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What Is Arrokoth and What Has New Horizons Discovered There?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

7 minutes before cloest aproach, taken at 6700 km mu69

7 minutes before cloest aproach, taken at 6700 km mu69

What Is a Kuiper Belt Object?

At the end of August 2015, NASA announced that the follow-up to Pluto would be 2014 MU69, an object over 1 billion miles past Pluto after it was passed for review on July 1, 2016. It was one of five objects spotted by Hubble in 2014 as the New Horizons team was searching for the next phase of the mission.

The object chosen was the one that would offer the best high-resolution pictures, would be able to map in both color and stereo imaging, get temperature readings on the surface, look for atmospheres or dust rings, and see the effects of the solar wind that far out (NASA "New Horizons Selects," Wenz "New Horizons," NASA "NASA Releases," Stern “Ultima” 23)

En route to MU69, data from an occultation hinted at MU69 being either a contact binary or a football shape. And on the route to MU69, a calibration phot taken using NGC 3532 was the furthest picture taken by a probe...until it took another photo of 2012 HZ84 and 2012 HE85 at about 3.79 billion miles from Earth.

NASA held a fan-naming competition, and the ultimate (though unofficial) name given to MU69 was “Ultima Thule” which is Latin for “a world beyond us.” Later, the official name of Arrokoth was chosen, a Native American term meaning “sky” or “cloud” (Ibid).

Flyby Time

As the final hours of 2018 rolled by, NASA was gearing up for its flyby. It had been years since New Horizons’ historic visit to Pluto, and this mission would be just as important to learning more about Kuiper-Belt Objects (KBOs). Finally, in the early hours of January 1, 2019, after years of traveling in the void, New Horizons zipped past Arrokoth, making that the farthest object ever visited by a probe at 4.1 billion miles from Earth. The closest approach was at 3,500 kilometers, with 900 images being taken and 50 gigabits of data to be transferred as late as September of 2020.

Natural coloring, the enhanced image, ad the hybrid between the two.

Natural coloring, the enhanced image, ad the hybrid between the two.


Pictures came in quickly, showing that Arrokoth is the first confirmed contact binary ever visited. This is a class of object that was originally two objects orbiting one another, but that slowly (2-3 kilometers an hour) fell into each other, finally merging into a snowman-like shape that is in contact with each other.

Of course, the object could have just naturally formed that way but that far out from the Sun means the temperatures are not sufficient to allow melting to occur. The larger lobe is named Ultima with a diameter of 12.1 miles while the smaller one is Thule with a diameter of 8.8 miles.

Totaling in at 21 miles long, Arrokoth is about 10 times bigger than a typical comet (and therefore about 100 times the volume), rotates once every 15.92 hours, and has an axis tilt of 99 degrees(!) and has a low-albedo surface (reflecting only 10% of the light hitting its surface) which isn’t helped by the 1,900 times fainter conditions around it compared to Earth (Parks, Eicher “Ultima”, Gohd “New Horizons survived,” Timmer “New,” Timmer “NASA,” Stern “Exclusive,” Eicher et al., Stern “Ultima” 23-5, Haynes “New Horizons,” Kohler).


Like many KBOs, it has a red surface likely from solar radiation or radioactive decays of organic materials. Interestingly, the surface is relatively smooth with little impact craters seen, meaning it's either young or that the Kuiper Belt isn’t full of small objects that can act as impactors (Ibid).

Around January 18 of 2019, a new image taken by the MVIC portion of the Ralph component was finally received, with a resolution of 135 meters a pixel. After cleaning it up, it was released to the public on January 25, 2019. It was taken just 7 minutes prior to the closest approach by New Horizons, at 6,700 kilometers from Arrokoth’s surface. New details emerged in this picture, including some pits on the surface 700 meters wide and a circular region of the Thule portion (Gohd “New Horizons latest,” Lovett “Craters,” Eicher et al., Eicher “New”).

At the time, whether it was from an impact or some sort of internal process was unknown. Also unknown was the reason for the alternating light/dark regions seen, especially the white band around the point of contact. This seems likely from “fine-grained particles that have slumped downward by gravity.” Many of the smaller regions on the surface seemed to be in a linear line, indicating something internally collapsing rather than an impact string (Ibid).

8,862 km behind UlitmaThule

8,862 km behind UlitmaThule

Then in February 2019, some surprising data was downloaded and released. An image taken by the Long-Range Reconnaissance Imager from 8,862 kilometers behind Arrokoth and 10 minutes post-flyby indicate that the KBO is rather flat instead of spherical hemispheres. This is based on how the light from stars around the sides of the KBO was curving around the terminus (plus after compiling multiple images a 3-D picture began to emerge). Ultima is the flatter of the two and has some commonality with a few moons of Saturn (Berger, Lovett “Ultima: less Star Wars,” Stern “Exclusive,” Klesman, Stern “Ultima” 25-6).


Whether this can be a useful factor remains unknown, for those moons are a part of the formation of Saturn while Arrokoth was under much different circumstances. Because of that lack of cratering on Arrokoth, this shape of it is likely from its formational period and not a recent event. This would put it as a member of cold classical KBOs, relatively untouched since the solar system’s formation (Ibid).


Other clues as to its old age are the tholins on its surface or chains of red organic molecules caused by UV bombardment. Spectroscopy points to water and methanol being present. And those flat shapes seem to be relatively in line with one another, with only at most a 10-degree difference between them. Rather than a chance merging, it shows how the two objects were tidally locked as gravity drew them in together (Lovett “Ultima: an ancient,” Haynes “New data”).

Because of a lack of stress marks on either Ultima or Thule, scientists estimate the rate of the merger was at about 2 meters per second. They formed together and merged later. This points to a direct collapse model for the solar system formation, with gases and dust clumping together and slowly building up the planets, moon, and other objects in the solar system. This is in direct contrast with the violent collisions some models predict could have occurred (Ibid).

The possible formation of Arrokoth, with the components of Ultima and Thule.

The possible formation of Arrokoth, with the components of Ultima and Thule.

A Third Flyby?

The successes of New Horizons at the Pluto system and at Arrokoth got scientists excited and gearing for more. The amount of fuel and power left on the probe after its Arrokoth flyby guaranteed another 15-20 years’ worth of use, with 10 of those traversing the Kuiper Belt. Could another unprecedented flyby occur? It’s up to NASA because the resources and the drive are there. Only permission is required, and perhaps if it happens then we will get another shot at excellence, something that Pluto and Arrokoth had demonstrated without a doubt (Timmer “NASA”).

Works Cited

Berger, Eric. “New images of the distant Ultima Thule object have surprised scientists.” Conte Nast., 08 Feb. 2019. Web. 14 Mar. 2019.

Eicher, David J. “New Horizons swings past Ultima Thule.” Astronomy Jun. 2019. Print. 33-4.

---. “Ultima Thule emerges as contact binary, “cosmic snowman,” in new spacecraft images.” Kalmbach Publishing Co., 02 Jan. 2019. Web. 13 Mar. 2019.

Eicher, David J., and Chelsea Gohd, Jake Parks. “Ultima Thule Revealed as Contact Binary.” Astronomy. Apr. 2019. Print. 14.

Gohd, Chelsea. “New Horizons’ latest images from Ultima Thule reveal new details.” Kalmbach Publishing Co., 25 Jan. 2019. Web. 14 Mar. 2019.

---. “New Horizons survived its flyby of Ultima Thule.” Kalmbach Publishing Co., 01 Jan. 2019. Web. 13 Mar. 2019.

Haynes, Korey. “New data from New Horizons’ Arrokoth flyby hints at how planets formed.” Kalmbach Publishing Co., 13 Feb. 2020. Web. 21 Sept. 2020.

---. “New Horizons reveals Ultima Thule’s quiet, lonesome past.” Kalmbach Publishing Co., 17 May 2019. Web. 21 Sept. 2020.

Klesman, Allison. “Ultima Thule reveals its flatter shape.” Astronomy Jun. 2019. Print. 13.

Kohler, Susanna. "Insights from MU69's (Lack of) Craters." American Astronomical Society, 18 Feb. 2019. Web. 02 Dec. 2022.

Lovett, Richard A. “Craters, bulgy mounds and a collar.” Cosmos, 29 Jan. 2019. Web. 14 Mar. 2019.

---. “Ultima Thule latest: an ancient red worldlet, dancing lobe to lobe.” Cosmos, Mar. 2019. Web. 21 Sept. 2020.

---. “Ultima Thule latest: less Star Wars, more Star Trek.” Cosmos, 13 Feb. 2019. Web. 14 Mar. 2019.

NASA. "NASA Releases Record-Breaking Photos From Beyond Pluto." Kalmbach Publishing Co., 14 Feb. 2018. Web. 20 Mar. 2018.

---. "New Horizons selects Kuiper Belt target." Kalmbach Publishing Co., 31 Aug. 2015. Web. 09 Sept. 2015.

Parks, Jake. “New Horizons’ first close-up pictures of Ultima Thule reveal a binary world.” Kalmbach Publishing Co., 02 Jan. 2019. Web. 13 Mar. 2019.

Stern, Alan. “Exclusive: New Horizons’ mission head on new discoveries at Ultima Thule.” Kalmbach Publishing Co., 22 Feb. 2019. Web. 14 Mar. 2019.

---. “Ultima Thule Revealed.” Astronomy Aug. 2019. Print. 23-6.

Timmer, John. “NASA probe finds a snowman-shaped relic of the early Solar System.” Conte Nast., 02 Jan. 2019. Web. 14 Mar. 2019.

---. “New Horizons has a successful flyby of the Kuiper Belt’s bowling pin.” Conte Nast., 01 Jan. 2019. Web. 14 Mar. 2019.

Wenz, John. "New Horizons May Visit Twice the Object for the Same Price." Kalmbach Publishing Co., 04 Aug. 2017. Web. 27 Nov. 2017.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Leonard Kelley