 KV2) and [[300P/Catalina]] (2005 JQ5).<ref name="removed"/>

== JPL SBDB comparison ==
The [[JPL Small-Body Database]] close approach table lists a [[Linearity|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.<ref name="Scout-intro"/> The impact risk assessment is rated on a scale of 0–4 (negligible, small, modest, moderate, or elevated).<ref group="note">"Negligible" (0) is for objects where no impacting solution was identified. "Small" (1) is for objects with a <0.1% chance of impact. "Modest" (2) is for a 0.1-1% chance. "Moderate" (3) is for a 1-10% chance, and "Elevated" (4) is for a >10% chance of impact.</ref> Scout is used to help identify imminent impactors. ESA's equivalent to Scout is Meerkat Asteroid Guard.

==See also==
* [[Asteroid impact prediction]]
* [[Earth-grazing fireball]]
* [[Impact event]]
* [[List of asteroid close approaches to Earth]]
* [[List of Earth-crossing asteroids]]
* [[Meteoroid]]
* {{annotated link|NEODyS}}
* [[Time-domain astronomy]]

==Notes==
{{reflist|group="note"}}

== References ==
{{Reflist
| refs =

<ref name="risk-table">{{cite web
  |title=Sentry Risk Table
  |publisher=NASA/JPL Near-Earth Object Program Office
  |url=https://cneos.jpl.nasa.gov/sentry/
  |access-date=2017-04-13}} (Click "Use Unconstrained Settings" to see how many objects are on the list; H<=22 for list of PHAs)</ref>

<ref name="Giorgini">{{cite web
  |date=30 July 2002
  |title=Understanding Risk Pages
  |publisher=Columbine, Inc. (hohmanntransfer)
  |author=Jon Giorgini
  |url=http://www.hohmanntransfer.com/by/giorgjon.htm
  |archive-url=https://web.archive.org/web/20021208202559/http://www.hohmanntransfer.com/by/giorgjon.htm
  |url-status=dead
  |archive-date=8 December 2002
  |access-date=2011-11-21}}</ref>

<ref name="intro">{{cite web
  |date=31 Aug 2005
  |title=IMPACT RISK ASSESSMENT: AN INTRODUCTION
  |publisher=NASA/JPL Near-Earth Object Program Office
  |url=http://neo.jpl.nasa.gov/risk/doc/sentry.html
  |archive-url=https://web.archive.org/web/20020419174057/http://neo.jpl.nasa.gov/risk/doc/sentry.html
  |url-status=dead
  |archive-date=19 April 2002
  |access-date=2011-10-14}}</ref>

<ref name="notes">{{cite web
  |title=Sentry Notes
  |publisher=NASA/JPL Near-Earth Object Program Office
  |date=2013-08-12
  |url=https://cneos.jpl.nasa.gov/sentry/notes.html
  |access-date=2017-04-13}}</ref>

<ref name="removed">{{cite web
 |title       = Removed Objects
 |publisher   = NASA/JPL Near-Earth Object Program Office
 |url         = http://cneos.jpl.nasa.gov/sentry/removed.html
 |archive-url  = https://archive.today/20170225212225/http://cneos.jpl.nasa.gov/sentry/removed.html
 |archive-date = 2017-02-25
 |url-status     = live
 |access-date  = 2017-02-16
}}(Search for "P/" to list comets removed.)</ref>

<ref name="Scout-intro">{{cite web
  |title=Scout: NEOCP Hazard Assessment
  |publisher=NASA/JPL Near-Earth Object Program Office
  |url=https://cneos.jpl.nasa.gov/scout/intro.html
  |access-date=2019-06-26}}</ref>

<ref name="Sentry2002">{{cite web
  |title=2002 Archive of Sentry Risk Table
  |url=http://neo.jpl.nasa.gov/risk/ |archive-url=https://web.archive.org/web/20020321092735/http://neo.jpl.nasa.gov/risk/ |url-status=dead |archive-date=2002-03-21 }}</ref>

