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{{short description|Oort cloud comet}}
{{For|other comets discovered by the Siding Spring Observatory|Comet Siding Spring (disambiguation)}}
{{Use dmy dates|date=September 2019}}
{{Infobox Comet
| name           = C/2013 A1 (Siding Spring)
| image          = NASA-14090-Comet-C2013A1-SidingSpring-Hubble-20140311.jpg
| caption        = Comet Siding Spring as seen by [[Hubble Space Telescope|Hubble]] on 11 March 2014
| discovery_ref  = {{r|MPEC2013-A14}}
| discoverer     =[[Siding Spring Observatory]]<br>0.5-m Schmidt ([[List of observatory codes#E00-E99|E12]])
| discovery_date = 3 January 2013
| orbit_ref      = {{r|jpldata|MPEC2014-L52|barycenter}}
| epoch          = 30 October 2014 ([[Julian day|JD]] 2456960.5)
| observation_arc= 3.53 years
| obs            = 449
| orbit          = [[Oort cloud]]
| aphelion       = 52000 [[Astronomical unit|AU]] (inbound)<br/>13000 AU (outbound)
| perihelion     = 1.39875 AU (q)
| eccentricity   = 1.00043
| period         = several million years inbound<br><small>(Barycentric solution for [[Epoch (astronomy)|epoch]] 1950)</small><br>~{{val|500000}} years outbound<br><small>(Barycentric solution for [[Epoch (astronomy)|epoch]] 2050)</small>
| inclination    = 129.033°
| asc_node       = 300.999°
| arg_peri       = 2.449°
| tjup           = –0.919
| Earth_moid     = 0.3836 AU
| Jupiter_moid   = 3.6748 AU
| last_p         = 25 October 2014
| physical_ref   = {{r|jpldata|HiRISE141021|Swift}}
| dimensions     = ~{{cvt|400–700|m|mi}}
| M1             = 8.5
| M2             = 12.0
}}

'''C/2013 A1''' ('''Siding Spring''') is an [[Oort cloud]] [[comet]] discovered on 3 January 2013 by [[Robert H. McNaught]] at [[Siding Spring Observatory]] using the {{convert|0.5|m|in|adj=on|sp=us}} [[Uppsala Southern Schmidt Telescope]].<ref name="MPEC2013-A14" /><ref name="NYT-20140804" />

At the time of discovery it was 7.2&nbsp;[[Astronomical unit|AU]] from the Sun and located in the [[constellation]] [[Lepus (constellation)|Lepus]]. Comet C/2013 A1 probably took millions of years to come from the Oort cloud. After leaving the planetary region of the [[Solar System]], the post-perihelion [[orbital period]] ([[Epoch (astronomy)|epoch]] 2050) is estimated to be roughly 1 million&nbsp;[[Julian year (astronomy)|years]].<ref name="barycenter" />

C/2013 A1 passed the planet [[Mars]] very closely on 19 October 2014, at a distance of {{convert|140496.6 ± 4.0|km|mi au|abbr=on|lk=off}}.<ref name="FarnocchiaChesley2016">{{cite journal|last1=Farnocchia|first1=D.|last2=Chesley|first2=S.R.|last3=Micheli|first3=M.|last4=Delamere|first4=W.A.|last5=Heyd|first5=R.S.|last6=Tholen|first6=D.J.|last7=Giorgini|first7=J.D.|last8=Owen|first8=W.M.|last9=Tamppari|first9=L.K.|title=High precision comet trajectory estimates: The Mars flyby of C/2013 A1 (Siding Spring)|journal=Icarus|volume=266|year=2016|pages=279–287|issn=0019-1035|doi=10.1016/j.icarus.2015.10.035|bibcode=2016Icar..266..279F|arxiv = 1507.01980 |s2cid=118874989}}</ref> After its discovery, there was thought to be a chance of a collision with Mars, but this possibility was excluded when its orbit was determined with about a 200-day [[observation arc]].<ref name=esa/>

All [[NASA]] Mars orbiters—including [[2001 Mars Odyssey]],<ref name="NASA-20141019-ODY" /> [[Mars Reconnaissance Orbiter]]<ref name="NASA-20141019-MRO" /> and [[MAVEN]]<ref name="NASA-20141019-MAV" />—as well as [[European Space Agency|ESA's]] orbiter, [[Mars Express]],<ref name="ESA-20141020" /> and [[Indian Space Research Organisation|ISRO's]] orbiter, the [[Mars Orbiter Mission]],<ref name="ISRO MOM safe after Mars comet flyby" /> reported a healthy status after the comet flyby on 19 October 2014.<ref name="NASA-20141019" /><ref name="NYT-20141019" /> During the flyby, orbiters around Mars detected thousands of kilograms per hour of [[comet dust]] composed of [[magnesium]], [[iron]], [[sodium]], [[potassium]], [[manganese]], [[nickel]], [[chromium]] and [[zinc]].<ref name="NYT-20141107-KC" /> In addition, the [[comet nucleus]] was determined to be between {{convert|400|and|700|m|mi|1|sp=us}},<ref name="HiRISE141021"/><ref name="Swift"/><ref name="NYT-20141107-KC" /> much smaller than originally assumed. The nucleus rotates once every eight hours.<ref name="JPL141107"/>

== Discovery ==

The comet was discovered on 3 January 2013 by professional astronomer Robert McNaught at the [[Siding Spring Observatory]] at [[Coonabarabran]] NSW [[Australia]] and received the official designation C/2013 A1.<ref name="MPEC2013-A14" /> It was named Siding Spring based on a tradition to identify the observatory that discovered it. Three images were obtained through the use of a CCD camera mounted on the [[Uppsala Southern Schmidt Telescope]] with a spherical mirror of 0.5 meters in diameter. Comet Siding Spring had an [[apparent magnitude]] of 18.4 to 18.6. At the time of its discovery, it was {{convert|7.2|AU|km mi|abbr=on|lk=on}} from the Sun.

