View source for Comet Ikeya–Seki

{{Short description|Great Comet of 1965}}
{{About|the "Great Comet of 1965"|the 1967–1969 comet of the same name|C/1967 Y1 (Ikeya–Seki)}}
{{For|other comets discovered by Kaoru Ikeya and Tsutomu Seki|Comet Ikeya|Comet Seki}}
{{Infobox Comet
| name            = C/1965 S1 (Ikeya–Seki)<br>(Great Comet of 1965)
| image           = Comet C 1965 S1 Ikeya-Seki.jpg
| caption         = Comet Ikeya–Seki photographed by Maynard Pittendreigh sometime in 1965
| discovery_ref   = {{r|IAUC_1921}}
| discoverer      = [[Kaoru Ikeya]]<br>[[Tsutomu Seki]]
| discovery_site  = [[Japan]]
| discovery_date  = 18 September 1965
| designations    = 1965 VIII, 1965f{{r|ICQ1}}
| orbit_ref       = {{r|jpl1|jpl2}}
| orbit           = [[Kreutz sungrazer]]
| epoch           = 7 October 1965 ([[Julian day|JD]] 2439040.5) 
| observation_arc = 115 days
| obs             = 101–119
| semimajor       = 91.6 AU (A)<br>103.7 AU (B)
| perihelion      = 0.007786 AU (A)<br>0.007778 AU (B)
| aphelion        = 183 AU (A)<br/>207 AU (B)
| eccentricity    = 0.999915 (A)<br>0.999925 (B)
| period=795 years (A){{r|barycenter-A}}<br/>946 years (B){{r|barycenter-B}}
| inclination     = 141.864° (A)<br>141.861° (B)
| asc_node        = 346.995° (A)<br>346.981° (B)
| arg_peri        = 69.049° (A)<br>69.034° (B)
| physical_ref    = {{r|ICQ2|Knight_2010}}
| mean_radius     = {{cvt|4.3|km|mi}}<br>{{small|(pre-perihelion)}}
| mass            = 1.20{{x10^|17}} kg
| magnitude       = –10.0<br>{{small|(1965 apparition)}}
| last_p          = 21 October 1965
| next_p          =
}}

'''Comet Ikeya–Seki''', [[Astronomical naming conventions#Comets|formally designated]] '''C/1965 S1''', '''1965 VIII''', and '''1965f''', was a [[List of non-periodic comets|long-period]] [[comet]] discovered independently by [[Kaoru Ikeya]] and [[Tsutomu Seki]]. First observed as a faint telescopic object on 18 September 1965, the first calculations of its orbit suggested that on October 21, it would pass just {{cvt|450,000|km}} above the [[Sun]]'s surface, and would probably become extremely bright.

Comets can defy such predictions, but Ikeya–Seki performed as expected. As it approached [[perihelion]] observers reported that it was clearly visible in the daytime sky next to the Sun. In [[Japan]], where it reached [[perihelion]] at local noon, it was seen shining at [[Apparent magnitude|magnitude]] −10.{{r|ICQ2}} It proved to be one of the brightest comets seen in the last thousand years, and is sometimes known as the ''[[Great Comet]] of 1965''.

The comet was seen to break into three pieces just before its perihelion passage. The three pieces continued in almost identical orbits, and the comet re-appeared in the morning sky in late October, showing a very bright tail. By early 1966, it had faded from view as it receded into the outer [[Solar System]].

Ikeya–Seki is a member of the [[Kreutz sungrazer]]s, which are suggested to be fragments of a large comet. 

