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The announcent of the massive trans-Neptunian object 2014 UN271 to be a comet is rather new (June 22nd, 2021) and really exciting. That comet is rather huge (~100km diameter) and "falling toward the sun for a 'approach' outside the orbit of Saturn in early 2031." (citing the spaceweather.com-newsletter from today).

The name of the object suggests that it has been first observed in 2014, but do I understand it correctly that only now we learned that the object is outgasing therefore deserves the classification "comet"?

Its perihelion of 10.95 AU will occur in late January 2031. Will it be more impressive to observe than Halley in February 1986? Halley's perihelion was 0.586 AU, but it has only 11km diameter, so I am wondering how much bigger (or smaller) 2014UN271 will appear for the observer on Earth.

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    $\begingroup$ The Wikipedia article you link to does mention “Once at perihelion, the suspected comet is not expected to get brighter than Pluto (mag 13–16) and is more likely to reach the brightness of Pluto's moon Charon (mag 16.8).” $\endgroup$ – Pierre Paquette Jun 23 at 20:58
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A comet's impressiveness is not so much determined by the nucleus size - it depends on its activity, and thus its coma and tail size. Activity is not easily relatable to the size of the comet, but there is a way:

As to the influence of the size of the comet: it has an influence on the tensile strength of the cometary material due to compaction. Following the Rosetta mission we find many evidence that comets form by a gentle gravitational collaps of a cloud of mm-sized pebbles in the protoplanetary nebula. Thus the tensile strength ist primarily determined by the nucleus size due to graviational compaction - and once compacted it will not become less compact.

A nucleus is active when the tensile strength can be overcome by the gas pressure and this gas drags the dust away from the nucleus to form the coma and tail. Thus for a large comet nucleus only the upper layers are weak enough that a gas pressure can overcome it while for smaller comets, the tensile strength is small enough at any depth to be overcome by the gas pressure during perihelion passage; this works best for comets smaller than somewhere around 5km nucleus size (see Gundlach & Blum (2006) in total, but especially fig 2).

While age and perihelion passages thus might not play a large role for small comets, it plays a huge role the larger the object is: the more perihelion passages, the depleted the nucleus might be in volatile materials, thus the activity and outgassing will be less and it will be less impressive as the then-present surface will be too strong to show considerable outgassing.

Thus in summary: if this object has not seen any perihelion passages we might get lucky at approach - but due to its size it will only have very little depth available to erosion, thus a spectacular view is quite unlikely and any volatile materials on the surface along with the dust quickly lost and the remaining material too strong to make a visual appealing comet.

That said, the distinction between comet and asteroid is certainly not as clear as one might sometimes hope - and it might actually just be a phenomenological distinction more than one based on physical properties or genesis. Quite a few asteroids might actually be dead comets where the volatile material on the surface was lost. And an icy asteroid from the outer solar system might show short-lived activity when entering the inner solar system before becoming inactive for the remainder of its life.

Adding to all the above: a perihelion of 10AU is far out - too far out for any object to form any significant outgassing. 67P Churyumov-Gerasimenko showed minimal first activity around 2...3 AU. Outgassing depends directly on insolation, thus a close-in perihelion is of absolute importance. Thus you can expect this object to be as well visible as any of the similarily-sized asteroids in the asteroid belts or Jupiter trojans: not much at all.

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