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The Tan Snowman

by Perry Pezzolanella

Watching the ball drop on New Year’s Eve as 2019 was about to arrive was exciting in the comfort of a warm living room, but over four billion miles away a spacecraft was on approach to a strange, dimly lit, tiny world bathed in mind-numbing cold. History was about to be made and to those back on Earth in charge of the spacecraft and others who were following the mission it was going to be a very special beginning to a New Year.

New Horizons is the spacecraft that made the historic encounter with Pluto and its moons on July 14, 2015. It was launched on January 19, 2006, with the goal of exploring Pluto and its moons and then continuing outward to encounter a Kuiperoid. New Horizons was steadily nearing Pluto in 2014 but there was still no known Kuiperoid it could reach. Time was running out as the spacecraft had to be properly targeted at Pluto to reach it. Searching with the Hubble Space Telescope was proving futile, but on June 26, 2014, a tiny world, hardly 20 miles across and shining faintly at magnitude +27 was discovered about 4 billion miles away. The Kuiperoid is so remote that it takes about 296 years to orbit once around the Sun. It was given the provisional designation, 2014 MU69, and informally called Ultima Thule, pronounced “ultima toolee”, which is a Latin phrase for “beyond the farthest frontiers”. It is now officially known as Arrokoth, a Native American term meaning “sky” in the Powhatan/Algonquian language. It was found to be deeply reddish, which means it is carbon-rich, coated with organic compounds, and very ancient. Occultation of stars from Earth on June 10 and July 17, 2017 revealed a strange shape for Arrokoth. It appeared to be bi-lobed, meaning it is two objects that are in close orbit with each other or barely touching, a contact binary. If they were touching, it would appear like a reddish peanut. One occultation hinted at the possibility of a tiny moon.

The main objectives for this historic first flyby of a Kuiperoid were to map its shape, geology, color, surface composition, and to search for satellites and rings. The secondary objectives included searching for an atmosphere and determining its composition along with its interaction with the solar wind; making stereoscopic maps of the surface to determine the topography; and measuring the temperature of the day and night sides of Arrokoth. The flyby occurred on time at 12:33 A.M. EST on January 1, 2019, when New Horizons flew as close as 2175 miles at a speed of nearly 32,000 miles per hour and found it to be a primordial contact binary. It is almost certainly a relic planetesimal that can provide valuable information on how Kuiperoids, comets, and dwarf planets formed within the Kuiper Belt.

Arrokoth looks like a tan snowman, but looks can be deceiving. The two lobes appear to be similar shapes. The larger lobe is 12.1 miles across and the smaller lobe is 8.8 miles across, but the larger lobe is shaped like a very thick hamburger patty while the smaller lobe resembles a dented walnut. The surface of both lobes shows hills, scarps, and either craters or collapse pits, or possibly both. The strangest features are the abutting, rounded terrain units, each about one or two miles across that cover the lobes like a network of cells bordered by brighter material in what may be troughs between the cells. The most prominent bright material is a narrow band where the two lobes are joined. How this formed is still a mystery. The rotation of Arrokoth was pinned down upon approach to be 15.9 hours and rotates like a propeller. The reddish or tan coloring is rather uniform and typical of Kuiperoids. No moons, rings, or atmosphere have been discovered and given its small size it was not expected, but many small Kuiperoids do have tiny moons and even rings, so Arrokoth may end up being unusual.

New Horizons made the stunning revelation that Arrokoth is the most prehistoric and pristine object ever explored. It answered the question about planetesimal origins by analyzing Arrokoth’s shape, geology, color, and composition, making a major advance in the understanding on how the planets formed. The uniform color and composition of Arrokoth is proof that it formed from a small, uniform cloud in the solar nebula rather than from a chaotic collection of debris from other areas of the nebula. With this data in hand, the formation of Arrokoth’s shape is now known to be the result of a gentle merger of two objects that formed close together during the collapse of a cloud of solid particles in the primordial solar nebula. The two objects slowly grew close together as they orbited each other and finally touched. The flattened shapes of both lobes along with the close alignment of their poles and equators are also proof of an orderly, gentle merger from a collapsing cloud. The smooth surface with very few craters indicates it is a well-preserved relic from the end of the planet formation era.

Plans are being drafted for a new mission to Pluto and the Kuiper Belt. Known as Persephone, this mission, based on a spacecraft design similar to New Horizons’, is being planned as a Pluto orbiter with multiple Kuiperoid flybys. Persephone will bristle with state-of-the-art technology to thoroughly explore Pluto, Charon, and the other moons along with several Kuiperoids. If approved, the mission could be launched in 2031 on a long, looping orbit where it would encounter a reasonably sized Kuiperoid about 30-60 miles across in 2050. Persephone would then travel to Pluto and arrive in 2058 where it would orbit for at least three years. It would leave Pluto in 2061 and fly past another decent size Kuiperoid about 50 miles across in 2069. It may be possible for Persephone to orbit that Kuiperoid depending on advancements in rocket propulsion technology. If the spacecraft remains healthy, it might be possible for it to study several other remote Kuiperoids as it races into the depths of space.

Thanks to the flawless performance of New Horizons and the wealth of data it gathered during the flyby of Arrokoth, an important question regarding its formation has been solved. There is still much to understand and learn from a tiny world that was not known to exist until 2014 nestled within a belt of distant objects unheard of until 1992. New Horizons will continue onward into the dark depths of space, never to return, but will continue to carry out observations of any Kuiperoids it passes. It also continues to map the charged particle-radiation and dust environment in the Kuiper Belt. These Kuiperoids are too far away for New Horizons to reveal any detail, but the shape and surface properties can be measured. The search continues for any potential Kuiperoid that may be close enough for New Horizons to fly close by if fuel allows. The legacy of New Horizons is that it will always be remembered as the first spacecraft to encounter Pluto, its moons, and a pristine relic of the Solar System.