
Sentry is a highly automated
impact prediction system operated by the
JPL Center for NEO Studies (CNEOS) since 2002. It continually monitors the most up-to-date
asteroid
An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic o ...
catalog for possibilities of future impact with
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...
over the next 100+ years.
Whenever a potential impact is detected it will be analyzed and the results immediately published by the
Center for Near-Earth Object Studies.
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 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.
For example, even though has a 1-in-500,000 chance of impacting Earth on 11 March 2023, it is expected to be farther than the Sun at the time.
Most objects on the Sentry Risk Table have an
observation arc
In observational astronomy, the observation arc (or arc length) of a Solar System body is the time period between its earliest and latest observations, used for tracing the body's path. It is usually given in days or years. The term is mostly use ...
of less than 14 days and have not been observed for years.
There are 1510 near-Earth asteroids listed on the risk table with 36,345 virtual impactor dates. For each asteroid listed on the risk table there are on average about 24 virtual impactors. Only about 17 objects on the risk list are large enough to be considered
potentially hazardous object
A potentially hazardous object (PHO) is a near-Earth object – either an asteroid or a comet – with an orbit that can make close approaches to the Earth and is large enough to cause significant regional damage in the event of impact. They ...
s with a diameter greater than about 140 meters. The average size of an object on the default page of Sentry is 120 meters with an average impact probability of 1:500. More
eccentric
Eccentricity or eccentric may refer to:
* Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal"
Mathematics, science and technology Mathematics
* Off- center, in geometry
* Eccentricity (graph theory) of a ...
orbits (such as ) that extend to nearly the orbit of
Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandt ...
can make
atmospheric entry
Atmospheric entry is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. There are two main types of atmospheric entry: ''uncontrolled entry'', such as the ...
at velocities of ~40 km/s.
[
]
Sentry Risk Table
The Impact Risk page lists a number of lost minor planet
A minor planet is "lost" when today's observers cannot find it, because its location is too uncertain to target observations. This happens if the orbital elements of a minor planet are not known accurately enough, typically because the observat ...
s that are, for all practical purposes, permanent residents of the risk page; their removal may depend upon a serendipitous
Serendipity is an unplanned fortunate discovery. Serendipity is a common occurrence throughout the history of product invention and scientific discovery.
Etymology
The first noted use of "serendipity" was by Horace Walpole on 28 January 1754. I ...
rediscovery. Lost asteroid 1979 XB
Events
January
* January 1
** United Nations Secretary-General Kurt Waldheim heralds the start of the ''International Year of the Child''. Many musicians donate to the ''Music for UNICEF Concert'' fund, among them ABBA, who write the song ...
has been on the list since the list's inception. and 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. was serendipitously rediscovered in 2006 after being lost for more than 8 years. was determined to be a harmless main belt
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
asteroid in 2014. Some objects on the Sentry Risk Table, such as , might even be artificial.
is the asteroid with greatest probability (10%) of impacting Earth, but is only ~7 meters in diameter. The only numbered objects with observation arc
In observational astronomy, the observation arc (or arc length) of a Solar System body is the time period between its earliest and latest observations, used for tracing the body's path. It is usually given in days or years. The term is mostly use ...
s of several years are (29075) 1950 DA
, provisional designation ', is a risk–listed asteroid, classified as a near-Earth object and potentially hazardous asteroid of the Apollo group, approximately in diameter. It once had the highest known probability of impacting Earth. In 2002 ...
and 101955 Bennu
101955 Bennu (provisional designation ) is a carbonaceous asteroid in the Apollo group discovered by the LINEAR Project on 11 September 1999. It is a potentially hazardous object that is listed on the Sentry Risk Table and has the highest cumu ...
. Notable asteroids removed from Sentry include (most recently removed listed first): 99942 Apophis
99942 Apophis is a near-Earth asteroid and potentially hazardous asteroid with a diameter of that caused a brief period of concern in December 2004 when initial observations indicated a probability up to 2.7% that it would hit Earth on April&nbs ...
, (410777) 2009 FD
(410777) 2009 FD is a carbonaceous sub-kilometer asteroid and binary system, classified as near-Earth object and potentially hazardous asteroid of the Apollo group, discovered on 24 February 2009 by astronomers of the Spacewatch program at K ...
, , , , , , , , , 367943 Duende
367943 Duende (provisional designation ) is a micro-asteroid and a near-Earth object of the Aten and Atira group, approximately in diameter. It was discovered by astronomers of the Astronomical Observatory of Mallorca at its robotic La Sagra Ob ...
, and .
Of the 145 asteroids with better than a 1-in-10,000 chance of impacting Earth only 101955 Bennu
101955 Bennu (provisional designation ) is a carbonaceous asteroid in the Apollo group discovered by the LINEAR Project on 11 September 1999. It is a potentially hazardous object that is listed on the Sentry Risk Table and has the highest cumu ...
is larger than 50 meters in diameter.
The soonest virtual impactor of an asteroid larger than 50 meters in diameter with a better than 1:1-million chance of impact is on 11 March 2023 with a 1:500000 chance of impact. It is estimated to be 54-meters in diameter, has a short observation arc of 3-days, and is expected to be from Earth on 11 March 2023. Since it has not been observed since 2005 and has an orbital period of , we do not know where on its orbit is.
The asteroid with the greatest chance of impacting Earth in 2023 is (4-meters in diameter) with less than a 1-day observation arc. It has a 1:53,000 chance of impact on 10 June 2023, but is expected to be around from Earth on that date. Such an impact would be similar to .
