There is a distant, dim domain of everlasting twilight in our Solar System’s outer limits, where our Sun can glow gently with only a faint, feeble fire. This faraway region is named the Kuiper belt, and it is the home of the dwarf planet Pluto, its large frigid moon, Charon, and a multitude of other frozen objects that include a dancing sea of icy comet nuclei. Fragile and ephemeral, comets that come streaking inward towards the brilliant light and melting heat of our Star, are the fleeing refugees from this remote region of perpetual dusk. In February 2020, planetary scientists from NASA’s New Horizons mission to Pluto, and beyond, announced that data collected from this mission are providing new insights into how planets and planetesimals–the building blocks of planets–were born in our primordial Solar System. The New Horizons spacecraft soared past the ancient Kuiper belt object (KBO) Arrokoth (2014 MU 69) on January 1, 2019, providing humanity with its very first close-up view of one of those mysterious icy relics of our Solar System’s birth almost five billion years ago.

Using detailed data on Arrokoth’s shape, geology, color and composition–collected during a record-setting flyby that took place more than 4,000,000,000 miles from Earth–planetary scientists announced in February 2020 that they have been able to answer a longstanding question about planetesimal origins, and therefore they have made a major advance in our understanding of how the planets themselves were born. The team reports those findings in a set of three papers published in the February 13, 2020 issue of the journal Science. The scientists also presented their findings at a media briefing held at the annual American Association for the Advancement of Science (AAAS) meeting in Seattle, Washington.

486958 Arrokoth is the provisional name of 2014 MU 69, a trans-Neptunian object. It is a contact binary, 22 miles long, made up of a duo of icy primordial planetesimals that are 13 miles and 9 miles across. The two planetesimals have been nicknamed Ultima and Thule, respectively, and they are bound together along their major axes. Ultima is flatter than Thule, and it is actually an aggregate of 8–or more–smaller objects, each approximately 3 miles across. These smaller objects are believed to have fused together before Ultima and Thule met up and merged together. Because there have apparently been no–or very few–catastrophic impacts on Arrokoth since it formed, the history of its formation has been preserved in our Solar System’s deep freeze, far from the melting heat of our roiling Star.

Arrokoth is both the most distant and most primitive object in our Solar System to be visited by a spacecraft. It was discovered on June 26, 2014 by Dr. Marc Bule and the New Horizons Search Team using the Hubble Space Telescope (HST) as part of a hunt for the KBO that would become the target of New Horizons in its first extended mission. This discovery required the use of the HST because, with an apparent magnitude of only 26, Arrokoth is far too faint to be observed–except by the most powerful telescopes. The HST also has the capability to conduct extremely precise astrometry. This means that it can provide a reliable determination of an orbit.

Arrokoth was chosen over two other candidate KBOs, and thus became the destination of New Horizons’ extended mission beyond Pluto. With an orbital period of about 298 years and a low orbital inclination and eccentricity, Arrokoth is designated a cold classical KBO.

The provisional name Arrokoth itself comes from the Powhatan region of Virginia and Maryland, where Arrokoth’s discovery occurred. The HST and Johns Hopkins Applied Physics Laboratory are both operated in Maryland and are primarily responsible for Arrokoth’s 2014 discovery.

Pluto And Beyond

On July 14, 2015, after its dangerous decade-long odyssey through space, New Horizons reached its primary target in the Kuiper belt–the ice dwarf planet Pluto and its family of five icy moons–including its large moon, Charon, which is almost half the size of Pluto. New Horizons, after successfully making its historic closest approach to the Pluto system, sent back to Earth some revealing images of this faraway small world with a big heart.

For most of the 20th century, astronomers considered Pluto to be a lonely little world, where it dwells in the outer domain of our Solar System. However, this viewpoint changed back in 1992, when a second KBO was discovered in this region. Because of this discovery, planetary scientists came to the realization that Pluto is far from alone in this remote frozen region of perpetual twilight. Indeed, Pluto orbits our Star in the company of myriad others of its mysterious frigid kind. Since 1992, a multitude of other icy, rocky little worlds, very similar to Pluto, have been discovered by astronomers searching for these distant bodies. Some of the other distant denizens of our Solar System’s deep freeze also orbit our Star in eccentric orbits–just like Pluto. The dwarf planet Eris is one of the most important of these scattered disc objects. Discovered back in 2005, Eris is a bit more massive than Pluto, and its discovery is what forced planetary scientists to realize that Pluto is merely one of many. As a result, at that time, poor little Pluto was unceremoniously evicted from the pantheon of major planets, and is now classified as an ice dwarf–a mere dwarf planet, but a planet nonetheless. Prior to that, Pluto was classified as the ninth major planet from our Star.