== Discovery and observations ==
[[File:Mensen op de brug bij de Amstel wachten tevergeefs op het zichtbaar worden van d, Bestanddeelnr 918-3461.jpg|thumb|left|Onlookers in [[Amsterdam]] awaiting the comet's apparition on 21 October 1965|alt=Black-and-white photograph of a crowd observing the comet]]
Comet Ikeya–Seki was independently discovered on 18 September 1965 by Japanese amateur astronomers [[Kaoru Ikeya]] and [[Tsutomu Seki]] within roughly 15&nbsp;minutes of each other. The recent passage of a [[typhoon]] had afforded [[Astronomical seeing|favorably clear]] conditions for identifying the comet.{{r|Hale_2020}} Upon discovery, the comet was an 8th-[[apparent magnitude|magnitude]]{{efn|Unless otherwise noted, all observations of comet magnitude refer to [[apparent magnitude]] in [[visible light]]. A more negative value for apparent magnitude indicates a brighter object.}} object visually located 10° west of [[Alphard|α Hydrae]],{{efn|Reported initial position upon discovery was: [[right ascension|α]] {{=}} {{RA|8|45.4}}, [[declination|δ]] {{=}} {{DEC|–8|38}}{{r|IAUC_1921}}}} moving east across the sky at around 1° per day;{{r|Hale_2020|Roemer_1965}} [[Proper motion|acceleration]] of the comet and significant brightening was observed shortly afterwards. Based on preliminary estimations of Ikeya–Seki's orbit, [[Fred Whipple]] of the [[Smithsonian Astrophysical Observatory]] conjectured that Ikeya–Seki was a [[sungrazing comet]]. Subsequent and more accurate computations of the comet's [[ephemerides|orbital properties]] showed a close similarity between Ikeya–Seki and the [[Great Comet of 1882]], including the calculated [[perihelion]] of 0.0079&nbsp;[[astronomical unit|AU]] (1.2&nbsp;million km, 0.7&nbsp;million mi). Leading up to perihelion, Ikeya–Seki's [[light curve]] was also similar to the 1882 comet.{{r|Roemer_1965}} The comet brightened to magnitude&nbsp;+4 by 7 October, with a [[tail (comet)|tail]] extending to over 1° in length.{{r|Roemer_1965}} By mid-October, the tail had extended to a length of 10°.{{r|Hale_2020}} The comet's approach of the Sun visually placed the comet at increasingly lower [[horizontal coordinate system|altitude]] and brighter skies, leading to greater difficulty in estimating the comet's brightness as perihelion drew nearer.{{r|Roemer_1966a}} Nonetheless, Ikeya–Seki's increasing luminosity remained apparent;{{r|Hale_2020}} in the Southern Hemisphere, where visibility of the comet was most favorable around the time of perihelion, observers reported Ikeya–Sekit to be as bright as magnitude&nbsp;0 by 18 October.{{r|Roemer_1966a}} The comet brightened considerably in the 60&nbsp;hours after 18 October. By 20 October, the comet had become easily visible with the [[naked eye]] in daylight. Ikeya–Seki continued to brighten as perihelion approached, becoming comparable in brightness to the [[full Moon]]. The comet also projected a slightly curved tail; 2° of the tail was sufficiently visible to the naked eye with manual obscuration of the Sun.{{r|Roemer_1966a}}
In October–November 1965 the observation was conducted from [[Mauna Kea, Hawaii]]. One of the key findings from this study was the documentation of the comet's rapid brightening and the detection of fragmentation in its nucleus. The images obtained provided evidence of the comet's intense interaction with solar radiation and its effects on the comet's physical structure.{{r|Herring_1966}}

Ikeya–Seki reached perihelion at 21:18&nbsp;[[Coordinated Universal Time|UTC]] on 21&nbsp;October.{{r|Roemer_1966a}} As viewed from Earth, the comet and the Sun were separated by only a few [[arcminute]]s.{{r|Hale_2020}} Observations indicated that the comet's [[comet nucleus|nucleus]] began to break apart near the time of perihelion,{{r|Roemer_1966a}} with Japanese observers noting two small fragments detaching from the primary nucleus that later evaporated soon after.{{r|Hale_2020|Hirayama_1965}} The comet faded after perihelion as it receded from the Sun, with the [[coma (comet)|coma]] dimming to magnitude&nbsp;+3 by 26&nbsp;October. However, its tail elongated, reaching a length of at least 15° by 26&nbsp;October and reaching a maximum of nearly 30° in early November&nbsp;1965. While the fragmented nucleus of Ikeya–Seki had hitherto remained close together, by 6&nbsp;November two primary components had become visually distinct in both separation and brightness. Though the comet's coma had dimmed to magnitude&nbsp;7.4 by 27&nbsp;November, a tail spanning 10° remained visible to the naked eye.{{r|Roemer_1966a}} The comet dimmed below naked-eye visibility by early December.{{r|Hale_2020}} The two components of Ikeya–Seki's fractured nucleus remained apparent with increasing visual separation, moving apart at approximately {{cvt|14|m/s|mph}}; one was brighter but more diffuse in appearance than the other. Extrapolation of the observed positions of the two nuclei calculated by [[Zdenek Sekanina]] suggested that the nuclei broke apart on 26&nbsp;October. Similar calculations by H. Pohn of the [[United States Geological Survey]] yielded 26&nbsp;October as the date of separation, though Sekanina believed Pohn's calculation used different cometary fragments.{{r|Roemer_1966b}} By 1966, the two fragments were separated by nearly a full arcminute.{{r|Hale_2020}} Although Ikeya–Seki's brightness closely paralleled the Great Comet of 1882, Ikeya–Seki dimmed much more rapidly after perihelion;{{r|Roemer_1966c}} while the 1882 comet was observed for up to eight months following perihelion,{{r|Roemer_1966c}} the last photographs of Ikeya–Seki were taken prior to mid-February 1966,{{r|Hale_2020}} after which the comet became fainter than magnitude&nbsp;+13.{{r|Roemer_1966b}} Ikeya–Seki was indiscernible in a 60-second [[Exposure (photography)|exposure]] using a 40-inch [[reflector telescope]] at the [[United States Naval Observatory Flagstaff Station]] in mid-March 1966.{{r|Roemer_1966c}}

