View source for 1620 Geographos

{{Short description|Asteroid}}
{{Use dmy dates|date=October 2019}}
{{Infobox planet
 | minorplanet     = yes
 | name            = 1620 Geographos
 | background      = #FFC2E0
 | image           = 1620Geographos (Lightcurve Inversion).png
 | image_scale      =
 | caption         = [[Lightcurve]]-based 3D-model of Geographos
 | discovery_ref   = <ref name="jpldata" />
 | discoverer      = [[Albert George Wilson|A. G. Wilson]]<br />[[Rudolph Minkowski|R. Minkowski]]
 | discovery_site  = [[Palomar Observatory]]
 | discovered      = 14 September 1951
 | mpc_name        = (1620) Geographos
 | alt_names       = 1951 RA{{·}}{{mp|1983 CY|3}}
 | pronounced      = {{IPAc-en|dʒ|iː|oʊ|ˈ|ɡ|r|æ|f|ɒ|s}}
 | named_after     = {{nowrap|[[National Geographic Society]]<ref name="springer" />}}
 | mp_category     = [[Apollo asteroid|Apollo]]{{·}}[[Near-Earth object|NEO]]{{·}}[[Potentially hazardous object|PHA]]<ref name="jpldata" /><ref name="MPC-object" />
 | orbit_ref       = <ref name="jpldata" />
 | epoch           = 4 September 2017 ([[Julian day|JD]] 2458000.5)
 | uncertainty     = 0
 | observation_arc = 65.37 yr (23,875 days)
 | earliest_precovery_date = 31 August 1951
 | aphelion        = 1.6629 [[Astronomical unit|AU]]
 | perihelion      = 0.8276 AU
 | semimajor       = 1.2453 AU
 | eccentricity    = 0.3354
 | period          = 1.39 [[Julian year (astronomy)|yr]] (508 days)
 | mean_anomaly    = 246.00[[Degree (angle)|°]]
 | mean_motion     = {{Deg2DMS|0.7093|sup=ms}} / day
 | inclination     = 13.337°
 | asc_node        = 337.20°
 | arg_peri        = 276.90°
 | moid            = 0.0301 AU{{·}}11.7 [[Lunar distance (astronomy)|LD]]
 | dimensions      = (5.0×2.0×2.1) ± 0.15 km<ref name="Hudson-1999" />
 | mean_diameter   = {{val|1.77|0.1}} [[Kilometre|km]]<ref name="SIMPS" /><br />{{val|1.87|0.05}} km<ref name="Nugent-2015" /><br />{{val|1.96|0.06}} km<ref name="Nugent-2016" /><br />{{val|2.5}} km<ref name="lcdb" /><ref name="Harris-1998a" /><br />{{val|2.56|0.15}} km<ref name="Hudson-1999" />
 | rotation        = {{val|5.2220|0.0003}} [[Hour|h]]<ref name="Higgins-2008b" /><br />{{val|5.22204|0.00007}} h{{efn|name=lcdb-Hamanowa-2011}}<br />{{val|5.22229|0.00004}} h<ref name="geneva-obs" /><br />{{val|5.223|0.003}} h<ref name="Skiff-2012a" /><br />{{val|5.223|0.005}} h<ref name="Warner-2016e" /><br />{{val|5.223|}} h<ref name="Dunlap-1974" /><br />{{val|5.223326}} h<ref name="Magnusson-1996" /><ref name="Kaasalainen-2001" /><br />{{val|5.223327}} h<ref name="Hudson-1999" /><br />{{val|5.223328}} h<ref name="Kwiatkowski-1995" /><ref name="Michalowski-1994" /><br />{{val|5.224|0.007}} h<ref name="Polishook-2009c" />
 | albedo          = {{val|0.26}}<ref name="lcdb" /><ref name="Harris-1998a" /><br />{{val|0.290|0.038}}<ref name="Nugent-2015" /><br />{{val|0.29|0.04}}<ref name="Nugent-2015" /><br />{{val|0.32|0.04}}<ref name="Nugent-2016" /><br />{{val|0.3258|0.051}}<ref name="SIMPS" />
 | spectral_type   = [[Tholen classification|Tholen]] {{=}} [[S-type asteroid|S]]<ref name="jpldata" /><br />[[SMASS classification|SMASS]] {{=}} [[S-type asteroid|S]]<ref name="jpldata" /><ref name="lcdb" />{{·}}[[S-type asteroid|S]]<ref name="Dandy-2003" /><br />[[Asteroid color indices|B–V]] {{=}} 0.862<ref name="jpldata" /><br />[[Asteroid color indices|B–V]] {{=}} {{val|0.896|0.009}}<ref name="Dandy-2003" /><br />[[Asteroid color indices|U–B]] {{=}} 0.471<ref name="jpldata" /><br />[[Asteroid color indices|V–R]] {{=}} {{val|0.462|0.005}}<ref name="Dandy-2003" /><br />[[Asteroid color indices|V–I]] {{=}} {{val|0.816|0.006}}<ref name="Dandy-2003" />
 | abs_magnitude   = 15.09<ref name="lcdb" /><ref name="Harris-1998a" />{{·}}15.41<ref name="Nugent-2016" />{{·}}{{val|15.55|0.37}}<ref name="Veres-2015" />{{·}}15.6<ref name="jpldata" /><ref name="SIMPS" /><ref name="Nugent-2015" />
}}

