Potentially hazardous object explained

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 0.05AU 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 are no impact threat over the next 100 years., 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. 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. 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 (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 (see bar charts below). 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.[1] [2] [3]

Overview

An object is considered a PHO if its minimum orbit intersection distance (MOID) with respect to Earth is less than 0.05abbr=onNaNabbr=on – approximately 19.5 lunar distances – and its absolute magnitude is brighter than 22, approximately corresponding to a diameter above 140m (460feet). 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

See main article: Torino scale and Palermo Technical Impact Hazard Scale.

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 comet

A potentially hazardous comet (PHC) is a short-period comet which currently has an Earth-MOID less than 0.05 AU. 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.

Numbers

In 2012 NASA estimated 20 to 30 percent of these objects have been found. 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.

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 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.[4] [5] [6] Although the early original criticism had not undergone peer review,[7] a more recent peer-reviewed study was subsequently published.[8] [9]

Largest PHAs

With a mean diameter of approximately 7 kilometers, 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.). The lowest numbered PHA is 1566 Icarus.

Designation !! colspan=3
Discovery !(H)
(mag)
D
(km)
Orbital description Remarks References
width=45 Year !width=115 Discoverer !Class a
i
q
Q
MOID
data-sort-value="4953" 1990413R. H. McNaught14.13APO1.6210.65824.40.5552.6870.02640
data-sort-value="3122" 3122 Florence1981413S. J. Bus14.15AMO1.7690.42322.21.0202.5180.04430
data-sort-value="16960" 1998704LINEAR14.34APO2.2030.85817.50.3134.0930.01443
data-sort-value="4183" 4183 Cuno1959074C. Hoffmeister14.44APO1.9820.6346.70.7253.2400.02825
data-sort-value="3200" 3200 Phaethon1983500IRAS14.65.8APO1.2710.89022.30.1402.4020.01945
data-sort-value="242450" 2004E12Siding Spring Survey14.73APO1.0840.47737.00.5671.6010.04686
data-sort-value="89830" 2002704LINEAR14.93.1AMO2.0770.50743.71.023 3.1310.02767
data-sort-value="137427" 1999704LINEAR15.12.9APO2.4480.61039.20.9553.9420.01787
data-sort-value="111253" 2001704LINEAR15.23APO1.7540.43942.00.9832.5240.02934
data-sort-value="53319" 1999704LINEAR15.27APO2.7260.64113.8 0.978 4.4740.02346Likely largest PHO
data-sort-value="1981" 1981 Midas1973675C. T. Kowal15.22APO1.7760.65039.80.6212.9310.00449
data-sort-value="2201" 2201 Oljato1947690H. L. Giclas15.252.1APO2.1750.7132.50.6243.7260.00305
data-sort-value="90075" 2002644NEAT15.32.2APO2.1340.5768.90.9043.3630.03010
data-sort-value="4179" 4179 Toutatis1989 010C. Pollas15.302.5APO2.5360.6290.40.9404.1320.00615
data-sort-value="159857" 2004704LINEAR15.43APO2.2640.59323.10.9203.6070.01682
data-sort-value="85713" 1998704LINEAR15.63.5APO1.9240.63910.80.6943.1540.00234
data-sort-value="4486" 4486 Mithra1987071E. W. Elst
V. G. Shkodrov
15.62APO2.2000.6633.00.7423.6580.04626
data-sort-value="1620" 1620 Geographos1951675A. G. Wilson
R. Minkowski
15.602.5APO1.2450.33513.30.8281.6630.03007
data-sort-value="415029" 2011703CSS15.72.5APO2.1220.65334.90.7363.5090.01925
data-sort-value="242216" 2003699LONEOS15.72.5AMO2.2310.54139.61.0243.4380.00956
data-sort-value="12923" 12923 Zephyr1999699LONEOS15.82APO1.9620.4925.30.9962.9270.02115
data-sort-value="12923" 1998566NEAT15.82APO2.3800.5735.91.0173.7430.01573

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.

Number! width=85
Namewidth=35 Yearwidth=35 (H)Refs
align=right 4179Toutatis198915.3
align=right 4953199014.9
align=right 7341199117.0
align=right 10115199217.2
align=right 39572199316.6
align=right 7482199416.7
align=right 243566199517.4
align=right 8566199616.3
align=right 35396199717.0
align=right 16960199814.4
align=right 137427199915.3
align=right 138095200016.0
align=right 111253200115.3
align=right 89830200215.0
align=right 242216200315.7
align=right 242450200414.6
align=right 308242200516.3
align=right 374851200616.7
align=right 214869200716.5
align=right 294739200817.1
align=right 523630200916.2
align=right 458122201017.6
align=right 415029201115.9
align=right 201217.9
align=right 507716201316.4
align=right 533671201416.1
align=right 201517.6
align=right 620095201617.6
align=right 201717.9
align=right 201817.7
align=right 201918.1
align=right 202017.7
align=right 202117.7
align=right 202217.3

See also

External links

Minor Planet Center

Notes and References

  1. News: McFall-Johnsen . Morgan . Woodward . Aylin . A NASA simulation revealed that 6 months' warning isn't enough to stop an asteroid from hitting Earth. We'd need 5 to 10 years.. 12 May 2021 . . 14 May 2021 .
  2. News: Bartels . Meghan . How did you spend your week? NASA pretended to crash an asteroid into Earth. . 1 May 2021 . . 14 May 2021 .
  3. News: Chodas . Paul . Khudikyan . Shakeh . Chamberlin . Alan . Planetary Defense Conference Exercise - 2021 Planetary Defense Conference (virtually) in Vienna, Austria, April 26–April 30, 2021. . 30 April 2021 . . 14 May 2021 .
  4. News: Chang . Kenneth . How Big Are Those Killer Asteroids? A Critic Says NASA Doesn't Know. . 23 May 2016 . . 24 May 2016 .
  5. Myhrvold . Nathan . Asteroid thermal modeling in the presence of reflected sunlight with an application to WISE/NEOWISE observational data . 23 May 2016 . 1605.06490v2. astro-ph.EP .
  6. Web site: Billings . Lee . For Asteroid-Hunting Astronomers, Nathan Myhrvold Says the Sky Is Falling . 27 May 2016 . . 28 May 2016 .
  7. News: NASA Administrator . NASA Response to Recent Paper on NEOWISE Asteroid Size Results . 25 May 2016 . . 29 May 2016 .
  8. Myhrvold . Nathan . Nathan Myhrvold . An empirical examination of WISE/NEOWISE asteroid analysis and results . . 314 . 64–97 . 10.1016/j.icarus.2018.05.004 . 2018Icar..314...64M . 2018 . free .
  9. News: Chang . Kenneth . Asteroids and Adversaries: Challenging What NASA Knows About Space Rocks - Two years ago, NASA dismissed and mocked an amateur's criticisms of its asteroids database. Now Nathan Myhrvold is back, and his papers have passed peer review. . 14 June 2018 . . 14 June 2018 .