== Structure and composition ==
[[File:Ikeyaseki nucleus 6Nov1965.jpg|thumb|A 6-minute exposure of Ikeya–Seki's nucleus on 6 November 1965, faintly resolving the two primary components of the comet's fractured nucleus]]
Observations obtained by the [[McMath–Pierce solar telescope]] at [[Kitt Peak National Observatory]] detected [[emission line]]s associated with ionized [[calcium]], [[chromium]], [[cobalt]], [[copper]], [[iron]], [[manganese]], [[nickel]], [[sodium]], [[vanadium]], and [[cyanide]] in Ikeya–Seki's coma. The properties of the ionized iron and nickel lines suggested Ikeya–Seki reached an effective temperature of around 4800&nbsp;[[Kelvin (unit)|K]] around perihelion.{{r|Slaughter_1969}}

It is estimated that its [[comet nucleus|nucleus]] had an effective radius of {{cvt|4.3|km|mi}} before disintegrating.{{r|Knight_2010}}

== Observing campaigns ==
Ikeya–Seki's perihelion presented a unique opportunity for astrophysical observations to be taken of a bright comet passing extremely close to the Sun.{{r|Roemer_1965|Roemer_1966a}} Additionally, the orientation of the comet's orbit with respect to Earth's was virtually ideal for observation of Ikeya–Seki.{{r|Hale_2020}} Several observatories{{snd}}including [[Kitt Peak National Observatory]], [[Lick Observatory]], and [[Haute-Provence Observatory]]{{snd}}performed [[spectrography|spectrographic observations]] of the comet near perihelion, documenting strong [[emission line]]s associated with ionized [[calcium]], [[iron]], [[sodium]], and other metals. [[Spectrogram]]s were also obtained by a rocket launched from the [[White Sands Missile Range]] to observe the comet in [[ultraviolet]].{{r|Roemer_1966b}} A pair of rocket launches from [[Wallops Island]] intended to provide similar data proved unsuccessful.{{r|Roemer_1966b}} Efforts at [[MIT]] and [[Harvard]] to detect [[radio emission]] from the comet yielded negative results. A [[Convair 990 Coronado|Convair 990]] operated by [[NASA]] out of [[Hawaii]] and a [[Boeing 707]] with scientists from [[Los Alamos National Laboratory]] were also involved in observational efforts targeting Ikeya–Seki. [[Gemini 6A]] was also planned to incorporate observation of the comet until the loss of the [[Agena target vehicle]] led to the cancellation of the original mission. [[Elizabeth Roemer]] remarked of the breadth of observational data in ''[[Publications of the Astronomical Society of the Pacific]]'' that "There seems no doubt that the appearance of Comet Ikeya–Seki will stand as a landmark in cometary physics."{{r|Roemer_1966b}}

== Gallery ==
<gallery>
1965 S1.jpg|Comet Ikeya–Seki, seen from Canberra, 31 October 1965. Drawing by David Nicholls.
Ikeyaseki tail 30Oct1965.jpg|Comet Ikeya–Seki, 30 October 1965. Photo by [[James W. Young]] ([[Table Mountain Observatory|TMO]]/JPL/NASA)
</gallery>

