What We're Learning About Arrokoth, Formerly Known as Ultima Thule

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An illustration of New Horizons' flyby of Arrokoth, a Kuiper Belt object that has changed little since its formation billions of years ago. (Photo: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Steve Gribben)

On Jan. 1, 2019, while the confetti was still fresh on the streets of Times Square, a space probe billions of miles from Earth made a historic flyby of an object dating back to the earliest days of our solar system.

Since named "Arrokoth" by NASA, replacing the earlier nickname "Ultima Thule," this celestial time capsule was visited by NASA's New Horizons spacecraft at about 12:33 a.m. EST on New Year's Day 2019. Unlike Pluto — which New Horizons also flew by, completely upending our knowledge of the dwarf planet in 2015 — Arrokoth is tiny, only 19 miles (31 kilometers) in diameter, compared with Pluto's diameter of more than 1,477 miles (2,377 km).

Despite its small size, Arrokoth is no ordinary space rock. As a resident of the Kuiper Belt — a location beyond Neptune containing early remnants from our solar system's formation — it has largely remained untouched for billions of years. It's also so far from the sun that temperatures there are nearly absolute zero, helping preserve ancient clues that might have otherwise been lost.

Information from the flyby has been trickling in, but since Arrokoth is more than 4 billion miles away, it's taking a while for all of the data to reach Earth. In February 2020, however, NASA unveiled "astonishing" new details about Arrokoth that seem to shed unprecedented light not just on this faraway rock, but on the formation of planets throughout our solar system.

"Arrokoth is the most distant, most primitive and most pristine object ever explored by spacecraft, so we knew it would have a unique story to tell," says New Horizons principal investigator Alan Stern in a statement. "It's teaching us how planetesimals formed, and we believe the result marks a significant advance in understanding overall planetesimal and planet formation."

This illustration shows how Arrokoth might have formed. (Photo: James Tuttle Keane/NASA)

There are two competing theories for how planet formation began in our solar system, where the young sun was initially ringed by a cloud of dust and gas called the solar nebula. In one theory, known as "hierarchical accretion," small bits of material whizzed around in space, sometimes colliding with enough force to stick together. Over millions of years, these violent crashes would produce planetesimals. In the other theory, known as "particle-cloud collapse," certain areas of the solar nebula had a higher density, causing them to gently clump together until they were big enough to "gravitationally collapse" into planetesimals.

Everything about Arrokoth — including its color, shape and composition — suggests it was born via cloud collapse rather than accretion, according to NASA, which outlined the new revelations with three separate papers published in the journal Science.

"Arrokoth has the physical features of a body that came together slowly, with 'local' materials in the solar nebula," says Will Grundy, New Horizons composition theme team lead from Lowell Observatory in Flagstaff, Arizona. "An object like Arrokoth wouldn't have formed, or look the way it does, in a more chaotic accretion environment."

"All of the evidence we've found points to particle-cloud collapse models, and all but rule out hierarchical accretion for the formation mode of Arrokoth, and by inference, other planetesimals," Stern adds.

More complex than expected

This composite image of Arrokoth combines enhanced color data (close to what the human eye would see) with detailed high-resolution panchromatic pictures. (Photo: Roman Tkachenko/NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

The New Horizons team released its initial results from the flyby in May 2019 in the journal Science. Analyzing just the first set of data, the team "quickly discovered an object far more complex than expected," according to a news release from NASA.

Arrokoth is a "contact binary," or a pair of small celestial objects that have gravitated toward one another until they touch, creating a two-lobed structure sort of like a peanut. The two lobes have very different shapes, NASA notes, with one large, oddly flat lobe linked to a smaller, slightly rounder lobe at a juncture nicknamed "the neck." These two lobes once orbited each other, until they were united in a "gentle" merger.

Researchers are also studying surface features on Arrokoth, including a variety of bright spots, hills, troughs, craters and pits. The largest depression is a crater measuring 5 miles (8 km) wide, likely formed by an impact, although some of the smaller pits may have formed in other ways. Arrokoth is also "very red," NASA adds, probably due to modification of organic materials on its surface. The flyby revealed evidence of methanol, water ice and organic molecules on the surface, which differs from what's been found on most icy objects explored by spacecraft, according to NASA.

"We're looking into the well-preserved remnants of the ancient past," Stern said in a statement, adding that he has no doubt the discoveries made from Arrokoth "are going to advance theories of solar system formation."

Origin of the name 'Arrokoth'

Reverend Nick Miles, Tecumseh Red Cloud, Pamunkey Tribe, performs a traditional Algonquian chant at a naming ceremony for Arrokoth in November 2019. (Photo: Aubrey Gemignani/NASA)

NASA had nicknamed the rock Ultima Thule, after a mythical northern land from classical and medieval European literature, but that name drew backlash due to Nazi connotations from the early 20th century, according to Agence France-Presse. NASA announced in November 2019 that Ultima Thule is now Arrokoth, a Native American term meaning "sky" in the Powhatan/Algonquian language. The name is being used with consent from Powhatan elders and representatives.

