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 which is large enough to cause significant regional damage in the event of impact. They are conventionally defined as having a minimum orbit intersection distance with Earth of less than and an absolute magnitude of 22 or brighter, the latter of which roughly corresponds to a size larger than 140 meters. More than 99% of the known potentially hazardous objects pose no impact threat over the next 100 years. , just 21 of the known potentially hazardous objects listed on the Sentry Risk Table could not be excluded as potential threats over the next hundred years. Over hundreds if not thousands of years, though, the orbits of some "potentially hazardous" asteroids can evolve to live up to their name.

Most of these objects are potentially hazardous asteroids (PHAs), and a few are comets. 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 . Most of the discovered PHAs are Apollo asteroids (1,965) and fewer belong to the group of Aten asteroids (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 surveys, the number of known PHAs has increased tenfold since the end of the 1990s . The Minor Planet Center'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, as most recently based on a simulated exercise conducted by the 2021 Planetary Defense Conference.

Overview

An object is considered a PHO if its minimum orbit intersection distance (MOID) with respect to Earth is less than approximately 19.5 lunar distancesand its absolute magnitude 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 in the case of an ocean impact. Such impact events occur on average around once per 10,000 years. NEOWISE 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 scale and the Torino scale.

Potentially hazardous comet

A potentially hazardous comet (PHC) is a comet which meets the definition for a PHO. Known PHCs include: 109P/Swift-Tuttle, 55P/Tempel–Tuttle, 15P/Finlay, 289P/Blanpain, 255P/Levy, 206P/Barnard–Boattini, 21P/Giacobini–Zinner, and 73P/Schwassmann–Wachmann. Halley's Comet fit the criteria before AD 837, when it passed the Earth at a distance of 0.033 AU. It now has an MOID of 0.075 AU.

Numbers

In 2012 NASA estimated 20 to 30 percent of these objects have been found. During an asteroid's close approaches to another planet it will be subject to gravitational perturbation, 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 astronomical survey projects such as Lincoln Near-Earth Asteroid Research, Catalina Sky Survey and Pan-STARRS continue to search for more PHOs. Each one found is studied by various means, including optical, radar, and infrared to determine its characteristics, such as size, composition, rotation state, and to more accurately determine its orbit. Both professional and amateur astronomers participate in such observation and tracking.

Size

Asteroids larger than approximately 35 meters (approximately 115 feet) 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 NASA and the Jet Propulsion Laboratory use the more practical measure of absolute magnitude (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 which is also not usually known for certain. The NASA near-Earth object program uses an assumed albedo of 0.14 for this purpose. In May 2016, the asteroid size estimates arising from the Wide-field Infrared Survey Explorer and NEOWISE 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 7kilometers (), Apollo asteroid is likely the largest known potentially hazardous object, despite its fainter absolute magnitude 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.).

