The largest comet ever seen
Credits: Nasa, Esa, Man-To Hui (Macau University of Science and Technology), David Jewitt (Ucla). Image processing: Alyssa Pagan (STScI)
A mammoth comet Comet C / 2014 UN271 was discovered by chance by the astronomers of the Dark Energy Survey at the Cerro Tololo Inter-American Observatory (Chile) Pedro Bernardinelli and Gary Bernstein In 2010 . Since that moment it has attracted many glances on itself: despite being at an abysmal distance from the Earth, its brightness is such as to suggest truly colossal dimensions. Measuring its core by separating it from the canopy of gas and dust that surrounds it is not so obvious. A lot of data has accumulated, including those collected by the instruments of the Atacama observatory in Chile, but there was no confirmation.
So the team of Man-To Hui and David Jewitt of UCLA asked for help to Hubble, the space telescope of NASA and ESA, which last January took 5 photographs of the comet: considering the distance, not even Hubble can focus on the nucleus alone, but the images have allowed scientists to develop a computer model capable of to separate the brightness of the comet's coma from the central nucleus, thus being able to estimate its size.
For researchers, the nucleus of comet Bernardinelli-Bernstein is the largest ever observed so far: 129 kilometers for a mass of around 500 thousand billion tons. A nucleus that is 50 times larger than that of other known comets. The data are consistent with those collected previously, and also add a note of color: the surface of the nucleus appears to be blacker than coal.
Comparison between the dimensions of the nucleus of comet Bernardinelli-Bernstein and that of others known comets. Credits: Nasa, Esa, Zena Levy (STScI)
From the Oort cloud, towards the Sun and back Comets are among the oldest objects in our Solar System, "fossils" from its dawn. They are amicably referred to by experts as big "dirty snowballs" because their core is made of ice mixed with rock debris and dust. Shot out of the Solar System billions of years ago by the gravitational force games of the outer system planets (such as Jupiter and Saturn, when they were still forming), they are thought to nestle in the so-called Oort Cloud, one of the hypothetical largest and most more distant than the solar system. There they tend to stay, at least until they receive a push from other gravitational forces that change their orbit.
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So it must to have been for the Bernardinelli-Bernstein which now travels along an elliptical orbit three million years long around the Sun. According to scientists it will touch the closest point to our star (half a light year) in 2031, while remaining very far away from ours planet. Then the return journey to the Oort Cloud will begin.