[[Precovery]] images by the [[Catalina Sky Survey]] from 8 December 2012 were found quickly and announced with the discovery giving Comet Siding Spring a 29-day [[observation arc]].<ref name="MPEC2013-A14" /> On 3 March 2013, [[Pan-STARRS]] precovery images from 4 October 2012 were announced that extended the observation arc to 148 days.<ref name="mpc" />
{{multiple image|align=center
|title=Comet Siding Spring Mars flyby on 19 October 2014 (artist's concepts) |total_width=600 |width1=2560 |height1=1600 |image1=PIA18611-Mars-CometSidingSpringFlyby-20141009.jpg |caption1=Orbiters take cover from Comet Siding Spring during Mars flyby |width2=786 |height2=385 |image2=CometSidingSpring-HeadingTowardsMars-ArtistConcept-20141006.jpg|caption2=Mars as seen from Comet Siding Spring |width3=1080 |height3=668 |image3=Comet-SidingSpring-Passing-PlanetMars-On-20141019-ArtistConcept-20140905.jpg |caption3=Comet Siding Spring as seen from Mars }}

== Encounter with Mars ==
[[File:Animation of C／2013 A1 orbit.gif|thumb|right|Animation of C/2013 A1 orbit<br>{{legend2|yellow|Sun}}{{·}}{{legend2|Gold|Mercury}}{{·}}{{legend2|OrangeRed|Venus }}{{·}}{{legend2|RoyalBlue|Earth }}{{·}}{{legend2|Lime|Mars}}{{·}}{{legend2|Magenta|C/2013 A1}}]]
Comet Siding Spring passed [[Astronomy on Mars#Comets and meteors|extremely close]] to [[Mars]] on 19 October 2014 at 18:28 ± 0:01 [[Coordinated Universal Time|UTC]].<ref name="jpl-close" /> Initial [[Astrometry|observations]] by [[Leonid Elenin]] on 27 February 2013, suggested that it might pass {{convert|0.000276|AU|km mi|abbr=on|lk=off}} from the center of Mars.<ref name="Elenin0227" /> With an observation arc of 733&nbsp;days, the [[Curve fitting|nominal]] pass is {{convert|0.000931|AU|km mi|abbr=on}} from the center-point of Mars and the uncertainty region shows that it would not come closer than {{convert|0.000927|AU|km mi|abbr=on|lk=off}}.<ref name="jpl-close" />

For comparison, Mars's outer moon [[Deimos (moon)|Deimos]] orbits it at a distance of {{convert|0.00016|AU|km mi|abbr=on}}. Due to the uncertainty region, there was the possibility that it could pass Mars as far away as {{convert|0.000934|AU|km mi|abbr=on|lk=off}}. It actually passed by at a distance of {{convert|140496.6 ± 4.0|km|mi au|abbr=on|lk=off}},<ref name="FarnocchiaChesley2016"/> at a relative velocity of {{convert|56|km/s|mi/s|abbr=on}}.<ref name="jpl-close" /> As seen from Mars, C/2013 A1 peaked at approximately [[apparent magnitude]] −6.<ref>{{cite web|url=https://twitter.com/SungrazerComets/status/498901731166924800|title=Sungrazing Comets 11 Aug 2014|access-date=28 October 2014}}</ref>{{dubious|date=March 2023}}
{{multiple image|align=center|caption_align=center|title=Comet Siding Spring encounter with Mars, 19 October 2014|total_width=600 |width1=1920 |height1=1080 |image1=Mars-C2013A1SidingSpring-Orbits-20141019.png |caption1=Projected position of Comet Siding Spring and its closest approach to Mars |width2=1655 |height2=1516 |image2=PIA17833-CometSidingSpring-C2013A1-MarsEncounter-20140128.png |caption2=Comet Siding Spring encounter }}

{| class="wikitable sortable collapsible collapsed" style="text-align:center; margin:0.2em auto; min-width:40em;"
! colspan=5 style="font-weight:normal" | [[JPL Small-Body Database]] – uncertainty region<br>for close approach to Mars on 19 October 2014
|-
! Observation<br>arc<br>(in days) !! Minimum<br>distance<br>(AU) !! Nominal<br>distance<br>(AU) !! Maximum<br>distance<br>(AU) !! Nominal<br>passage<br>time (UT)
|-
| &nbsp; 44 || 0 || 0.0059 &nbsp; || 0.024 &nbsp; &nbsp; || 10:26
|-
| &nbsp; 58 || 0 || 0.0025 &nbsp; || 0.012 &nbsp; &nbsp; || 15:50
|-
| &nbsp; 74 || 0 || 0.00070 || 0.0079 &nbsp; || 21:00
|-
| 148 || 0 || 0.00035 || 0.0021 &nbsp; || 19:28
|-
| 154 || 0 || 0.00074 || 0.0023 &nbsp; || 18:50
|-
| 162<ref>{{cite web |url=http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2013A1;cad=1 |title=Webcite capture on 2013-Mar-23 of JPL solution #10 (2013-Mar-18; arc=162 days) for C/2013 A1 |access-date=28 October 2014 |url-status=dead |archive-url=https://web.archive.org/web/20170312234833/http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2013A1;cad=1 |archive-date=12 March 2017 }}</ref> || 0 || 0.00079 || 0.0023 &nbsp; || 18:45
|-
| 171 || 0 || 0.00080 || 0.0022 &nbsp; || 18:44
|-
| 185 || 0.00006 &nbsp; || 0.00076 || 0.0020 &nbsp; || 18:51
|-
| 201 || 0.000068 || 0.00080 || 0.0019 &nbsp; || 18:45
|-
| 211 || 0.000209 || 0.00079 || 0.0018 &nbsp; || 18:45
|-
| 244 || 0.000098 || 0.00082 || 0.0016 &nbsp; || 18:41
|-
| 293 || 0.000252 || 0.00082 || 0.0014 &nbsp; || 18:41
|-
| 341 || 0.000443 || 0.00089 || 0.0013 &nbsp; || 18:32
|-
| 360 || 0.000546 || 0.00089 || 0.0012 &nbsp; || 18:32
|-
| 369 || 0.000607 || 0.00093 || 0.0012 &nbsp; || 18:28
|-
| 379 || 0.000595 || 0.00088 || 0.0012 &nbsp; || 18:34
|-
| 388 || 0.000690 || 0.00090 || 0.0011 &nbsp; || 18:31
|-
| 394 || 0.000775 || 0.00095 || 0.0011 &nbsp; || 18:26
|-
| 399 || 0.000801 || 0.00095 || 0.0011 &nbsp; || 18:26
|-
| 415 || 0.000853 || 0.00096 || 0.0011 &nbsp; || 18:25
|-
| 428 || 0.000868 || 0.00096 || 0.0010 &nbsp; || 18:25
|-
| 465 || 0.000875 || 0.00092 || 0.0010 &nbsp; || 18:28
|-
| 493 || 0.000867 || 0.00090 || 0.00093 || 18:31
|-
| 612 || 0.000858 || 0.00088 || 0.00091 || 18:32
|-
| 662 || 0.000858 || 0.00088 || 0.00090 || 18:33
|-
| 687 || 0.000926 || 0.00093 || 0.00094 || 18:28
|-
| 694 || 0.000916 || 0.00093 || 0.00094 || 18:28
|-
| 733<ref>{{cite web |url=http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2013A1;cad=1 |title=Webcite capture on 2014-Oct-09 of JPL solution #97 (2014-Oct-07; arc=2.01 yr) for C/2013 A1 |access-date=28 October 2014 |url-status=dead |archive-url=https://web.archive.org/web/20170312234833/http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2013A1;cad=1 |archive-date=12 March 2017  }}</ref> || 0.000927 || 0.00093 || 0.00093 || 18:28
|}