== References ==
=== Notes ===
{{Notelist}}

=== Citations ===
{{Reflist|refs=

<ref name="barycenter-A">{{cite web
| author1= [[JPL Horizons On-Line Ephemeris System|Horizons]] output
| title= Barycentric Osculating Orbital Elements for Comet Ikeya-Seki (C/1965 S1-A)
| url= https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%271965+S1-A%27&TABLE_TYPE=%27ELEMENTS%27&START_TIME=%272200-01-01%27&STOP_TIME=%272400-01-01%27&STEP_SIZE=%27300%20years%27&CENTER=%27@0%27&OUT_UNITS=%27AU-D%27
| access-date= 26 August 2023 }} (Solution using the Solar System's [[barycenter]] (Sun+Jupiter). Select Ephemeris Type:Elements and Center:@0)</ref>

<ref name="barycenter-B">{{cite web
| author1= [[JPL Horizons On-Line Ephemeris System|Horizons]] output
| title= Barycentric Osculating Orbital Elements for Comet Ikeya-Seki (C/1965 S1-B)
| url= https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%271965+S1-B%27&TABLE_TYPE=%27ELEMENTS%27&START_TIME=%272200-01-01%27&STOP_TIME=%272400-01-01%27&STEP_SIZE=%27300%20years%27&CENTER=%27@0%27&OUT_UNITS=%27AU-D%27
| access-date= 26 August 2023 }} (Solution using the Solar System's [[barycenter]] (Sun+Jupiter). Select Ephemeris Type:Elements and Center:@0)</ref>

<ref name="Hale_2020">{{cite web
| author1= A. Hale
| title= Comet of the Week: Ikeya-Seki 1965F 
| url= https://www.rocketstem.org/2020/10/24/ice-and-stone-comet-of-week-44/
| website= RocketSTEM.org
| date= 24 October 2020
| access-date= 15 May 2023 }}
</ref>

<ref name="Herring_1966">{{cite journal 
| author1= A. Herring
| title= Observations of Comet Ikeya-Seki (1965f) from Mauna Kea, Hawaii
| url= https://sic.lpl.arizona.edu/sites/sic.lpl.arizona.edu/files/collection/journal/067_Herring%20_CommLPL_1966.pdf
| journal= Communications of the Lunar and Planetary Laboratory
| volume= 4
| pages= 141–145
| year= 1966 }}
</ref>

<ref name="Hirayama_1965">{{cite journal
| author1= T. Hirayama
| author2= F. Moriyama
| title= Observations of Comet Ikeya-Seki (1965f)
| url= https://ui.adsabs.harvard.edu/link_gateway/1965PASJ...17..433H/ADS_PDF
| journal= Publications of the Astronomical Society of Japan
| publisher=Astronomical Society of Japan
| year= 1965
| volume= 17
| issue= 4
| pages= 433–466
| bibcode= 1965PASJ...17..433H }}
</ref>

<ref name="IAUC_1921">{{cite journal
| author1= O. Gingerich
| title= Comet Ikeya-Seki (1965f)
| url= http://www.cbat.eps.harvard.edu/IAUCs/IAUC1921.jpg
| journal= Central Bureau for Astronomical Telegrams
| volume= 1921
| date= 20 September 1965 }}
</ref>

<ref name="ICQ1">{{cite web
| title= Comet Names and Designations
| url= http://www.icq.eps.harvard.edu/names1.html
| website= International Comet Quarterly
| access-date= 15 December 2024 }}
</ref>

<ref name="ICQ2">{{cite web
| title= Brightest comets seen since 1935
| url= http://www.icq.eps.harvard.edu/brightest.html
| website= International Comet Quarterly
| access-date= 12 October 2023 }}
</ref>

<ref name="jpl1">{{cite web
| title= C/1965 S1-A (Ikeya–Seki) – JPL Small-Body Database Lookup
| url= https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=1965S1-A
| website= ssd.jpl.nasa.gov
| publisher= [[Jet Propulsion Laboratory]]
| access-date= 26 August 2023 }}
</ref>

<ref name="jpl2">{{cite web
| title= C/1965 S1-B (Ikeya–Seki) – JPL Small-Body Database Lookup
| url= https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=1965S1-B
| website= ssd.jpl.nasa.gov
| publisher= [[Jet Propulsion Laboratory]]
| access-date= 15 December 2024 }}
</ref>