<ref name="NEODyS2023">{{cite web
 |title=2005ED224 Ephemerides for March 2023
 |publisher=NEODyS (Near Earth Objects{{Snd}} Dynamic Site)
 |url=https://newton.spacedys.com/neodys/index.php?pc=1.1.3.1&n=2005ED224&oc=500&y0=2023&m0=3&d0=10&h0=0&mi0=0&y1=2023&m1=3&d1=12&h1=0&mi1=0&ti=1.0&tiu=days
 |access-date=2021-03-26}} (Having not been observed since 2005 with an orbital period of 2.6±0.3 years, we do not know where on its orbit 2005 ED224 is.)</ref>

<ref name=velocity>{{cite web
  |title=Sentry Risk Table: 2015 RD36
  |type=using 2022-Sep-13 solution
  |publisher=NASA JPL [[Center for Near-Earth Object Studies|CNEOS]]
  |url=https://cneos.jpl.nasa.gov/sentry/details.html#?des=2015%20RD36
  |access-date=2022-10-02 }}</ref>

<ref name="VIs">{{cite web
  |title=VI (Virtual Impactor) Data
  |publisher=NASA/JPL Near-Earth Object Program Office
  |url=https://cneos.jpl.nasa.gov/sentry/vi.html
  |access-date=2022-11-09}} Unconstrained Settings, Sort by Date. For objects larger than 50 meters and odds of impact better than 1:1 million also include Impact Probability ≥ 1e-6 and H≤24.</ref>

<ref name="Horizons2025">{{cite web
  |title=Horizons Batch for 2025-11-06 Virtual Impactor
  |publisher=[[JPL Horizons On-Line Ephemeris System|JPL Horizons]]
  |url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%272009+VA%27&START_TIME=%272025-11-06%27&STOP_TIME=%272025-11-07%27&STEP_SIZE=%272%20day%27&QUANTITIES=%2720,39%27
  |accessdate=2025-02-03}} RNG_3sigma = uncertainty range in km. (JPL#7/Soln.date: 2021-Apr-15 generates RNG_3sigma = {{val|934114563|u=km}} for 2025-Nov-06.)</ref>

<ref name=Horizons2022PX1>{{cite web
  |title=JPL Horizons: 2022 PX1 geocentric distance and uncertainty on 11 August 2040
  |publisher=JPL Horizons
  |url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%272022+PX1%27&START_TIME=%272040-Aug-11%27&STOP_TIME=%272040-Aug-12%27&STEP_SIZE=%272%20day%27&QUANTITIES=%2720,39%27
  |accessdate=2025-03-13}}</ref>

<ref name="Sentry-II">{{cite web
  |title=NASA's Next-Generation Asteroid Impact Monitoring System Goes Online
  |publisher=NASA/JPL Near-Earth Object Program Office
  |url=https://www.jpl.nasa.gov/news/nasas-next-generation-asteroid-impact-monitoring-system-goes-online
  |date=2021-12-06}}</ref>

}} <!-- end of reflist -->

==External links==
* [https://cneos.jpl.nasa.gov/sentry/intro.html Sentry: Introduction]
* [https://cneos.jpl.nasa.gov/sentry/ Sentry: Impact Risk Data table]
* List of [https://cneos.jpl.nasa.gov/sentry/removed.html objects for which all previously detected potential impacts have been eliminated]
* Similar lists: [https://newton.spacedys.com/neodys/index.php?pc=4.1 NEODyS CLOMON2] / [https://neo.ssa.esa.int/risk-list ESA NEO Risk List] / [http://www.brera.mi.astro.it/sormano/teca.html Sormano Observatory TECA] / [https://robertinventor.online/booklets/sentry_table_simplified.htm Simplified List]
* [https://www.youtube.com/watch?v=8M_27st1ZCI&t=142 Asteroid Hazards, Part 3: Finding the Path] – [[Minor Planet Center]] on YouTube
* [https://cneos.jpl.nasa.gov/scout/intro.html Scout: NEOCP Hazard Assessment: Introduction]

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