Three articles – available here, here, and here – published in the journal “Science” report various aspects of a research on the origins and characteristics of Arrokoth, the Kuiper Belt object classified as 2014 MU69 and for some time known by the nickname Ultima Thule. Different teams of researchers with various members in common used data collected by NASA’s New Horizons space probe to study it from various points of view. One of the conclusions concerns its origin, which might have occurred following the collapse of a cloud of solid particles in the primordial solar nebula and not following the process known as hierarchical accretion, a process that has high-speed collisions between planetesimals.
The Arrokoth flyby performed on January 1, 2019 by the New Horizons space probe was an extraordinary moment for astronomy. The first mission of New Horizons with the dwarf planet Pluto flyby was more spectacular for the discovery of a magnificent small world and its moons but the second mission took it to the most distant object ever visited by a space probe. Arrokoth is a primordial object that has changed very little after its formation, a sort of fossil very interesting from a scientific point of view for the possible secrets it holds about the processes that led to the formation of the solar system.
The discovery that Arrokoth is made up of two lobes suggested from the outset that originally there were two objects that collided at low speeds, allowing them to merge into what’s technically called a contact binary asteroid. Various computer simulations have been conducted to try to understand the details of that process and the conclusions are interesting regarding planet formation. The most likely scenario has two asteroids born in the same area in the same period that started orbiting each other until they merged after getting closer, perhaps after being slowed down by the collision with the original nebula’s materials.
Arrokoth’s origin process indicates that it formed following the collapse of a cloud of solid particles in the primordial solar nebula, a growth process in which the particles merge slowly. This means ruling out the theory of formation following the process known as hierarchical growth, where there are high-speed collisions between planetesimals. That type of collision would have brought various parts of the primordial nebula to collide and would have led to the formation of an object with different colors, not of uniform color like Arrokoth.
The homogeneous surface of Arrokoth, with few impact craters, indicates that it remained in a quiet area of the solar system where it went through few changes. Its formation may have occurred about four billion years ago, at the end of the solar system’s formation process. That’s probably why no rings or satellites were found around it, although it can’t be ruled out that they weren’t detected during the New Horizons space probe’s flyby.
Arrokoth’s surface color is reddish, typically attributed to tholins, organic compounds found in various places in the solar system and formed thanks to the action of ultraviolet rays from the Sun and cosmic rays on simpler compounds. The same mechanism led to the formation of methanol, whose ice covers this object’s surface, starting from methane and water. Actually, no significant presence of water was detected on Arrokoth but it could be hidden under its surface.
The New Horizons space probe keeps on transmitting the data collected during its Arrokoth flyby because, due to the distance and the amount of those data, the total estimated time to complete their transmission is about 20 months. This means that key information for astronomical research might yet to be received.
Meanwhile, New Horizons is also studying the Kuiper Belt by detecting the presence of charged particles and dust by observing other small objects from a distance. It will be impossible to obtain observations comparable to those of Arrokoth but any information on the asteroids in that area can be useful. If an object is discovered in a position that New Horizons can reach with the fuel still available, NASA may decide to set up a third flyby to obtain even more information on Kuiper Belt objects.
