Potentially hazardous object

The asteroid Toutatis is listed as a potentially hazardous near-Earth asteroid, yet poses no immediate threat to Earth. (Radar image taken by GDSCC in 1996.)

A potentially hazardous object (PHO) is a near-Earth object – either an asteroid or a comet – with an orbit that can make exceptionally close approaches to the Earth and large enough to cause significant regional damage in the event of impact.^[1]

Most of these objects are potentially hazardous asteroids (PHAs), defined as having a minimum orbital intersection distance with Earth of less than 0.05 astronomical units (19.5 lunar distances) and an absolute magnitude of 22 or brighter.^[2] As of January 2018 there are 1,885 known PHAs (about 11% of the total near-Earth population), of which 157 are estimated to be larger than one kilometer in diameter (see list of largest PHAs below).^{[3][4][lower-alpha 1]} Most of the discovered PHAs are Apollo asteroids (1,601) and fewer belong to the group of Aten asteroids (169).^[5][6]

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. Potentially hazardous asteroids are normally only a hazard on a time scale of hundreds of years as the known orbit becomes more divergent. After several astronomical surveys, the number of known PHAs has increased tenfold since the end of the 1990s (see bar charts below).^[3] The Minor Planet Center's website List of the Potentially Hazardous Asteroids also publishes detailed information for these objects.^[7]

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 of at least 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.^[8]

Numbers

Detected NEAs by various projects. The broader class of NEAs includes all PHAs as a subset.^[3]

LINEAR   NEAT   Spacewatch   LONEOS   CSS

Pan-STARRS   NEOWISE   ATLAS   Others

In 2012 NASA estimated 20 to 30 percent of these objects have been found.^[8] 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.

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.

Largest PHAs

With a mean-diameter of approximately 7 kilometers, the 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 objects brightness and its (assumed) albedo. They are only an approximation.). The lowest numbered PHA is 1566 Icarus.^[7]

Gallery

• 
  Radar image of the 360-meter PHA (308635) 2005 YU55

See also

• Asteroid impact avoidance
• Earth-grazing fireball
• Global catastrophic risk
• List of asteroid close approaches to Earth
• Scout (NASA program), short term warnings
• Sentry (monitoring system)
• Spaceguard

Notes

References

External links

• Sentry Risk Table (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