A potentially hazardous object (PHO) is a

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). ...

– either an

asteroid An asteroid is a minor planet of the 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 or icy bodies with no atmosphere. ...

or a

comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...

– with an

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

that can make close approaches to the

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 surfa ...

and is large enough to cause significant regional damage in the event of impact. They are defined as having a

minimum orbit intersection distance Minimum orbit intersection distance (MOID) is a measure used in astronomy to assess potential close approaches and collision risks between astronomical objects. It is defined as the distance between the closest points of the osculating orbits o ...

with Earth of less than and an

absolute magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse Logarithmic scale, logarithmic Magnitude (astronomy), astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent mag ...

of 22 or brighter. More than 99% of the known potentially hazardous objects are not an impact threat over the next 100 years. , only 17 potentially hazardous objects are listed on the

Sentry Risk Table 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 catalog for possibilities of future impact with Earth over the next 100+ y ...

as objects that are known not to be a threat over the next hundred years are excluded. Over hundreds if not thousands of years, "potentially hazardous" asteroids have the potential for their orbits to evolve to live up to their namesake. Most of these objects are potentially hazardous asteroids (PHAs), and a few are

s. there are 2,304 known PHAs (about 8% of the total near-Earth population), of which 153 are estimated to be larger than one kilometer in diameter ''(see list of largest PHAs below)''. Most of the discovered PHAs are

Apollo asteroids The Apollo asteroids are a group of near-Earth asteroids named after 1862 Apollo, discovered by German astronomer Karl Reinmuth in the 1930s. They are Earth-crossing asteroids that have an orbital semi-major axis greater than that of the Eart ...

(1,965) and fewer belong to the group of

Aten asteroid The Aten asteroids are a dynamical group of asteroids whose orbits bring them into proximity with Earth. By definition, Atens are Earth-crossing asteroids . The group is named after 2062 Aten, the first of its kind, discovered on 7 January ...

s (185). A potentially hazardous object can be known not to be a threat to Earth for the next 100 years or more, if its orbit is reasonably well determined. Potentially hazardous asteroids with some threat of impacting Earth in the next 100 years are listed on the Sentry Risk Table. , only 17 potentially hazardous asteroids are listed on the Sentry Risk Table. Most potentially hazardous asteroids are ruled out as hazardous to at least several hundreds of years when their competing best orbit models are sufficiently constrained, but recent discoveries whose orbital constraints are little-known have divergent or incomplete mechanical models until observation yields further data. After several

astronomical survey An astronomical survey is a general map or image of a region of the sky (or of the whole sky) that lacks a specific observational target. Alternatively, an astronomical survey may comprise a set of images, spectra, or other observations of obje ...

s, the number of known PHAs has increased tenfold since the end of the 1990s ''(see bar charts below)''. The

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 ...

's website ''List of the Potentially Hazardous Asteroids'' also publishes detailed information for these objects. In May 2021, NASA astronomers reported that 5 to 10 years of preparation may be needed to avoid a potential impactor based on a simulated exercise conducted by the 2021 Planetary Defense Conference.

Overview

An object is considered a PHO if its

(MOID) with respect to Earth is less than – approximately 19.5 lunar distances – and its

is brighter than 22, approximately corresponding to a diameter above . This is big enough to cause regional devastation to human settlements unprecedented in human history in the case of a land impact, or a major

tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater explo ...

in the case of an

ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...

impact. Such impact events occur on average around once per 10,000 years.

NEOWISE Wide-field Infrared Survey Explorer (WISE, observatory code C51, Explorer 92 and SMEX-6) is a NASA infrared astronomy space telescope in the Explorers Program. It was launched in December 2009, and placed in hibernation mode in February 2011, ...

data estimates that there are 4,700 ± 1,500 potentially hazardous asteroids with a diameter greater than 100 meters.

Levels of hazard

The two main scales used to categorize the impact hazards of asteroids are the Palermo Technical Impact Hazard Scale and the Torino scale.

Potentially hazardous comets

Short-period comets currently with an Earth-

MOID Minimum orbit intersection distance (MOID) is a measure used in astronomy to assess potential close approaches and collision risks between astronomical objects. It is defined as the distance between the closest points of the osculating orbits of ...

less than 0.05 AU include: 109P/Swift-Tuttle, 55P/Tempel–Tuttle, 15P/Finlay, 289P/Blanpain, 255P/Levy, 206P/Barnard–Boattini, 21P/Giacobini–Zinner, and

73P/Schwassmann–Wachmann 73P/Schwassmann–Wachmann, also known as Schwassmann–Wachmann 3, is a periodic comet that has a 5.4 year orbital period and that has been actively disintegrating since 1995. It last came to perihelion (closest approach to the Sun) in March 20 ...

