|
|
| Line 1: |
Line 1: |
| [[File:The VLT images the very faint Near-Earth Object 2009 FD.jpg|thumb|[[Very Large Telescope|VLT]] image of the very faint near-Earth asteroid [[2009 FD]].<ref>{{cite news|title=ESA/ESO Collaboration Successfully Tracks Its First Potentially Threatening Near-Earth Object|url=http://www.eso.org/public/announcements/ann14004/|accessdate=22 January 2014|newspaper=ESO Announcement}}</ref> ]]
| | The author is named Solomon but he hardly ever really liked that title. Tennessee is where his property is and his family loves it. Finished he loves many will be to model trains and he'll be starting something else together with it. For many years he's been functioning as an information specialist and he'll be offered quickly.<br><br>my web site; [http://es.wikipedia.org/wiki/Usuario:Juan-Pablo-Schiappacasse-Canepa jaun pablo schiappacasse canepa] |
| | |
| A '''near-Earth object''' ('''NEO''') is a [[Solar System]] object whose orbit brings it into proximity with [[Earth]]. All NEOs have a closest approach to the Sun ([[Apsis|perihelion]]) of less than 1.3 [[Astronomical unit|AU]].<ref name=NEO-groups/> They include a few thousand near-Earth asteroids (NEAs), near-Earth [[comet]]s, a number of solar-orbiting [[spacecraft]], and [[meteoroid]]s 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 planet.<ref name = "BROWN02">{{cite web|url = http://www.sciencenews.org/pages/sn_arc97/75th/rm_essay.htm|title = The Call of Catastrophes|author = Richard Monastersky|publisher = Science News Online|date = March 1, 1997|accessdate = 2007-10-23}}</ref> NEOs have become of increased interest since the 1980s because of increased awareness of the potential danger some of the [[asteroids]] or [[comets]] pose to the Earth, and active mitigations are being researched.<ref name=IAU-NEOs/>
| |
| | |
| Those NEOs that are [[asteroid]]s (NEA) have orbits that lie partly between 0.983 and 1.3 [[astronomical unit]]s away from the Sun.<ref name = "MorbidelliAstIII" /> When an NEA is detected it is submitted to the [[IAU]]'s [[Minor Planet Center]] (located at the [[Harvard–Smithsonian Center for Astrophysics]]) for cataloging. Some near-Earth asteroids' orbits intersect that of Earth's so they pose a collision danger.<ref name = "CHAPMAN04">{{cite journal|title = The hazard of near-Earth asteroid impacts on earth|author = Clark R. Chapman|journal = Earth and Planetary Science Letters|volume = 222|issue = 1|pages = 1–15|date=May 2004|doi = 10.1016/j.epsl.2004.03.004|bibcode = 2004E&PSL.222....1C}}</ref> The [[United States]], [[European Union]], and other nations are currently scanning for NEOs<ref name=shigad/> in an effort called [[Spaceguard]].
| |
| | |
| In the United States, [[NASA]] has a [[United States Congress|congressional]] mandate to catalogue all NEOs that are at least 1 kilometer wide, as the impact of such an object would be catastrophic. {{As of|2012|August}}, there had been 848 near-Earth asteroids larger than 1 km discovered, of which 154 are [[potentially hazardous asteroid]]s (PHAs).<ref name=nasa_neo/> It was estimated in 2006 that 20% of the mandated objects have not yet been found.<ref name=shigad>{{cite news | last = Shiga | first = David | title = New telescope will hunt dangerous asteroids | publisher = New Scientist | date = 2006-06-27 | url = http://www.newscientist.com/article/dn9403 | accessdate = 2008-11-15 }}</ref><!--800 from this reference, 200 from 2004 estimate of ca. 1000 large NEOs needs check.--> As a result of [[NEOWISE]] in 2011, it is estimated that 93% of the NEAs larger than 1 km have been found and that only about 70 remain to be discovered.<ref name="pia14734"/>
| |
| | |
| [[Potentially hazardous object]]s (PHOs) are currently defined based on parameters that measure the object's potential to make threatening close approaches to the Earth.<ref name="pha">{{cite web |title=Potentially Hazard Asteroids |publisher=NASA/JPL Near-Earth Object Program Office |url=http://neo.jpl.nasa.gov/neo/pha.html |accessdate=2011-05-05}}</ref> Mostly objects with an Earth minimum orbit intersection distance (MOID) of 0.05 AU or less and an absolute magnitude (H) of 22.0 or brighter (a rough indicator of large size) are considered PHOs. Objects that cannot approach closer to the Earth (i.e. MOID) than {{convert|0.05|AU|km mi|abbr=on|lk=on}}, or are smaller than about 150 m (500 ft) in diameter (i.e. H = 22.0 with assumed [[albedo]] of 13%), are not considered PHOs.<ref name=NEO-groups>{{cite web |title=NEO Groups |publisher=NASA/JPL Near-Earth Object Program Office |url=http://neo.jpl.nasa.gov/neo/groups.html |accessdate=2012-06-04}}</ref> The NASA Near Earth Object Catalog also includes the approach distances of asteroids and comets measured in [[Lunar distance (astronomy)|lunar distances]],<ref>[http://neo.jpl.nasa.gov/ca/ NEO Earth Close Approaches] at NASA/JPL Near-Earth Object Program Office</ref> and this usage has become a common unit of measure used by the news media in discussing these objects.
| |
| | |
| Some NEOs are of high interest because they can be physically explored with lower [[mission velocity]] even than the Moon, due to their combination of low velocity with respect to Earth ([[delta-V|ΔV]]) and small gravity, so they may present interesting scientific opportunities both for direct geochemical and astronomical investigation, and as potentially economical sources of extraterrestrial materials for human exploitation.<ref>{{cite news|url = http://www.usatoday.com/tech/science/space/2007-02-12-asteroid_x.htm|title = Near-Earth asteroids could be 'steppingstones to Mars'|author = Dan Vergano|publisher = USA Today|date = February 2, 2007|accessdate = 2007-10-22}}</ref> This makes them an attractive target for exploration.<ref>{{cite journal|title = Design and optimization of trajectory to Near-Earth asteroid for sample return mission using gravity assists|author = Rui Xu, Pingyuan Cui, Dong Qiao and Enjie Luan|journal = Advances in Space Research|volume = 40|issue = 2|pages = 200–225|date = 18 March 2007|doi = 10.1016/j.asr.2007.03.025|bibcode = 2007AdSpR..40..220X}}</ref> As of 2012, three near-Earth objects have been visited by spacecraft: [[433 Eros]], by [[NASA]]'s [[Near Earth Asteroid Rendezvous]] probe,<ref>{{cite web|url = http://nssdc.gsfc.nasa.gov/planetary/news/near_descent_pr_20010131.html|title = NEAR Mission Completes Main Task, Now Will Go Where No Spacecraft Has Gone Before|author = Donald Savage and Michael Buckley|publisher = National Aeronautics and Space Administration|date = January 31, 2001|accessdate = 2007-10-22}}</ref> [[25143 Itokawa]], by the [[JAXA]] [[Hayabusa (spacecraft)|Hayabusa]] mission,<ref>{{cite web|url = http://neo.jpl.nasa.gov/missions/hayabusa.html|title = Hayabusa's Contributions Toward Understanding the Earth's Neighborhood|author = Don Yeomans|date = August 11, 2005|publisher = National Aeronautics and Space Administration|accessdate = 2007-10-22}}</ref> and [[4179 Toutatis]], by [[China National Space Administration|CNSA's]] [[Chang'e 2]] spacecraft.<ref name=IAU-NEOs/><ref>{{cite web|url = http://www.planetary.org/blogs/emily-lakdawalla/2012/12141551-change-2-imaging-of-toutatis.html|title = Chang'e 2 imaging of Toutatis|author = Emily Lakdawalla}}</ref>
| |
| | |
| == History of human awareness of NEOs ==
| |
| [[File:ESO-Asteroid Toutatis-phot-28c-04-normal.jpg|thumb|Asteroid Toutatis from [[Paranal Observatory|Paranal]].]]