=== Predicted effects ===
[[File:PIA18593-Mars-CometSidingSpring-NEOWISE-20140728.jpg|thumb|left|C/2013 A1 – four images]]

The main body of the [[Comet tail|comet's tail]] was projected to miss Mars by some 10 Mars diameters.<ref>
{{cite journal
 |last1=Ye |first1=Quan-Zhi
 |last2=Hui |first2=Man-To
 |year=2014
 |title=An Early Look of Comet C/2013 A1 (Siding Spring): Breathtaker or Nightmare?
 |journal=[[The Astrophysical Journal]]
 |volume=787 |issue=2 |pages=115
 |arxiv=1403.7128
 |bibcode=2014ApJ...787..115Y
 |doi=10.1088/0004-637X/787/2/115
|s2cid=119105054
 }}</ref> As a result, only higher-than-average-velocity [[meteoroid]] dust, ejected earlier in the approach of the comet, allow for impacts on Mars, its moons, and orbiting spacecraft. Dust particles ejected from the nucleus of the comet, at more than double the expected velocity when the comet was 3 AU from the Sun, could reach Mars approximately 43 to 130 min after the closest approach of the comet.<ref name="Farnocchia2014" /> There is a possibility for millimeter- to centimeter-size particles released more than 13 AU from the Sun, however, this is considered unlikely,<ref name="Farnocchia2014" /> although massive ejections from farther out have been deduced.<ref>
{{cite arXiv
 |last1=Sekanina |first1=Zdenek
 |last2=Kracht |first2=Rainer
 |title=Disintegration of Comet C/2012 S1 (ISON) Shortly Before Perihelion: Evidence from Independent Data Sets
 |eprint=1404.5968
 |class=astro-ph.EP
|year=2014
 }}</ref>

In 2013 it was thought possible that Comet Siding Spring would create a [[meteor shower]] on Mars or be a threat to the spacecraft in Mars orbit.<ref name="NS-20131206" /> Studies in 2014 showed the threat to orbiting spacecraft to be minimal.<ref name="CIOC140802" /> The greatest threat would be about 100 minutes after closest approach.<ref name="CIOC140802" /> Mars passed about {{convert|27000|km|abbr=on}} from the comet's orbit around 20:10 UT.<ref name="Kelley2014" />

The [[Coma (cometary)|coma]] of the comet is projected to more than double the amount of hydrogen in the high atmosphere for a period of several tens of hours and to warm it by about 30 K for a few hours—the combination increasing the effect of [[atmospheric drag]] on the [[Mars Reconnaissance Orbiter]] and [[MAVEN]] spacecraft causing a measurable increase in [[orbital decay]] because of atmospheric [[ram pressure]].<ref>
{{cite journal
 |last1=Yelle |first1=Roger V.
 |last2=Mahieux |first2=Arnaud
 |last3=Morrison |first3=Sarah
 |last4=Vuitton |first4=Véronique
 |last5=Hörst |first5=Sarah M.
 |year=2014
 |title=Perturbation of the Mars Atmosphere by the Near-Collision with Comet C/2013 A1 (Siding Spring)
 |journal=[[Icarus (journal)|Icarus]]
 |volume=237 |pages=202–210
 |bibcode=2014Icar..237..202Y
 |doi=10.1016/j.icarus.2014.03.030
}}</ref> These spacecraft will be approaching Mars to minimum altitudes of 250&nbsp;km and 150&nbsp;km and orbital periods of 3 and 4 hours, respectively. The amount of drag cannot be narrowed down greatly until the production rate of the comet is known, but it could be from 1.6 to 40 times normal drag. MAVEN, in particular, also has instruments to observe any changes to the gas composition of the atmosphere. The closer moon of Mars, [[Phobos (moon)|Phobos]], orbits far higher, at a minimum distance of {{convert|9234.42|km|mi|abbr=on}},<ref name="NASA">{{cite web
 |date         = 30 September 2003
 |title        = Mars: Moons: Phobos
 |url          = http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mar_Phobos&Display=Facts
 |publisher    = [[NASA]]
 |access-date   = 2 December 2013
 |archive-url  = https://web.archive.org/web/20131019162634/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mar_Phobos&Display=Facts
 |archive-date = 19 October 2013
 |url-status     = dead
 |df           = dmy-all
}}</ref> more than 10 times the height of Mars's atmosphere.<ref>
{{cite web
 |last1=Withers |first1=Paul
 |last2=Pätzold |first2=Martin
 |last3=Witasse |first3=Olivier
 |date=15 November 2012
 |title=New views of the Martian ionosphere
 |url=http://sci.esa.int/mars-express/51056-new-views-of-the-martian-ionosphere/
 |publisher=[[ESA]]
 |access-date=10 May 2014
}}</ref>

Estimates for the diameter of the nucleus have varied from {{convert|1|to|50|km|0|abbr=on}},<ref name="Elenin0225" /><ref name="nasa-26mar" /> but now the nucleus is known to be only approximately {{convert|400–700|m|mi|1|sp=us}} in diameter,<ref name="HiRISE141021"/><ref name="Swift" /> roughly the diameter of asteroid {{mpl|2010 XG|11}} that approached Mars on 29 July 2014.<ref>{{cite sbdb |title=2010 XG11 |id=3553153 |cad=yes}}</ref> Based on early upper-limit size estimates, the resulting upper-limit energy of a hypothetical impact with Mars was 24&nbsp;billion [[TNT equivalent|megatons]].<ref name="Elenin0225" /><ref name="Bell" /> The diameter of such a hypothetical [[impact crater]] would be roughly ten times the diameter of the comet's [[Comet nucleus|nucleus]].<ref name="Elenin0225" /> A 700-meter impactor would create around a {{convert|7–10|km|mi|0|abbr=on}} crater.

