Some very recent exciting news from the Dark Energy Survey collaboration (https://minorplanetcenter.net/mpec/K21/K21M53.html)
This new object, 2014 UN271, is not just unusual, but radically exceptional among all known bodies in the Solar System to date. Discovered about 29 AU out from the Sun and currently around 22 away, its orbit takes it from just beyond the orbit of Saturn (10.9 AU) all the way out to the Oort Cloud - no, not the Hill Cloud. the Oort Cloud. The incoming barycentric aphelion was 39,400 +/- 1200 AU, and outgoing it will be an even huger 54,600 +/- 2300 AU (!!)
Based on the given absolute magnitude, and given how exceptionally red it looked in 2014 precovery images from CFHT (the g - r color was 0.9, and r - i was 0.5!) I would estimate at an albedo of 0.01-0.08 a diameter of 130-370 kilometers (nominally 160) which puts it on a similar scale, if not larger than, Sarabat’s huge comet C/1729 P1, and almost undoubtedly the largest Oort Cloud object ever discovered - almost in dwarf planet territory!
Sam Deen
Exciting indeed! This newly discovered TNO is larger than most known comets, with the possible exception of Sarabat’s comet, and potentially nearly the size of major main belt asteroids – and comparable to the largest centaur Chiron. It can certainly make you wonder what other planetoids lie undiscovered on the outskirts of our solar system. As 2014 UN271 is currently on an approaching trajectory, astronomers will have plenty of opportunities to refine its orbit and study it during its perihelion passage, currently estimated for January 2031, and beyond. Some have reported observations of a cometary coma around it already, but its closest approach brings it just outside the orbit of Saturn, so it will be too far from the Sun to be visible to the naked eye, or to generate a sizeable coma.
As a side note, I has no idea Comet Hale–Bopp was so large, with an estimated diameter of 40–80 km (2014 UN271 is at least twice larger). Hale–Bopp was the first comet I ever saw, during my high-school years.
This close passage of a remote Oort Cloud object through the orbits of the outer planets could provide an excellent opportunity to launch a mission to study this unique planetoid up close – for reference, Cassini travelled for about 7 years from Earth to Saturn, so reaching 2014 UN271 in August 2033, as it passes through the ecliptic plane, seems feasible. A probe fitted with ion propulsion, like Hayabusa and Dawn, could reduce the journey time even further.
I would be even more ambitious and aim for an orbiter mission like Rosetta. This would allow scientists to study the composition of the object, as well as its potential gas and particle emissions, for a longer time, up to a couple of years, depending on how long the probe keeps functioning properly. As 2014 UN271 recedes into the huge unexplored parts of the outer solar system, an orbiting probe could also use cameras to scan the skies around it for other slow-moving, dim objects, from a vastly different vantage point than Earth’s.
Update: after confirmation of cometary activity, the object has received the official cometary designation C/2014 UN271 (Bernardinelli-Bernstein).
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