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.[1] They are conventionally defined as having a minimum orbit intersection distance with Earth of less than 0.05 astronomical units (19.5 lunar distances) and an absolute magnitude of 22 or brighter, the latter of which roughly corresponds to a size larger than 140 meters.[2] More than 99% of the known potentially hazardous objects are no impact threat over the next 100 years.[3] As of September 2022[update], just 17 of the known potentially hazardous objects listed on the Sentry Risk Table could not be excluded as potential threats over the next hundred years.[4] Over hundreds if not thousands of years though, the orbits of some "potentially hazardous" asteroids can evolve to live up to their namesake.
Most of these objects are potentially hazardous asteroids (PHAs), and a few are comets. As of November 2022[update] 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).[5][6][a] Most of the discovered PHAs are Apollo asteroids (1,965) and fewer belong to the group of Aten asteroids (185).[7][8]
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. As of September 2022[update], only 17 potentially hazardous asteroids are listed on the Sentry Risk Table.[4] 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 (see bar charts below).[5] The Minor Planet Center's website List of the Potentially Hazardous Asteroids also publishes detailed information for these objects.[9]
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.[10][11][12]
Overview
Plot of orbits of known potentially hazardous asteroids, with sizes over 140 metres (460 ft) and that pass within 7.6 million kilometres (4.7×10^6 mi) of Earth's orbit. Epoch as of early 2013.
An object is considered a PHO if its minimum orbit intersection distance (MOID) with respect to Earth is less than 0.05 AU (7,500,000 km; 4,600,000 mi) – approximately 19.5 lunar distances – and its absolute magnitude is brighter than 22, approximately corresponding to a diameter above 140 meters (460 ft).[1][2] 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.[13]
In 2012 NASA estimated 20 to 30 percent of these objects have been found.[13] During an asteroid's close approaches to planets or moons other than the Earth, 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.
Asteroids larger than approximately 35 meters across can pose a threat to a town or city.[14] 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.[2]
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.[15][16][17] Although the early original criticism had not undergone peer review,[18] a more recent peer-reviewed study was subsequently published.[19][20]
Largest PHAs
With a mean diameter of approximately 7 kilometers, Apollo asteroid (53319) 1999 JM8 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.). The lowest numbered PHA is 1566 Icarus.[9]
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.[5] PHAs brighter than absolute magnitude 17.75 are likely larger than 1 km in size.
Brightest PHA discoveries of each calendar year since 1989 [9]
PHA-KM: potentially hazardous asteroids larger than 1 kilometer since 1999 – Cumulative number of discovered PHA by end of year (first of December). As of September 2020, there are 157 known PHAs larger than one kilometer.[5]
500
1,000
1,500
2,000
2,500
3,000
prev.
1999
2001
2003
2005
2007
2009
2011
2013
2015
2017
2019
PHA: total number of potentially hazardous asteroids since 1999 – Cumulative number of all discovered PHA by end of year (first of December). As of September 2020, there are 2115 PHAs.[5]
Scout (NASA program) – JPL program to monitor the Minor Planet Center's catalog for Earth impactsPages displaying short descriptions of redirect targets; short term warnings
Spaceguard – Efforts to study asteroids that might impact Earth
Notes
^An object's calculated mean-diameter is only a rough estimate. It is inferred from the object's varying brightness—observed and measured at various times—and the assumed, yet often unknown reflectivity of its surface. NASA's Asteroid Size Estimator is a tool for a generic absolute magnitude-to-diameter conversion for an assumed geometric albedo.
^Myhrvold, Nathan (23 May 2016). "Asteroid thermal modeling in the presence of reflected sunlight with an application to WISE/NEOWISE observational data". arXiv:1605.06490v2 [astro-ph.EP].