The odds of an impact with Mars were 1 in 1250 in March 2013,<ref name="Elenin0303" /> 1 in 2000 in late March 2013,<ref name="nasa-26mar" /> 1 in 8000 by April 2013,<ref name="CBS2013-04" /> and 1 in 120,000 by 8 April 2013.<ref name="NASA2013-081" /> The 8 April 2013 [[JPL Small-Body Database]] [[3-sigma]] solution was the first estimate to show that the minimum approach by Comet Siding Spring would miss Mars.<ref name="jpl-close" /><ref name="Farnocchia2014" />

=== Actual effects ===

[[MAVEN]] detected an intense meteor shower.<ref name="JPL141107"/> Comet Siding Spring has a rotation period of approximately 8 hours.<ref name="JPL141107"/>{{clarify|date=August 2023|reason=Clarify how the comet's rotation period would be relevant to its effects on Mars.}} Debris from Comet Siding Spring added a temporary, but strong layer of ions to Mars's ionosphere (the first time such a phenomenon has been observed on any planet), and 85 tonnes of cometary dust were vaporized high in Mars's atmosphere. Magnesium, iron, and other metals were observed to have been deposited.<ref name="NYT-20141107-KC" /> An observer on the surface would have seen a few tens of meteors during the plane crossing.<ref name="CIOC141110"/>

During the flyby of Mars at a proximity of 140,000&nbsp;km, Comet Siding Spring's magnetic field, generated by its interaction with the solar wind, caused a violent turmoil that lasted for several hours, long after its flyby. Its coma washed over Mars with the dense inner coma, reaching or almost reaching the planet's surface. The cometary magnetic field temporarily merged with and overwhelmed Mars'  weak magnetic field.<ref>{{cite journal |first1= Jared R. |last1= Espley |first2= Gina A. |last2= DiBraccio |first3= John E. P. |last3= Connerney |first4= David |last4= Brain |first5= Jacob |last5= Gruesbeck |first6= Yasir |last6= Soobiah |first7= Jasper |last7= Halekas |first8= Michael |last8= Combi |first9= Janet |last9= Luhmann |first10= Yingjuan |last10= Ma |first11= Yingdong |last11= Jia |first12= Bruce |last12= Jakosky |title= A comet engulfs Mars: MAVEN observations of comet Siding Spring's influence on the Martian magnetosphere |doi= 10.1002/2015GL066300 |journal= Geophysical Research Letters |volume= 42 |issue= 21 |date= Nov 2015 |pages= 8810&ndash;8818 |bibcode = 2015GeoRL..42.8810E |doi-access= free |url= https://zenodo.org/records/1229058/files/article.pdf }}</ref>

== Observation ==
[[File:PIA17833-CometSidingSpring-C2013A1-NEOWISE-20140128.jpg|thumb|Comet Siding Spring as seen by [[NEOWISE]] on 16 January 2014]]

As seen from Earth, on 19 October 2014, Mars was in the [[constellation]] [[Ophiuchus (constellation)|Ophiuchus]], near [[globular cluster]] [[NGC 6401]], and 60&nbsp;degrees from the Sun. Mars and C/2013 A1 were {{convert|1.6|AU|km mi|abbr=on|lk=off}} from Earth.<ref name="MPC-eph" /> As of October 2014, C/2013 A1 had an [[apparent magnitude]] of roughly 11 and was the third-brightest comet in the sky at that time.<ref name="Yoshida141011" /><ref name="MPEC2014-T44" /> At an apparent magnitude of 0.9, Mars was estimated to be about 11,000 times brighter than the diffuse-looking comet with a low-surface brightness.<ref>[[Apparent magnitude#Calculations|Math]]: <math>(\sqrt[5]{100})^{11-0.9}\approx 10964</math></ref> To observe C/2013 A1 visually from Earth would have required a telescope with an optical mirror at least {{convert|0.2|m|in|0|adj=on|sp=us}} in diameter. By November 2014 the comet had dimmed to magnitude 11.6 and was only around the fifth-brightest comet in the sky.<ref name="Yoshida141102"/>

Mars and Comet Siding Spring were visible to the [[STEREO]]-A spacecraft during the 2014 encounter.<ref name="STEREO-HI2" /> In orbit around Mars were the spacecraft [[Mars Reconnaissance Orbiter]], [[2001 Mars Odyssey]], [[ESA]]'s [[Mars Express]], [[MAVEN]], and the Indian [[Mars Orbiter Mission|Mars Orbiter Mission (Mangalyaan)]]. The last two missions had arrived less than one month before the closest approach of C/2013 A1 to Mars. All these artificial satellites may have been exposed to potentially damaging particles.<ref name="Moorhead-Jan. 2014" /><ref name="evaluating" /> The level of exposure will not be known for months, but NASA had taken several "precautionary measures" as it prepared to study C/2013 A1.<ref name="NASA-20140725">{{cite web |last1=Brown |first1=Dwayne |last2=Webster |first2=Guy |title=NASA Mars Spacecraft Prepare for Close Comet Flyby |url=http://www.jpl.nasa.gov/news/news.php?release=2014-244 |date=25 July 2014 |work=[[NASA]] |access-date=25 July 2014 }}</ref> Two key strategies to lessen the risk were to place the orbiters on the opposite side of Mars at the time of the highest risk and to orient the orbiters so that their most vulnerable parts were not in the line of impact.<ref name="evaluating" /> On the ground, the [[Curiosity (rover)|Curiosity]] and [[Opportunity (rover)|Opportunity]] rovers obtained images as well. Results from the observations will be discussed during a special session "Comet C/2013 A1 Siding Spring at Mars" at the 2014 [[American Geophysical Union|AGU]] Fall Meeting in San Francisco on 18 December 2014.{{Update inline|date=September 2015}}