<ref name="Knight_2010">{{cite journal
| author1= M. M. Knight
| author2= M. F. A'Hearn
| author3= D. A. Biesecker
| author4= G. Faury
| author5= D. P. Hamilton
| author6= P. Lamy
| author7= A. Llebaria
| display-authors= 4
| title= Photometric Study of the Kreutz Comets Observed by SOHO from 1996 to 2005
| journal= The Astronomical Journal
| year= 2010
| volume= 139
| issue= 3
| pages= 926–949
| bibcode= 2010AJ....139..926K
| doi= 10.1088/0004-6256/139/3/926
| doi-access= free }}
</ref>

<ref name="Roemer_1965">{{cite journal
| author1= E. Roemer
| title= Comet Notes
| journal= Publications of the Astronomical Society of the Pacific
| publisher= Astronomical Society of the Pacific 
| year= 1965
| volume= 77
| issue= 459
| pages= 475–477
| bibcode= 1965PASP...77..475R
| jstor= 40674261
| doi= 10.1086/128264
| doi-access= free }}
</ref>

<ref name="Roemer_1966a">{{cite journal
| author1= E. Roemer
| title= Comet Notes
| url= https://iopscience.iop.org/article/10.1086/128303/pdf
| journal= Publications of the Astronomical Society of the Pacific
| publisher= Astronomical Society of the Pacific 
| date= February 1966
| volume= 78
| issue= 460
| pages= 83–91
| bibcode= 1966PASP...78...83R
| jstor= 40674283
| doi= 10.1086/128303
| doi-access= | url-access= subscription
}}
</ref>

<ref name="Roemer_1966b">{{cite journal 
| author1= E. Roemer
| title= Comet Notes
| url= https://iopscience.iop.org/article/10.1086/128325/pdf
| journal= Publications of the Astronomical Society of the Pacific
| publisher= Astronomical Society of the Pacific 
| date= April 1966
| volume= 78
| issue= 461
| pages= 178–179
| bibcode= 1966PASP...78..178R
| jstor= 40674308
| doi= 10.1086/128325
| doi-access= free | url-access= subscription
}}
</ref>

<ref name="Roemer_1966c">{{cite journal
| author1= E. Roemer
| title= Comet Notes
| url= https://iopscience.iop.org/article/10.1086/128364/pdf
| journal= Publications of the Astronomical Society of the Pacific
| publisher= Astronomical Society of the Pacific 
| date= August 1966
| volume= 78
| issue= 463
| pages= 348–350
| bibcode= 1966PASP...78..348R
| jstor= 40674376
| doi= 10.1086/128364
| doi-access= free }}
</ref>

<ref name="Slaughter_1969">{{cite journal
| author1= C. D. Slaughter
| title= The Emission Spectrum of Comet Ikeya–Seki 1965-f at Perihelion Passage
| url= https://ui.adsabs.harvard.edu/link_gateway/1969AJ.....74..929S/ADS_PDF
| journal= The Astronomical Journal
| publisher= American Astronomical Society 
| year= 1969
| volume= 74
| issue= 7
| pages= 929–943 
| bibcode= 1969AJ.....74..929S
| doi= 10.1086/110884
| doi-access= free }}
</ref>

}}

== External links ==
{{Commons category-inline|Comet Ikeya-Seki}}
*{{JPL small body|name=C/1965 S1-A}}
*{{JPL small body|name=C/1965 S1-B}}
* [https://hdr-astrophotography.com/simulations-of-past-comets/ HDR Astrophotography: Simulations Atlas of Past Comets (1900 to 1999)] by Nicolas Lefaudeux

{{Comets}}
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{{DEFAULTSORT:Ikeya-Seki, 1965 S1}}
[[Category:Long-period comets]]
[[Category:Kreutz Sungrazers]]
[[Category:Sungrazing comets]]
[[Category:Destroyed comets]]
[[Category:Split comets]]
[[Category:Great comets]]
[[Category:Science and technology in Japan]]
[[Category:Astronomical objects discovered in 1965|19650918]]
[[Category:Discoveries by Tsutomu Seki|#]]
[[Category:Discoveries by amateur astronomers]]