Potentially hazardous object

Overview

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

Main article: Torino scale, Palermo scale

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 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. 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 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 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.).

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.

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Gallery

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

Notes

References

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References

1. (12 May 2021). "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.". [[Business Insider]].
2. Bartels, Meghan. (1 May 2021). "How did you spend your week? NASA pretended to crash an asteroid into Earth.". [[Space.com]].
3. (30 April 2021). "Planetary Defense Conference Exercise – 2021 Planetary Defense Conference (virtually) in Vienna, Austria, April 26–April 30, 2021.". [[NASA]].
4. Chang, Kenneth. (23 May 2016). "How Big Are Those Killer Asteroids? A Critic Says NASA Doesn't Know.". The New York Times.
5. Myhrvold, Nathan. (23 May 2016). "Asteroid thermal modeling in the presence of reflected sunlight with an application to WISE/NEOWISE observational data".
6. Billings, Lee. (27 May 2016). "For Asteroid-Hunting Astronomers, Nathan Myhrvold Says the Sky Is Falling". [[Scientific American]].
7. NASA Administrator. (25 May 2016). "NASA Response to Recent Paper on NEOWISE Asteroid Size Results". [[NASA]].
8. Myhrvold, Nathan. (2018). "An empirical examination of WISE/NEOWISE asteroid analysis and results". [[Icarus (journal).
9. Chang, Kenneth. (14 June 2018). "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.". The New York Times.
10. (17 [[Sentry (monitoring system)#Numbers. Sentry "risk-listed"]] PHAs / [https://cneos.jpl.nasa.gov/stats/totals.html 2289 known PHAs]) ≈ 0.74%