NASA’s Hubble Space Telescope has set its sights on the largest icy comet nucleus ever observed. The telescope has determined that the comet is approximately 128 km (80 mi) in diameter and its nucleus is about 50 times larger than most known comets, although the nucleus could be as large as 137 km (85 mi). The comet is larger than the state of Rhode Island. The comet’s mass is a ‘staggering’ 500 trillion tons, per NASA, which is ‘a hundred thousand times greater than the mass of a typical comet found much closer to the Sun.’
The giant comet, officially known as C/2014 UN271 (Bernardinelli-Bernstein), is moving in our direction at about 34,400 kph (22,000 mph) from the edge of the solar system. NASA says that it will never get closer than about a billion miles away from the Sun, further away than even Saturn. The comet will be at its closest – but still far – distance in 2031.
While the comet poses no threat, it is well worth paying close attention to due to its incredible size. The previous ‘biggest comet’ record holder, discovered in 2002 by the Lincoln Near-Earth Asteroid Research (LINEAR) project, is comet C/2002 VQ94. Its nucleus is about 96 km (60 mi) across.
The new record holder, C/2014 UN271, was discovered by astronomers Pedro Bernardinelli and Gary Bernstein using archival images from the Dark Energy Survey at the Cerro Tololo Inter-American Observatory in Chile. The duo first observed it in November 2010 when it was 4.8 billion kilometers (3 billion miles) from the Sun. Since that initial observation, which NASA describes as serendipitous, the comet has been extensively investigated using telescopes both on the ground and in space.
C/2014 UN271 proved challenging to measure. It’s difficult to ‘discriminate the solid nucleus from the huge dusty coma enveloping it.’ Even with Hubble’s awesome observational power, the comet is still too far away to resolve accurately visually. Hubble’s photograph of the comet shows a bright spike and glow. In a new paper published in The Astrophysical Journal Letters, lead author Man-To Hui of the Macau University of Science and Technology in Taipa, Macau and co-author professor David Jewitt of the University of California, Los Angeles developed a computer model of the coma surrounding the comet and adjusted it to fit Hubble’s real-world observation. Then, the team subtracted the glow of the coma, leaving behind the comet’s nucleus.
‘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,’ said Jewitt, a professor of planetary science and astronomy at the University of California, Los Angeles (UCLA). ‘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.’
Hui added, ‘This is an amazing object, given how active it is when it’s still so far from the Sun. We guessed the comet might be pretty big, but we needed the best data to confirm this.’ Hui and his team used Hubble to capture five photos of the C/2014 UN271 on January 8, 2022.
Hui and his team then compared the brightness of the nucleus against earlier radio observations from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. By combining the data, they were able to constrain the diameter and reflectivity of the nucleus. The new Hubble measurements are similar to earlier radio-based size estimates. However, the new observations suggest a darker nucleus surface. ‘It’s big and it’s blacker than coal,’ said Jewitt.
C/2014 UN271 (Bernardinelli-Bernstein) has been falling toward the Sun for over a million years. It’s believed to have originated in the Oort Cloud, a ‘nesting ground of trillions of comets.’ The Oort Cloud was first hypothesized in 1950 by Jan Oort, a Dutch astronomer. It’s technically still theoretical, as its comets are too faint and too distant to be observed. NASA’s Voyager spacecraft won’t reach the inner realm of the Oort Cloud for another 300 years, and it could take ten times longer to pass through it.
The comets in the proposed Oort Cloud formed closer to the Sun, but were flung to the outer reaches of the solar system billions of years ago by gravitational ‘pinball’ as the orbits of the massive outer planets, like Jupiter and Saturn, were still evolving. The comets only depart the Oort Cloud when the gravitational pull of passing stars yanks them out.
NASA writes, ‘Comet Bernardinelli-Bernstein follows a 3-million-year-long elliptical orbit, taking it as far from the Sun as roughly half a light-year. The comet is now less than 2 billion miles from the Sun, falling nearly perpendicular to the plane of our solar system. At that distance temperatures are only about minus 348 degrees Fahrenheit. Yet that’s warm enough for carbon monoxide to sublimate off the surface to produce the dusty coma.’