Editing Near-Earth object (section)

=== Near-Earth comets<!--'Near-Earth comet' and 'Near-Earth comets' redirect here--> ===

[[File:Halley's Comet - May 29 1910.jpg|thumb|[[Halley's Comet]] during its 0.10&nbsp;AU<ref>{{cite web |first=Donald K. |last=Yeomans |title=Great Comets in History |date=April 2007 |publisher=NASA/JPL |url=https://ssd.jpl.nasa.gov/sb/great_comets.html |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20240126111145/https://ssd.jpl.nasa.gov/sb/great_comets.html |archive-date=January 26, 2024}}</ref> approach of Earth in May 1910]]

'''Near-Earth comets'''<!--boldface per WP:R#PLA--> (NECs) are objects in a near-Earth orbit with a tail or coma made up of dust, gas or ionized particles emitted by a solid nucleus. Comet nuclei are typically less dense than asteroids but they pass Earth at higher relative speeds, thus the impact energy of a comet nucleus is slightly larger than that of a similar-sized asteroid.<ref name="NEOSDT2003"/> NECs may pose an additional hazard due to fragmentation: the meteoroid streams which produce meteor showers may include large inactive fragments, effectively NEAs.<ref name="Jenniksens2005">{{Cite conference |last=Jenniksens |first=Peter |title=Meteor Showers from Broken Comets |date=September 2005 |conference=Workshop on Dust in Planetary Systems (ESA SP-643) |volume=643 |pages=3–6 |bibcode=2007ESASP.643....3J}}</ref> 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]].<ref>{{cite journal |last=Kresak| first=L'.l |title=The Tunguska object – A fragment of Comet Encke |bibcode=1978BAICz..29..129K |volume=29 |date=1978 |pages=129 |journal=Bulletin of the Astronomical Institutes of Czechoslovakia}}</ref>

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.<ref name="TaskForceReport"/> 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 since the start of precise observations, thus their approaches cannot be predicted well in advance.<ref name="TaskForceReport">{{cite book |author=<!--Staff writer(s); no by-line.--> |title=Report of the Task Force on potentially hazardous Near Earth Objects |location=London |publisher=British National Space Centre |date=September 2000 |url=https://spaceguardcentre.com/wp-content/uploads/2014/04/full_report.pdf |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241208153948/https://spaceguardcentre.com/wp-content/uploads/2014/04/full_report.pdf |archive-date=December 8, 2024}}</ref> 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 have consistently focused on asteroids and short-period comets.<ref name="Vulcano1995"/><ref name="NEOSDT2003">{{cite book |author=<!--Staff writer(s); no by-line.--> |title=Study to Determine the Feasibility of Extending the Search for Near-Earth Objects to Smaller Limiting Diameters |date=August 22, 2003 |publisher=NASA |url=https://cneos.jpl.nasa.gov/doc/neoreport030825.pdf |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241212090955/https://cneos.jpl.nasa.gov/doc/neoreport030825.pdf |archive-date=December 12, 2024}}</ref> Both NASA's CNEOS<ref name="CNEOS-NEO-groups"/> and ESA's NEOCC<ref name="NEOCC-DA"/> restrict their definition of NECs to short-period comets. {{As of|2024|12|30|df=US}}, 123 such objects have been discovered.<ref name="neo-jpl-stats" />

[[Comet Swift-Tuttle|Comet 109P/Swift–Tuttle]], which is also the source of the [[Perseids|Perseid meteor shower]] every year in August, has a roughly 130-year orbit that 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) had been identified, and the longer observation arc eliminated the impact risk. 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.<ref>{{cite web |first=Sally | last=Stephens |title=What about the comet that's supposed to hit the Earth in 130 years? |date=1993 |work=Cosmic Collisions |publisher=[[Astronomical Society of the Pacific]] |url=https://astrosociety.org/file_download/inline/245c66de-59dd-49e9-8773-16ef08de09ff |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20231003194758/https://www.astrosociety.org/file_download/inline/245c66de-59dd-49e9-8773-16ef08de09ff |archive-date=October 3, 2023}}</ref>