Largest Comet Ever Is Approaching Our Sun. Here's How It Compares to Other Objects

A near 100 mile-wide comet from the outer edges of the solar system is headed toward the sun and the inner solar system where Earth dwells.

At an estimated 93 miles wide, the Bernardinelli-Bernstein Comet is believed to be the largest comet ever discovered. It will make a close approach to Saturn's orbit in a decade.

The comet, officially designated C/2014 UN271, has been labeled the first mega comet and it's estimated to be around seven times as large as Mars' moon Phobos.

The average diameter of comets is around six miles, and the largest comet discovered before the Bernardinelli-Bernstein Comet, Hale-Bopp, had a diameter of around 37 miles. That means the Bernardinelli-Bernstein Comet could be as much as 17 times larger than an average comet, and just under three times the size of Hale Bopp.

The comet is five and a half times as long as the solar system's tallest mountain, Olympus Mons, found on the surface of Mars.

The mega comet was first discovered in 2014 by University of Washington researcher Pedro Bernardinelli and University of Pennsylvania Professor of Physics and Astronomy Gary Bernstein, who confirmed its existence using images from the Dark Energy Survey. It was only confirmed as a new discovery by the Minor Planet Center in Cambridge, Massachusetts in June of this year.

The Bernardinelli-Bernstein Comet is believed to have originated from the Oort cloud, a spherical shell of icy bodies that exists on the outskirts of the solar system. Little is known about the Oort cloud as until objects in the cometary shell start to diffuse gas they are difficult to spot.

Science journalist and astronomer Will Gater compared the size of the Bernardinelli-Bernstein Comet to other bodies in the solar system and shared it on his Twitter feed.

Reading @phbernardinelli's tweets this morning I was trying to get my head around just how big Comet Bernardinelli-Bernstein (aka C/2014 UN271) is.

So here's a graphic I've just made to show an (edited) @NOIRLabAstro illo of it in comparison to some other Solar System objects.

— Will Gater (@willgater) September 22, 2021

The Mega comet seems to be so large that when it was initially spotted astronomers believed it was a dwarf planet. This misidentification was only overturned when scientists realized its extreme orbit means it originated trillions of miles from the sun.

When examining the mega comet with NASA's exoplanet-hunting telescope, Transiting Exoplanet Survey Satellite (TESS), the authors also noticed activity at the surface of the Bernardinelli-Bernstein Comet that's common to comets.

This activity normally happens when comets pass close to a star as the ice that forms them begins to boil away. Not only does this give the comet a haze that surrounds it, called a coma, it also forms the distinctive tail of such bodies.

The data from TESS revealed that the Bernardinelli-Bernstein Comet has a diffuse cometary tail of gas and dust and was releasing gas while it was 2.4 billion miles from the sun.

Not only is its visible coma something of a mystery, but when the mega comet gets closer to our star it could be as bright in the night sky as Saturn's largest moon, Titan.

Its closest approach to the sun and the inner planets will occur 10 years from now, in 2031, with it brushing the orbit of Saturn. There is no danger of the comet striking Earth as it will maintain a distance of around 1 billion miles.

It remains to be seen how long the Bernardinelli-Bernstein Comet can hang on to its title of the largest comet. When the Vera C.Rubin Observatory begins its Legacy Survey of Space and Time part of its mission will be to look for comets, meaning it could turn up one that challenges this mega comet in terms of sheer size.

Additionally, the comet's 2031 close approach will allow astronomers to more accurately calculate its size. Current estimates of the comet's nucleus were made using the mega comet's coma which can be imprecise, meaning it could turn out to be smaller than currently estimated.

the Bernardinelli-Bernstein Comet
An illustration of the Bernardinelli-Bernstein Comet. The comet is belived to be almost 100 miles wide and will approach Saturn in 2031. NSF/AURA/J. da Silva Spaceengine/NOIRLab