Forty years ago this month, a heavenly sight-stopping spectacle came along, literally a bolt from the blue.
A 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 of the earth to the moon.
In fact, when the comet was first sighted on April 25, 1983, it was not with human eyes or a telescope, but from a satellite: IRAS, an acronym for InfraRed Astronomical Satellite, launched from the then Vandenberg Air Force Base on former. January and placed in a 560 mile (900 km) orbit around Earth. The satellite was a joint venture of Britain, the Netherlands and the United States and was the first space telescope to conduct an all-sky survey at infrared wavelengths. Their main goal 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 the ‘dirty snowballs’ of space
First seen by 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 informed the Tokyo Observatory of the discovery of a new comet in the constellation Draco the Dragon. This was followed by an observation by George Alcock, a well-known British comet spotter, who was scanning the sky with 15 x 80 binoculars. Surprisingly, Alcock, who had previously discovered four other comets, was inside your house and looking through a closed window, when he ran into the comet that Araki had sighted just seven hours before!
It soon became more and more evident that the object that IRAS had discovered was not really an asteroid, but the same comet found by both Araki and Alcock. Therefore, it was considered appropriate to name the comet IRAS-Araki-Alcock. When Araki and Alcock sighted it, the comet was bright at 6th magnitude, the threshold of visibility for someone without using any optical aids under a clear, dark sky.
Getting bright… and close!
Once a preliminary orbit for the comet had been worked out, two things were determined.
First, intrinsically, this was a relatively small comet, probably no more than 2 or 3 miles (3 or 5 km) across. And yet, within the next week, it is predicted to rapidly increase in brightness by more than 60 times, possibly to a second magnitude, as bright as Polaris, the North Star.
But for something like that to happen, it would have to get 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 comet ever observed, except for another comet called Lexell, and that was in the year 1770!
Although IRAS-Araki-Alcock would make its closest approach to the sun (called perihelion) on May 21, 1983, at a point just inside Earth’s orbit, it was during the time period from May 4 to its approach. closest to Earth ( perigee) on May 11 that the comet aroused 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 dark sky (new moon was May 12), while arcing near a series of familiar and easy-to-find celestial landmarks on successive nights, it did very well with the mainstream media.
Busy busy busy!
In hindsight, maybe a little very good . . .
At the Central Bureau of Astronomical Telegrams (CBAT) in Cambridge, Massachusetts, the clearinghouse for astronomical discoveries worldwide, news of the IRAS-Araki-Alcock comet spread like wildfire. According to the bureau’s director, Dr. Brian G. Marsden (1937-2010), he and his small staff were “absolutely overwhelmed” 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 on the approaching comet. In his time at the helm of CBAT, Dr. Marsden clearly considered the passage of this comet to be “the busiest time in the bureau’s history.”
Probably the question most frequently asked by reporters was, “Are we in immediate danger of a collision?” (No!).
A timeline of the close encounter
May 9, 1983: The comet, now shining at 3rd magnitude, could be found passing close to the bright orange star Kochab in Little Dipper’s bowl; the comet’s motion relative to the star was clearly obvious. In a span of less than two hours, IRAS-Araki-Alcock appeared to approach Kochab, finally passing within half a degree of the star, and then gradually moving away from it. It was like looking at the minute hand on a clock. From everywhere north of the Tropic of Cancer the comet was circumpolar, that is, it was visible in the sky all night. In essence, we were looking directly from Earth at the “bottom” 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 the Big Dipper, and appeared high in the north-northwest sky to US observers. Keen-eyed skygazers could find the comet without binoculars less than an hour after sunset.
May 11, 1983: On the day of its closest approach to Earth, it revealed that the comet was 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 gas tail was recorded in many photographs, but visually through binoculars and telescopes only the diffuse head of the comet (called the coma) was visible. And seen against a dark sky it seemed absolutely enormous, measuring approximately three degrees across; equal in apparent size to about 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 bounce radar signals. Both the 1,000-foot (305-meter) radio telescope in Arecibo, Puerto Rico, and NASA’s Jet Propulsion Laboratory in Goldstone, California, were able to obtain such radar echoes, which were used to provide details about the radius, rotation, and the composition of the comet nucleus.
May 12, 1983: Now rapidly moving away from Earth, the comet, making its farewell appearance to observers in the northern hemisphere, can be found low in the southwestern sky after sunset, having rapidly dipped in brightness to about a third magnitude. . The next night it was sinking below the horizon before the end of evening twilight. The show was over almost as quickly as it started.
Our next chance?
Will we ever get another chance to see a comet pass so close to Earth in the foreseeable future?
Maybe.
Close approaches of comets to Earth are quite rare. A comet’s approach to within 9 million miles (14.5 million km) of our planet occurs, on average, about once every 30 to 40 years. For a comet passing within 5 million miles (8 million kilometers) of Earth, such a close approach is even rarer, 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 case of IRAS-Araki-Alcock.
Interestingly, however, since 1983, there have been several comets, or comet fragments, that may have come even closer to Earth. A small comet, P/SOHO 5, “may” have come within 1.1 million miles (1.7 million km) of our planet on June 12, 1999, although this value is considered highly uncertain.
Another, 55P/Tempel-Tuttle, the comet that produces the annual Leonid meteor shower, was recently determined to have passed within 2.1 million miles (3.4 million km) of Earth on October 26, 1366.
It would seem that only small, faint comets pass exceptionally close to Earth, but with one notable exception: Halley’s Comet.
On April 10, 837, this most famous of all comets passed just 4.9 million kilometers (3.1 million miles) from Earth. Viewed from China, Japan and Europe, the comet shone as brightly as Venus, accompanied by a tail that extended more than 90 degrees across the sky.
Oh to see a comet like that In our life!
And looking into the future, until May 7, 2134, Halley’s Comet will pass within 8.6 million miles (13.8 million km) of Earth, likely shining as bright as Jupiter and again displaying a spectacularly long tail.
Something our great, great, great, great, great grandchildren can look forward to.
Joe Rao is an instructor and guest speaker at New York’s hayden planetarium. write about astronomy natural history magazinehe farmers almanac and other publications.
