 Comet Ikeya–Seki photographed by Maynard Pittendreigh sometime in 1965 | |
| Discovery[1] | |
|---|---|
| Discovered by | Kaoru Ikeya Tsutomu Seki |
| Discovery site | Japan |
| Discovery date | 18 September 1965 |
| Designations | |
| 1965 VIII, 1965f[2] | |
| Orbital characteristics[5][6] | |
| Epoch | 7 October 1965 (JD 2439040.5) |
| Observation arc | 115 days |
| Number of observations | 101–119 |
| Orbit type | Kreutz sungrazer |
| Aphelion | 183 AU (A) 207 AU (B) |
| Perihelion | 0.007786 AU (A) 0.007778 AU (B) |
| Semi-major axis | 91.6 AU (A) 103.7 AU (B) |
| Eccentricity | 0.999915 (A) 0.999925 (B) |
| Orbital period | 795 years (A)[3] 946 years (B)[4] |
| Inclination | 141.864° (A) 141.861° (B) |
| 346.995° (A) 346.981° (B) | |
| Argument of periapsis | 69.049° (A) 69.034° (B) |
| Last perihelion | 21 October 1965 |
| Physical characteristics[7][8] | |
Mean radius | 4.3 km (2.7 mi) (pre-perihelion) |
| Mass | 1.20×1017 kg |
| –10.0 (1965 apparition) | |
Comet Ikeya–Seki,formally designatedC/1965 S1,1965 VIII, and1965f, was along-periodcomet discovered independently byKaoru Ikeya andTsutomu 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 450,000 km (280,000 mi) above theSun's surface, and would probably become extremely bright.
Comets can defy such predictions, but Ikeya–Seki performed as expected. As it approachedperihelion observers reported that it was clearly visible in the daytime sky next to the Sun. InJapan, where it reachedperihelion at local noon, it was seen shining atmagnitude −10.[7] It proved to be one of the brightest comets seen in the last thousand years, and is sometimes known as theGreat 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 outerSolar System.
Ikeya–Seki is a member of theKreutz sungrazers, which are suggested to be fragments of a large comet.
Comet Ikeya–Seki was independently discovered on 18 September 1965 by Japanese amateur astronomersKaoru Ikeya andTsutomu Seki within roughly 15 minutes of each other. The recent passage of atyphoon had affordedfavorably clear conditions for identifying the comet.[9] Upon discovery, the comet was an 8th-magnitude[a] object visually located 10° west ofα Hydrae,[b] moving east across the sky at around 1° per day;[9][10]acceleration of the comet and significant brightening was observed shortly afterwards. Based on preliminary estimations of Ikeya–Seki's orbit,Fred Whipple of theSmithsonian Astrophysical Observatory conjectured that Ikeya–Seki was asungrazing comet. Subsequent and more accurate computations of the comet'sorbital properties showed a close similarity between Ikeya–Seki and theGreat Comet of 1882, including the calculatedperihelion of 0.0079 AU (1.2 million km, 0.7 million mi). Leading up to perihelion, Ikeya–Seki'slight curve was also similar to the 1882 comet.[10] The comet brightened to magnitude +4 by 7 October, with atail extending to over 1° in length.[10] By mid-October, the tail had extended to a length of 10°.[9] The comet's approach of the Sun visually placed the comet at increasingly loweraltitude and brighter skies, leading to greater difficulty in estimating the comet's brightness as perihelion drew nearer.[11] Nonetheless, Ikeya–Seki's increasing luminosity remained apparent;[9] 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 0 by 18 October.[11] The comet brightened considerably in the 60 hours after 18 October. By 20 October, the comet had become easily visible with thenaked eye in daylight. Ikeya–Seki continued to brighten as perihelion approached, becoming comparable in brightness to thefull 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.[11]In October–November 1965 the observation was conducted fromMauna 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.[12]
Ikeya–Seki reached perihelion at 21:18 UTC on 21 October.[11] As viewed from Earth, the comet and the Sun were separated by only a fewarcminutes.[9] Observations indicated that the comet'snucleus began to break apart near the time of perihelion,[11] with Japanese observers noting two small fragments detaching from the primary nucleus that later evaporated soon after.[9][13] The comet faded after perihelion as it receded from the Sun, with thecoma dimming to magnitude +3 by 26 October. However, its tail elongated, reaching a length of at least 15° by 26 October and reaching a maximum of nearly 30° in early November 1965. While the fragmented nucleus of Ikeya–Seki had hitherto remained close together, by 6 November two primary components had become visually distinct in both separation and brightness. Though the comet's coma had dimmed to magnitude 7.4 by 27 November, a tail spanning 10° remained visible to the naked eye.[11] The comet dimmed below naked-eye visibility by early December.[9] The two components of Ikeya–Seki's fractured nucleus remained apparent with increasing visual separation, moving apart at approximately 14 m/s (31 mph); one was brighter but more diffuse in appearance than the other. Extrapolation of the observed positions of the two nuclei calculated byZdenek Sekanina suggested that the nuclei broke apart on 26 October. Similar calculations by H. Pohn of theUnited States Geological Survey yielded 26 October as the date of separation, though Sekanina believed Pohn's calculation used different cometary fragments.[14] By 1966, the two fragments were separated by nearly a full arcminute.[9] Although Ikeya–Seki's brightness closely paralleled the Great Comet of 1882, Ikeya–Seki dimmed much more rapidly after perihelion;[15] while the 1882 comet was observed for up to eight months following perihelion,[15] the last photographs of Ikeya–Seki were taken prior to mid-February 1966,[9] after which the comet became fainter than magnitude +13.[14] Ikeya–Seki was indiscernible in a 60-secondexposure using a 40-inchreflector telescope at theUnited States Naval Observatory Flagstaff Station in mid-March 1966.[15]
Observations obtained by theMcMath–Pierce solar telescope atKitt Peak National Observatory detectedemission lines associated with ionizedcalcium,chromium,cobalt,copper,iron,manganese,nickel,sodium,vanadium, andcyanide in Ikeya–Seki's coma. The properties of the ionized iron and nickel lines suggested Ikeya–Seki reached an effective temperature of around 4800 K around perihelion.[16]
It is estimated that itsnucleus had an effective radius of 4.3 km (2.7 mi) before disintegrating.[8]
Ikeya–Seki's perihelion presented a unique opportunity for astrophysical observations to be taken of a bright comet passing extremely close to the Sun.[10][11] Additionally, the orientation of the comet's orbit with respect to Earth's was virtually ideal for observation of Ikeya–Seki.[9] Several observatories – includingKitt Peak National Observatory,Lick Observatory, andHaute-Provence Observatory – performedspectrographic observations of the comet near perihelion, documenting strongemission lines associated with ionizedcalcium,iron,sodium, and other metals.Spectrograms were also obtained by a rocket launched from theWhite Sands Missile Range to observe the comet inultraviolet.[14] A pair of rocket launches fromWallops Island intended to provide similar data proved unsuccessful.[14] Efforts atMIT andHarvard to detectradio emission from the comet yielded negative results. AConvair 990 operated byNASA out ofHawaii and aBoeing 707 with scientists fromLos 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 theAgena target vehicle led to the cancellation of the original mission.Elizabeth Roemer remarked of the breadth of observational data inPublications 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."[14]
Media related toComet Ikeya-Seki at Wikimedia Commons
.png)