| |
| | |
| Human [[perception]] of near-Earth objects as benign objects of fascination or killer objects with high [[risk]] to [[human society]] have ebbed and flowed in the short period of human history that NEOS have been scientifically observed.<ref name=wired20130323>
| |
| {{cite news |last=Portree|first=David S. |title=Earth-Approaching Asteroids as Targets for Exploration (1978) |url=http://www.wired.com/wiredscience/2013/03/earth-approaching-asteroids-as-targets-for-exploration-1978/ |accessdate=2013-03-27 |newspaper=Wired |date=2013-03-23 |quote=''People in the early 21st century have been encouraged to see asteroids as the interplanetary equivalent of sea monsters. We often hear talk of “killer asteroids,” when in fact there exists no conclusive evidence that any asteroid has killed anyone in all of human history. ... In the 1970s, asteroids had yet to gain their present fearsome reputation ... most astronomers and planetary scientists who made a career of studying asteroids rightfully saw them as sources of fascination, not of worry.''}}</ref>
| |
| | |
| ===Risk===
| |
| [[File:Toutatis.jpg|thumb|Asteroid [[4179 Toutatis]] is a [[potentially hazardous object]] that has passed within 2.3 [[Lunar distance (astronomy)|lunar distances]].]]
| |
| | |
| More recently, 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]]. "NEOs have been understood differently throughout history." Each time an NEO is observed, "a different risk was posed, and throughout time that risk perception has evolved. It is not just a matter of scientific knowledge."<ref name=tsr20120514>
| |
| {{cite journal |last=Fernández Carril|first=Luis |title=The evolution of near Earth objects risk perception |journal=The Space Review |date=2012-05-14 |url=http://www.thespacereview.com/article/2080/1 |accessdate=2012-05-25 }}</ref>
| |
| | |
| Such perception of risk is thus "a product of religious belief, philosophic principles, scientific understanding, technological capabilities, and even economical resourcefulness."<ref name=tsr20120514/>
| |
| | |
| ==== Risk scales ====
| |
| There are two schemes for the scientific classification of impact hazards from NEOs:
| |
| * the simple [[Torino Scale]], and
| |
| * the more complex [[Palermo Technical Impact Hazard Scale]]
| |
| | |
| The annual background frequency used in the Palermo scale for impacts of energy greater than ''E'' [[megatonne]]s is estimated as:<ref name="palermo"/>
| |
| | |
| :<math>f_B = 0.03E^{-0.8} \;</math>
| |
| | |
| For instance, this formula implies that the [[expected value]] of the time from now until the next impact greater than 1 megatonne is 33 years, and that when it occurs, there is a 50% chance that it will be above 2.4 megatonnes. This formula is only valid over a certain range of ''E''.
| |
| | |
| However, another paper<ref>
| |
| | |
| "The flux of small near-Earth objects colliding with the Earth" by P. Brown et al., [[Nature (journal)|Nature]], 420, pp. 294–6, November 2002.
| |
| | |
| </ref> published in 2002 – the same year as the paper on which the Palermo scale is based – found a power law with different constants:
| |
| | |
| :<math>f_B = 0.00737 E^{-0.9} \;</math>
| |
| | |
| This formula gives considerably lower rates for a given ''E''. For instance, it gives the rate for bolides of 10 megatonnes or more (like the [[Tunguska explosion]]) as 1 per thousand years, rather than 1 per 210 years as in the Palermo formula. However, the authors give a rather large uncertainty (once in 400 to 1800 years for 10 megatonnes), due in part to uncertainties in determining the energies of the atmospheric impacts that they used in their determination.
| |
| | |
| ====Highly rated risks====
| |
| [[File:Potentially Hazardous Asteroids 2013.png|thumb|Plot of orbits of known [[potentially hazardous asteroid]]s (size over {{convert|460|ft|m}} and passing within {{convert|4.7|e6mi|e6km}} of Earth's orbit) as of early 2013]]
| |
| On 24 December 2004, minor planet [[99942 Apophis]] (at the time known by its provisional designation {{mp|2004 MN|4}}) was assigned a 4 on the Torino scale, the highest rating ever achieved. There was a 2.7% chance of Earth impact on 13 April 2029. However, on 28 December 2004, the risk of impact dropped to zero for 2029, but future potential impact solutions were still rated 1 on the Torino scale. The 2036 risk was lowered to a Torino rating of 0 in August 2006. The Palermo rating is −3.2.<ref name=Current_Impact_Risks>{{cite news |title = Current Impact Risks| url = http://neo.jpl.nasa.gov/risk | accessdate = 2010-05-28 }}</ref>
| |
| | |
| The only known NEO with a Palermo scale value currently greater than zero is [[(29075) 1950 DA]], which may pass very close to or collide with the Earth (probability ≤ 0.003) in the year 2880. Depending on the unknown orientation of its axis of rotation, it will either miss the Earth by tens of millions of kilometers, or have a 1 in 300 chance of hitting the Earth. However, humanity has over 800 years to refine the orbit of (29075) 1950 DA, and to deflect it, if necessary.<ref name="NEO-1950DA">{{cite web
| |
| |title=Asteroid 1950 DA
| |
| |publisher=NASA/JPL Near-Earth Object Program Office
| |
| |url=http://neo.jpl.nasa.gov/1950da/
| |
| |accessdate=2011-10-14}}</ref>
| |
| | |
| ====List of current threats====
| |
| [[NASA]] maintains a continuously updated [[Sentry Risk Table]] of the most significant NEO threats in the next 100 years.<ref name=Current_Impact_Risks/> All or nearly all of the objects are highly likely to eventually drop off the list as more observations come in, reducing the uncertainties and enabling more accurate orbital predictions. (The list does not include [[(29075) 1950 DA|1950 DA]], because that will not strike for at least 800 years.)<ref name=IAU-NEOs/><ref name="NEO-1950DA"/>
| |
| | |
| ===History of NEO science and exploratory mission proposals===
| |
| In a 2013 article in [[Wired Science]], David Portree provides an overview of NEO science and proposed asteroidal missions, with an emphasis on the outcome of two conferences held in the 1970s. The [[International Astronomical Union]] minor planets workshop was held in [[Tucson, Arizona]] in March 1971 and a consensus "[[emergence|emerged]] that launching spacecraft to asteroids would be 'premature'."<ref name=wired20130323/>
| |
| "In January 1978, NASA’s Office of Space Science held a workshop at the [[University of Chicago]] to "assess the state of asteroid studies and consider options for the future."<ref name=wired20130323/>
| |
| | |
| Of all of the near-Earth asteroids (NEA) that had been discovered by mid-1977, it was estimated that spacecraft could [[Space rendezvous|rendezvous]] with and return from only about one in 10 using less [[Spacecraft propulsion|propulsive energy]] than is necessary to reach [[Mars]]. "Because even the most massive NEA—{{convert|35|km}}-wide [[1036 Ganymed]], discovered in 1924, has a very low surface gravity—landing and takeoff would need very little [[delta V|energy]]. This meant that a single spacecraft could sample multiple sites on any given NEA."<ref name=wired20130323/>
| |
| Overall, it was estimated that about one percent of all NEAs might provide opportunities for [[human spaceflight|human-crewed]] missions, or no more than about ten known NEAs. Therefore, unless the NEA discovery rate were "immediately increased five-fold, no opportunity to launch 'astronaut-scientists' to an NEA was likely to occur within a decade of the Chicago workshop."<ref name=wired20130323/>
| |
| | |
| == Number and classification of near-Earth objects ==
| |
| <div style="float:right;">
| |
| {| class="wikitable sortable"
| |
| |+NEAs with [[Absolute magnitude#Solar System bodies (H)|(H)]] < 16 discovered since 2008:<ref name="JPL-NEO-H16"/>
| |
| !Name/Year!![[Absolute magnitude#Solar System bodies (H)|(H)]]
| |
| |-
| |
| | {{mp|2008 EJ|1}} || 15.8
| |
| |-
| |
| | {{mp|(243298) 2008 EN|82}} || 15.6
| |
| |-
| |
| | {{mpl|2011 UL|21}} || 15.8
| |
| |-
| |
| | {{mp|2012 SF|51}} || 15.4
| |
| |-
| |
| | {{mp|2012 US|136}} || 15.9
| |
| |-
| |
| | {{mp|2013 GJ|35}} || 15.8
| |
| |-
| |
| | {{mp|2013 UQ|4}} || 12.8
| |
| |}
| |
| </div>
| |
| Near-Earth objects are classified as [[meteoroid]]s, [[asteroid]]s, or [[comet]]s depending on size and composition. Asteroids can also be members of an [[asteroid family]], and comets create meteoroid streams that can generate [[meteor shower]]s.