== Gallery ==

=== Before comet flyby ===
{{multiple image
| align     = center
| direction = horizontal

| image1    = Mars Fleet and Comet Siding Spring.webm
| width1    = 200
| alt1      =
| caption1  = Comet Siding Spring passes Mars on 19 October 2014.

| image2    = Sun view of Comet Siding Spring and the Mars orbiting fleet.ogg
| width2    = 200
| alt2      =
| caption2  = Orbiters hide as Comet Siding Spring passes Mars.
}}
<br>
{{multiple image
| align     = center
| direction = horizontal

| image1    = MAVEN and Comet Siding-Spring encountering Mars.webm
| width1    = 200
| alt1      =
| caption1  = [[MAVEN]] and Comet Siding Spring encounter Mars.

| image2    = MAVEN and Comet Siding-Spring encountering Mars fixed cam.webm
| width2    = 200
| alt2      =
| caption2  = MAVEN and Comet Siding Spring encountering Mars – fixed-camera view.

| image3    = Mars orbiters landers rovers during Comet Siding Spring.webm
| width3    = 200
| alt3      =
| caption3  = Comet Siding Spring – watched by several landers and orbiting spacecraft.
}}
<br>
{{wide image|Comet Siding Spring Close Approach.png|600px|align-cap=center|Closest approach of Comet Siding Spring to Mars – maximum and minimum distances noted}}

=== During comet flyby ===
{{wide image|Comet-C2013A1-SidingSpring-NearMars-Hubble-20141019.jpg|600px|align-cap=center|Close encounter of Comet Siding Spring with the planet Mars<br>([[composite image]]; [[Hubble Space Telescope|Hubble ST]]; 19 October 2014).}}
{{Gallery
| align = center
| title  =
| width  = 180
| height = 140
|Image:Comet C-2013 A1 Siding Spring flyby from Mars Orbiter.jpg|Images taken by the [[Mars Reconnaissance Orbiter]] of Comet Siding Spring on 19 October 2014
|Image:Cometa C2014 A1 en su acercamiento al planeta Marte.JPG|Image by the Observatorio de la Asociación [[Entre Ríos Province|Entrerriana]]<!---[[:es:Entrerriana|Entrerriana]]---> de Astronomía from Earth on 19 October 2014
||Image by a 90-mm (3.5-inch) telescope from Earth on 19 October 2014, about 9 hours after closest approach.
}}
<br>
{{Gallery
| align = center
| title  =
| width  = 180
| height = 140
|Image:MER Opportunity Comet Siding Spring Blink.gif|Images taken by the [[Opportunity (rover)|Opportunity Rover]] of Comet Siding Spring from the Martian surface on 19 October 2014
|Image:PIA18591-MarsOpportunityRover-CometSidingSpringFlyby-20141019.jpg|Image taken by the [[Opportunity (rover)|Opportunity Rover]] of Comet Siding Spring from the Martian surface on 19 October 2014
|Image:PIA18592-MarsOpportunityRover-CometSidingSpringFlyby-20141019.jpg|Image taken by the [[Opportunity (rover)|Opportunity Rover]] of Comet Siding Spring from the Martian surface on 19 October 2014
}}
{{wide image|PIA19043-MarsCuriosityRover-CometSidingSpring-20141019.gif|600px|align-cap=center|The [[Curiosity (rover)|''Curiosity'' rover]] views Comet C/2013 A1 during flyby on 19 October 2014.}}

== References ==
{{reflist|refs=

<ref name="jpldata">{{cite web
| title= C/2013 A1 (Siding Spring) – JPL Small-Body Database Lookup
| url= https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=2013A1
| website= ssd.jpl.nasa.gov
| publisher= [[Jet Propulsion Laboratory]]
| access-date= 12 November 2024 }}
</ref>

<ref name="NASA-20141019-ODY">{{cite web
  |last1=Webster |first1=Guy
  |last2=Brown |first2=Dwayne
  |title=NASA's Mars Odyssey Orbiter Watches Comet Fly Near
  |url=http://www.jpl.nasa.gov/news/news.php?feature=4346
  |date=19 October 2014
  |work=NASA
  |access-date=20 October 2014}}</ref>

<ref name="NASA-20141019-MRO">{{cite web
  |last1=Webster |first1=Guy
  |last2=Brown |first2=Dwayne
  |title=NASA's Mars Reconnaissance Orbiter Studies Comet Flyby
  |url=http://www.jpl.nasa.gov/news/news.php?feature=4344
  |date=19 October 2014
  |work=NASA
  |access-date=20 October 2014}}</ref>

<ref name="NASA-20141019-MAV">{{cite web
  |last1=Jones |first1=Nancy
  |last2=Steigerwald |first2=Bill
  |last3=Webster |first3=Guy
  |last4=Brown |first4=Dwayne
  |title=NASA's MAVEN Studies Passing Comet and Its Effects
  |url=http://www.jpl.nasa.gov/news/news.php?feature=4345
  |date=19 October 2014
  |work=NASA
  |access-date=20 October 2014}}</ref>

<ref name="NASA-20141019">{{cite web
  |last1=Webster |first1=Guy
  |last2=Brown |first2=Dwayne
  |last3=Jones |first3=Nancy
  |last4=Steigerwald |first4=Bill
  |title=All Three NASA Mars Orbiters Healthy After Comet Flyby
  |url=http://www.jpl.nasa.gov/news/news.php?feature=4347
  |date=19 October 2014
  |work=NASA
  |access-date=20 October 2014}}</ref>

<ref name="NYT-20141019">{{cite news
  |author=Agence France-Presse
  |title=A Comet's Brush With Mars
  |url=https://www.nytimes.com/2014/10/19/science/a-comets-brush-with-mars.html
  |date=19 October 2014
  |work=[[The New York Times]]
  |access-date=20 October 2014}}</ref>

