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Rick Robinson

Jul 6th 2022

Giant Comet Bernardinelli-Bernstein: What Comes Around Goes Around

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An enormous comet is gliding through the outer reaches of the solar system, currently passing between the orbits of Uranus and Neptune.

We are accustomed to thinking of these distant worlds as orbiting in the outer reaches of the solar system. But from the perspective of the newly discovered Comet Bernardinelli-Bernstein — the largest comet ever seen so far from the sun — the current stage of its journey, per Space.com, is more like a fast, scorching swing by the fire. This will culminate in 2031 with the comet’s closest approach to the sun, passing just beyond the orbit of Saturn, according to Sci-news.

And the sun is already warming it up, after millions of years of interstellar cold. Early measurements of its brightness indicated that the nucleus of Comet Bernadinelli-Bernstein (commonly called Comet B-B for short) might be about 60-125 miles in diameter — one of the largest solid comet nuclei ever observed.

But, as ScienceAlert shares, further observations have indicated that B-B is an “active” comet. Already brightening, its solid surface is evaporating to form a coma and tail, which is unusual for a comet of any size so far from the sun. Its nucleus may thus be smaller than first estimated, but it’s still a giant among comets.

A Long, Long Journey

And after its closest approach? Comet Bernardinelli-Bernstein will continue in its orbit: a jaunt of four million years or more that will take it out to its native haunts in the true outer reaches of the solar system — more than 40,000 times Earth’s distance from the sun or two thirds of a light year.

This vast, distant region surrounding the familiar solar system is the domain of the Oort Cloud. It comprises billions of comets and fragments of early solar-system material that were caught up by early orbit changes of the planets and hurled out into this remote deep freeze.

There, they remain all but untouched by time. Well, except for some that have been perturbed through the ages by passing stars or galactic tides and sent dropping back toward the inner solar system to become the comets that have been observed throughout history.

An Emissary From Deep History

If Comet Bernardinelli-Bernstein is an emissary from the early history of the solar system, its discovery also reflects the long history of astronomy and comets.

Early astronomers were mainly concerned with the regularity of the heavens. They determined the length of the year, the complex cycles of the moon and visible planets, and they even learned to predict rare events such as a total eclipse of the sun.

However, comets seemed to completely violate this regularity. Bright comets appeared on rare occasions unpredictably, moving and changing from night to night before finally fading away and disappearing. Early observers were not even sure whether they were celestial events or phenomena of the upper air.

As History.com recounts, all that changed thanks to Edmund Halley. He shared how a number of bright comets that were observed over the centuries had timing and paths through the sky that neatly matched according to Newton’s laws, showing them to be the same object — just like the one now called Halley’s Comet.

While astronomers now suspect that all comets originated in the Oort Cloud, so-called periodic comets — such as Halley’s Comet — were deflected into shorter, closer orbits during an earlier pass. These periodic comets now come near the sun every few years, decades or centuries, instead of only after many thousands or even millions of years like visitors such as Comet B-B that have come directly from the Oort Cloud.

Halley did not live to see his prediction proven, but the return of Halley’s Comet in 1758 made comets the hottest topic in astronomy. This was a big change, not just in astronomy but in the popular culture.

The people of ancient civilizations hadn’t just been perplexed by comets; they had been terrified of them. As Nature reports, the possibility that a meteor strike — or comet impact — actually destroyed a Bronze Age city in the Jordan Valley may explain the dire reputation of comets as portents of disaster.

But since Halley’s day, comets have mostly enjoyed a much better reputation as rare but sometimes spectacular sky shows. In popular culture, astronomers don’t go hunting for obscure objects called TNOs or even spookier-sounding “dark matter.” In movies like Roxanne and Local Hero, astronomers — both professional and amateur — are shown hunting for comets.

Indeed, co-discoverer Gary Bernstein noted how happy his mother was about his discovery, perhaps because she could finally tell her friends something about her son’s work that they’d instantly appreciate.

