C/1983 H1 (IRAS–Araki–Alcock)

For the other comets discovered by George E. D. Alcock and the Infrared Astronomical Satellite, see Comet Alcock and Comet IRAS.

Comet IRAS–Araki–Alcock (formal designation C/1983 H1, formerly 1983 VII) is a long-period comet that, in 1983, made the closest known approach to Earth of any comet in the last 200 years, at a distance of about 0.0312 AU (4.67 million km; 2.90 million mi).^[3]

C/1983 H1
(IRAS–Araki–Alcock)

Comet IRAS–Araki–Alcock photographed by Russell E. Milton on 11 May 1983
Discovery[1][2]
Discovered by	IRAS George E. D. Alcock Genichi Araki
Discovery date	25 April 1983
Designations
Alternative designations	1983d[3] 1983 VII
Orbital characteristics[4]
Epoch	13 May 1983 (JD 2445467.5)
Observation arc	160 days
Number of observations	347
Aphelion	195.08 AU
Perihelion	0.9913 AU
Semi-major axis	98.034 AU
Eccentricity	0.98989
Orbital period	970.49 years
Inclination	73.251°
Longitude of ascending node	49.102°
Argument of periapsis	192.85°
Last perihelion	21 May 1983
TJupiter	0.408
Earth MOID	0.0058 AU
Jupiter MOID	3.0838 AU
Physical characteristics[4][5]
Dimensions	9.2 km (5.7 mi)
Synodic rotation period	51.3±0.3 hours
Geometric albedo	0.02
Comet total magnitude (M1)	12.6
Apparent magnitude	3–4 (1983 apparition)

Discovery and observations

A false colour image of Comet IRAS–Araki–Alcock in 1983, viewed in infrared light by the Infrared Astronomical Satellite (IRAS)

This figure combines an International Ultraviolet Explorer FES image showing its diffuse tail, and the long-wavelength redundant (LWR) spectrum depicting the molecular emission lines of sulfur (S₂) and hydroxyl (OH).

The comet was named after its discoverers – the Infrared Astronomical Satellite and two amateur astronomers, George Alcock of the United Kingdom and Genichi Araki of Japan. Both men were schoolteachers by profession, although Alcock was retired. Alcock had made his discovery simply by observing through the window of his home, using binoculars.^[6][7] During the closest approach, the comet appeared as a circular cloud about the size of the full moon, having no discernible tail, and shining at a naked eye magnitude of 3–4. It swept across the sky at an angular speed of about 30 degrees per day. On May 11 the comet was detected on radar by Arecibo Observatory and Goldstone Solar System Radar making it the first comet detected by two different radar systems.^[8] A second detection was made by Goldstone on 14 May.^[8]

It is a long-period comet, with an orbital period of about 970 years,^[4] and is the parent comet of the minor Eta Lyrid meteor shower.^[9] This shower's radiant lies between Vega and Cygnus and produces 1 or 2 meteors an hour in mid-May with a peak between 9 May and 11 May.

Flyby

The comet passed from north to south between 9 May to 13 May, passing through Ursa Major towards Cancer. Its motion is marked every 2 hours here.

Moving north to south, it crossed just inside the Earth's orbit on 11 May.

Comet IRAS–Araki–Alcock made its closest approach to Earth in 1983, at a distance of about 0.0312 AU (4.67 million km; 2.90 million mi). It was the closest approach up to that time of any comet in the last 200 years; only Lexell's Comet (1770) and 55P/Tempel–Tuttle (1366) are thought to have come closer.^[4][10] Subsequently, on 12 June 1999, the small comet P/1999 J6 (SOHO) passed about 0.012 AU (1.8 million km; 1.1 million mi) from Earth.^[11] What was thought to be a small fragment of 252P/LINEAR, 460P/PANSTARRS, passed at a distance of 0.0237 AU (3.55 million km; 2.20 million mi) on 22 March 2016.^[12]

See also

• C/1983 J1 (Sugano–Saigusa–Fujikawa) – another comet which passed close to Earth in 1983