Forty years ago this month, a show came that stopped the celestial spectacle—literally a bolt from the blue.
A brand-new comet has been making headlines for days around the world because of its exceptionally close pass close to Earth: a distance of less than 3 million miles (4.8 million kilometers), or about 12 times the distance from Earth to the Moon.
In fact, when the comet was first seen on April 25, 1983, it was not with human eyes or a telescope, but from a satellite: IRAS, short for InfraRed Astronomical Satellite, launched from the former Vandenberg Air Force Base. January and is placed in an orbit of 560 miles (900 km) around Earth. The satellite was a joint project of Great Britain, the Netherlands, and the United States and was the first space telescope to scan the entire sky in infrared wavelengths. Its main purpose was to catalog the heat “signatures” of asteroids as well as to monitor 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 satellite
When the IRAS satellite captured 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 by George Alcock, a well-known British comet observer, who was scanning the sky with 15 x 80 binoculars. Surprisingly, Alcock – who had previously discovered four other comets – was inside his house And looking through a closed window, When he stumbled across the comet Araki had seen just seven hours earlier!
It soon became more and more clear that the object IRAS had detected, was not actually an asteroid, but the same comet that Araki and Alcock had found. It was therefore deemed appropriate to name the comet IRAS-Araki-Alcock. When seen by Araki and Alcock, the comet shone with a sixth power — the threshold of being seen by a person without the use of any optical aid under a clear, dark sky.
Get bright…and close!
Once a comet’s preliminary orbit was determined, two things were determined.
First, in substance, this was a relatively small comet, perhaps no more than 2 or 3 miles (3 or 5 km) across. However, within the next week, it was expected to brighten more than 60 times faster, possibly to a second magnitude, as bright as Polaris, the North Star.
But for something like that For this to happen, it must get very close to the ground. 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 Lexell – and that was in 1770!
Although IRAS-Araki-Alcock will make its closest approach to the Sun (called perigee) on May 21, 1983, at a point just inside Earth’s orbit, it was during the time frame from May 4 to its closest approach to Earth (perigee) on May 11 May that the comet has garnered a lot of attention around the world.
In a way, it was a call to arms for astronomers. The combination of a comet passing close to Earth and appearing in a dark sky (the new moon was on May 12), while arching closely through a series of familiar and easy-to-find celestial features on successive nights, passed very well in the mainstream media.
Busy busy busy!
Later, maybe a little bit very good . . .
At the Central Bureau of Astronomical Telegrams (CBAT) in Cambridge, Massachusetts — the clearinghouse for astronomical discoveries worldwide — news of comet IRAS-Araki-Alcock spread like wildfire. According to the office’s director, Dr. Brian J. Marsden (1937-2010), he and his small team were “inundated with hundreds of calls from reporters, planetarium staff, professional and amateur astronomers, and even the ‘street’ curious guy, all asking for the latest information on a comet’s approach. In his time at the helm of CBAT Dr. Marsden regarded this comet’s passage as “the busiest time ever in the history of the Bureau”.
Perhaps the question journalists asked the most was: “Are we in immediate danger of a collision?” (no!).
Close meeting schedule
May 9, 1983: The comet, now shining as bright as a 3rd magnitude, can be found near the bright orange star Kochab in the bowl of the Little Dipper; The comet’s motion relative to the star was clearly visible. Over a period of less than two hours, IRAS-Araki-Alcock appeared to approach Kochab, eventually passing less than half a degree from the star, and then gradually moving away from it. It was like watching the minute hand of a clock. From everywhere north of the Tropic of Cancer the comet was circumpolar, meaning it was visible in the sky all night long. In essence, we were looking directly from Earth at the “underside” of the comet.
May 10, 1983: It formed a broad, somewhat equilateral triangle with Dubhe and Merak, the famous “pointer stars” in the Big Dipper bowl, and appeared high in the northwest sky for American observers. Sharp sky watchers can find the comet without binoculars less than an hour after sunset.
May 11, 1983: On the day of its approach to Earth, the comet revealed itself strikingly close to the famous Beehive star cluster in the constellation of Cancer, although the comet was incomparably brighter, peaking at around +1.5. A narrow gas tail has been recorded on many images, but visually through binoculars and telescopes, only the diffuse head of the comet (called the coma) was visible. And, looking up against a dark sky, it seemed very huge, being about three degrees in diameter; Approximately equal to its apparent size Six full moons! Through large telescopes, fantastic structures have emerged that illuminate the inner coma.
With IRAS-Araki-Alcock now close to Earth, there has been interest in trying to bounce radar signals off it. Both the 1,000-foot (305-meter) radio telescope in Arecibo, Puerto Rico, and NASA’s Jet Propulsion Laboratory in Goldstone, California, acquired these radar echoes, which were used to provide details about their radius, rotation, and composition. Comet nucleus.
May 12, 1983: Now the comet is rapidly receding from Earth, making its farewell appearance to northern hemisphere observers — it can be found low in the southwestern sky after sunset, having rapidly decreased in brightness to a third magnitude. By the evening of the next day, it was sinking below the horizon just before the end of evening twilight. The show ended almost as quickly as it began.
Our next chance?
Will we have another chance to see a comet pass very close to Earth in the foreseeable future?
Close approaches of comets to Earth are rare. The comet comes within 9 million miles (14.5 million km) of our planet — on average — about once every 30 to 40 years. For a comet passing less than 5 million miles (8 million km) from Earth, such a close approach is rare, occurring about once every 80 or 90 years.
So, you can see how unusual the approach to Earth of less than 3 million miles (4.8 million km) was in the case of IRAS-Araki-Alcock.
Interestingly, since 1983, there have been many comets – or comet fragments – that may have approached Earth even more closely. One small 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 value is considered highly uncertain.
Another, 55P/Tempel-Tuttle — the comet that produces the annual Leonid meteor shower — was recently identified as passing 2.1 million miles (3.4 million kilometers) from Earth on October 26, 1366.
Only small, dim comets appear to be making exceptionally close passes to Earth, but with one notable exception: Halley’s Comet.
On April 10, 837, this most famous comet passed 3.1 million miles (4.9 million kilometers) from Earth. In China, Japan and Europe, the comet shone brightly like the planet Venus, accompanied by a tail that stretched more than 90 degrees across the sky.
Oh, to see like a comet that in our lives!
Looking far into the future, until May 7, 2134, Halley’s comet will pass within 8.6 million miles (13.8 million kilometers) of Earth, likely to be as bright as Jupiter and once again displaying an astonishingly long tail.
Something our great, great, great grandchildren can look forward to.
Joe Rao is a teacher and guest lecturer in New York Hayden Planetarium. He writes about astronomy for Natural History Journalthe Farmers’ almanac and other publications.
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