Monitoring Potential Asteroid Impacts

A near-Earth object (NEO) is any small Solar System body whose orbit can bring it into proximity with Earth. By definition, a Solar System body is a NEO if its closest approach to the Sun (perihelion) is less than 1.3 astronomical unit (AU). If a NEO's orbit crosses that of the Earth's and the object is larger than 140 meters across, it is considered a potentially hazardous object (PHO). Most known PHOs and NEOs are asteroids.

Known NEOs include more than seventeen thousand near-Earth asteroids (NEAs), more than one hundred near-Earth comets (NECs), and a number of solar-orbiting spacecraft and meteoroids, large enough to be tracked in space before striking the Earth. It is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of the Earth. NEOs have become of increased interest since the 1980s because of increased awareness of the potential danger some of the asteroids or comets pose, and methods of mitigation are being researched.

Based on the orbit calculations of NEOs, the risk of future impacts is assessed on two scales, the Torino scale and the more complex Palermo scale, both of which rate a risk of any significance with values above 0. Some NEOs have had positive initial Torino or Palermo scale ratings after their discovery, but as of January 2018, more precise calculations based on subsequent observations led to a reduction of the rating to or below 0 in all cases.

The United States, European Union, and other nations are currently scanning for NEOs in an effort called Spaceguard. In the United States and since 1998, NASA has a congressional mandate to catalogue all NEOs that are at least 1 kilometer (km) wide, as the impact of such an object would be globally catastrophic. In 2006, it was estimated that 20% of the mandated objects had not yet been found. In 2011, NASA estimated that, largely as a result of its NEOWISE survey program, 93% of the NEAs larger than 1 km had been found and that only about 70 remained to be discovered. As of January 5, 2018, 886 NEAs larger than 1 km have been discovered, of which 157 are potentially hazardous. The inventory is much less complete for smaller objects, which still have potential for large scale, though not global, damage. To improve on that, NASA's Spaceguard mandate was extended in 2005 to objects at least 140 meters in diameter, and since 2016, NASA's Planetary Defense Coordination Office aims to track NEOs larger than 30 to 50 meters in diameter.

Due to their Earth-like orbits and low surface gravity, some NEOs can be approached by spacecraft with a relatively low energy (and thus fuel) expenditure. Since the 1970s, both manned and unmanned missions have been proposed. As of January 2018, three near-Earth asteroids have been visited by spacecraft, and two more are en route for future rendezvous. Plans to mine NEAs commercially have been picked up by a private company.

Risk as a Near Earth Object Frame of Reference

From the late 1990s, a typical frame of reference for looking at NEOs has been through the scientific concept of risk. In this frame, the risk that any near-Earth object poses is typically seen through a lens that is a function of both the culture and the technology of human society. Through history, humans have associated NEOs with changing risks, based on religious, philosophical or scientific views, as well as humanity's technological or economical capability to deal with risks. Thus, NEOs have been seen as omens of natural disasters or wars, harmless spectacle in an unchanging universe, the source of era-changing cataclysms, the source of potentially poisonous fumes (during Earth's passage through the tail of Halley's Comet in 1910), and finally as the causes of crater-forming impacts that can even cause extinction.

For near-Earth comets, the potential of catastrophic impacts was recognised as soon as orbit calculations have been made: in 1694, Edmond Halley presented a theory that Noah's flood in the Bible was caused by a comet impact. Human perception of near-Earth asteroids as benign objects of fascination or killer objects with high risk to human society has ebbed and flowed in the short period of human history that NEAs have been scientifically observed. The modern awareness of the threat of impacts that create craters much bigger than the impacting bodies and have indirect effects on an even wider area arose in the 1990s, after the confirmation of a theory that the Cretaceous–Paleogene extinction event (in which dinosaurs died out) 65 million years ago was caused by a large asteroid impact, and the observation of the impact of the fragments of Comet Shoemaker–Levy 9 into Jupiter in July 1994.

                               https://en.wikipedia.org/wiki/Near-Earth_object

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See Also

Asteroid Impact Avoidance at https://en.wikipedia.org/wiki/Asteroid_impact_avoidance