Huge, Distant Comet Already Active with a Coma

Astronomers analyze data from NASA’s TESS spacecraft and find that giant Comet Bernardinelli-Bernstein is already active and has a coma.

Beth Johnson
2 min readDec 10, 2021


IMAGE: The Comet Bernardinelli-Bernstein (BB), represented in this artist rendition as it might look in the outer Solar System, is estimated to be about 1000 times more massive than a typical comet. The largest comet discovered in modern times, it is among the most distant comets to be discovered with a coma, which means ice within the comet is vaporizing and forming an envelope of dust and vapor around the comet’s core. CREDIT: NOIRLab/NSF/AURA/J. da Silva/Spaceengine

Astronomers are still working to understand that huge comet that was recently discovered and led Beth to yell at headline writers. Comet Bernardinelli-Bernstein was discovered back in June of this year in data from the Dark Energy Survey, which has been working to survey the Southern Hemisphere’s sky.

While most comets are about one kilometer across, this particular comet is estimated at 100 kilometers across, making it the largest comet ever discovered. Right now, comet BB is out past the orbit of Uranus, but it’s expected to come close to Saturn’s orbit before heading back out toward the Kuiper Belt. So it’s not a threat to Earth, or even Saturn, but its size has made it an unusual subject of interest for planetary scientists.

We’ve talked about the differences between an asteroid and a comet on this show, and on Astronomy Cast, before, but the basic point remains that they are essentially the same type of rocky body. Comets, however, are considered active when the ices in their structure begin to vaporize as they approach the Sun and get warmer. These ices can be a variety of types: water, carbon dioxide, carbon monoxide, etc. Each type of ice vaporizes at a different temperature. And with comet BB being so far away but still so large, astronomers led by Tony Farnham wondered if the comet was already active despite its distance.

Unfortunately, the Dark Energy Survey images did not have the resolution necessary to say for certain one way or another if comet BB is vaporizing, so the team turned to data gathered by NASA’s TESS spacecraft. They stacked the images taken, confirmed their accuracy, and found that yes, comet BB is surrounded by a hazy glow indicating the sublimation of ice. And due to the distance, the ice being sublimated is likely carbon monoxide. The results were published in The Planetary Science Journal.

More Information

University of Maryland press release

Early Activity in Comet C/2014 UN271 Bernardinelli–Bernstein as Observed by TESS,” Tony L. Farnham, Michael S. P. Kelley and James M. Bauer, 29 November 2021, The Planetary Science Journal

This story was written for the Daily Space podcast/YouTube series. Want more news from myself, Dr. Pamela Gay, and Erik Madaus? Check out



Beth Johnson

Planetary scientist, podcast host. Communication specialist for SETI Institute and Planetary Science Institute. Buy me a coffee: