Free Astronomy Magazine September-October 2021

22 SEPTEMBER-OCTOBER 2021 ASTRO PUBLISHING T his image from the Dark Energy Survey (DES) is composed of some of the discovery exposures showing Comet Bernardinelli-Bern- stein collected by the 570-mega- pixel Dark Energy Camera (DECam) mounted on the Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile. These images show the comet in October 2017, when it was 25 au away [Dark Energy Survey/ DOE/ FNAL/DECam/CTIO/ NOIRLab/ NSF/AURA/P. Bernardinelli & G. Bernstein (UPenn)/DESI Legacy Im- aging Surveys. − Acknowledge- ment: T.A. Rector (University of Alaska Anchorage/NSF’s NOIRLab), M. Zamani (NSF’s NOIRLab) & J. Miller (NSF’s NOIRLab)] Its current inward journey began at a distance of over 40,000 astronom- ical units (au) from the Sun — in other words 40,000 times farther from the Sun than Earth is, or 6 tril- lion kilometers away (3.7 trillion miles or 0.6 light-years — 1/7 of the distance to the nearest star). For comparison, Pluto is 39 au from the Sun, on average. This means that Comet Bernardinelli-Bernstein orig- inated in the Oort Cloud of objects, ejected during the early history of the Solar System. It could be the largest member of the Oort Cloud ever detected, and it is the first comet on an incoming path to be detected so far away. Comet Bernardinelli-Bernstein is currently much closer to the Sun. It was first seen by DES in 2014 at a distance of 29 au (4 billion kilome- ters or 2.5 billion miles, roughly the distance of Neptune), and as of June 2021, it was 20 au (3 billion kilome- ters or 1.8 billion miles, the distance of Uranus) from the Sun and cur- rently shines at magnitude 20. The comet’s orbit is perpendicular to the plane of the Solar System and it will reach its closest point to the Sun (known as perihelion) in 2031, when it will be around 11 au away (a bit more than Saturn’s distance from the Sun) — but it will get no closer. Despite the comet’s size, it is cur- rently predicted that skywatchers will require a large amateur tele- scope to see it, even at its brightest. “We have the privilege of having discovered perhaps the largest comet ever seen — or at least larger than any well-studied one — and caught it early enough for people to watch it evolve as it approaches and warms up,” said Gary Bernstein. “It has not visited the Solar System in more than 3 million years.” Comet Bernardinelli-Bernstein will be followed intensively by the astro- nomical community, including with NOIRLab facilities, to understand the composition and origin of this massive relic from the birth of our own planet. Astronomers suspect that there may be many more undis- covered comets of this size waiting in the Oort Cloud far beyond Pluto and the Kuiper Belt. These giant comets are thought to have been scattered to the far reaches of the Solar System by the migration of Jupiter, Saturn, Uranus, and Nep- tune early in their history. “This is a much needed anchor on the unknown population of large objects in the Oort Cloud and their connection with early migration of the ice/gas giants soon after the Solar System was formed,” said NOIRLab astronomer Tod Lauer. “These observations demonstrate the value of long-duration survey observations on national facilities like the Blanco telescope , ” says Chris Davis, National Science Foun- dation Program Director for NOIR- Lab. “Finding huge objects like Comet Bernardinelli-Bernstein is crucial to our understanding of the early history of our Solar System.” It is not yet known how active and bright it will become when it reaches perihelion. However, Bernardinelli says that Vera C. Rubin Observatory, a future Program of NOIRLab, “will continuously measure Comet Ber- nardinelli-Bernstein all the way to its perihelion in 2031, and probably find many, many others like it,” al- lowing astronomers to characterize objects from the Oort Cloud in much greater detail. !