| |
| | |
| {{As of|2013|2}}, 9,683 NEOs have been discovered:<ref name=nasa_neo/> 93 near-Earth comets and 9,590 near-Earth Asteroids. Of those there are 751 [[Aten asteroid]]s, 3,613 [[Amor asteroid]]s, and 5,214 [[Apollo asteroid]]s. There are 1,360 NEOs that are classified as [[potentially hazardous asteroid]]s (PHAs). Currently, 155 PHAs and 861 NEAs have an [[absolute magnitude]] of 17.75 or brighter, which roughly corresponds to at least 1 [[kilometer|km]] in size.<ref name=nasa_neo>{{cite web
| |
| |title=NEO Discovery Statistics
| |
| |url=http://neo.jpl.nasa.gov/stats/
| |
| |accessdate=2012-05-17}}</ref>
| |
| | |
| {{As of|2013|2}}, there are 430 NEAs on the [[Sentry (monitoring system)|Sentry impact risk page]] at the [[NASA]] website.<ref name=Current_Impact_Risks/> A significant number of these NEAs – 215 {{as of|2010|5|lc=on}} – are equal to or smaller than 50 meters in diameter and none of the listed objects are placed even in the "yellow zone" ([[Torino Scale]] 2), meaning that none warrant the attention of general public.<ref>[http://neo.jpl.nasa.gov/torino_scale.html The Torino Impact Hazard Scale]</ref> {{As of|2013|1}}, only asteroid {{mpl|2007 VK|184}} is listed as having a Torino Scale of 1. The [[JPL Small-Body Database]] lists 1,518 near Earth asteroids with an [[Absolute magnitude#Solar System bodies (H)|absolute magnitude]] (H) dimmer than 25 (roughly 50 meters in diameter).<ref name="JPL-50meter"/>
| |
| | |
| Near-Earth asteroids smaller than ~1 meter are near-Earth [[meteoroids]] and are listed as asteroids on most asteroid tables. The smallest known near-Earth meteoroid is {{mp|2008 TS|26}} with an absolute magnitude of 33 and estimated size of only 1 meter.<ref name="JPL-50meter"/>
| |
| | |
| === Near-Earth asteroids ===
| |
| [[File:NEO by size.png|thumb|Near-Earth asteroids by size.]]
| |
| [[File:Neas.svg|thumb|Near-Earth asteroids classification.]]
| |
| [[File:Known NEAs.png|thumb|Cumulative discoveries of near-Earth asteroids known by size, 1980–2013.]]
| |
| [[File:WISE Finds Fewer Asteroids near Earth.ogv|thumb|There are significantly fewer near-Earth asteroids in the mid-size range than previously thought.]]
| |
| These are objects in a near-Earth orbit without the tail or coma of a comet. {{As of|2013|June}}, 9,991 near-Earth asteroids are known,<ref name=nasa_neo/> ranging in size from 1 meter up to ~32 kilometers ([[1036 Ganymed]]). The number of near-Earth asteroids over one kilometer in diameter is estimated to be about 981.<ref name="pia14734"/><ref>{{cite web|url = http://www.jpl.nasa.gov/releases/2000/neat.html|title = Asteroid Population Count Slashed|date = January 12, 2000|author = Jane Platt|publisher = National Aeronautics and Space Administration|accessdate = 2007-10-22}}</ref><ref name = "Rabinowitzetal00">{{cite journal|title = A reduced estimate of the number of kilometer-sized near-Earth asteroids|journal = Nature|author = David Rabinowitz, Eleanor Helin, Kenneth Lawrence and Steven Pravdo|date = 13 January 2000|volume = 403|pages = 165–166|accessdate = 2007-10-22|url = http://www.nature.com/nature/journal/v403/n6766/full/403165a0.html|doi = 10.1038/35003128|pmid = 10646594|issue = 6766|bibcode = 2000Natur.403..165R }}</ref> The composition of near-Earth asteroids is comparable to that of asteroids from the asteroid belt, reflecting a variety of [[asteroid spectral types]].<ref>{{cite journal|title = On the Origins of Earth-Approaching Asteroids|author = D.F. Lupishko and T.A. Lupishko|journal = Solar System Research|volume = 35|issue = 3|pages = 227–233|date=May 2001 | doi = 10.1023/A:1010431023010|bibcode = 2001SoSyR..35..227L}}</ref>
| |
| | |
| NEAs survive in their orbits for just a few million years.<ref name = "MorbidelliAstIII">{{cite journal|url = http://www.boulder.swri.edu/~bottke/Reprints/Morbidelli-etal_2002_AstIII_NEOs.pdf|title = Origin and Evolution of Near-Earth Objects|author = A. Morbidelli, W. F. Bottke Jr., Ch. Froeschlé, P. Michel|journal = Asteroids III|editor = W. F. Bottke Jr., A. Cellino, P. Paolicchi, and R. P. Binzel|pages = 409–422|date=January 2002|publisher = University of Arizona Press|format=PDF |bibcode=2002aste.conf..409M|last2 = Bottke|last3 = Froeschlé|last4 = Michel}}</ref> They are eventually eliminated by planetary [[Perturbation (astronomy)|perturbations]], causing ejection from the Solar System or a collision with the Sun or a planet. With orbital lifetimes short compared to the age of the Solar System, new asteroids must be constantly moved into near-Earth orbits to explain the observed asteroids. The accepted origin of these asteroids is that [[Asteroid belt|asteroid-belt asteroids]] are moved into the inner Solar System through [[orbital resonance]]s with [[Jupiter]]. The interaction with Jupiter through the resonance [[Perturbation (astronomy)|perturb]]s the asteroid's orbit and it comes into the inner Solar System. The asteroid belt has gaps, known as [[Kirkwood gap]]s, where these resonances occur as the asteroids in these resonances have been moved onto other orbits. New asteroids migrate into these resonances, due to the [[Yarkovsky effect]] that provides a continuing supply of near-Earth asteroids.<ref>{{cite journal|title = The Yarkovsky-driven origin of near-Earth asteroids|author = A. Morbidelli, D. Vokrouhlický|journal = Icarus|volume = 163|issue = 1|pages = 120–134|date=May 2003 | doi = 10.1016/S0019-1035(03)00047-2|bibcode = 2003Icar..163..120M}}</ref>
| |
| | |
| A small number of NEOs are [[extinct comets]] that have lost their volatile surface materials, although having a faint or intermittent comet-like tail does not necessarily result in a classification as a near-Earth comet, making the boundaries somewhat fuzzy. The rest of the near-Earth asteroids are driven out of the asteroid belt by gravitational interactions with [[Jupiter]].<ref name = "MorbidelliAstIII" /><ref>{{cite journal|title = What the physical properties of near-Earth asteroids tell us about sources of their origin?|author = D.F. Lupishko, M. di Martino and T.A. Lupishko|journal = Kinematika i Fizika Nebesnykh Tel Supplimen|volume = 3|issue = 3|pages = 213–216|date=September 2000|bibcode = 2000KFNTS...3..213L|last2 = Di Martino|last3 = Lupishko}}</ref>
| |
| | |
| Near-Earth asteroids are divided into groups based on their [[semi-major axis]] (a), [[Apsis|perihelion]] distance (q), and [[Apsis|aphelion]] distance (Q):<ref name=NEO-groups/><ref name = "MorbidelliAstIII" />
| |
| | |
| * The ''[[Apohele asteroid|Atiras]]'' or ''Apohele asteroids'' have orbits strictly inside Earth's orbit: an Atira asteroid's aphelion distance (Q) is smaller than Earth's perihelion distance (0.983 AU). That is, Q < 0.983 AU. (This implies that the asteroid's semi-major axis is also less than 0.983 AU.)
| |
| * The ''[[Aten asteroid|Atens]]'' have a semi-major axis of less than 1 AU and cross Earth's orbit. Mathematically, a < 1.0 AU and Q > 0.983 AU.