+ Brightest Potentially Hazardous Asteroids Designation Discovery (H)(mag) D(km) Orbital description Remarks References Year Place Discoverer Class a e i q Q MOID 1990 413 R. H. McNaught 14.1 3 APO 1.621 0.658 24.4 0.555 2.687 0.02640 — 3122 Florence 1981 413 S. J. Bus 14.1 5 AMO 1.769 0.423 22.2 1.020 2.518 0.04430 — 1998 704 LINEAR 14.3 4 APO 2.203 0.858 17.5 0.313 4.093 0.01443 — 4183 Cuno 1959 074 C. Hoffmeister 14.4 4 APO 1.982 0.634 6.7 0.725 3.240 0.02825 — 3200 Phaethon 1983 500 IRAS 14.6 5.8 APO 1.271 0.890 22.3 0.140 2.402 0.01945 — 2004 E12 Siding Spring Survey 14.7 3 APO 1.084 0.477 37.0 0.567 1.601 0.04686 — 2002 704 LINEAR 14.9 3.1 AMO 2.077 0.507 43.7 1.023 3.131 0.02767 — 1999 704 LINEAR 15.1 2.9 APO 2.448 0.610 39.2 0.955 3.942 0.01787 — 2001 704 LINEAR 15.2 3 APO 1.754 0.439 42.0 0.983 2.524 0.02934 — 1999 704 LINEAR 15.2 7 APO 2.726 0.641 13.8 0.978 4.474 0.02346 Likely largest PHO 1981 Midas 1973 675 C. T. Kowal 15.2 2 APO 1.776 0.650 39.8 0.621 2.931 0.00449 — 2201 Oljato 1947 690 H. L. Giclas 15.25 2.1 APO 2.175 0.713 2.5 0.624 3.726 0.00305 — 2002 644 NEAT 15.3 2.2 APO 2.134 0.576 8.9 0.904 3.363 0.03010 — 4179 Toutatis 1989 010 C. Pollas 15.30 2.5 APO 2.536 0.629 0.4 0.940 4.132 0.00615 — 2004 704 LINEAR 15.4 3 APO 2.264 0.593 23.1 0.920 3.607 0.01682 — 1998 704 LINEAR 15.6 3.5 APO 1.924 0.639 10.8 0.694 3.154 0.00234 — 4486 Mithra 1987 071 E. W. ElstV. G. Shkodrov 15.6 2 APO 2.200 0.663 3.0 0.742 3.658 0.04626 — 1620 Geographos 1951 675 A. G. WilsonR. Minkowski 15.60 2.5 APO 1.245 0.335 13.3 0.828 1.663 0.03007 — 2011 703 CSS 15.7 2.5 APO 2.122 0.653 34.9 0.736 3.509 0.01925 — 2003 699 LONEOS 15.7 2.5 AMO 2.231 0.541 39.6 1.024 3.438 0.00956 — 12923 Zephyr 1999 699 LONEOS 15.8 2 APO 1.962 0.492 5.3 0.996 2.927 0.02115 — 1998 566 NEAT 15.8 2 APO 2.380 0.573 5.9 1.017 3.743 0.01573 —

Statistics

Below is a list of the largest PHAs (based on absolute magnitude,) discovered in a given year. Historical data of the cumulative number of discovered PHA since 1999 are displayed in the bar chartsone 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. + Brightest PHA discoveries of each calendar year since 1989 Number Name Year (H) Refs 4179 Toutatis 1989 15.3 4953 1990 14.9 7341 1991 17.0 10115 1992 17.2 39572 1993 16.6 7482 1994 16.7 243566 1995 17.4 8566 1996 16.3 35396 1997 17.0 16960 1998 14.4 137427 1999 15.3 138095 2000 16.0 111253 2001 15.3 89830 2002 15.0 242216 2003 15.7 242450 2004 14.6 308242 2005 16.3 374851 2006 16.7 214869 2007 16.5 294739 2008 17.1 523630 2009 16.2 458122 2010 17.6 415029 2011 15.9 746849 2012 17.9 507716 2013 16.4 533671 Nabu 2014 16.1 802885 2015 17.6 — 620095 2016 17.6 – 2017 17.9 — 756316 2018 17.7 – 2019 18.1 — – 2020 17.7 — – 2021 17.7 — – 2022 17.3 — 887030 2023 18.3 — – 2024 19.0 —

Gallery

See also

Asteroid impact avoidance Earth-grazing fireball Global catastrophic risk List of asteroid close approaches to Earth ; short term warnings

Notes

References (Click "Use Unconstrained Settings" and select "H<=22" for list of PHAs)

(17 Sentry "risk-listed" PHAs / 2289 known PHAs) ≈ 0.74%

External links Sentry: Earth Impact Monitoring (current) (NASA NEO Program) Very Close Approaches (<0.01 AU) of PHAs to Earth 1900-2200 TECA Table of Asteroids Next Closest Approaches to the Earth, Sormano Astronomical Observatory 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 List of the Potentially Hazardous Asteroids (PHAs)