Numbers

In 2012

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

estimated 20 to 30 percent of these objects have been found. During an asteroid's close approaches to planets or moons other than the

, it will be subject to gravitational

perturbation 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 * Perturbat ...

, modifying its orbit, and potentially changing a previously non-threatening asteroid into a PHA or vice versa. This is a reflection of the dynamic character of the Solar System. Several

projects such as

Lincoln Near-Earth Asteroid Research The Lincoln Near-Earth Asteroid Research (LINEAR) project is a collaboration of the United States Air Force, NASA, and the Massachusetts Institute of Technology's Lincoln Laboratory for the systematic detection and tracking of near-Earth objects ...

Catalina Sky Survey Catalina Sky Survey (CSS; obs. code: 703) is an astronomical survey to discover comets and asteroids. It is conducted at the Steward Observatory's Catalina Station, located near Tucson, Arizona, in the United States. CSS focuses on the search ...

and

Pan-STARRS The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1; List of observatory codes, obs. code: IAU code#F51, F51 and Pan-STARRS2 obs. code: IAU code#F52, F52) located at Haleakala Observatory, Hawaii, US, consists of astronomical c ...

continue to search for more PHOs. Each one found is studied by various means, including optical,

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 ...

, and

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 around ...

to determine its characteristics, such as size, composition, rotation state, and to more accurately determine its orbit. Both professional and

amateur astronomer Amateur astronomy is a hobby where participants enjoy observing or imaging celestial objects in the sky using the unaided eye, binoculars, or telescopes. Even though scientific research may not be their primary goal, some amateur astronomers ...

s participate in such observation and tracking.

Size

Asteroids larger than approximately 35 meters across can pose a threat to a town or city. However the diameter of most small asteroids is not well determined, as it is usually only estimated based on their brightness and distance, rather than directly measured, e.g. from radar observations. For this reason

and the

Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States. Founded in the 1930s by Caltech researchers, JPL is owned by NASA an ...

use the more practical measure of

(''H''). Any asteroid with an absolute magnitude of 22.0 or brighter is assumed to be of the required size. Only a coarse estimation of size can be found from the object's magnitude because an assumption must be made for its

albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, 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 ...

which is also not usually known for certain. The NASA

program uses an assumed

of 0.14 for this purpose. In May 2016, the asteroid size estimates arising from the

Wide-field Infrared Survey Explorer Wide-field Infrared Survey Explorer (WISE, observatory code C51, Explorer 92 and SMEX-6) is a NASA infrared astronomy space telescope in the Explorers Program. It was launched in December 2009, and placed in hibernation mode in February 2011, ...

and

missions have been questioned. Although the early original criticism had not undergone peer review, a more recent peer-reviewed study was subsequently published.

Largest PHAs

With a mean diameter of approximately 7 kilometers, Apollo asteroid is likely the largest known potentially hazardous object, despite its fainter

of 15.2, compared to other listed objects in the table below ''(note: calculated mean-diameters in table are inferred from the object's brightness and its (assumed) albedo. They are only an approximation.)''. The lowest numbered PHA is 1566 Icarus.

Statistics

Below is a list of the largest PHAs (based on absolute magnitude H) discovered in a given year. Historical data of the cumulative number of discovered PHA since 1999 are displayed in the bar charts—one for the total number and the other for objects larger than one kilometer. PHAs brighter than absolute magnitude 17.75 are likely larger than 1 km in size.

Gallery

File:2005YU55-20111107.jpg, Radar image of the 350-meter PHO

Notes

References

External links

Sentry: Earth Impact Monitoring (current)
(

NEO Program)
Very Close Approaches (<0.01 AU) of PHAs to Earth 1900-2200

''

Sormano Astronomical Observatory The Sormano Astronomical Observatory ( it, Osservatorio Astronomico Sormano, obs. code: 587) is an astronomical observatory north of Milan, Italy. Located near the Swiss border at 1000 meters elevation at the mountain village of Sormano in the p ...

''
MBPL - Minor Body Priority List (PHA Asteroids)
''Sormano Astronomical Observatory''
Responding to the Threat of Potentially-Hazardous Near Earth Objects
(PDF)
Risk of comet hitting Earth is greater than previously thought, say researchers
''The Guardian'', 22 December 2015
Conversion of Absolute Magnitude to Diameter
''Minor Planet Center''

Minor Planet Center

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