| |
| * The ''[[List of Apollo asteroids|Apollos]]'' have a semi-major axis of more than 1 AU and cross Earth's orbit. Mathematically, a > 1.0 AU and q < 1.017 AU. (1.017 AU is Earth's aphelion distance.)
| |
| * The ''[[Amor asteroid|Amors]]'' have orbits strictly outside Earth's orbit: an Amor asteroid's perihelion distance (q) is greater than Earth's aphelion distance (1.017 AU). Amor asteroids are also near-earth objects so q < 1.3 AU. In summary, 1.017 AU < q < 1.3 AU. (This implies that the asteroid's semi-major axis (a) is also larger than 1.017 AU.) Some Amor asteroid orbits cross the orbit of Mars.
| |
| | |
| (Note: Some authors define the Atens group differently: they define it as being all the asteroids with a semi-major axis of less than 1 AU. That is, they consider the Atiras to be part of the Atens. Historically, until 1998, there were no known or suspected Atiras, so the distinction wasn't necessary.)
| |
| | |
| Because the Atens and all Apollos have orbits that cross Earth's orbit, they might impact the Earth. Atiras and Amors do not cross the Earth's orbit and are not immediate impact threats, but their orbits may change to become Earth-crossing orbits in the future.
| |
| | |
| === Near-Earth comets ===
| |
| {{As of|2013|2}}, 93 near-Earth comets have been discovered.<ref name=nasa_neo/> 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| title=The Tunguska object – A fragment of Comet Encke| publisher=Astronomical Institutes of Czechoslovakia| bibcode=1978BAICz..29..129K| author1=Kresak| first1=L'.| volume=29| year=1978| pages=129| journal=Astronomical Institutes of Czechoslovakia}}</ref> Cometary fragmenting may also be responsible for some impacts from near-Earth objects. It is rare for a comet to pass within {{convert|0.1|AU|km mi|abbr=on|lk=off}} of Earth.<ref name="closest"/>
| |
| | |
| These near-Earth objects were probably derived from the [[Kuiper belt]], beyond the orbit of [[Neptune]].
| |
| | |
| == Impact rate ==
| |
| Stony asteroids with a diameter of {{convert|4|m|sp=us}} impact Earth approximately once per year.<ref name="Earth-impact"/> Asteroids with a diameter of roughly 7 meters enter Earth's atmosphere with as much energy as [[Little Boy]] (the atomic bomb dropped on [[Atomic bombings of Hiroshima and Nagasaki|Hiroshima]], approximately 15 kilotonnes of TNT) about every 5 years.<ref name="Earth-impact"/> These ordinarily explode in the [[Mesosphere|upper atmosphere]], and most or all of the solids are [[Evaporation|vaporized]].<ref>{{Cite journal|author = Clark R. Chapman & David Morrison|title = Impacts on the Earth by asteroids and comets: assessing the hazard|journal = Nature|volume =367|issue = 6458|pages=33–40|date=January 6, 1994|bibcode = 1994Natur.367...33C|doi = 10.1038/367033a0|postscript = <!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}}}</ref> Every 2000–3000 years, objects produce explosions of 10 [[megatons]] comparable to the one observed at [[Tunguska event|Tunguska]] in 1908.<ref name=Asher2005/> Objects with a diameter of one kilometer hit the Earth an average of twice every million year interval.<ref name = "MorbidelliAstIII" /> Large collisions with five kilometer objects happen approximately once every twenty million years.<ref name="Earth-impact"/>
| |
| | |
| Assuming that these rates will continue for the next billion years, there exist at least 2,000 objects of diameter greater than 1 km that will eventually hit Earth. However, most of these are not yet considered potentially hazardous objects because they are currently orbiting between [[Mars]] and [[Jupiter]]. Eventually they will change orbits and become NEOs. Objects spend on average a few million years as NEOs before hitting the Sun, being ejected from the Solar System, or (for a small proportion) hitting a planet.<ref name = "MorbidelliAstIII"/>
| |
| | |
| == Historic impacts ==
| |
| [[File:Impact event.jpg|thumb|Illustration of the impact of an asteroid a few kilometers across. Such impacts are expected to occur less often than every 100 million years.]]
| |
| | |
| The general acceptance of the [[Alvarez hypothesis]], explaining the [[Cretaceous–Paleogene extinction event]] as the result of a large object [[impact event]], raised the [[awareness]] of the possibility of future Earth impacts with other objects that cross the Earth's orbit.<ref name="BROWN02"/>
| |
| | |
| === 1908 Tunguska event ===
| |
| {{main|Tunguska event}}
| |
| | |
| It is now highly believed that on 30 June 1908 a [[Meteorite classification|stony asteroid]] exploded over Tunguska with the energy of 10 [[megatons]] of TNT. The explosion occurred at a height of 8.5 [[kilometers]]. The object that caused the explosion has been estimated to have had a diameter of 45–70 meters.<ref>{{cite journal|author = Christopher F. Chyba, Paul J. Thomas & Kevin J. Zahnle|title = The 1908 Tunguska explosion: atmospheric disruption of a stony asteroid|journal = Nature|date = January 7, 1993|volume = 361|issue = 6407|pages = 40–44|doi = 10.1038/361040a0|bibcode = 1993Natur.361...40C}}</ref>
| |
| | |
| === 1930 Brazilian event ===
| |
| On August 13, 1930 the area near [[Curuçá River]] at latitude 5° S and longitude 71.5° W experienced a [[meteor]]ic [[air burst]], also known as the "Brazilian Tunguska event".<ref name="Reza" /><ref name="Reza et al" />
| |
| | |
| The mass of the meteorite was estimated at between 1,000 and 25,000 tons, with an energy release estimated between 0.1 and 5 [[TNT equivalent|megatons]], significantly smaller than the Tunguska Event.<ref name="Reza">Reza, Ramiro de la. [http://www.comciencia.br/reportagens/espaco/espc17.htm O evento do Curuçá: bólidos caem no Amazonas (The Curuçá Event: Bolides Fall in the Amazon)] {{Pt icon}}, Rio de Janeiro: National Observatory. Retrieved from the Universidade Estadual de Campinas website.</ref>
| |
| <ref name="Reza et al">Reza, Ramiro de la; Martini, P. R.; Brichta, A.; Lins de Barros, H.; Serra, P.R.M. [http://www.lpi.usra.edu/meetings/metsoc2004/pdf/5150.pdf The Event Near The Curuçá River], presented at Rio de Janeiro, Brazil: 67th Annual [[Meteoritical Society|Meteoritical Society Meeting]], August 2–6, 2004. Retrieved from [[Universities Space Research Association]] (USRA) website, Columbia, MD.</ref><ref name="Lienhard">Lienhard, John H. [http://www.uh.edu/engines/epi1102.htm Meteorite at Curuçá], The Engines of Our Ingenuity, [[University of Houston]] with KUHF-FM Houston.</ref><ref name="McFarland">McFarland, John. [http://star.arm.ac.uk/impact-hazard/Brazil.html The Day the Earth Trembled], Armagh, Northern Ireland: [[Armagh Observatory]] website, last revised on November 10, 2009.</ref>
| |
| | |
| === 1979 Vela Incident ===
| |
| {{main|Vela Incident}}
| |
| | |
| On the 22 of September 1979 event recorded as occurring near the junction of the South Atlantic and the Indian Ocean was possibly a low-yield nuclear test, but was also initially thought to have been caused by the possible impact of an extraterrestrial object. The event, which became known as the [[Vela Incident]], was identified by a U.S. [[Vela (satellite)|Vela]] defense satellite in Earth orbit. The event alarm triggered multi-year investigations by several organizations which could not conclusively determine if the explosion was of nuclear or non-nuclear origin.
