The comet will make its closest approach to Earth in 2031, and it’s already attracting major attention from astronomers.

Bernardinelli-Bernstein is officially the largest comet ever discovered, according to updated observations of the inbound object.

Oort Cloud comet C/2014 UN271, also known as Comet Bernardinelli-Bernstein, measures some 85 miles (137 km) in diameter, give or take 10.5 miles (17 km), reports a research team led by astronomer Emmanuel Lellouch of the Paris Observatory. Their new paper on the mega comet has been accepted for publication in Astronomy and Astrophysics Letters, and you can sneak a peak of the preprint at the arXiv.

These latest observations confirm that Comet Bernardinelli-Bernstein is the largest Oort Cloud object ever detected, as it’s nearly twice as big as comet Hale-Bopp (observed in 1997), the nucleus of which measured between 25 and 50 miles (40 and 80 km) wide. It’s also bigger than Comet Sarabat (observed in 1729), which had a nucleus measuring somewhere around 62 miles (100 km) in diameter.

Comet Bernardinelli-Bernstein is currently inbound from the Oort Cloud, a distant region of the solar system known for packing billions and possibly trillions of icy objects. The comet will make its closest approach to Earth in 2031, when it will come to within 11 au of the Sun (1 billion miles), in which 1 au is the average distance from Earth to the Sun. The comet, coming no closer than Saturn, won’t likely be visible to the unaided eye, but astronomers will be keeping a close watch, as it’s turning out to be a rather extraordinary object.

Named after its discoverers, Pedro Bernardinelli and Gary Bernstein from the Dark Energy Survey, the comet is special for several reasons. Astronomers first detected the inbound object when it was still very far away—some 29 au from the Sun (2.7 billion miles). That’s as far out as the orbit of Neptune, but astronomers didn’t appreciate its significance until it came to within 24 au of the Sun (2.2 billion miles), at which time it began to display distinctive cometary activity. Researchers with Las Cumbres Observatory confirmed its cometary nature in June 2021. Its remarkable brightness indicated an object of enormous size, with preliminary estimates pointing to an object between 62 and 230 miles (100 and 370 kilometers) wide.

For the new study, Lellouch and his colleagues used the Atacama Large Millimeter Array (ALMA) in Chile to refine the comet’s size and reflectivity, or albedo. They did so on August 8, 2021, when Bernardinelli-Bernstein was 20 au from the Sun (1.86 billion miles). The team honed in on microwave radiation leaking out from the comet’s nucleus, while taking care to exclude radiation produced by the surrounding cloud of dust.

These thermal emissions pointed to the 85-mile (137 km) diameter, with a lower bound of 75 miles (120 km) and an upper bound of 96 miles (154 km). The large error bar is on account of uncertainties having to do with the object’s shape and reflectivity. Future observations should refine these estimates further.

The estimated albedo of 5.3% now represents the most distant measurement yet of a comet’s reflectivity. With the size of the nucleus now better defined, astronomers will be able to measure how much material the comet will lose during its trip around the Sun.

Bernardinelli-Bernstein is not the 230-mile behemoth suggested by preliminary measurements, but it’s still gigantic. As it nears the Sun, volatiles on its surface, especially ice, will increasingly sublimate, turning directly from solid into gas. This could give the comet a distinctive coma and tail, but we’ll have to wait a few more years to know for sure. We’ll be watching.

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