KBOs are generally believed to be made up mainly of a combination of ice and rock. The Kuiper belt extends out from the orbit of Neptune to approximately 50 astronomical units (AU). One AU is equal to the mean distance between Earth and Sun, which is about 93,000,000 miles. Neptune’s average distance from our Star is approximately 30.1 AU–its perhihelion (when it is closest to our Star) is 29.8 AU, while its aphelion (when it is farthest from our Star) is 30.4 AU.

After completing its primary mission of a flyby over Pluto and its moons, the New Horizon spacecraft was maneuvered for its future flight over the distant Arrokoth.

Arrokoth And Its Many Mysteries

Arrokoth, as the most remote, primitive, and purest object ever explored by a spaceship, has a unique story to tell us. This remote little object has enabled planetary scientists to make important progress in their understanding of how planets formed in the primordial Solar System. The first post-flyby images obtained from New Horizons in 2019 revealed that Arrokoth is actually composed of two interconnected blades with smooth surfaces, as well as the same composition. This indicates that it is most likely a pristine object and, as such, it provides valuable information about how it was formed.

These interesting new findings contribute to the many historic accomplishments of an important mission. NASA’s discoveries obtained from New Horizons have enhanced our knowledge and understanding of how planetary bodies were born in our Solar System–as well as in distant planetary systems beyond our own.

In the future, the New Horizons team plans on using data with greater resolution. The scientists are also planning to use sophisticated supercomputer simulations in order to create models of how Arrokoth may have formed billions of years ago.

Their analysis has already revealed that Arrokoth’s “binary contact” lobes were originally two separate objects, orbiting close to each other at slow speeds. Ultimately, the duo gently collided and merged together to form the 22 mm body observed by New Horizons.

This information indicates that Arrokoth was born during the gravitational collapse of solid particle clouds in the original primordial solar nebula that gave birth to our Sun and its family of planets, moons, and other objects, about 4.56 billion years ago. In dramatic contrast to a high-speed collision and hierarchical accumulation, the particles that created Arrokoth merged together gently when the cloud collapsed. This indicates that Arrokoth, and other similar objects inhabiting the ancient Solar System, gradually increased in size to form ever larger and larger bodies. In a way that has been compared to how fossils show the way species evolved on our own planet, lingering planetesimals like Arrokoth reveal how planets evolved in our own Solar System–and beyond.

Arrokoth would not look the way it does if it had formed as the result of a violent collision in our primordial Solar System. Instead, evidence indicates that it formed as the result of a more complex and gentle ballet in which the two dancing planetesimals slowly orbited one another before their dance gravitationally drew them both together, thus forming the two lobed single planetesimal observed today.

Two other important lines of evidence support this scenario. The uniform surface composition and color of Arrokoth reveal that it formed from materials dancing closely to one another, as predicted by local collapsing patterns within clouds. This contrasts with a model indicating that it formed from material located in more widely separated regions of the primordial solar nebula.

In addition, the flattened shape of each of Arrokoth’s two lobes, as well as its very narrow polar alignment and equator, indicate that it was born in an orderly cradle. Also, Arrokoth’s smooth surface suggests that its face has been well preserved in our Solar System’s dusky deep freeze since the end of the era of planet formation. This means that Arrokoth displays physical properties indicating that it formed as the result of gradually and gently merging objects that formed near one another in the solar nebula.

The three papers published in the February 13, 2020 issue of the journal Science are based on ten times more data than the first reports that were published about the new findings. Together the papers provide a more complete portrayal of Arrokoth’s mysterious origins.

New Horizons continues to observe objects dancing in the distant Kuiper belt. In the summer of 2020, the New Horizons team plans to use a large telescope on Earth to hunt for additional KBOs that can be studied–and also search for other potential targets for the spacecraft.

As of this writing, the New Horizons spacecraft is 7.1 billion kilometers from Earth. It is still operating normally, as it soars through this unexplored, frigid twilight region at speeds of almost 50,400 kilometers per hour.

New Horizons Principal Investigator Dr. Alan Stern explained their reasons for giving Arrokoth its name:

“The name ‘Arrokoth’ reflects the inspiration of looking to the skies, and wondering about the stars and worlds beyond our own. That desire to learn is at the heart of the New Horizons’ mission, and we’re honored to join with the Powhatan community and people of Maryland in this celebration of discovery.”

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