| |
| | |
| === 2002 Eastern Mediterranean event ===
| |
| {{main|Eastern Mediterranean event}}
| |
| | |
| On 6 June 2002 an object with an estimated diameter of 10 meters collided with Earth. The collision occurred over the [[Mediterranean Sea]], between [[Greece]] and [[Libya]], at approximately 34°N 21°E and the object exploded in mid-air. The energy released was estimated (from infrasound measurements) to be equivalent to 26 [[kiloton]]s of TNT, comparable to a small [[nuclear weapon]].<ref>{{cite journal|title = The flux of small near-Earth objects colliding with the Earth|url = http://www.astro.uwo.ca/~pbrown/documents/flux-final.pdf|author = P. Brown, R.E. Spalding, D.O. ReVelle, E. Tagliaferri and S.P. Worden|journal = Nature|date = 21 November 2002|volume = 420|issue = 6913|pages = 294–296|accessdate = 2007-10-23 | doi = 10.1038/nature01238|format=PDF|pmid = 12447433}}</ref>
| |
| | |
| === 2008 Sudan event ===
| |
| :<div class="noprint relarticle mainarticle">''Main article: {{mpl|2008 TC|3}}</div>
| |
| | |
| On 6 October 2008, scientists calculated that a small near-Earth asteroid, {{mpl|2008 TC|3}}, just sighted that night, would impact the Earth on 7 October over Sudan, at 0246 UTC, 5:46 local time.<ref>{{cite web |title=Small Asteroid Predicted to Cause Brilliant Fireball over Northern Sudan |author=Don Yeomans |publisher=NASA/JPL Near-Earth Object Program Office |date=October 6, 2008 |accessdate=2008-10-09 |url=http://neo.jpl.nasa.gov/news/news159.html }}</ref><ref>{{cite web |title=FLASH! Meteor to Explode Tonight |author=Richard A. Kerr |publisher= ScienceNOW Daily News |date=6 October 2008 |accessdate=2008-10-09 |url=http://sciencenow.sciencemag.org/cgi/content/full/2008/1006/2 }}</ref> The asteroid arrived as predicted.<ref>{{cite web |title=Impact of Asteroid 2008 TC3 Confirmed |author=Don Yeomans |publisher=NASA/JPL Near-Earth Object Program Office |date=October 7, 2008 |accessdate=2008-10-09 |url=http://neo.jpl.nasa.gov/news/news160.html }}</ref><ref>{{cite web |title=Asteroid Watchers Score a Hit |author=Richard A. Kerr |publisher=ScienceNOW Daily News |date=8 October 2008 |url=http://sciencenow.sciencemag.org/cgi/content/full/2008/1008/1 |accessdate=2008-10-09}}</ref> This is the first time that an asteroid impact on Earth has been accurately predicted. While hundreds of rock fragments have subsequently been recovered on ground,<ref name="Nature0309">{{Cite journal|author=P. Jenniskens et al.|url=http://www.nature.com/nature/journal/v458/n7237/abs/nature07920.html|title=The impact and recovery of asteroid 2008 TC3 |journal=[[Nature (journal)|Nature]]|date=2009-03-26|pages=485–488|doi=10.1038/nature07920|archiveurl=http://www.webcitation.org/5hDZOG4BF|archivedate=2009-06-01|deadurl=no|accessdate=2009-04-04|issue= 7237|pmid=19325630|volume=458|bibcode=2009Natur.458..485J}} Published in Letters to Nature</ref> no reports of the actual impact have so far been published since it occurred in a very sparsely populated area.<ref>[http://www.skyandtelescope.com/community/skyblog/newsblog/30686199.html Little Asteroid Makes a Big Splash] Sky and Telescope, October 9, 2008.</ref>
| |
| | |
| === 2009 Indonesia event ===
| |
| A large fireball was observed in the skies near Bone, Indonesia on October 8, 2009. This was thought to be caused by an asteroid approximately 10 meters in diameter. The fireball contained an estimated energy of 50 kilotons of TNT, or about twice the [[Fat Man|Nagasaki atomic bomb]]. No injuries were reported.<ref name="NEO165">{{cite web |date=October 23, 2009 |title=Asteroid Impactor Reported over Indonesia |publisher=NASA/JPL Near-Earth Object Program Office |author=Don Yeomans, Paul Chodas and Steve Chesley |url=http://neo.jpl.nasa.gov/news/news165.html |accessdate=2009-10-30}}</ref>
| |
| | |
| === 2013 Chelyabinsk meteor ===
| |
| {{main|Chelyabinsk meteor}}
| |
| On 15 February 2013, an [[Apollo asteroid|Apollo]] [[Near-Earth asteroids|asteroid]] entered Earth's atmosphere over [[Russia]] at about 09:20 [[Yekaterinburg Time|YEKT]] (03:20 [[Coordinated Universal Time|UTC]])<ref name="NASA20130215">{{cite web |last=Agle |first=D. C. |title=Russia Meteor Not Linked to Asteroid Flyby |work=NASA news |publisher=NASA |date=13 February 2013 |url=http://www.nasa.gov/mission_pages/asteroids/news/asteroid20130215.html |accessdate=15 February 2013 }}</ref><ref name="USAtoday1921991">{{cite web |url=http://www.usatoday.com/story/news/world/2013/02/15/russia-meteorite/1921991/ |title=Meteor in central Russia injures at least 500 |first1=Anna |last1=Arutunyan |first2=Marc |last2=Bennetts |work=[[USA Today]] |accessdate=15 February 2013 }}</ref><ref name="Mercury22596238">{{cite web |title=100 injured by meteorite falls in Russian Urals |url=http://www.mercurynews.com/breaking-news/ci_22596238/100-injured-by-meteorite-falls-russian-urals |publisher=Mercury News |first1=Jim |last1=Heintz |first2=Vladimir |last2=Isachenkov |agency=Associated Press |date=15 February 2013 |accessdate=15 February 2013 }}</ref><ref name="Universe99982">{{cite web |last=Major |first=Jason |publisher=Universe Today |title=Meteor Blast Rocks Russia |url=http://www.universetoday.com/99982/meteor-blasts-rock-russia/ |date=15 February 2013 |accessdate=15 February 2013 }}</ref> with an estimated speed of 18 km/s (40,000 mph);<ref name="NASA20130215" /> it became a brilliant [[superbolide]] [[meteor]]<!-- the terminology "superbolide meteor" is the result of two separate Talk page consensus-building discussions: 18-28 Feb 2013 AND 4-6 Mar 2013; please do not change it without first discussing on the Talk page --> over the southern [[Ural (region)|Ural region]].<ref name="IAUCBET3423">{{cite web |url=http://www.cbat.eps.harvard.edu/iau/cbet/003400/CBET003423.txt |publisher=International Astronomical Union |work=Astronomical Telegrams |title=CBET 3423 : 20130223 : Trajectory and Orbit of the Chelyabinsk Superbolide |date=2013-02-23 |archiveurl=http://www.webalice.it/mizar02/articoli/Meteorb.dat |archivedate=2013-02-23}}{{registration required}}</ref> The dazzling light of the meteor was bright enough to cast moving shadows during the morning daylight in [[Chelyabinsk]] and was observed from [[Sverdlovsk Oblast|Sverdlovsk]], [[Tyumen Oblast|Tyumen]], [[Orenburg Oblast]]s, the [[Republic of Bashkortostan]], and in [[Kazakhstan]]. Eyewitnesses also felt intense heat from the fireball.<ref name="YT-9Iq2h2DIqt8">{{cite web |title=Russian Meteor strike eyewitnesses speak |url=https://www.youtube.com/watch?v=9Iq2h2DIqt8 |date=15 February 2013 |publisher=YouTube |accessdate=15 February 2013 |quote=In Russian, with translation voiceover in English}}</ref>
| |
| | |
