Distant comet C/2014 UN271 sets the record for the largest comet nucleus seen.


Comet C/2014 UN271 – HST images versus computer models. Credit: NASA/ESA/Man-to Hui/Macau University/David Jewitt/UCLA/Alyssa Pagan/STScI

Be thankful that a monster of a comet is staying out of the inner solar system. Recently, astronomers had a chance to turn the aging Hubble Space Telescope on distant comet C/2014 UN271 (Bernardinelli-Bernstein)… and what they saw was amazing.

Discovered in October 2014 by astronomers Gary Bernstein and Pedro Bernardinelli in archival images in the Dark Energy Survey, C/2014 UN271 was a whopping 29 Astronomical Units (AU) or 4.3 billion kilometers distant at the time of discovery, near the edge of Neptune’s orbit. That alone set a record for the most distant comet discovery to date.

The orbit of C/2014 UN271 is also intriguing. On a 2.75 million year orbit inbound, the comet is definitely a denizen of the distant Oort Cloud, with an aphelion of 39,300 AU distant, or just under 1/7th the distance to Proxima Centauri. Like many comets entering the solar system, C/2014 UN271 gets its orbit tweaked slightly on its outbound leg.

C/2014 UN271 reaches perihelion 10.9 AU (1.6 billion kilometers) from the Sun just under a decade from now in January 2031, just exterior to the orbit of Saturn. C/2014 UN271 is no threat to Earth or any planet in the inner solar system, and will only reach magnitude +13 near perihelion, visible only in a large amateur telescope.

A distant comet discovery always gives astronomers pause, suggesting an intrinsically large object. Hubble snapped the recent series of five images of the comet early this year on January 8th 2022, when the comet was 21.4 AU or 3.2 billion kilometers distant. The images were done using Hubble’s Wide Field Camera 3.

Measuring the true size of such a distant object is tricky; though already active, Comet C/2014 UN271 currently appears as a fuzzy, +19th magnitude point of light. For the study, the team isolated the nucleus by comparing the bright ‘spike’ the center, versus a computer model of the surrounding coma. Then, the team compared the brightness seen in the Hubble images earlier radio images constructed by the Atacama Large Millimeter Array (ALMA).


A comparison of the most massive comets known. Credit: NASA/ESA/Zena Levy/STScI

“This is an amazing object, given how active it is when it’s still so far from the Sun,” Says Man-To Hui of Macau University of Science and Technology in a recent press release. “We guessed the comet might be pretty big, but we needed the best data to confirm this.”

These measurements suggest a large and—like Comet 67P Churyumov-Gerasimenko seen up close by the European Space Agency’s Rosetta mission—an exceptionally ‘black’ object. The nucleus of Comet C/2014 UN271 is an estimated 137 kilometers across, handily beating out the former record holder C/2002 VQ94 by 40 kilometers. UN 271 is actually bigger than some moons, including Saturn’s moon Epimetheus.


Saturn’s moon Epimetheus: a rough ‘twin’ of comet UN 271? Credit: NASA/Cassini/Space Science Institute.

Objects such as C/2014 UN 271 offer a valuable glimpse of the distant Oort Cloud, to include a sampling of the size and distribution of objects. The Oort Cloud was first hypothesized by Dutch astronomer Van Oort in 1950, as a vast reservoir of comets spanning a sphere around the solar system 2,000 to 200,000 AU from the Sun.

Be thankful that C/2014 UN 271 isn’t Earthbound; for context, the Chicxulub impactor the struck the Yucatan peninsula 66 million years ago was ‘only’ an estimated 10 kilometers across. A strike from a UN 271-sized object would spell a bad day for the Earth, a definite extinction level event.

Thankfully, we can simply enjoy Comet C/2014 UN 271 from a distance. Hopefully, the soon to be commissioned James Webb Space Telescope will also give us some images of the comet in the infrared this coming summer.

Read the study: Hubble Space Telescope Detection of the Nucleus of Comet C/2014 UN271 (Bernardinelli-Bernstein) in The Astrophysical Journal Letters.