'''1620 Geographos''' ({{IPAc-en|dʒ|iː|oʊ|ˈ|ɡ|r|æ|f|ɒ|s}}; [[provisional designation]] '''{{mp|1951 RA}}''') is a highly elongated, stony [[asteroid]], [[near-Earth object]] and [[potentially hazardous asteroid]] of the [[Apollo asteroid|Apollo group]], with a [[mean diameter]] of approximately {{convert|2.5|km|abbr=on}}. It was discovered on 14 September 1951, by astronomers [[Albert George Wilson]] and [[Rudolph Minkowski]] at the [[Palomar Observatory]] in California, United States.<ref name="MPC-object" /> The asteroid was named in honor of the [[National Geographic Society]].<ref name="springer" />

== Orbit and classification ==
[[File:Animation of 1620 Geographos orbit.gif|thumb|left|Animation of 1620 Geographos{{'s}} orbit 2010-2020<br />{{legend2| yellow| Sun}}{{·}}{{legend2| Lime| Venus}}{{·}}{{legend2| RoyalBlue| Earth}}{{·}}{{legend2| OrangeRed|Mars}}{{·}}{{legend2| Magenta| 1620 Geographos }}]]
Geographos orbits the Sun at a distance of 0.8–1.7&nbsp;[[Astronomical unit|AU]] once every 1 years and 5 months (508 days). Its orbit has an [[orbital eccentricity|eccentricity]] of 0.34 and an [[orbital inclination|inclination]] of 13[[Degree (angle)|°]] with respect to the [[ecliptic]].<ref name="jpldata" /> Its orbit is well-determined for the next several hundred years. Due to its high eccentricity, Geographos is also a [[Mars-crosser asteroid]]. The body's [[observation arc]] begins at Palomar, two weeks prior to its official discovery observation.<ref name="MPC-object" />

=== Close approaches ===
As a [[potentially hazardous asteroid]], Geographos has a [[minimum orbital intersection distance]] (MOID) with Earth of less than 0.05&nbsp;AU and a diameter of greater than 150 meters. The Earth-MOID is currently {{convert |0.0301 |AU |km |abbr=on |lk=off |sigfig=3}}, which translates into 11.7 [[Lunar distance (astronomy)|lunar distances]].<ref name=jpldata /> In 1994, Geographos made its closest approach to Earth in two centuries at 5.0 Gm – which will not be bettered until 2586.<ref name="Ostro-1996" />

== Failed ''Clementine'' mission ==
Geographos was to be explored by the U.S.'s [[Clementine mission]] which was launched in January 1994. However, a malfunction in the spacecraft ended the mission before it could approach the asteroid.<ref>[http://www.nasa.gov/mission_pages/LCROSS/searchforwater/clementine.html NASA - Clementine]</ref> 

== Physical characteristics ==

=== Spectral type ===
In the [[Tholen classification|Tholen]] and [[SMASS classification]], Geographos is an [[S-type asteroid]].<ref name="jpldata" /> This means that it is highly reflective and composed of nickel-iron mixed with iron- and [[magnesium]]-[[silicates]].