| The object exploded in an [[air burst]] over [[Chelyabinsk Oblast]] at a height of about {{convert|15|to|25|km|mi|abbr=on}},<ref name="NASA20130215" /><ref name="NASA1360985685055">{{cite web |url=http://blogs.nasa.gov/cm/blog/Watch%20the%20Skies/posts/post_1360985685055.html |title=Orbit of the Russian Meteor | work=NASA blogs | date=15 February 2013 |accessdate=17 February 2013 |first=William |last=Cooke }}</ref> with {{convert|23.3|km|mi|abbr=on}} being the most recent official burst height.<ref name="JPL20130301" /> It exploded with the generation of a bright flash, small fragmentary [[meteorite]]s and a powerful [[shock wave]]. The atmosphere absorbed most of the object's energy,<ref name="RT-284">{{cite news |title=Meteor threat wasn't expected for another 2,000 years – Russian Emergency Minister |date=22 February 2013 |url=http://rt.com/news/meteor-attack-not-expected-284/ |publisher=[[RT (TV Network)|RT]] |accessdate=22 February 2013}}</ref> with a total [[kinetic energy]] before atmospheric impact equivalent to ~ 440 [[TNT equivalent|kilotons of TNT]] {{nowrap|(~ 1.8 [[Joule|PJ]]),}}.<ref name="NASA20130215" /><ref name="JPL20130301">{{cite web |date=1 March 2013 |title=Additional Details on the Large Fireball Event over Russia on Feb. 15, 2013 |publisher=NASA/JPL Near-Earth Object Program Office |first1=Don |last1=Yeomans |first2=Paul |last2=Chodas |url=http://neo.jpl.nasa.gov/news/fireball_130301.html |accessdate=2013-03-02}}</ref><ref name="RT-283">{{cite news | url=http://rt.com/news/meteorite-crash-urals-chelyabinsk-283/ | title=Meteorite hits Russian Urals: Fireball explosion wreaks havoc, up to 1,200 injured (PHOTOS, VIDEO) | publisher=[[RT (TV Network)|RT]] | date=15 February 2013}}</ref><ref name="RT-337">{{cite web | url=http://rt.com/news/scientists-explain-chelyabinsk-bolide-337/ | title=Russian meteorite blast explained: Fireball explosion equal to 20 Hiroshimas | publisher=[[RT (TV Network)|RT]] | date=15 February 2013}}</ref> This is about 20–30 times more [[energy]] than was released from the [[Atomic bombings of Hiroshima and Nagasaki|atomic bombs detonated at Hiroshima and Nagasaki]],<ref name="NASA20130215" /><ref name="RT-283" /><ref name="RT-337" /><ref name="Telegraph9874662">{{cite web |url=http://www.telegraph.co.uk/science/space/9874662/Russian-meteor-hit-atmosphere-with-force-of-30-Hiroshima-bombs.html |title=Russian meteor hit atmosphere with force of 30 Hiroshima bombs |publisher=The Telegraph |date=16 February 2013 |accessdate=16 February 2013 }}</ref> but much less than the {{convert|50|to|57|MtonTNT|abbr=on}} [[Tsar bomb]].<ref>{{cite web|url=http://www.nuclearweaponarchive.org/Russia/TsarBomba.html|title=The Tsar Bomba ("King of Bombs")|accessdate=2005-01-01-2006}}</ref> Also, unlike a [[nuclear explosion]] the object and its fragments [[power (physics)|did not release all of its energy at once]], with the total radiated [[energy]] of the [[fireball]], which generated the main explosion, predicted to have emitted an energy equivalent to ~ 90 [[TNT equivalent|kilotons of TNT]] {{nowrap|(~ 0.4 [[Joule|PJ]]),}} according to [[NASA]]'s [[Jet Propulsion Laboratory]].<ref name="JPL20130301" />
| |
| | |
| About 1,500 people were injured, two seriously. All of the injuries were due to indirect effects rather than the meteor itself, mainly from broken glass from windows that were blown in when the [[shock wave]] arrived, several minutes after the bright flash.<ref name="RT-283" /><ref name="Canada7968297">{{cite web |url=http://www.canada.com/news/Meteor+explodes+over+Russia+Ural+Mountains+injured+shock+wave+blasts+windows/7968297/story.html | title=Meteor explodes over Russia's Ural Mountains; 1,100 injured as shock wave blasts out windows | date=15 February 2013 | first1=Jim | last1=Heintz | first2=Vladimir | last2=Isachenkov | accessdate=4 March 2013 | agency=The Associated Press | publisher=Postmedia Network Inc | quote=Emergency Situations Ministry spokesman Vladimir Purgin said many of the injured were cut as they flocked to windows to see what caused the intense flash of light, which momentarily was brighter than the sun.}}</ref> Initially 4,300,<ref name="WSJ74597722">{{cite web |last1=Marson |first1=James |title=Falling Meteor Explodes Over Russia |url=http://online.wsj.com/article/SB10001424127887324162304578305163574597722.html |publisher=Wall Street Journal |date=15 February 2013 |accessdate=15 February 2013 |first2=Gautam |last2=Naik }}</ref><ref name="Forbes20130215">{{cite web |last=Ewalt |first=David M |title=Exploding Meteorite Injures A Thousand People in Russia |url=http://www.forbes.com/sites/davidewalt/2013/02/15/exploding-meteorite-injures-a-thousand-people-in-russia/ |publisher=Forbes |date=15 February 2013 |accessdate=15 February 2013 }}</ref> later 7,200 buildings in six cities across the region were reported to have been damaged by the explosion.
| |
| | |
| With an estimated initial mass of 11,000 tonnes, and measuring approximately 17 to 20 meters across,<ref name="JPL20130301" /> the Chelyabinsk meteor is the largest object<ref name="nature-12438">{{cite web |last=Brumfiel |first=Geoff |title=Russian meteor largest in a century |url=http://www.nature.com/news/russian-meteor-largest-in-a-century-1.12438 |publisher=Nature | doi=10.1038/nature.2013.12438 | date=15 February 2013 |accessdate=15 February 2013 }}</ref> to have entered Earth's atmosphere since the 1908 [[Tunguska event]] and the [[Curuçá River|1930 Brazilian event]],<ref>{{cite web | url=http://www.uh.edu/engines/epi1102.htm | title=Meteorite at CURUÇA | first=John H | last=Lienhard | date= | accessdate=4 March 2013 | quote=This explosion was only 1/10 as powerful as the Tunguska explosion. Yet that's still 50 times as powerful as the Hiroshima bomb.}}</ref> and it is the only meteor known to have resulted in a large number of injuries.<ref name="Forbes20130215" /> The object had not been detected before [[atmospheric entry]].<ref name="Today50820935">{{cite web |url=http://www.today.com/video/today/50820935 |title=Neil deGrasse Tyson: Radar could not detect meteor |work=Today |accessdate=15 February 2013 }}</ref>
| |
| | |
| == Close approaches ==
| |
| [[File:Asteroid 2004 FH.gif|framed|right|Flyby of asteroid [[2004 FH]] (centre dot being followed by the sequence). The other object that flashes by is an artificial [[satellite]].]]
| |
| {{main|List of asteroid close approaches to Earth}}
| |
| {{see also|Potentially hazardous object}}
| |
| On August 10, 1972 a meteor that became known as [[1972 Great Daylight Fireball]] was witnessed by many people moving north over the [[Rocky Mountains]] from the U.S. Southwest to Canada. It was an Earth-grazing meteoroid that passed within 57 kilometers (about 34 miles) of the Earth's surface. It was filmed by a tourist at the [[Grand Teton National Park]] in [[Wyoming]] with an 8-millimeter color movie camera.<ref>[http://www.youtube.com/watch?v=7M8LQ7_hWtE Grand Teton Meteor Video], [[Youtube]]</ref>
| |
| | |
| On March 23, 1989 the 300-meter (1,000-foot) diameter Apollo asteroid [[4581 Asclepius]] (1989 FC) missed the Earth by {{convert|700,000|km|mi|sp=us}} passing through the exact position where the Earth was only 6 hours before. If the asteroid had impacted it would have created the largest explosion in recorded history, 12 times as powerful as the [[Tsar Bomba]], the most powerful nuclear bomb ever exploded. It attracted widespread attention as early calculations had its passage being as close as {{convert|64,000|km|mi|abbr=on}} from the Earth, with large uncertainties that allowed for the possibility of it striking the Earth.<ref>{{cite web|url = http://www.cfa.harvard.edu/iau/pressinfo/1997XF11Globe.html|title=HOW THE ASTEROID STORY HIT: AN ASTRONOMER REVEALS HOW A DISCOVERY SPUN OUT OF CONTROL|author = Brian G. Marsden|publisher = The Boston Globe|date = 1998-03-29|accessdate = 2007-10-23}}</ref>
| |
| | |
| On March 18, 2004, [[LINEAR]] announced a 30-meter asteroid, [[2004 FH]], which would pass the Earth that day at only {{convert|42,600|km|mi|abbr=on}}, about one-tenth the distance to the Moon, and the closest miss ever noticed. They estimated that similar-sized asteroids come as close about every two years.<ref>{{cite web|url = http://neo.jpl.nasa.gov/news/news142.html|title = Recently Discovered Near-Earth Asteroid Makes Record-breaking Approach to Earth|author = Steven R. Chesley and Paul W. Chodas|publisher = National Aeronautics and Space Administration|date = March 17, 2004|accessdate = 2007-10-23}}</ref>
| |
| | |
| On March 31, 2004, two weeks after 2004 FH, [[meteoroid]] {{mpl|2004 FU|162}} set a new record for closest recorded approach, passing Earth only {{convert|6,500|km|mi|abbr=on}} away (about one-sixtieth of the distance to the Moon). Because it was very small (6 meters/20 feet), FU<sub>162</sub> was detected only hours before its closest approach. If it had collided with Earth, it probably would have harmlessly disintegrated in the atmosphere.
