40 years ago, a comet came out of the blue in a surprise Earth flyby. Here's what we know now.

Forty years ago this month, there came a show stopping celestial sight — literally a bolt out of the blue.

A brand-new comet, that for several days made headlines around the world due to its exceptionally close passage near the Earth: A distance amounting to 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, the acronym for InfraRed Astronomical Satellite, launched from the then-Vandenberg Air Force Base the previous January and placed in a 560-mile (900 km) orbit around Earth. The satellite was a joint undertaking by Great Britain, the Netherlands and the United States and was the first space telescope to perform a survey of the entire sky at infrared wavelengths. Its main purpose 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

Seen first by a satellite

TOP TELESCOPE PICK:

Want to hunt for comets on your own? We recommend the Celestron Astro Fi 102 as the top pick in our best beginner's telescope guide.

When the IRAS satellite picked up on a fast-moving object on April 25, it was first assumed that it was 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 of 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. Amazingly, Alcock — who had previous discovered four other comets — was inside his house and looking through a closed window, when he stumbled across the comet that Araki had sighted just seven hours before!

It soon became more and more apparent that the object that IRAS had discovered, was in reality not an asteroid, but was the very same comet found by both Araki and Alcock. It was thus deemed appropriate to name the comet IRAS-Araki-Alcock. When Araki and Alcock sighted it, the comet was shining at sixth magnitude — the threshold of visibility for someone without using any optical aid under a dark, clear sky.

Getting bright ... and close!

Once a preliminary orbit for the comet had been worked out, two things had been determined.

First, intrinsically, this was a relatively small comet, probably measuring no more than 2 or 3 miles (3 or 5 km) wide. And yet, within the next week, it was forecast to rapidly brighten more than 60-fold, possibly to second magnitude, as bright as Polaris, the North Star.

But in order for something like that to happen, it would have to approach very close to Earth. And indeed, calculations indicated that it was destined to miss our planet by only 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 by the name of 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 of the Earth's orbit, it was during the time frame from May 4 to its closest approach to Earth (perigee) on May 11 that the comet garnered tremendous interest worldwide.

In a way, it was like a call to arms for astronomers. The combination of a comet passing exceedingly close to the Earth and appearing in a dark sky (the new moon was on May 12), while arching closely past a series of familiar and easy to find celestial landmarks on successive nights, went over very well with the mainstream news media.

Busy, busy, busy!

In retrospect, maybe a bit too well . . .

At the Central Bureau for Astronomical Telegrams (CBAT) in Cambridge, Massachusetts — the clearing house for astronomical discoveries worldwide — news of Comet IRAS-Araki-Alcock spread like wildfire. According to the bureau's director, Dr. Brian G. Marsden (1937-2010), he and his small staff were "absolutely swamped" with hundreds of calls from reporters, planetarium personnel, professional and amateur astronomers, and even the curious "man on the street," all requesting the very latest information on the approaching comet. In his time at the helm of the CBAT, Dr. Marsden considered the passage of this comet as clearly, "the busiest time ever in the bureau's history."

Probably the question that reporters asked the most was: "Are we in immediate danger of a collision?" (Nope!).

A timeline of the close encounter

May 9, 1983: The comet, now shining as bright as third magnitude, could be found passing near to the bright orange star Kochab in the Little Dipper's bowl; the comet's movement relative to the star was plainly obvious. Over a span of less than two hours, IRAS-Araki-Alcock appeared to approach Kochab, ultimately passing less than a half degree from the star, and then gradually moved 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, that is, it was visible in the sky all night. In essence, we were looking directly up from Earth at the "underside" of the comet.

May 10, 1983: It formed a wide, more-or-less equilateral triangle with Dubhe and Merak, the famous "pointer stars" in the bowl of the Big Dipper, and appeared high in the north-northwest sky for American observers. Sharp-eyed skywatchers could find the comet without binoculars less than an hour after sunset.

May 11, 1983: The day of its closest approach to the Earth — revealed the comet strikingly near to the popular Beehive star cluster in the Cancer constellation, though the comet was incomparably brighter, peaking at around magnitude +1.5. A narrow gas tail was recorded on many photographs, but visually through binoculars and telescopes only the comet's diffuse head (called the coma) was visible. And viewed against a dark sky it appeared absolutely enormous, measuring roughly three degrees across; equal in apparent size to roughly six full moons! Through large telescopes, fascinating structures appeared to light up the inner coma.

With IRAS-Araki-Alcock now so close to the Earth, there was interest in trying to bounce radar signals off of it. Both, the 1,000-foot (305-meter) radio telescope at Arecibo, Puerto Rico and NASA's Jet Propulsion Laboratory at Goldstone, California, were successful in obtaining such radar echoes, which were used to provide details on the radius, rotation and composition of the comet's nucleus.

May 12, 1983: Now rapidly receding from Earth, the comet — making its farewell appearance for Northern Hemisphere observers — could be found low in the southwest sky after sunset, having rapidly diminished in brightness to third magnitude. By the following evening it was sinking below the horizon before the end of evening twilight. The show had ended almost as fast as it started.

Our next chance?

Will we ever have another chance to see a comet pass so very close to Earth in the foreseeable future?

Maybe.

Close approaches of comets to the Earth are rather infrequent. A comet's approach to within 9 million miles (14.5 million km) of our planet comes along — on average — about once every 30 to 40 years. For a comet passing to within less than 5 million miles (8 million km) of Earth, such a very close approach is even more infrequent, 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 approached Earth even more closely. One tiny comet, P/SOHO 5, "may" have come within 1.1 million miles (1.7 million km) of our planet on June 12, 1999, though 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 2.1 million miles (3.4 million km) from Earth on October 26, 1366.

It would seem that only small, dim comets ever make exceptionally close passes to the Earth, but with one outstanding exception: Halley's Comet.

RELATED STORIES:

—  The Greatest Comet Close Encounters of All Time

— A comet coming in 2024 could outshine the stars - if we're lucky

— Comets vs asteroids: How do these rocky objects compare?

On April 10 in the year 837, this most famous of all comets passed just 3.1 million miles (4.9 million km) from Earth. Seen from China, Japan and Europe, the comet shone as brilliantly as Venus, accompanied by a tail that stretched for over 90 degrees across the sky.

Oh, to see a comet like that in our lifetime!

And looking far ahead, to May 7, 2134, Halley's Comet will pass to 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 that our great, great, great, great grandchildren can look forward to.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications.