☄ IRAS-Araki-Alcock C/1983 H1

How to follow comet IRAS-Araki-Alcock live

The panel above recomputes the position of IRAS-Araki-Alcock every second in your browser: its distance from the Sun and from Earth, its position in the sky (right ascension and declination). It runs on the same kind of engine observatories use, a Kepler solver applied to the JPL osculating orbital elements, so the numbers are not a static snapshot, they keep ticking.

Just below, the top-down map of the Solar System shows exactly where IRAS-Araki-Alcock is right now among the planets. You can fast-forward time with the day slider, zoom and pan, compare its distance to another body with a click, and press "Next event" to jump straight to perihelion. It is the most direct way to grasp the orbit of IRAS-Araki-Alcock with no math at all.

Comet fact sheet

About IRAS-Araki-Alcock

C/1983 H1 holds the record for the closest cometary approach to Earth since 1770, when Comet Lexell passed at about 2.2 million km. On 11 May 1983, IRAS-Araki-Alcock came within 0.0313 astronomical units (roughly 4.68 million km) of Earth's centre. Despite the extraordinary proximity, its small nucleus and favourable orbit made the object completely harmless; brightness reached magnitude 3 to 4, naked-eye visible under a dark sky but without the spectacle of larger, brighter comets.

The comet earned its triple name by being discovered independently and almost simultaneously by three different sources: the infrared satellite IRAS (Infrared Astronomical Satellite), Japanese amateur astronomer Genichi Araki, and British amateur astronomer George Alcock. It is one of the most perfect examples of independent multiple discovery in the history of modern astronomy, and illustrates how the same object can be found by radically different technologies at the same time.

History and discovery

The IRAS satellite, launched jointly by NASA, ESA and the Netherlands in January 1983, was the first space observatory dedicated exclusively to infrared observations. On 25 April 1983, during routine sky surveys, its detectors registered a moving object with a thermal signature consistent with a cometary nucleus. Detection was automatic and the data were reported to the project science team.

Two days later, on 27 April, Japanese amateur astronomer Genichi Araki, observing the night sky from Osaka with high-power binoculars, visually spotted the same object. Less than 24 hours after that, on 28 April, British amateur astronomer George Alcock, from Peterborough in England, made the third independent discovery, also visually with binoculars.

The International Astronomical Union credited all three discoveries, giving the comet the triple name IRAS-Araki-Alcock. The official designation is C/1983 H1. The speed of visual detection by Araki and Alcock shows the comet was already binocular-visible two weeks before closest approach, and both found it independently within 48 hours of each other.

Orbit and physical nature

C/1983 H1 has a very long period, near-parabolic orbit, with perihelion at 0.99 AU on 21 May 1983, almost exactly at Earth's solar distance. The high orbital inclination of 73 degrees and moderate relative velocity allowed the closest Earth approach to occur before perihelion, on 11 May, a geometry that rarely combines so favourably for a near-Earth passage.

The nucleus was estimated at about 5 to 8 km in diameter from radar data obtained at Arecibo and Goldstone. The dust production rate was low, explaining the absence of a prominent visible tail despite the enormous proximity. The comet appeared during the approach as a diffuse patch of uniform brightness, without a tail, with an apparent size comparable to the lunar disc, something radically different from what people expect of a "close" comet.

What makes it unique

Three characteristics make IRAS-Araki-Alcock singular in cometary history. First, the proximity of the passage: at 0.031 AU, it was the closest comet to Earth in 213 years, and any comet approaching within 0.05 AU is exceptional. Second, the angular velocity across the sky during the approach: the object moved so fast that binocular observers could detect the displacement within minutes, something essentially never seen in Solar System objects by casual observers. Third, it was the first comet detected simultaneously by two different radar systems, marking a milestone in the application of planetary radar to comets.

The angular velocity of 30 to 40 degrees per day (roughly 1 to 1.5 degrees per hour) caused the comet to sweep several degrees of sky per night, making it a target that stayed in no telescope field for long. Paradoxically, this complicated prolonged observation with narrow-field instruments: the comet moved too fast to remain in the eyepiece.