| |
| | |
| On March 2, 2009, near-Earth asteroid {{mpl|2009 DD|45}} flew by Earth at about 13:40 UT. The estimated distance from Earth was {{convert|72,000|km|mi|abbr=on}}, approximately twice the height of a geostationary communications satellite. The estimated size of the space rock was about 35 meters (115 feet) wide.<ref>[http://news.yahoo.com/s/space/20090302/sc_space/asteroidfliespastearth Yahoo News – 20090302 – Asteroid Flies Past Earth]</ref>
| |
| | |
| On January 13, 2010 at 12:46 UT, near-Earth asteroid {{mpl|2010 AL|30}}<ref>[http://neo.jpl.nasa.gov/news/news167.html Small Asteroid 2010 AL30 Will Fly Past The Earth]. [[NASA]]/[[JPL]] Near-Earth Object Program, January 12, 2010.</ref> passed at about {{convert|122,000|km|mi|abbr=on}}. It was approximately {{convert|10|-|15|m|ft|abbr=on}} wide. If {{mp|2010 AL|30}} had entered the Earth's atmosphere, it would have created an [[air burst]] equivalent to between 50 kt and 100 kt ([[TNT equivalent|kilotons of TNT]]). The Hiroshima "[[Little Boy]]" atom bomb had a yield between 13-18 kt.<ref>[http://epod.usra.edu/blog/2010/03/nearearth-object-2010-al30.html Near-Earth Object 2010 AL30]. [[NASA]] Earth Science Picture of the Day March 06, 2010.</ref>
| |
| | |
| On June 28, 2011 an asteroid designated [[2011 MD]], estimated at {{convert|5|-|20|m|ft|abbr=on}} in diameter, passed within {{convert|20,000|km|mi|abbr=on}} of the Earth, passing over the Atlantic Ocean.<ref>{{cite news|title=Asteroid soars over Atlantic Ocean|url=http://www.abc.net.au/news/stories/2011/06/28/3255543.htm|publisher=[[Australian Broadcasting Corporation]]|date=June 28, 2011|accessdate=2011-07-03}}</ref>
| |
| | |
| On November 8, 2011 {{mpl|2005 YU|55}} (at about 400m diameter) passed within {{convert|324600|km|abbr=on}} (0.85 [[lunar distance (astronomy)|lunar distance]]s) of Earth. Ten weeks later, on January 27, 2012, the 10-metre wide asteroid {{mpl|2012 BX|34}} passed a mere {{convert|60000|km|abbr=on}} from Earth.<ref>{{cite news|title=Asteroid makes near-miss fly-by|url=http://www.bbc.co.uk/news/science-environment-16756450|publisher=[[British Broadcasting Corporation]]|date=27 January 2012|accessdate=2012-01-28}}</ref>
| |
| | |
| On February 15, 2013, {{mpl|2012 DA|14}} passed approximately 27,700 km (17,200 mi) above the surface of Earth. This was closer than satellites in geosynchronous orbit. The asteroid was not visible to the unaided eye.
| |
| | |
| == Future impacts ==
| |
| [[File:1950da color 150.jpg|frame|left|Radar image of [[asteroid 1950 DA]].]]
| |
| {{main|Asteroid impact avoidance}}
| |
| Although there have been a few false alarms, a number of objects have been known to be threats to the Earth. {{mpl|(89959) 2002 NT|7}} was the first asteroid with a positive rating on the [[Palermo Technical Impact Hazard Scale]], with approximately one in a million on a potential impact date of February 1, 2019; it is now known that on January 13, 2019, {{mp|2002 NT|7}} will safely pass {{convert|0.4078|AU|km mi|abbr=on|lk=on}} from the Earth.<ref name=jpl-NT7>{{cite web
| |
| |date=2011-09-12 last obs (arc=57 years)
| |
| |title=JPL Close-Approach Data: 89959 (2002 NT7)
| |
| |url=http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2002NT7;cad=1#cad
| |
| |accessdate=2011-11-04}}</ref>
| |
| | |
| Asteroid [[(29075) 1950 DA]] was lost after its discovery in 1950, since not enough observations were made to allow plotting of its orbit, and then rediscovered on December 31, 2000. The chance it will impact Earth on March 16, 2880, during its close approach has been estimated as 1 in 300. This chance of impact for such a large object is roughly 50% greater than that for all other such objects combined up until 2880.<ref>{{cite journal |last=Giorgini |first=J. D. |last2=Ostro |first2=S. J. |last3=Benner |first3=L. A. M. |last4=Chodas |first4=P. W. |last5=Chesley |first5=S. R. |last6=Hudson |first6=R. S. |last7=Nolan |first7=M. C. |last8=Klemola |first8=A. R. |last9=Standish |first9=E. M. |date=April 5, 2002 |title=Asteroid 1950 DA's Encounter with Earth in 2880: Physical Limits of Collision Probability Prediction |journal=[[Science (journal)|Science]] |volume=296 |pages=132–136 |url=http://neo.jpl.nasa.gov/1950da/1950da.pdf |format=PDF |accessdate=January 11, 2010 |doi=10.1126/science.1068191 |pmid=11935024 |issue=5565|bibcode = 2002Sci...296..132G }}</ref> It has a diameter of about a kilometer (0.6 miles). The next radar opportunity for 1950 DA is in 2032,<ref name=Giorgini2002/> and our knowledge of the orbit won't improve considerably until then.
| |
| | |
| Only the asteroids [[99942 Apophis]] (provisionally known as 2004 MN4) and {{mpl|(144898) 2004 VD|17}} have briefly had above-normal rankings on the [[Torino Scale]].
| |
| | |
| {{clear}}
| |
| | |
| == Projects to minimize the threat ==
| |
| {{main|Asteroid-impact avoidance}}
| |
| [[File:Neo-chart.png|thumb|Number of Near Earth Asteroids detected by various projects.]]
