page_title: Near-Earth object
section_title: Near-Earth comets
hierarchical_section_title: Near-Earth object / Number and classification / Near-Earth comets
caption_attribution_description: English: An image of Halley's Comet, taken on May 29, 1910.
context_page_description: A near-Earth object 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 is less than 1.3 astronomical units. If a NEO's orbit crosses the Earth's, and the object is larger than 140 meters across, it is considered a potentially hazardous object. Most known PHOs and NEOs are asteroids, but a small fraction are comets.
  There are over 20,000 known near-Earth asteroids, over a hundred short-period near-Earth comets, and a number of solar-orbiting 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 this 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 if they impact a continent or tsunamis if they impact sea.
context_section_description: Near-Earth comets (NECs) are objects in a near-Earth orbit with a tail or coma. Comet nuclei are typically less dense than asteroids but they pass Earth at higher relative speeds, thus the impact energy of comet nucleus is slightly larger than that of a similar-sized asteroid. NECs may pose an additional hazard due to fragmentation: the meteoroid streams which produce meteor showers may include large inactive fragments, effectively NEAs. Although no impact of a comet in Earth's history has been conclusively confirmed, the Tunguska event may have been caused by a fragment of Comet Encke.
  Comets are commonly divided between short-period and long-period comets. Short-period comets, with an orbital period of less than 200 years, originate in the Kuiper belt, beyond the orbit of Neptune; while long-period comets originate in the Oort Cloud, in the outer reaches of the Solar System. The orbital period distinction is of importance in the evaluation of the risk from near-Earth comets because short-period NECs are likely to have been observed during multiple apparitions and thus their orbits can be determined with some precision, while long-period NECs can be assumed to have been seen for the first and last time when they appeared during the Age of Science, thus their approaches cannot be predicted well in advance. Since the threat from long-period NECs is estimated to be at most 1% of the threat from NEAs, and long-period comets are very faint and thus difficult to detect at large distances from the Sun, Spaceguard efforts consistently focused on asteroids and short-period comets. CNEOS even restricts its definition of NECs to short-period comets—as of May 10, 2018, 107 such objects have been discovered.
  As of March 2018, only 20 comets have been observed to pass within 0.1 AU (15,000,000 km; 9,300,000 mi) of Earth, including 10 which are or have been short-period comets. Two of these comets, Halley's Comet and 73P/Schwassmann–Wachmann, have been observed during multiple close approaches. The closest observed approach was 0.0151 AU (5.88 LD) for Lexell's Comet on July 1, 1770. After an orbit change due to a close approach of Jupiter in 1779, this object is no longer a NEC. The closest approach ever observed for a current short-period NEC is 0.0229 AU (8.92 LD) for Comet Tempel–Tuttle in 1366. This comet is the parent body of the Leonid meteor shower, which also produced the Great Meteor Storm of 1833. Orbital calculations show that P/1999 J6 (SOHO), a faint sungrazing comet and confirmed short-period NEC observed only during its close approaches to the Sun, passed Earth undetected at a distance of 0.0121 AU (4.70 LD) on June 12, 1999.
  Comet 109P/Swift–Tuttle, which is also the source of the Perseid meteor shower every year in August, has a roughly 130-year orbit which passes close to the Earth. During the comet's September 1992 recovery, when only the two previous returns in 1862 and 1737 had been identified, calculations showed that the comet would pass close to Earth during its next return in 2126, with an impact within the range of uncertainty. By 1993, even earlier returns (back to at least 188 AD) have been identified, and the longer observation arc eliminated the impact risk, and the comet will pass Earth in 2126 at a distance of 23 million kilometers. In 3044, the comet is expected to pass Earth at less than 1.6 million kilometers.
caption: Halley's Comet during its 0.10 AU[114] approach of Earth in May 1910
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