Comet Ikeya–Seki

Lore & Background

Comet Ikeya–Seki was independently discovered on 18 September 1965 by Japanese amateur astronomers Kaoru Ikeya and Tsutomu Seki within roughly 15 minutes of each other, aided by clear conditions after a typhoon. Initially an 8th-magnitude object, it brightened rapidly as it approached the Sun. Fred Whipple of the Smithsonian Astrophysical Observatory first suggested it was a sungrazing comet, and subsequent calculations showed its orbit closely resembled that of the Great Comet of 1882, with a perihelion of 0.0079 AU. The comet brightened to magnitude +4 by 7 October, and by 20 October it was easily visible with the naked eye in daylight. It reached perihelion at 21:18 UTC on 21 October, at which point it was separated from the Sun by only a few arcminutes and shone at magnitude −10 in Japan, where it occurred at local noon.

The comet broke into three pieces just before perihelion, and Japanese observers noted two small fragments detaching from the primary nucleus that later evaporated. After perihelion, the comet re-appeared in the morning sky in late October with a very bright tail. The fragmented nucleus remained close together initially, but by 6 November two primary components became visually distinct. Calculations by Zdenek Sekanina and H. Pohn both suggested the nuclei broke apart on 26 October, though Sekanina believed Pohn used different fragments. The two components moved apart at about 14 m/s, and by 1966 were separated by nearly a full arcminute. The comet faded below naked-eye visibility by early December, and the last photographs were taken before mid-February 1966, after which it became fainter than magnitude +13. It was indiscernible in a 60-second exposure using a 40-inch reflector telescope at the United States Naval Observatory Flagstaff Station in mid-March 1966.

Reader's Guide

Comet Ikeya–Seki's significance lies in its exceptional brightness and its role as a member of the Kreutz sungrazers, a group of comets thought to be fragments of a larger comet that broke apart. Its perihelion passage provided a unique opportunity for astrophysical observations, as the comet passed extremely close to the Sun and its orbit was virtually ideally oriented for observation from Earth. Several observatories—including Kitt Peak National Observatory, Lick Observatory, and Haute-Provence Observatory—performed spectrographic observations near perihelion, documenting strong emission lines of ionized calcium, iron, sodium, and other metals. A rocket launched from the White Sands Missile Range obtained ultraviolet spectrograms, though a pair of rocket launches from Wallops Island were unsuccessful. Efforts at MIT and Harvard to detect radio emission yielded negative results. NASA operated a Convair 990 out of Hawaii, and Los Alamos National Laboratory scientists used a Boeing 707 for observations. Gemini 6A was planned to observe the comet until the loss of the Agena target vehicle canceled the original mission. Elizabeth Roemer remarked that the breadth of observational data made the comet 'a landmark in cometary physics.' The comet's brightness closely paralleled the Great Comet of 1882, but Ikeya–Seki dimmed much more rapidly after perihelion, being observed for only about five months compared to the 1882 comet's eight months.

Did You Know?

• Comet Ikeya–Seki was discovered independently by two Japanese amateur astronomers within about 15 minutes of each other on 18 September 1965.
• It reached a peak magnitude of −10 and was visible in the daytime sky next to the Sun.
• The comet broke into three pieces just before perihelion, and two primary fragments later became visually distinct.
• Its nucleus had an estimated effective radius of 4.3 km before fragmenting.