=== Rotation period ===
Since the 1970s, several rotational [[lightcurve]] of Geographos have been obtained from photometric observations. Lightcurve analysis gave a [[rotation period]] (retrograde sense of rotation) between 5.222 and 5.224 hours with a very high brightness variation between 1.02 and 2.03 [[Magnitude (astronomy)|magnitude]] ({{small|[[LCDB quality code|U=3/3/3/2/3/3/3]]}}).<ref name="Higgins-2008b" /><ref name="geneva-obs" /><ref name="Skiff-2012a" /><ref name="Warner-2016e" /><ref name="Dunlap-1974" /><ref name="Polishook-2009c" />{{efn|name=lcdb-Hamanowa-2011}}

The [[Yarkovsky effect]] is causing a decrease in the orbital [[semimajor axis]] of {{Val|27.4|5.7|u=m yr<sup>−1</sup>}},<ref name="Rozitis-2014" /> while the [[Yarkovsky–O'Keefe–Radzievskii–Paddack effect|YORP]] effect is increasing the asteroid's rotation at the rate of {{Val|1.14e−3|0.03|u=rad yr<sup>−2</sup>}}.<ref>{{cite journal|doi=10.1051/0004-6361/202141844|title=Rotation acceleration of asteroids (10115) 1992 SK, (1685) Toro, and (1620) Geographos due to the YORP effect|year=2022|last1=Ďurech|first1=J.|last2=Vokrouhlický|first2=D.|last3=Pravec|first3=P.|last4=Krugly|first4=Yu. N.|last5=Kim|first5=M.-J.|last6=Polishook|first6=D.|last7=Ayvazian|first7=V. V.|last8=Bonev|first8=T.|last9=Choi|first9=Y.-J.|last10=Datashvili|first10=D. G.|last11=Donchev|first11=Z.|last12=Ehgamberdiev|first12=S. A.|last13=Hornoch|first13=K.|last14=Inasaridze|first14=R. Ya.|last15=Kapanadze|first15=G. V.|last16=Kim|first16=D.-H.|last17=Kučáková|first17=H.|last18=Kusakin|first18=A. V.|last19=Kušnirák|first19=P.|last20=Lee|first20=H.-J.|last21=Molotov|first21=I. E.|last22=Moon|first22=H.-K.|last23=Mykhailova|first23=S. S.|last24=Nikolenko|first24=I. V.|last25=Novichonok|first25=A.|last26=Oey|first26=J.|last27=Omarov|first27=Ch. T.|last28=Pollock|first28=J. T.|last29=Reva|first29=I. V.|last30=Rumyantsev|first30=V. V.|journal=Astronomy & Astrophysics|volume=657|pages=A5|arxiv=2110.06548|bibcode=2022A&A...657A...5D|s2cid=238744034|display-authors=1}}</ref>

=== Spin axis ===
Several lightcurve were also modeled from the abundant photometric observations. In 1994 and 1995, Polish astronomers obtained a concurring period 5.223328 hours and found a [[Poles of astronomical bodies|spin axis]] of (54.0°, −52.0°) in [[Ecliptic coordinate system|ecliptic coordinates]] (λ, β) ({{small|[[LCDB quality code|Q=3/3]]}}).<ref name="Michalowski-1994" /><ref name="Kwiatkowski-1995" /> Radiometric observations gave a period of 5.223327 hours and a pole of (55.0°, −46.0°).<ref name="Hudson-1999" /> Two other international studies obtained a period of 5.223326 hours and a pole at (56.0°, −47.0°) and (55.0°, −45.0°), respectively ({{small|[[LCDB quality code|Q=3/3]]}}).<ref name="Magnusson-1996" /><ref name="Kaasalainen-2001" />

=== Shape and structure ===
The [[light curve]] shows a high amplitude, indicative of its elongated shape, measuring 5.0×2.0×2.1 kilometers, which corresponds to a mean-diameter of 2.5&nbsp;km.<ref name="Hudson-1999" />

The interior of the asteroid probably has a [[Rubble pile|rubble-pile structure]]. The asteroid's high thermal inertia indicates the surface is most likely a mix of fine grains and large rocks and boulders.<ref name="Rozitis-2014" /> During the asteroid's close approach to Earth in 1994, a [[radar]] study of it was conducted by the [[Deep Space Network]] at the [[Goldstone Observatory]], California. The resultant images show Geographos to be the most elongated object in the [[Solar System]].<ref name="Ostro-1996" />

=== Diameter and albedo ===
According to the observations with the [[Goldstone Observatory]] and the space-based surveys carried out by the Infrared Astronomical Satellite [[IRAS]] and the [[NEOWISE]] mission of the [[Wide-field Infrared Survey Explorer]], Geographos measures between 1.77 and 2.56 kilometers in diameter  and its surface has an [[astronomical albedo|albedo]] between 0.26 and 0.3258.<ref name="SIMPS" /><ref name="Nugent-2015" /><ref name="Nugent-2016" /><ref name="Harris-1998a" /><ref name="Hudson-1999" /> The ''Collaborative Asteroid Lightcurve Link'' adopts an albedo of 0.26 and a diameter of 2.5 kilometers based on an [[absolute magnitude]] of 15.09.<ref name="lcdb" />

