“Mega Comet” heading towards us is possibly the biggest ever

In June of this year, two astronomers discovered possibly the largest comet ever, an object so large that there was initial debate as to whether it could really be a dwarf planet in a similar orbit. to a comet. This “mega comet” is on an inward trajectory of the outer solar system.

Now, its discoverers and many co-authors are reporting the results of three months spent learning more about this exceptional object. An article accepted by Astrophysical Journal Letters (preprint on ArXiv.org) reveals a lot that we didn’t know about this world when its existence made headlines.

Anyone learning about C / 2014 UN₂₇₁ (Bernardinelli-Bernstein) for the first time, don’t worry, though. Even at its closest approach in 2031, the UN₂₇₁ will be farther away than Saturn by about 11 astronomical units ((1 AU = distance from Earth to Sun), frustrating astronomers who would like to take a closer look at something so unusual.

A₂₇₁The orbit of has been drawn and its last approach to the Sun dates back to about 3.5 million years. On this occasion, he moved away only 18 astronomical units, almost double the distance he will reach this time and roughly the distance from Uranus. It will probably soon be the closest to the Sun it has ever been, making it the most pristine comet we’ve ever seen, a true throwback to the origins of the solar system.

The first figures for the UN₂₇₁the size of were inevitably imprecise. However, the authors have now stopped at an estimate 150 kilometers (100 miles) in diameter. This gives it a volume thousands of times that of a typical comet, and at least 10 times the size of a giant like Hale-Bopp. For comparison, Comet 67P, which Rosetta studied, is only 4.3 kilometers in diameter, and Arrakoth, the most distant world humanity has ever explored, is 22 miles (35 kilometers) long and 12 miles (20 kilometers) wide.

–

There were hopes of getting more certainty about the size when the UN₂₇₁ passed a star seen from eastern Australia, but clouds covered the whole area from where the event could have been seen. However, regardless of its size, the comet will not be visible to the naked eye when it comes close.

One of the first things astronomers wanted to know about the UN₂₇₁ was whether it was already showing cometary activity, that is, whether matter turned into gas and formed a coma. UN₂₇₁ had been photographed both by TESS, NASA’s planet hunter, and by the Dark Energy Survey (DES) in 2018 without anyone noticing its importance. So the authors checked the previous images to see if they could find any telltale signs of blurring.

They found a gap in the measurements, with TESS reporting an object almost twice as bright. Turns out the reason was that DES was only looking at a small area around the UN₂₇₁, as the TESS gathered over a larger space, including a large but weak coma, indicating that material had long since escaped. The composition of the coma cannot yet be detected, but it is believed that carbon dioxide carrying dust grains with it when it escapes is very likely.

A₂₇₁the tail of is even weaker, requiring the combination of many images to detect it.

“It is generally a losing proposition to speculate on the future behavior of comets,” admits the newspaper, but nonetheless projects that at its brightest UN₂₇₁ should be around magnitude 9 – visible to hobbyists with small telescopes under dark skies.

Some astronomers want a UN mission₂₇₁, the calculation of the best time for a flyby is 2033, which would require a launch by 2028.