Astronomical scientists at the University of Maryland have discovered that comet Bernardinelli-Bernstein (BB), C/2014 UN271, the largest known comet, has begun to show signs of activity at a record distance from the Sun. This fact gives scientists an opportunity to better investigate this comet, to study its composition, and possibly to obtain answers to some of the questions about the early periods of the formation and existence of the solar system.
Note that in the history of astronomy, scientists have only once been able to detect comet activity at such a great distance from the Sun. Activity in this case refers to the beginning of the formation of a shell of dust and gas around the comet's nucleus, known as a coma. However, in the very first case, activity was observed in a comet which is much smaller than Bernardinelli-Bernstein, which has a diameter of about 100-200 kilometres. For comparison, the diameter of the notorious Halley's Comet is 15 kilometres, while the vast majority of comets in the solar system have a nucleus around one kilometre in diameter.
Comets, often called 'mucky snowballs' or 'lumps of frozen mud', are accumulations of dust and ice from the early years of the solar system. As a comet approaches the Sun, the surface of its nucleus begins to heat up, the ice melts and evaporates to form a coma, taking dust and small particles with it. Comet activity can start at different distances from the Sun and this depends on the composition of the comet's core ice, which can be frozen water, carbon dioxide, carbon monoxide or a more complex mixture of different chemical compounds.
Comet Bernardinelli-Bernstein was detected beyond Uranus' orbit in October 2014 in data collected by the Dark Energy Survey. The images showed the nucleus of this comet bright enough, but the resolution of the images themselves was low enough to see the beginnings of activity - the beginning of the formation of the gas-dust envelope of the nucleus.
Interested in the discovery, scientists suggested that higher-quality images of comet Bernardinelli-Bernstein must be contained in the data of the TESS (Transient Exoplanet Survey Satellite) space telescope, which observes a portion of the sky for 28 days continuously.
Indeed, thousands of images of comet BB, taken between 2018 and 2020, have been found in data from the TESS exoplanet-hunting telescope. Combining all these images into a single one has drastically increased the latter's resolution and provided a clearer picture of the processes taking place at the comet's core.
To combine the images, scientists had to develop a number of special algorithms, which aligned many of the features and eliminated some of the defects of individual frames. As a result, the final image already showed quite clearly the scattered light from the forming gas-dust envelope of the comet's nucleus, which was an indisputable proof that the comet Bernardinelli-Bernstein became active, being at a record distance from the Sun.