With a 24-day observation arc
In observational astronomy, the observation arc (or arc length) of a Solar System body is the time period between its earliest and latest observations, used for tracing the body's path. It is usually given in days or years. The term is mostly use ...
, has the most virtual impactors with 1244 virtual impactor dates.Sentry: 2017 SA20
(1244 VIs) using 2022-Aug-30 solution
The diameter of most near-Earth asteroids that have not been studied by
radar
Radar is a detection system that uses radio waves to determine the distance ('' ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
or
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from aroun ...
can generally only be estimated within about a factor of 2 based on the asteroid's
absolute magnitude
Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it we ...
(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
Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refl ...
of 0.15.
In August 2013, the Sentry Risk Table started using planetary
ephemeris
In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly ...
(
DE431) for all NEO orbit determinations.
DE431 (
JPL small-body perturber ephemeris: SB431-BIG16) better models the gravitational
perturbations
Perturbation or perturb may refer to:
* Perturbation theory, mathematical methods that give approximate solutions to problems that cannot be solved exactly
* Perturbation (geology), changes in the nature of alluvial deposits over time
* Perturbatio ...
of the planets and includes the 16 most massive
main-belt asteroid
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, cal ...
s.
In April 2021, Sentry transitioned to
DE441 which removed the very low impact probability of short-arc
2014 MV67
(also written 2014 MV67) was a lost asteroid with an observation arc less than 1 day. It had an assumed orbital eccentricity and a very poorly constrained orbit. Depending on the orbit, it could have been a potentially hazardous asteroid, ...
which had been less than 1:1-billion. The switch to DE441 also briefly added in the harmless Jupiter trojan
2014 ES57
is a Greek camp Jupiter trojan roughly in diameter that was briefly listed on the Sentry Risk Table in April 2021 when JPL transitioned to DE441. Once listed on the Sentry Risk Table additional archived observations were quickly located tha ...
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
The Yarkovsky effect is a force acting on a rotating body in space caused by the anisotropic emission of thermal photons, which carry momentum. It is usually considered in relation to meteoroids or small asteroids (about 10 cm to 10 km ...
that can significantly change a small asteroids path over decades and centuries.
Sentry-II defaults to a 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

, there are over 31,121
near-Earth object
A near-Earth object (NEO) is any small Solar System body whose orbit brings it into proximity with Earth. By convention, a Solar System body is a NEO if its closest approach to the Sun (perihelion) is less than 1.3 astronomical units (AU) ...
s of which roughly 1,510 near-Earth asteroids are listed on the risk table.
Only around 17 objects on the risk table are large enough to qualify as potentially hazardous objects with a diameter greater than 140 meters (
absolute magnitude
Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it we ...
brighter than 22). About 99% of the objects on the risk table are less than roughly 140 meters in diameter. Roughly 1,100 of these risk-listed near-Earth asteroids are estimated to be about the size of the
Chelyabinsk meteor
The Chelyabinsk meteor was a superbolide that entered Earth's atmosphere over the southern Ural region in Russia on 15 February 2013 at about 09:20 YEKT (03:20 UTC). It was caused by an approximately near-Earth asteroid that entered the ...
(H>26), which killed no one but had 1,491 non-direct injuries; or smaller. More than 3,051 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 (2003 KV2) and
300P/Catalina (2005 JQ5).
JPL SBDB comparison
The
JPL Small-Body Database
The JPL Small-Body Database (SBDB) is an astronomy database about small Solar System bodies. It is maintained by Jet Propulsion Laboratory (JPL) and NASA and provides data for all known asteroids and several comets, including orbital parameters an ...
close approach table lists a
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.
The impact risk assessment is rated on a scale of 0–4 (negligible, small, modest, moderate, or elevated).
["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.] Scout is used to help identify imminent impactors.
See also
*
Asteroid impact prediction
Asteroid impact prediction is the prediction of the dates and times of asteroids impacting Earth, along with the locations and severities of the impacts.
The process of impact prediction follows three major steps:
# Discovery of an asteroid and ...
*
Earth-grazing fireball
*
Impact event
An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or ...
*
List of asteroid close approaches to Earth
This is a list of examples where an asteroid or meteoroid travels close to the Earth. Some are regarded as potentially hazardous objects if they are estimated to be large enough to cause regional devastation.
Near-Earth object detection technol ...
*
List of Earth-crossing asteroids
*
Meteoroid
A meteoroid () is a small rocky or metallic body in outer space.
Meteoroids are defined as objects significantly smaller than asteroids, ranging in size from grains to objects up to a meter wide. Objects smaller than this are classified as mic ...
*
NEODyS
NEODyS (Near Earth Objects Dynamic Site) is an Italian service that provides information on near-Earth objects with a Web-based interface. It is based on a continually and (almost) automatically maintained database of near earth asteroid orbits. T ...
*
Time-domain astronomy
Time-domain astronomy is the study of how astronomical objects change with time. Though the study may be said to begin with Galileo's ''Letters on Sunspots'', the term now refers especially to variable objects beyond the Solar System. This may be d ...
Notes
References
External links
Sentry: IntroductionSentry: Impact Risk Data table* List o
* Similar lists:
ttps://newton.spacedys.com/neodys/index.php?pc=4.1 NEODyS CLOMON2ESA NEO Risk ListAsteroid Hazards, Part 3: Finding the Path–
Minor Planet Center
The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union (IAU). Founded in 1947, it operates at the Smithsonian Astrophysical Observatory.
Function
...
on YouTube
{{DEFAULTSORT:Sentry (Monitoring System)
Near-Earth object tracking
Planetary defense
Space program of the United States
JPL online services