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A potentially hazardous object (PHO) is a near-Earth asteroid or comet with an orbit such that it has the potential to make close approaches to the Earth and is of a size large enough to cause significant regional damage in the event of impact. A potentially hazardous object with a reasonably well determined orbit can be known not to be a threat to Earth for the next 100 years or more. Potentially hazardous asteroids with a threat of impacting Earth in the next 100 years are listed on the Sentry Risk Table. NASA had listed a cumulative total of 1,639 discovered potentially hazardous asteroids (PHAs)〔〔 and only 211 have an observation arc less than 30 days. Among these, there are 153 PHAs believed to be larger than one kilometer in diameter.〔 However a calculated diameter is only a rough estimate, as it is inferred from the object's varying brightness—observed and measured at various times—and the assumed, yet unknown reflectivity of its surface (albedo).〔 Most of the discovered PHAs are Apollo asteroids (1,363)〔 and fewer belong to the group of Aten asteroids (147).〔 Due to several astronomical surveys, the number of known PHAs has increased tenfold since the end of the 1990s ''(see bar charts below)''. These surveys have led to a total number of 12,933 discovered near-Earth Objects, most of them being asteroids and some 85 near-Earth comets (NECs).〔 The Minor Planet Center's website ''Unusual Minor Planets'' also publishes detailed statistics for these objects.〔 == 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 distances—and its diameter is at least —corresponding to absolute magnitude ''H'' < 22. 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.〔 As of 2012, an estimated 20 to 30 percent of these objects have been found.〔 Asteroids larger than 35 meters across can pose a threat to a town or city.〔 The diameter of most small asteroids is not well known and can only be estimated based on their brightness and distance. 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,〔〔 although 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 0.13 for this purpose.〔 Several astronomical survey projects such as Lincoln Near-Earth Asteroid Research and Catalina Sky Survey 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 monitoring. During an asteroid's close approaches to planets or moons 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. The two main scales used to categorize the impact hazards of asteroids are the Palermo Technical Impact Hazard Scale and the Torino Scale. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Potentially hazardous object」の詳細全文を読む スポンサード リンク
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