Near-Earth objects | Wikipedia audio article
Channel: wikipedia tts
Date: 2019-06-09

This is an audio version of the Wikipedia Article:
https://en.wikipedia.org/wiki/Near-Earth_object


00:02:37 1 Definitions
00:04:50 2 History of human awareness of NEOs
00:05:06 2.1 Risk
00:05:17 2.1.1 Risk scales
00:08:51 2.1.2 Magnitude of risk
00:08:59 2.1.3 Highly rated risks
00:11:12 2.2 Projects to minimize the threat
00:11:58 3 Number and classification
00:13:51 3.1 Observational biases
00:18:17 3.2 Near-Earth asteroids
00:22:03 3.2.1 Size distribution
00:23:17 3.2.2 Orbital classification
00:25:31 3.2.3 Co-orbital asteroids
00:28:01 3.3 Meteoroids
00:32:01 3.4 Near-Earth comets
00:32:44 3.5 Artificial near-Earth objects
00:34:01 4 Impacts
00:38:03 5 Close approaches
00:39:06 6 Exploratory missions
00:43:08 6.1 Missions to NEAs
00:45:10 6.2 Missions to NECs
00:49:37 7 See also
00:53:10 8 References
00:53:47 9 External links
00:57:36 Missions to NECs



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SUMMARY
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A near-Earth object (NEO) is any small Solar System body whose orbit brings it to proximity with Earth. By convention, a Solar System body is a NEO if its closest approach to the Sun (perihelion) is less than 1.3 astronomical units (AU). If a NEO's orbit crosses the Earth's and the object is larger than 140 meters (460 ft) across, it is considered a potentially hazardous object (PHO). Most known PHOs and NEOs are asteroids, but a small fraction are comets.There are over 20,000 known near-Earth asteroids (NEAs), over a hundred short-period 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 greater awareness of the potential danger.  Asteroids as small as 20 m can damage the local environment and populations. Larger asteroids penetrate the atmosphere to the surface of the Earth, producing craters or tsunamis.  Asteroid impact avoidance by deflection is possible in principle, and methods of mitigation are being researched.Based on how well constrained the orbit calculations of identified NEOs are, two scales, the Torino scale and the more complex Palermo scale, rate a risk.  Some NEOs have had temporarily positive Torino or Palermo scale ratings after their discovery, but as of March 2018, more precise calculations based on longer observation arcs led to a reduction of the rating to or below 0 in all cases.Since 1998, the United States, the European Union, and other nations are scanning for NEOs in an effort called Spaceguard. The initial US Congress mandate to NASA of cataloging at least 90% of NEOs that are at least 1 kilometre (0.62 mi) in diameter, which could cause a global catastrophe, had been met by 2011.  In later years, the survey effort has been expanded to smaller objects having the potential for large-scale, though not global, damage.
NEOs have low surface gravity, and many have Earth-like orbits making them easy targets for spacecraft. As of January 2019, five near-Earth comets and five near-Earth asteroids have been visited by spacecraft.  A small sample was returned to Earth in 2010, and similar missions are in progress. Preliminary plans for commercial asteroid mining have been drafted by private companies.