<ref name="ESA-20141020">{{cite web
  |last=Denis |first=Michel
  |title=Spacecraft in great shape – our mission continues
  |url=http://blogs.esa.int/mex/category/comet-siding-spring/
  |date=20 October 2014
  |work=[[European Space Agency]]
  |access-date=21 October 2014}}</ref>

<ref name="ISRO MOM safe after Mars comet flyby">{{cite news
  |author=Staff
  |title=I'm safe and sound, tweets MOM after comet sighting
  |url=http://www.thehindu.com/sci-tech/science/im-safe-and-sound-tweets-mom-after-comet-sighting/article6520803.ece
  |date=21 October 2014
  |work=[[The Hindu]]
  |access-date=21 October 2014}}</ref>

<ref name="MPEC2013-A14">{{cite web
  | date=5 January 2013
  | title=MPEC 2013-A14 : Comet C/2013 A1 (Siding Spring)
  | publisher=IAU Minor Planet Center
  | url=http://www.minorplanetcenter.net/mpec/K13/K13A14.html
  | archive-url=https://web.archive.org/web/20130301024411/http://www.minorplanetcenter.net/mpec/K13/K13A14.html
  | archive-date=1 March 2013
  | url-status=live}} (CK13A010)</ref>

<ref name="HiRISE141021">{{cite web
  | author=Alfred McEwen
  | title=Images of Comet C/2013 A1 Siding Spring
  | publisher=NASA/JPL/University of Arizona
  | url=http://www.uahirise.org/releases/siding-spring/
  | access-date=21 October 2014 }}</ref>

<ref name="MPEC2014-L52">{{cite web
  | date=10 June 2014
  | url=http://www.minorplanetcenter.net/mpec/K14/K14L52.html
  | title=MPEC 2014-L52: Observations and Orbits of Comets
  | publisher=IAU Minor Planet Center
  | archive-url=https://web.archive.org/web/20140615193947/http://www.minorplanetcenter.net/mpec/K14/K14L52.html | archive-date=15 June 2014 | url-status=live }}</ref>

<ref name="jpl-close">{{cite sbdb |title=C/2013 A1 (Siding Spring) |id=1003228|cad=yes}}</ref>

<ref name="barycenter">{{cite web
  | author=[[JPL Horizons On-Line Ephemeris System|Horizons]] output
  | url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%272013+A1%27&TABLE_TYPE=%27ELEMENTS%27&START_TIME=%271950-01-01%27&STOP_TIME=%272050-01-01%27&STEP_SIZE=%27100%20years%27&CENTER=%27@0%27&OUT_UNITS=%27AU-D%27
  | title=Barycentric Osculating Orbital Elements for Comet C/2013 A1 (Siding Spring) }} Solution using the Solar System [[Barycenter]]. Ephemeris Type:Elements and Center:@0 (To be outside planetary region, inbound epoch 1950 and outbound epoch 2050. For epoch 2050-Jan-01 orbit period is "PR= 1.9E+08 / 365.25 days" = ~520,000 years)</ref>

<ref name="mpc">{{cite web
  | date=1 March 2013
  | title=C/2013 A1 (Siding Spring) Orbit
  | publisher=[[Minor Planet Center]]
  | url=http://www.minorplanetcenter.net/db_search/show_object?object_id=CK13A010 }}</ref>

<ref name="Bell">{{cite web
  | date=26 February 2013
  | title=Comet C/2013 A1 (Siding Spring)
  | first=Charles | last=Bell
  | website=[[Facebook]]
 | url=https://www.facebook.com/photo.php?fbid=10151255803941688 }}</ref>

<ref name="Swift">{{cite web
  | date=19 June 2014
  | title=NASA's Swift Satellite Tallies Water Production of Mars-bound Comet
  | publisher=NASA
  | work=[[Swift satellite]]
  | url=http://www.nasa.gov/content/goddard/nasas-swift-satellite-tallies-water-production-of-mars-bound-comet
  | access-date=20 June 2014 }}</ref>

<ref name="Elenin0225">{{cite web
  | date=25 February 2013
  | title=Comet C/2013 A1 (Siding Spring) – a possible collision with Mars
  | publisher=SpaceObs.org blog (ISON-NM)
  | first=Leonid | last=Elenin | author-link=Leonid Elenin
  | url=http://spaceobs.org/en/2013/02/25/comet-c2013-a1-siding-spring-a-possible-collision-with-mars/
  | archive-url=https://web.archive.org/web/20130302152456/http://spaceobs.org/en/2013/02/25/comet-c2013-a1-siding-spring-a-possible-collision-with-mars/ | archive-date=2 March 2013 | url-status=live }}</ref>

<ref name="Elenin0227">{{cite web
  | date=27 February 2013
  | title=New data concerning the close approach of comet C/2013 A1 to Mars
  | publisher=SpaceObs.org blog (ISON-NM)
  | first=Leonid | last=Elenin
  | url=http://spaceobs.org/en/2013/02/27/new-data-concerning-the-close-approach-of-comet-c2013-a1-to-mars/
  | archive-url=https://web.archive.org/web/20130304074649/http://spaceobs.org/en/2013/02/27/new-data-concerning-the-close-approach-of-comet-c2013-a1-to-mars/ | archive-date=4 March 2013 | url-status=live }}</ref>

<ref name="Elenin0303">{{cite web
  | date=3 March 2013
  | title=Close approach to Mars. Up-to-date analysis
  | publisher=SpaceObs.org blog (ISON-NM)
  | first=Leonid | last=Elenin
  | url=http://spaceobs.org/en/2013/03/03/probability-of-collision-with-mars/
  | archive-url=https://web.archive.org/web/20130307081417/http://spaceobs.org/en/2013/03/03/probability-of-collision-with-mars/ | archive-date=7 March 2013 | url-status=live }}</ref>

<ref name="STEREO-HI2">{{cite web
  | date=28 February 2013
  | title=Hit or Miss? Either way, NASA/STEREO-A spacecraft has a ring-side seat of "Siding Spring v's Mars" next year
  | publisher=Sungrazing Comets
  | first=Bill | last=Thompson
  | url=http://twitpic.com/c7jfp3
  | archive-url=https://web.archive.org/web/20130303051916/http://twitpic.com/c7jfp3 | archive-date=3 March 2013 | url-status=live }}</ref>