What Is This Thing?

But the co-discoverers of B-B, Bernardinelli and Bernstein, were not looking for comets.

Pedro Bernardinelli was a grad student a week away from formally defending his PhD dissertation on TNOs — Trans-Neptunian Objects, the most familiar of these being Pluto — while Gary Bernstein was investigating the outer solar system for gravitational hints of dark matter. But then, their investigation turned up a surprise.

Contrary to another popular image, neither Bernardinelli nor Bernstein were looking through a telescope. They were working with data collected several years earlier by an automated survey — their discovery sitting in the data, waiting for someone to notice.

The data showed an object that was in roughly the right place to be a TNO — 29 Astronomical Units (AU), or 29 times Earth’s distance from the sun, when the search criteria were for objects at least 30 AU from the sun. And it was impressively bright, enough to trigger detection by the search software even though it didn’t quite match the search constraints.

It looked like a bright but otherwise unremarkable TNO find, except for its orbit — which, according to the results, showed that it reached its maximum distance from the sun not at a typical 40 or 50 AU but at least 40,000 AU. That placed its origin far out into the Oort Cloud, fully a sixth as far away as Proxima Centauri, the nearest star beyond the sun. As Space.com reports, Bernardinelli recounted thinking, “This is weird — what is this thing?”

Without intending to, Bernardinelli and Bernstein had discovered a comet — the largest comet ever seen coming directly from the Oort Cloud.

Almost everything about Comet B-B is surprising, and the fact that it was discovered by professional astronomers is just one more surprise. Indeed, until the recent development of automated sky surveys, comet hunting and comet discoveries had become largely the domain of amateur astronomers.

This is for a few reasons. The most important is what puzzled (and perhaps alarmed) ancient observers: the unpredictability of comet discoveries. A new comet can appear in any part of the sky — meaning that there is really no “efficient” way to search for one. You just have to spend a lot of time looking. Professional astronomers, who typically concentrate on specific research projects, rarely have that luxury. (Automated search telescopes have modified this limitation.)

Moreover, most newly discovered comets never become spectacular sights in the night sky or even bright enough to see without a telescope. And most do not become targets of special scientific study, either. They are simply cataloged as one more data point in our knowledge of comets. But the thrill of discovery is enough to keep skilled amateurs on the hunt.

Back in the 18th century, after Halley’s successful prediction made comets a hot topic, the story was different. Professional astronomy as we know it today hardly existed. A handful of national observatories compiled navigational sky charts, but otherwise, nearly all astronomers were amateurs.

From Comets to Galaxies — and Back?

The stars and other distant celestial objects (what modern astronomers — professional and amateurs alike — call the deep sky) was regarded as little more than a distant backdrop: very useful for measurements, but not something we would ever be able to know much about.

In the 1750s, according to Seasky.org, a French astronomer named Charles Messier became an enthusiastic comet hunter. To make the task easier, he compiled a list of common comet hunters’ mistakes — objects in the sky that tricked comet hunters because they looked like comets through a telescope. He listed nearly a hundred such celestial objects, and later observers added a few more to his list.

Messier’s comet discoveries are long forgotten, but his “don’t be fooled” list of star clusters, nebulae and galaxies remains one of the most familiar guides to the visible universe, familiar to amateur and professional astronomers alike.

As for Comet Bernardinelli-Bernstein, it will probably not put on a spectacular show in Earth’s sky, since it will never come anywhere near as close to the sun as dramatic comets do. It will probably only be visible in larger amateur telescopes. On the other hand, its early indications of large size and unusual brightness could mean more surprises in store.

What is not in doubt is that its early discovery — nearly a decade before its closest approach to the sun — will give astronomers plenty of time to study it in the years before it once again recedes into the far depths of space.

Anything they miss this time will just have to go on the bucket list for Comet B-B’s next close pass by the sun, sometime after the year 4,000,000.

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