On October 20, 2014, astronomers Pedro Bernardinelli and Gary Bernstein discovered the largest comet ever known using archival images from the Dark Energy Survey [see notes 1]. Comet Bernardinelli-Bernstein’s estimated diameter is approximately 85 miles (136.7 km) across, making it larger than the state of Rhode Island. According to NASA, its nucleus is about 50 times larger than that found at the heart of most known comets. [Source] Its mass is estimated to be a staggering 500 trillion tons – about 100,000 times greater than the mass of a typical comet found much closer to the Sun.

October 20 story of what happened this day in Science, Technology, Astronomy, and Space Exploration history.

Comet Bernardinelli-Bernstein: the largest comet ever known

Comet C/2014 UN271 (Bernardinelli–Bernstein) was first imaged on October 20, 2014, during the Dark Energy Survey (DES), although its significance as a comet was not immediately recognized. Using the 4-meter Victor M. Blanco Telescope in Chile, the DES was focused on investigating dark energy and cosmic structure. The survey’s wide-field camera captured vast amounts of celestial data, including images of this object.

Its discovery as a comet came years later when astronomers Pedro Bernardinelli and Gary Bernstein were analyzing the DES data specifically for trans-Neptunian objects. They noticed an object with a slow movement and faint, diffuse appearance, different from typical asteroids. This indicated its cometary nature, leading to its identification as C/2014 UN271. The comet, with an estimated nucleus size of approximately 85 miles (136.7 km), stands out as the largest comet ever discovered, an extraordinary find made possible by revisiting and analyzing previously captured astronomical data.

The comet’s discovery was announced by the Minor Planet Center on June 19, 2021. The Minor Planet Center (MPC) is an international organization responsible for collecting and cataloging observational data for minor planets, comets, and outer irregular natural satellites. It operates under the auspices of the International Astronomical Union (IAU).

Largest comet ever known: Comet Bernardinelli-Bernstein
This sequence shows how the nucleus of Comet C/2014 UN271 (Bernardinelli-Bernstein) was isolated from a vast shell of dust and gas surrounding the solid icy nucleus. On the left is a photo of the comet taken by the NASA Hubble Space Telescope’s Wide Field Camera 3 on January 8, 2022. A coma (middle panel) model was obtained utilizing fitting the surface brightness profile assembled from the observed image on the left. This allowed for the coma to be subtracted, unveiling the point-like glow from the nucleus. Combined with radio telescope data, astronomers arrived at a precise measurement of the nucleus size. That’s no small feat from something about 2 billion miles away. Though the nucleus is estimated to be as large as 85 miles (136.7 km) across, making it the largest comet ever discovered, it is so far away Hubble cannot resolve it. Its size is derived from its reflectivity as measured by Hubble. The nucleus is estimated to be as black as charcoal. The nucleus area is gleaned from radio observations. NASA, ESA, Man-To Hui (Macau University of Science and Technology), David Jewitt (UCLA); Image processing: Alyssa Pagan (STScI)

Before Comet C/2014 UN271 was identified, the record for the largest comet nucleus was held by C/2002 VQ94, estimated to be about 60 miles (97 km) in diameter. This comet was discovered in 2002 by the Lincoln Near-Earth Asteroid Research (LINEAR) project, a collaborative venture involving the United States Air Force, NASA, and MIT’s Lincoln Laboratory. The LINEAR project’s primary goal is to detect and track near-Earth asteroids, contributing significantly to our understanding of these celestial bodies and enhancing planetary defense efforts.

Not visible to the naked eye

Despite being the largest comet ever discovered, and despite having a relatively high brightness, Bernardinelli-Bernstein is not visible to the naked eye. When first imaged by the DES, its distance was 29 AU (4.3 billion km; 2.7 billion miles) from the Sun, which is nearly the average distance to Neptune. [An Astronomical Unit (AU) is a standard unit of measurement in astronomy, primarily used to express distances within our solar system. It is defined as the average distance between the Earth and the Sun. This average distance is approximately 93 million miles or 150 million kilometers.]

According to NASA, Comet Bernardinelli-Bernstein will never get closer than 1 billion miles (1.6 billion km) away from the Sun, which is slightly farther than the distance of the planet Saturn. And that won’t be until the year 2031.

Even when it’s at perihelion (the closest to the Sun), the comet is not expected to get brighter than Pluto (mag 13-16). It is more likely to reach the brightness of Charon, Pluto’s moon (mag 16.8). Bernardinelli-Bernstein will not enter the inner Solar System where comets become notably more active and more visible (when comets get close to the sun, they heat up and we can see their glow and long tails). Even if it reaches the magnitude of Pluto, it will require about a 200 mm telescope to be visually seen.

According to scientists, its nucleus is darker than coal.

Largest comet ever known: Comet Bernardinelli-Bernstein
Color composite image of C/2014 UN271 (Comet Bernardinelli-Bernstein), the largest comet ever discovered, from the Dark Energy Survey in October 2017. Image by Dark Energy Survey/DOE/FNAL/DECam/CTIO/NOIRLab/NSF/AURA/P. P. Bernardinelli & G. Bernstein (UPenn)/DESI Legacy Imaging Surveys – https://noirlab.edu/public/images/noirlab2119b/, CC BY 4.0, Link

Orbital characteristics of Comet Bernardinelli-Bernstein

  • Aphelion [closest to the Sun]: ~39,600 AU (inbound), ~55,000 AU (outbound)
  • Perihelion [farthest to the Sun]: 10.9502 AU (1.64 billion km)[c]
  • Semi-major axis: ~19,800 AU (inbound), ~27,500 AU (outbound)
  • Eccentricity (how much an orbit deviates from being a perfect circle): 0.99945 (inbound), 0.99960 (outbound)
  • Orbital period: ~2.79 million years (inbound), ~4.56 million years (outbound)
  • Inclination (the tilt of an object’s orbital plane relative to a reference plane, usually the plane of the equator of the primary body it orbits or the ecliptic plane in the Solar System): 95.466° (inbound), 95.460° (outbound)
  • Next perihelion: January 23, 2031
Comet nuclei size comparison
This diagram compares the size of the icy, solid nucleus of comet C/2014 UN271 (Bernardinelli-Bernstein) to several other comets, including Halley’s comet, the most famous comet ever. The majority of comet nuclei observed are smaller than Halley’s comet. They are typically a mile across or less. Comet C/2014 UN271 is currently the record-holder for big comets. And, it may be just the tip of the iceberg, astronomers think. NASA suggests There could be many more monsters out there for astronomers to identify as sky surveys improve in sensitivity. Though astronomers know this comet must be big to be detected so far out to a distance of over 2 billion miles (3.2 billion km) from Earth, only the Hubble Space Telescope has the sharpness and sensitivity to make a definitive estimate of nucleus size. Illustration: NASA, ESA, Zena Levy (STScI).

Notes

  1. The Dark Energy Survey (DES) is a significant astronomical project aimed at understanding the mysterious force known as dark energy, which is believed to accelerate the universe’s expansion. Utilizing a 570-megapixel camera mounted on the Blanco Telescope in Chile, DES has surveyed the sky since 2013, capturing detailed images of galaxies to study cosmic structures and their evolution over billions of years.

Sources

  1. “Hubble Confirms Largest Comet Nucleus Ever Seen” on the NASA Science website [April 12, 2022]
  2. C/2014 UN271 (Bernardinelli-Bernstein) on Wikipedia
M. Özgür Nevres
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