<ref name="nasa-26mar">{{cite web
  | url=https://science.nasa.gov/science-news/science-at-nasa/2013/26mar_marscomet/
  | title=Collision Course? A Comet Heads for Mars
  | first=Tony | last=Phillips
  | date=27 March 2013 | work=Science News | publisher=[[NASA]]
  | archive-url=https://web.archive.org/web/20130405065241/https://science.nasa.gov/science-news/science-at-nasa/2013/26mar_marscomet/ | archive-date=5 April 2013 | url-status=live }}</ref>

<ref name="NASA2013-081">{{cite web
  | date=12 April 2013
  | title=Comet to Make Close Flyby of Red Planet in October 2014
  | publisher=NASA/JPL
  | first=David C. |last=Agle
  | url=http://www.jpl.nasa.gov/news/news.php?release=2013-081
  | archive-url=https://web.archive.org/web/20130308140413/http://www.jpl.nasa.gov/news/news.php?release=2013-081 | archive-date=8 March 2013 | url-status=live }}</ref>

<ref name="CBS2013-04">{{cite web
  | date=1 April 2013
  | title=Mars vs. comet in 2014: Scientists prepare for red planet sky show
  | work=[[CBS News]]
  | url=https://www.cbsnews.com/news/mars-vs-comet-in-2014-scientists-prepare-for-red-planet-sky-show/
  | archive-url=https://web.archive.org/web/20130402213942/http://www.cbsnews.com/8301-205_162-57577242/mars-vs-comet-in-2014-scientists-prepare-for-red-planet-sky-show/ | archive-date=2 April 2013 | url-status=live }}</ref>

<ref name="NS-20131206">{{cite web
  | date=6 December 2013 | work=[[New Scientist]]
  | last=Grossman | first=Lisa
  | title=Fiercest meteor shower on record to hit Mars via comet
  | url=https://www.newscientist.com/article/dn24715-fiercest-meteor-shower-on-record-to-hit-mars-via-comet.html
  | archive-url=https://web.archive.org/web/20131212093435/http://www.newscientist.com/article/dn24715-fiercest-meteor-shower-on-record-to-hit-mars-via-comet.html | archive-date=12 December 2013 | url-status=live }}</ref>

<ref name="Moorhead-Jan. 2014">
{{cite journal
 | last1=Moorhead  | first1=Althea V.
 | last2=Wiegert  | first2=Paul A. |author2-link=Paul Wiegert
 | last3=Cooke  | first3=William J.
 | year=2014
 | title=The meteoroid fluence at Mars due to Comet C/2013 A1 (Siding Spring)
 | journal=[[Icarus (journal)|Icarus]]
 | volume=231 | pages=13–21
 | bibcode=2014Icar..231...13M
 | doi=10.1016/j.icarus.2013.11.028
| hdl=2060/20140010989
 | hdl-access=free
 }}</ref>

<ref name="evaluating">{{cite news
  | title=NASA Preparing for 2014 Comet Watch at Mars
  | date=28 January 2014
  | publisher=JPL
  | url=http://www.jpl.nasa.gov/news/news.php?release=2014-026
  | work=NASA
  | archive-url=https://web.archive.org/web/20140202232622/http://www.jpl.nasa.gov/news/news.php?release=2014-026 | archive-date=2 February 2014 | url-status=live }}</ref>

<ref name="Farnocchia2014">{{cite journal
  |last1=Farnocchia |first1=Davide
  |last2=Chesley |first2=Steven R.
  |last3=Chodas |first3=Paul W.
  |last4=Tricarico |first4=Pasquale
  |last5=Kelley |first5=Michael S .P.
  |last6=Farnham |first6=Tony L.
  |year=2014
  |title=Trajectory analysis for the nucleus and dust of comet C/2013~A1 (Siding Spring)
  |journal=The Astrophysical Journal
  |volume=790
  |issue=2
  |pages=114
  |arxiv=1404.4640
  |doi=10.1088/0004-637X/790/2/114
  |bibcode=2014ApJ...790..114F
 |s2cid=27067196
 }}</ref>

<ref name="CIOC140802">{{cite web
  | url=http://cometcampaign.org/karl/siding-spring-risk-assessment
  | title=Comet Siding Spring: Risk Assessment
  | first=Karl |last=Battams
  | date=2 August 2014
  | access-date=12 August 2014}}</ref>

<ref name="Kelley2014">{{cite journal
  |last1=Kelley |first1=Michael S. P.
  |last2=Farnham |first2=Tony L.
  |last3=Bodewits |first3=Dennis
  |last4=Tricarico |first4=Pasquale
  |last5=Farnocchia |first5=Davide
  |year=2014
  |title=A Study of Dust and Gas at Mars from Comet C/2013 A1 (Siding Spring)
  |arxiv=1408.2792
  |doi=10.1088/2041-8205/792/1/L16 |volume=792 |issue=1
  |journal=The Astrophysical Journal |page=L16|bibcode = 2014ApJ...792L..16K |s2cid=6907213
 }}</ref>

<ref name="NYT-20140804">{{cite news
  |last=Kaufmann
  |first=Marc
  |title=A Celestial Traveler Closes on Mars
  |url=https://www.nytimes.com/2014/08/05/science/space/celestial-traveler-closing-on-mars.html
  |date=4 August 2014
  |work=The New York Times
  |access-date=4 August 2014}}</ref>

<ref name="MPC-eph">{{cite web
 |url         = http://scully.cfa.harvard.edu/cgi-bin/returnprepeph.cgi?d=c&o=CK13A010
 |title       = Elements and Ephemeris for C/2013 A1 (Siding Spring)
 |publisher   = IAU Minor Planet Center
 |archive-url  = https://web.archive.org/web/20141012120502/http://scully.cfa.harvard.edu/cgi-bin/returnprepeph.cgi?d=c&o=CK13A010
 |archive-date = 12 October 2014
 |url-status     = dead
 |df          = dmy-all
 |access-date = 8 February 2013
}}</ref>