| |
| [[File:NEA 1 km or more.png|thumb|Number of Near Earth Asteroids detected by various projects, 1 km or more]]
| |
| | |
| Several surveys have undertaken "[[Spaceguard]]" activities (an umbrella term), including [[Lincoln Near-Earth Asteroid Research]] (LINEAR), [[Spacewatch]], [[Near-Earth Asteroid Tracking]] (NEAT), [[Lowell Observatory Near-Earth-Object Search]] (LONEOS), [[Catalina Sky Survey]], [[Campo Imperatore Near-Earth Object Survey]] (CINEOS), [[Japanese Spaceguard Association]], and [[Asiago-DLR Asteroid Survey]]. In 1998, the [[United States Congress]] mandated the Spaceguard Survey – detection of 90% of near-earth asteroids over 1 km diameter (which threaten global devastation) by 2008. In 2005, this was extended by the George E. Brown, Jr. Near-Earth Object Survey Act, which calls for NASA to detect 90% of NEOs with diameters of 140 meters or greater, by 2020.<ref>Public Law 109–155, Section 321 (d). [http://www.gpo.gov/fdsys/pkg/PLAW-109publ155/pdf/PLAW-109publ155.pdf]</ref>
| |
| | |
| As of 2011, 911 of the largest (>1 km diameter) near-Earth asteroids have been found, with an estimate of 70 yet to be found.<ref name="pia14734"/>
| |
| | |
| == See also ==
| |
| * [[6Q0B44E]], in Earth orbit with a period of 80 days
| |
| * [[Asteroid mining]]
| |
| * [[Bolide]]
| |
| * [[BRAMS]] (Belgian Radio Meteor Stations); study of meteor population<ref>[http://brams.aeronomie.be/ Belgian RAdio Meteor Stations]</ref>
| |
| * [[Co-orbital configuration]]
| |
| * [[CubeSat]]
| |
| * [[Euronear]]
| |
| * [[J002E3]], probably the third stage of the [[Apollo 12]] [[Saturn V]]
| |
| * [[List of Earth-crossing minor planets]]
| |
| * [[Mission Marco Polo]]
| |
| * {{mpl|2010 SO|16}}
| |
| * [[Near Earth Object Camera]]
| |
| * [[NEODyS]]
| |
| * [[Orbit@home]]
| |
| * [[Potentially hazardous object]]
| |
| * [[Other moons of Earth#Quasi-satellites and trojans|Quasi-satellites and trojans]]
| |
| * [[Space debris]]
| |
| * [[Spaceguard]]
| |
| | |
| == References ==
| |
| {{reflist
| |
| | colwidth=30em
| |
| | refs =
| |
| | |
| <ref name="palermo">{{cite web
| |
| |date=31 August 2005
| |
| |title=THE PALERMO TECHNICAL IMPACT HAZARD SCALE
| |
| |publisher=NASA/JPL Near-Earth Object Program Office
| |
| |url=http://neo.jpl.nasa.gov/risk/doc/palermo.html
| |
| |accessdate=2008-07-27}}</ref>
| |
| | |
| <ref name="pia14734">{{cite web
| |
| |date=2011-09-29
| |
| |title=WISE Revises Numbers of Asteroids Near Earth
| |
| |publisher=NASA/JPL
| |
| |url=http://www.nasa.gov/mission_pages/WISE/multimedia/gallery/neowise/pia14734.html
| |
| |accessdate=2012-05-17}} [http://www.nasa.gov/mission_pages/WISE/news/wise20110929.html (NASA Space Telescope Finds Fewer Asteroids Near Earth)]</ref>
| |
| | |
| <ref name="JPL-50meter">{{cite web
| |
| |title=JPL Small-Body Database Search Engine: asteroids and NEOs and H > 25 (mag)
| |
| |publisher=[[JPL Small-Body Database]]
| |
| |url=http://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=neo;obj_kind=ast;obj_numbered=all;OBJ_field=0;ORB_field=0;c1_group=OBJ;c1_item=Ai;c1_op=%3E;c1_value=25;table_format=HTML;max_rows=50;format_option=comp;c_fields=AcBhBgBjBiBnBsCkCqAi;.cgifields=format_option;.cgifields=ast_orbit_class;.cgifields=table_format;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=com_orbit_class&query=1&c_sort=AiD
| |
| |accessdate=2012-05-18}}</ref>
| |
| | |
| <ref name="JPL-NEO-H16">{{cite web
| |
| |title=JPL Small-Body Database Search Engine: asteroids and NEOs and H < 16 (mag)
| |
| |publisher=JPL Solar System Dynamics
| |
| |url=http://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=neo;obj_kind=ast;obj_numbered=all;OBJ_field=0;ORB_field=0;c1_group=OBJ;c1_item=Ai;c1_op=%3C;c1_value=16;table_format=HTML;max_rows=500;format_option=comp;c_fields=AcBhBgBjBiBnBsCkCqAi;.cgifields=format_option;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=ast_orbit_class;.cgifields=table_format;.cgifields=com_orbit_class&query=1&c_sort=AcA
| |
| |accessdate=2013-04-11}}</ref>
| |
| | |
| <ref name="closest">{{cite web
| |
| |title=Closest Approaches to the Earth by Comets
| |
| |publisher=Minor Planet Center
| |
| |url=http://www.minorplanetcenter.org/iau/lists/ClosestComets.html
| |
| |accessdate=2012-06-28}}</ref>
| |
| | |
| <ref name="Earth-impact">{{cite web
| |
| |year=2010
| |
| |title=Earth Impact Effects Program
| |
| |publisher=Imperial College London / Purdue University
| |
| |author=Robert Marcus, H. Jay Melosh, and Gareth Collins
| |
| |url=http://impact.ese.ic.ac.uk/ImpactEffects/
| |
| |accessdate=2013-02-04}} (solution using 2600kg/m^3, 17km/s, 45 degrees)</ref>
| |
| | |
| <ref name=Asher2005>{{cite journal
| |
| |last=Asher |first=D. J.
| |
| |coauthors=Bailey, M.; Emel'Yanenko, V.; Napier, W.
| |
| |title=Earth in the Cosmic Shooting Gallery
| |
| |journal=The Observatory
| |
| |volume=125 |issue=2 |pages=319–322 |year=2005
| |
| |bibcode=2005Obs...125..319A
| |
| |url=http://www.arm.ac.uk/preprints/455.pdf
| |
| |last2=Bailey
| |
| |last3=Emel'Yanenko
| |
| |last4=Napier}}</ref>
| |
| | |
| <ref name=Giorgini2002>{{cite journal
| |
| |last=Giorgini |first=J. D.
| |
| |coauthors=Ostro, S. J; Benner, L. A. M.; Chodas, P.W.; Chesley, S.R.; et al.
| |
| |title=Asteroid 1950 DA's Encounter With Earth in 2880: Physical Limits of Collision Probability Prediction
| |
| |journal=Science
| |
| |volume=296 |issue=5565 |pages=132–136 |year=2002
| |
| |url=http://neo.jpl.nasa.gov/1950da/1950da.pdf
| |
| |doi=10.1126/science.1068191
| |
| |arxiv=
| |
| |bibcode= 2002Sci...296..132G|pmid=11935024}}</ref>
| |
| | |
| <ref name=IAU-NEOs>{{cite web
| |
| |title=The IAU and Near Earth Objects
| |
| |url=https://www.iau.org/public/themes/neo/
| |
| |accessdate=6 November 2013}}</ref>
| |
| | |
| }}
| |
| | |
| == External links ==
| |
| {{Commons category|Near-Earth asteroids}}
| |
| * [http://neo.jpl.nasa.gov Near earth object program] — [[Jet Propulsion Laboratory]]
| |
| * [http://www.lowell.edu/LARI_display_asteroid_targets.php Observable Near-Earth Asteroids] — [[Lowell Observatory]]
| |
| * [http://www.brera.mi.astro.it/sormano/teca.html Sormano Astronomical Observatory: Table of Asteroids Next Closest Approaches to the Earth] — Sormano Astronomical Observatory
| |
| * {{citation|title=Earth In The Cosmic Shooting Gallery|url=http://www.arm.ac.uk/preprints/455.pdf|first=D.J.|last=Asher|first2=M.|last2=Bailey|first3=V.|last3=Emel’yanenko|first4=B.|last4=Napier|year=2005|journal=[[The Observatory (journal)|The Observatory]]|volume=125|pages=319–322|bibcode = 2005Obs...125..319A }}
| |
| * [http://smallbodies.ru/en/ Catalogue of the Solar System Small Bodies Orbital Evolution] — Samara State Technical University
| |
| * [http://www.nasa.gov/mission_pages/WISE/news/wise20110929.html NASA Space Telescope Finds Fewer Asteroids Near Earth] — [[NASA]]
| |
| * [http://szyzyg.arm.ac.uk/~spm/neo_map.html Current Map Of The Solar System] — [[Armagh Observatory]]
| |
| | |
| {{Modern impact events}}
| |
| {{Planetary defense}}
| |
| {{Small Solar System bodies}}
| |
| {{Asteroids}}
| |
| | |
| {{DEFAULTSORT:Near-Earth Object}}
| |
| [[Category:Near-Earth objects| ]]
| |
| [[Category:Planetary defense]]
| |
| | |
| [[it:Oggetto Near-Earth]]
| |
| [[nl:Aardscheerder]]
| |
| [[th:ดาวเคราะห์น้อยใกล้โลก]]
| |
| [[tr:Dünya'ya yakın asteroit#Dünya'ya Yakın Asteroitler]]
| |