Exploring Arrokoth: Insights into Planet Formation from Space
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Chapter 1: Introduction to Arrokoth
In 2015, after the successful flyby of Pluto by NASA’s New Horizons probe, scientists faced the decision of where to direct the spacecraft next. They chose Arrokoth (formerly known as Ultima Thule), an unusual, red, double-lobed rock situated around 4.1 billion miles from Earth.
Though Arrokoth might lack a significant atmosphere and varied geology, its peculiar shape resembles a whimsical cosmic snowman—measuring approximately 22 miles in length, 12 miles in width, and 6 miles in thickness, it has a flattened, pancake-like structure. This unique formation results in an unconventional gravitational field and rotation. As planetary scientist William McKinnon from Washington University in St. Louis remarks, “It doesn’t resemble any other celestial body we’ve encountered.”
Recent studies indicate that Arrokoth is far more intriguing than initially believed, with its formation potentially reflecting the broader processes that shaped our solar system.
Section 1.1: New Horizons and the Flyby
On New Year's Day last year, New Horizons raced past Arrokoth at speeds exceeding 31,500 miles per hour, coming within 2,200 miles of the rock. A trio of new research papers published in Science this past Thursday analyzes data from this flyby, revealing insights into how Arrokoth formed, which may also illuminate the origins of other planetesimals—the tiny building blocks of planets—in the solar system.
Theories on planetesimal formation generally fall into two categories: hierarchical accretion, where smaller objects collide at high speeds to create larger ones, and local cloud collapse, where gravitational forces cause local concentrations of matter to merge into larger entities at lower velocities.
Subsection 1.1.1: Supporting Local Cloud Collapse
The three studies examine various facets of Arrokoth: one investigates its geological and geophysical properties, another its material composition, and the last its likely formation. Collectively, they bolster the local cloud collapse theory. According to Alan Stern, a planetary scientist at the Southwest Research Institute and the New Horizons mission leader, “We’ve decisively solved a multi-decade debate about how planetesimals form,” noting that the outcome was unexpected yet compelling.
Using the new data, McKinnon and his team conducted simulations to explore the most plausible models for Arrokoth's creation, strongly favoring this planetesimal evolution process. They propose that Arrokoth’s two lobes formed separately from the collapse of the same material cloud, which accounts for their uniform red hue. As these bodies were already in proximity, they began to orbit each other, eventually merging gently at speeds of just a few miles per hour. “We believe we have a clear narrative for Arrokoth, and it likely applies to the rest of the solar system,” McKinnon states.
Section 1.2: New Discoveries from the Studies
The recent research also reveals several key findings:
- The bodies are rounder than early images suggested.
- The smooth surface, free from fractures and stress, indicates low-speed formation.
- The number of craters implies Arrokoth formed over 4 billion years ago during the solar system's infancy.
- Complex organic molecules are present on the surface, which is common among many solar system objects.
- Traces of methanol ice have been discovered, but the lack of water raises questions about its origins.
Chapter 2: Future of New Horizons
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It will take approximately another year to thoroughly download and analyze all data collected by New Horizons during its flyby of Arrokoth. Meanwhile, Stern reports that the spacecraft remains in excellent health, with about one-eighth of its fuel still available. It appears likely that New Horizons has at least one more flyby ahead. Over the coming summers, when conditions for observation are ideal, the mission team plans to use some of the largest Earth-based telescopes to identify promising targets for New Horizons. “We’re just beginning this process, and we’re eagerly anticipating what lies ahead,” he concludes.