<ref name="Yoshida141011">{{cite web
  |date=11 October 2014
  |title=Weekly Information about Bright Comets (2014 Oct. 11: South)
  |first=Seiichi |last=Yoshida
  |publisher=aerith.net
  |url=http://www.aerith.net/comet/weekly/20141011s.html
  |access-date=11 October 2014}}</ref>

<ref name="Yoshida141102">{{cite web
  |date=2 November 2014
  |title=Weekly Information about Bright Comets (2014 Nov. 1: North)
  |first=Seiichi |last=Yoshida
  |publisher=aerith.net
  |url=http://www.aerith.net/comet/weekly/20141101n.html
  |access-date=10 November 2014}}</ref>

<ref name="MPEC2014-T44">{{cite web
  |date=6 October 2014
  |title=MPEC 2014-T449 : Observations and Orbits of Comets
  |publisher=IAU Minor Planet Center
  |url=http://www.minorplanetcenter.net/mpec/K14/K14T44.html
  |access-date=11 October 2014}} (lookup as CK13A010)</ref>

<ref name="JPL141107">{{cite web
  |url=http://www.jpl.nasa.gov/news/news.php?feature=4366
  |title=Mars Spacecraft Reveal Comet Flyby Effects on Martian Atmosphere
  |date=7 November 2014
  |publisher=JPL }}</ref>

<ref name="CIOC141110">{{cite web
  |url=http://cometcampaign.org/tony/orbiter-results
  |title=Comet Dust in Mars' Atmosphere: How Do the Orbiter Observations Relate to the Siding Spring Dust Models?
  |first=Tony |last=Farnham
  |date=9 November 2014
  |access-date=10 November 2014}}</ref>

<ref name="NYT-20141107-KC">{{cite news
  |last=Chang |first=Kenneth
  |title=Opportunity, Curiosity, but No View of Mars Sky Show
  |url=https://www.nytimes.com/2014/11/08/science/space/bright-specks-of-comet-dust-light-up-martian-sky.html
  |date=7 November 2014
  |work=The New York Times
  |access-date=7 November 2014}}</ref>

<ref name=esa>{{cite web
  |title=How to determine the orbit of a comet?
  |publisher=esa
  |quote=It took 44 days of observation to achieve even a semblance of an orbit determination – one that was still all over the place
  |date=2014-03-07
  |url=http://blogs.esa.int/mex/2014/03/07/how-to-determine-the-orbit-of-a-comet/
  |accessdate=2022-01-08 }}</ref>

}}  <!-- end of reflist -->

== External links ==
{{Commons category}}
* {{JPL small body}}
* [http://scully.cfa.harvard.edu/cgi-bin/returnprepeph.cgi?d=c&o=CK13A010 C/2013 A1 (Siding Spring)] – Elements and Ephemeris – [[Minor Planet Center]]
* [http://www.aerith.net/comet/catalog/2013A1/2013A1.html C/2013 A1 (Siding Spring)] – Seiichi Yoshida @ Aerith.net
* [http://cometbase.net/en/observation/listObserv/85 C/2013 A1 (Siding Spring)] – CometBase
* [http://www.virtualtelescope.eu/2014/10/19/comet-c2013-a1-siding-spring-planet-mars-history-unique-image/ C/2013 A1 Siding Spring and Mars ] – [[Virtual Telescope Project]] (19 October 2014)
* [http://mars.nasa.gov/comets/sidingspring/ NASA – Official WebSite] – Comet Siding Spring.
* [https://www.youtube.com/watch?v=Oq8lEKAY_fI NASA – VIDEO (02:19)] – Overview of Comet Flyby of Mars (9 October 2014).
* [https://www.youtube.com/watch?v=yICO-Fy3RQ0 NASA – VIDEO (56:15)] – Press Briefing of Comet Flyby of Mars (9 October 2014).
* [http://www.iceinspace.com.au/forum/attachment.php?s=c104f3d68b3549220fa18b9427f7ef2b&attachmentid=171685 Comet C/2013 A1 and Mars] – 80mm f4 [[refractor]]/[[Pentax K-5]] (17 October 2014).
* [http://s727.photobucket.com/user/Rob_Kau/media/C2013A116amp17Oct2014200mmtextsmall.jpg.html Comet C/2013 A1 and Mars] – 200mm f5.6 [[telephoto]]/[[Canon 650D]] ([[Compositing|composite]]; 17 October 2014).
* [http://www.brera.mi.astro.it/sormano/C2013A1/C2013A1.html Comet C/2013 A1 and Mars] – [[List of asteroid-discovering observatories#Osservatorio Astronomico Sormano|Sormano Astronomical Observatory]] (19 October 2014).
* [http://planetary.s3.amazonaws.com/assets/images/4-mars/2014/20141103_15599878605_1be102f3bc_o.jpg Comet C/2013 A1 passes Mars] – [[Curiosity (rover)|''Curiosity'' rover view]] (enlarged upper-left; 19 October 2014).
* [http://apod.nasa.gov/apod/ap141020.html Comet C/2013 A1 passes Mars] – [[APOD|NASA APOD]] (20 October 2014).
* [https://web.archive.org/web/20141030044929/http://www.latestspacenews.com/comet-siding-spring-c-2013-a1-seen-next-to-mars.html Comet C/2013 A1 passes Mars] – [[Hubble Space Telescope]] (22 October 2014).
* [http://www.planetary.org/blogs/emily-lakdawalla/2014/11031353-seven-mars-spacecraft.html Comet C/2013 A1 passes Mars] – Collection of images (posted 3 November 2014).
{{Comets}}
{{Mars}}
{{2013 in space}}
{{2014 in space}}

{{DEFAULTSORT:Siding Spring, 2013 A1}}
[[Category:Long-period comets]]
[[Category:Non-periodic comets]]
[[Category:Astronomical objects discovered in 2013|20130103]]
[[Category:Discoveries by SSS]]
[[Category:Comets in 2013|20130103]]
[[Category:Comets in 2014|20141025]]
[[Category:Mars impact events|20141019]]
[[Category:Oort cloud|2013A01]]
[[Category:Articles containing video clips]]
[[Category:Predicted impact events]]

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