== Naming ==
This [[minor planet]] was named after the [[National Geographic Society]], in recognition of its contribution to astronomy by supporting the [[National Geographic Society – Palomar Observatory Sky Survey]] (NGS-POSS), which produced a photographic atlas of the entire northern sky in the 1950s. NGS-POSS was headed by the second discoverer, [[Rudolph Minkowski]]. The Greek word {{lang|el|geographos}} means [[geographer]] (from ''geo–'' 'Earth' + {{lang|el|graphos}} 'drawer/writer').<ref name="springer" /> The official {{MoMP|1620|naming citation}} was published by the [[Minor Planet Center]] in August 1956 ({{small|[[Minor Planet Circulars|M.P.C.]] 1468}}).<ref name="DoMP-Circular-dates" />

== Notes ==
{{notelist|refs=

{{efn|name=lcdb-Hamanowa-2011|1=Hamanowa (2011): [https://web.archive.org/web/20120216100727/http://www2.ocn.ne.jp:80/~hamaten/01620geographos-lc.htm lightcurve plot of (1620) Geographos], by H. & H. Hamanowa at the Hamanowa Astronomical Observatory ([[IAU code#D91|D91]]). Obs. date: 2008-02-10; rotation period {{val|5.22204}} hours (0.217585 days) with a brightness amplitude of {{val|1.39}} mag. Quality Code of 3. Summary figures at the [http://www.minorplanet.info/PHP/generateOneAsteroidInfo.php?AstInfo=1620%7CGeographos LCDB]}}

}} <!-- end of notelist -->

== References ==
{{reflist|30em|refs=

<ref name="jpldata">{{cite web
  |type       = 2017-01-11 last obs.
  |title      = JPL Small-Body Database Browser: 1620 Geographos (1951 RA)
  |url        = https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2001620
  |publisher  = [[Jet Propulsion Laboratory]]
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<ref name="springer">{{cite book
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  |doi       = 10.1007/978-3-540-29925-7_1621 |chapter       = (1620) Geographos }}</ref>

<ref name="MPC-object">{{cite web
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<ref name="DoMP-Circular-dates">{{cite book
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<ref name="geneva-obs">{{cite web
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<ref name="Rozitis-2014">{{Cite journal
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<ref name="lcdb">{{cite web
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<ref name="Dunlap-1974">{{Cite journal
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}} <!-- end of reflist -->

==External links==
* [http://echo.jpl.nasa.gov/asteroids/1620_Geographos/geographos.html NASA Asteroid Radar Research - Radar-Detected Asteroids: 1620 Geographos]
* [http://tech.groups.yahoo.com/group/mpml/message/21379 Large amplitude fast rotator]{{dead link|date=January 2025|bot=medic}}{{cbignore|bot=medic}} ([[Yahoo Groups]])
* [http://www.minorplanet.info/PHP/lcdbsummaryquery.php Asteroid Lightcurve Database (LCDB)], query form ([http://www.minorplanet.info/lightcurvedatabase.html info] {{Webarchive|url=https://web.archive.org/web/20171216050541/http://www.minorplanet.info/lightcurvedatabase.html |date=16 December 2017 }})
* [https://books.google.com/books?id=aeAg1X7afOoC&pg Dictionary of Minor Planet Names], Google books
* [http://obswww.unige.ch/~behrend/page_cou.html Asteroids and comets rotation curves, CdR] – Observatoire de Genève, Raoul Behrend
* {{NeoDys|1620}}
* {{ESA-SSA|1620|Geographos}}
* {{JPL small body}}

{{Minor planets navigator |1619 Ueta |number=1620 |1621 Druzhba}}

{{Authority control}}

{{DEFAULTSORT:Geographos}}
[[Category:Apollo asteroids|001620]]
[[Category:Discoveries by Albert George Wilson]]
[[Category:Named minor planets]]
[[Category:Radar-imaged asteroids|001620]]
[[Category:Potentially hazardous asteroids|001620]]
[[Category:S-type asteroids (Tholen)|001620]]
[[Category:S-type asteroids (SMASS)|001620]]
[[Category:Astronomical objects discovered in 1951|19510914]]