Hubble confirms the largest comet ever seen with a diameter of 80 miles

NASA's Hubble Space Telescope mapped out the size of the Bernardinelli-Bernstein comet, confirming that it is the largest icy comet nucleus ever seen by astronomers, a report by NASA reveals. 

The Bernardinelli-Bernstein) comet — also known as C/2014 UN271 — is traveling in Earth's approximate direction at 22,000 miles per hour (35,400 kph), though it will never fly closer than 1 billion miles (1.6 billion km) from the Sun, meaning we won't be in harm's way.

The nucleus is estimated to have a diameter of approximately 80 miles (128.7 km) across and a mass of 500 trillion tons, making it approximately a hundred thousand times greater in mass than the average comet found closer to our Sun. The findings are presented in a new study in The Astrophysical Journal Letters.

"This comet is literally the tip of the iceberg for many thousands of comets that are too faint to see in the more distant parts of the solar system," explained David Jewitt, a professor at the University of California, Los Angeles (UCLA), and co-author of the new study. "We've always suspected this comet had to be big because it is so bright at such a large distance. Now we confirm it is."

The previous record-holder is C/2002 VQ94, which was estimated to have a diameter of 60 miles ( 96.5 km) across when it was discovered in 2002.

Stripping a comet down to its nucleus

Comet C/2014 UN27 was discovered by astronomers Pedro Bernardinelli and Gary Bernstein in November 2010 when they were studying archival images from the Dark Energy Survey at the Cerro Tololo Inter-American Observatory in Chile.

The team behind the new study set out to confirm predictions of Bernardinelli-Bernstein's immense size. They used the Hubble Space Telescope to take five photos of the comet on January 8, 2022. Using advanced computer modeling, they were able to measure the vast dusty coma enveloping the comet from the comet's nucleus. 

"This is an amazing object, given how active it is when it's still so far from the Sun," said the paper's lead author Man-To Hui of the Macau University of Science and Technology, Taipa, Macau. "We guessed the comet might be pretty big, but we needed the best data to confirm this."

To reach their findings, Hui and his team compared their data with that of earlier radio observations from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. The new data is close to those earlier data, but it suggests that Bernardinelli-Bernstein has a much darker nucleus surface than once thought. The researchers believe their work provides valuable insight into the Oort Cloud, as it is believed the comet originated in this theorized spherical shell of space rocks surrounding our solar system.

Since it was launched in 1990, Hubble has made over 1.5 million observations and counting, leading to the publication of 18,000 scientific papers on papers ranging from dark energy to black holes and neutron stars. However, a series of technical problems in recent months and years, mean that the space observatory's confirmation of Bernardinelli-Bernstein's size might be one of its final missions. It's hard to predict exactly when Hubble will give in, and the iconic telescope still has some life left.