by Forty years ago this month, there was a show that interrupted the celestial vision – literally a bolt of lightning from nowhere.
A brand-new comet, which for several days made headlines around the world due to its exceptionally close pass to Earth: a distance of less than 3 million miles (4.8 million km), or about 12 times the distance from earth to the moon.
In fact, when the comet was first sighted on April 25, 1983, it wasn’t with human eyes or a telescope, but from a satellite: IRAS, short for InfraRed Astronomical Satellite, launched from what was then Vandenberg Air Force Base the previous year. January and placed in a 560 mile (900 km) orbit around the Earth. The satellite was a joint venture of Great Britain, the Netherlands and the United States and was the first space telescope to conduct an all-sky survey in infrared wavelengths. Its main objective was to catalog the heat “signatures” of asteroids, as well as to observe the processes involved in the birth and death of stars.
Related: Comets: Everything you need to know about space’s ‘dirty snowballs’
First seen from a satellite
When the IRAS satellite detected a fast-moving object on April 25, it was first assumed to be an asteroid. But then, just over a week later, on May 3, Japanese amateur astronomer Genichi Araki reported the discovery of a new comet in the constellation Draco the Dragon to the Tokyo Observatory. This was followed by an observation made by George Alcock, a well-known British comet observer, who was scanning the sky with 15 x 80 binoculars. Surprisingly, Alcock – who had already discovered four other comets – was inside his house and looking through a closed window, when he stumbled upon the comet that Araki had spotted just seven hours earlier!
It soon became increasingly apparent that the object that IRAS had discovered was not actually an asteroid, but the same comet found by Araki and Alcock. Thus, it was considered appropriate to name the comet IRAS-Araki-Alcock. When Araki and Alcock spotted it, the comet was glowing at sixth magnitude – the threshold of visibility for someone without the use of optical aids under a clear dark sky.
Getting bright… and close!
Once a preliminary orbit for the comet was worked out, two things were determined.
First, intrinsically, this was a relatively small comet, probably measuring no more than 2 or 3 miles (3 or 5 km) across. And yet, next week, a rapid brightness of more than 60 times, possibly to second magnitude, as bright as Polaris, the North Star, was predicted.
But for something like what for that to happen, it would have to come very close to Earth. And indeed, calculations indicated that it was destined to miss our planet by just 2.88 million miles (4.63 million km) on May 11, 1983, making it the closest approach of any comet ever observed, except for another comet called Lexell – and that was in the year 1770!
Although IRAS-Araki-Alcock made its closest approach to the sun (called perihelion) on May 21, 1983, at a point just inside Earth’s orbit, it was during the period from May 4 to its closest approach to Earth. (perigee) on May 11 that the comet attracted great interest around the world.
In a way, it was like a call to arms for astronomers. The combination of a comet passing extremely close to Earth and appearing in a darkened sky (the new moon was on May 12th), while passing a series of familiar and easy-to-find celestial landmarks on successive nights, went very well with the large news media.
Busy busy busy!
In retrospect, perhaps a little too well . . .
At the Central Bureau for Astronomical Telegrams (CBAT) in Cambridge, Massachusetts – the clearinghouse for astronomical discoveries worldwide – news of comet IRAS-Araki-Alcock spread like wildfire. According to the director of the department, Dr. Brian G. Marsden (1937-2010), he and his small team were “absolutely inundated” with hundreds of calls from reporters, planetarium staff, professional and amateur astronomers, and even the curious “man in the street,” all requesting the latest information about the approaching comet. In his time ahead of CBAT, Dr. Marsden clearly considered the passage of this comet “the busiest moment in the bureau’s history”.
Probably the question most reporters asked was, “Are we in immediate danger of a collision?” (No!).
A close encounter timeline
May 9, 1983: The comet, now third-magnitude brightness, can be found passing close to the bright orange star Kochab in the bowl of Ursa Minor; the comet’s motion relative to the star was clearly obvious. Over a period of less than two hours, IRAS-Araki-Alcock appeared to approach Kochab, passing within half a degree of the star, and then gradually moving away from it. It was like watching the minute hand of a clock. Of all places north of the Tropic of Cancer, the comet was circumpolar, meaning it was visible in the sky all night. In essence, we were looking directly from Earth at the “underside” of the comet.
May 10, 1983: It formed a broad, roughly equilateral triangle with Dubhe and Merak, the famous “indicator stars” in the bowl of Ursa Major, and appeared high in the north-northwest sky for American observers. Attentive skywatchers could spot the comet without binoculars less than an hour after sunset.
May 11, 1983: The day of its closest approach to Earth – revealed the comet surprisingly close to the popular Beehive star cluster in the constellation Cancer, although the comet was incomparably brighter, peaking at around magnitude +1.5. A narrow tail of gas was recorded in many photographs, but visually through binoculars and telescopes only the comet’s fuzzy head (called the coma) was visible. And seen against a dark sky it looked absolutely huge, measuring about three degrees in diameter; equal in apparent size to approx. six full moons! Through large telescopes, fascinating structures appeared to illuminate the inner coma.
With IRAS-Araki-Alcock now so close to Earth, there was interest in trying to send radar signals away from Earth. Both the 1,000-foot (305-meter) radio telescope in Arecibo, Puerto Rico, and NASA’s Jet Propulsion Laboratory in Goldstone, California, were successful in obtaining these radar echoes, which were used to provide details about the radius, rotation and composition of the comet nucleus.
May 12, 1983: Now rapidly moving away from Earth, the comet – making its farewell appearance to Northern Hemisphere observers – can be found low in the southwestern sky after sunset, having rapidly dimmed in brightness to the third magnitude. By the following night, it was sinking below the horizon before late evening twilight. The show ended almost as quickly as it started.
Our next chance?
Will we have another chance to see a comet pass this close to Earth in the foreseeable future?
Perhaps.
Close approaches of comets to Earth are quite rare. The approach of a comet 14.5 million kilometers from our planet occurs – on average – about once every 30 to 40 years. For a comet passing within 8 million kilometers of Earth, such a close approach is even less frequent, occurring about once every 80 or 90 years.
So you can see how unusual the very close approach of less than 3 million miles (4.8 million km) to Earth was in the IRAS-Araki-Alcock case.
Interestingly, however, since 1983, there have been several comets – or comet fragments – that may have come even closer to Earth. A tiny comet, P/SOHO 5, “may” have come within 1.1 million miles (1.7 million kilometers) of our planet on June 12, 1999, although this figure is considered highly uncertain.
Another, 55P/Tempel-Tuttle – the comet that produces the annual Leonid meteor shower – was recently determined to have passed 2.1 million miles (3.4 million km) from Earth on October 26, 1366.
It seems that only small, faint comets make exceptionally close passes of Earth, but with one notable exception: Halley’s Comet.
On April 10, 837, the most famous of all comets passed just 4.9 million kilometers from Earth. Seen from China, Japan and Europe, the comet shone as brightly as Venus, accompanied by a tail that stretched more than 90 degrees across the sky.
Oh, to see a comet like what in our life!
And looking ahead, on May 7, 2134, Halley’s Comet will pass within 8.6 million miles (13.8 million km) of Earth, likely to be as bright as Jupiter and again displaying a spectacularly long tail.
Something our great-grandchildren can look forward to.
Joe Rao serves as an instructor and guest speaker at New York’s Hayden Planetarium. He writes about astronomy for natural history magazineO Farmers’ Almanac and other publications.