Observations and science produced

The most dramatic aspect of the passage was the angular speed: near closest approach, the object moved about 30 to 40 degrees per hour, visible to the naked eye as a point that shifted noticeably over minutes. Binocular observers could detect the motion in real time. This is exceedingly rare for any Solar System object outside the Moon itself.

The proximity allowed radar observations using the Arecibo radio telescope in Puerto Rico and the Deep Space Network tracking antenna in Goldstone, California. It was the first time a long-period comet had been simultaneously detected by two different radar systems. The radar data provided:

• Nucleus diameter: 5 to 8 km (first direct radar measurement of a cometary nucleus with two-station baseline).
• Rotation period: ~51 hours (derived from radar echo intensity variation).
• Detection of a dense cloud of centimetre-sized particles surrounding the nucleus, extending to at least 800 km.
• Non-spherical nucleus shape, consistent with the irregular shapes of cometary nuclei later confirmed by probes such as Giotto (Halley, 1986) and Deep Impact (Tempel 1, 2005).

In addition to radar, radio telescopes detected OH emission and the IRAS satellite obtained high-quality infrared data of the coma. The proximity of the passage made IRAS-Araki-Alcock an ideal target for virtually every type of instrument available in 1983.

Debates and historical context

The triple discovery of IRAS-Araki-Alcock was discussed as an example of how different technologies can converge on the same discovery almost simultaneously. In 1983, the IRAS sky survey was the only space infrared mission, and there was no coordination with amateur astronomers. The coincidence of three independent detections within 72 hours illustrates that the object sat right at the detectability threshold from April 1983 onward.

The approach raised questions about the risk of cometary impact on Earth. While 4.7 million km is a comfortably safe distance (the Moon sits at about 384,000 km), the passage was unexpected and detected only two weeks in advance. In 1983, no early-warning system for near-Earth objects was operational. The episode contributed, along with other events of the decade, to increasing interest in asteroid and comet surveillance programmes formalised in the following decades.

The comparison with Comet Lexell of 1770 deserves attention. Lexell passed at about 2.2 million km, much closer than IRAS-Araki-Alcock. Had it been as close in 1983 it would have been visible at first magnitude. Retroactive orbital calculations show that Lexell was ejected into a hyperbolic orbit by a Jupiter perturbation in 1779 and never returned to the inner Solar System, making the direct comparison between the two comets historically fascinating.

Trivia

• George Alcock (1912-2000), one of the co-discoverers, was a primary-school teacher in Peterborough, England, who discovered 5 comets and 5 novae (temporarily brightening stars) visually during his lifetime, all without a camera, using only binoculars and a memorised map of up to 30,000 star positions. He was one of the last great visual discoverers before the era of automated surveys.
• IRAS ended its mission in November 1983 when the liquid-helium cryogenic coolant needed to cool its infrared detectors ran out. In its short operational window of under a year, it catalogued more than 250,000 infrared sources, discovered 6 comets including C/1983 H1, and identified the infrared excess around Vega, the first evidence of protoplanetary discs around stars beyond the Sun.
• The absence of a prominent dust tail and extended coma despite the closeness suggests the nucleus may be partially dormant, with a hardened outer crust limiting sublimation of more volatile ices. This behaviour has been linked to very long-period comets making rare visits to the inner Solar System.
• Comet Lexell of 1770 still holds the all-time record for closest cometary approach to Earth. IRAS-Araki-Alcock came closer than any other comet in the 213 years that followed. The third on the historical list is Comet Tempel-Tuttle, which passed at about 0.0229 AU in 1366, but that figure is a retroactive calculation.
• The angular speed of IRAS-Araki-Alcock during the approach was equivalent to crossing the full Moon's diameter in about 30 minutes. Observers who tried to photograph it needed exposures of seconds, not minutes, to avoid excessive trailing.
• Genichi Araki, the Japanese amateur astronomer, is one of the rare comet discoverers who used conventional binoculars without a motorised mount. The Japanese practice of systematic visual sky sweeping produced several notable comet discoverers in the 1970s to 1990s, before automated surveys came to dominate the field.

Other comets

See the full comet catalogue.