This links the object with native people from the region where it was discovered, NASA explained in a statement, since the New Horizon team is based in Maryland, part of the Chesapeake Bay region. "We graciously accept this gift from the Powhatan people," said Lori Glaze, director of NASA's Planetary Science Division. "Bestowing the name Arrokoth signifies the strength and endurance of the indigenous Algonquian people of the Chesapeake region. Their heritage continues to be a guiding light for all who search for meaning and understanding of the origins of the universe and the celestial connection of humanity."

A rendezvous far from home

New Horizons' journey through space has covered more than 4 billion miles so far. (Photo: NASA)

When New Horizons made its rendezvous with Arrokoth, it was more than 4.1 billion miles (6.6 billion km) from Earth and traveling faster than 32,000 miles per hour (51,500 kph). In fact, when it launched in 2006, the space probe set a record for the fastest spacecraft — with an Earth and Sun escape trajectory of 36,373 mph (58,537 kph). This excessive speed is one reason the spacecraft will only briefly analyze the object it has been chasing these last several years.

"Are there debris in the way? Will the spacecraft make it? I mean, you know, you can't get any better than that," Jim Green, director of NASA's planetary science division, said of the building drama. "And, we'll get spectacular images on top of that. What's not to like?"

History-making images

At left is a composite of two images taken by New Horizons' high-resolution Long-Range Reconnaissance Imager (LORRI), which sheds light on Arrokoth's size and shape. An artist's impression at right illustrates one possible appearance of Arrokoth, based on the actual image at left. The direction of Arrokoth's spin axis is indicated by the arrows. (Photo: NASA/JHUAPL/SwRI; sketch courtesy of James Tuttle Keane)

On Dec. 28, 2018, New Horizons approached within 2,200 miles (3,540 km) of Arrokoth and recorded images along the way. Within just 10 hours, the data were sent to John Hopkins Applied Physics Laboratory. While the spacecraft has continued to collect data and images in the ensuing months, NASA quickly released the first composite of two images, which showed Arrokoth is shaped roughly like a bowling pin and approximately 20 miles by 10 miles (32 km by 16 km).

A mystery frozen in time

An illustration of the surface of Arrokoth. (Photo: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker)

While Arrokoth's appearance and environment have been shrouded in mystery, scientists did know one thing going in: It's cold. Really cold, with average temperatures maybe only 40 to 50 degrees above absolute zero (minus 459.67 degrees Fahrenheit, or minus 273.15 Celsius). As such, mission planners see Arrokoth as a frozen time capsule from the solar system's earliest days.

"It's a big deal because we're going 4 billion years into the past," Stern said in 2018. "Nothing that we've ever explored in the entire history of space exploration has been kept in this kind of deep freeze the way Ultima has."

The mission team hopes to learn a lot about this Kuiper Belt enigma: Why do objects in the Kuiper Belt tend to exhibit a dark red color? Does Arrokoth have any active geology occurring? Dust rings? Maybe even its own moon? Is it possibly a dormant comet? Researchers are now answering some of these questions, although data from the flyby will continue to arrive well into 2020.

A mission steeped in patience

At its closest, New Horizons came within 2,200 miles of the surface of Arrokoth. (Photo: NASA)

Before New Horizons intercepted Arrokoth on Jan. 1, the spacecraft passed considerably closer than its flyby of Pluto in 2015. Whereas that historic encounter occurred at 7,750 miles (12,472 km) from the surface, this one took place from a distance of only 2,200 miles (3,540 km). This allowed the various cameras on New Horizons to capture superb details of Arrokoth's surface, with some geologic mapping images as fine as 110 feet (34 meters) per pixel.

According to Stern, a total of 50 gigabits of information was captured by New Horizons during its flyby. Because of its distance from Earth, data transmission rates average about 1,000 bits per second and can take upwards of six hours to reach home.

"This limitation, and the fact that we share NASA’s Deep Space Network of tracking and communication antennas with over a dozen other NASA missions, means that it will take 20 months or more, until late in 2020, to send all of the data about Ultima and its environment back to Earth," Stern wrote on Sky and Telescope.

To infinity and beyond

New Horizons' instruments could function to 2038 and beyond. (Photo: NASA/JPL)

While New Horizon's extended mission is expected to formally end on April 30, 2021, the mission team is hinting there may yet be another object out there worth visiting.

Looking beyond the early 2020s, NASA engineers estimate New Horizon's radioisotope thermoelectric generator will keep the spacecraft's instruments functioning until at least 2026. During this time, as it passes through the outer solar system, the probe likely will send back valuable data on the heliosphere –– the bubble-like region of space composed of solar wind particles emanating from the sun. As NASA announced in 2018, the spacecraft already detected the presence of a glowing "hydrogen wall" at the edge of the solar system.

"I think New Horizons has a bright future, continuing to do planetary science and other applications," Stern said at a conference in 2017. "There's fuel and power onboard the spacecraft to operate it for another 20 years. That's not going to be a concern even for a third or fourth extended mission."