NASA's New Horizons made history when it flew by "Ultima Thule" on New Year's Day, but the science return is only beginning. Here's the latest from mission principal investigator Alan Stern.

After more than three years of continuous preparations, it’s hard for me to believe that NASA’s New Horizons flyby of Kuiper Belt object 2014 MU69, nicknamed Ultima Thule, is now history — but it is.

Flyby Success
Scientists on the New Horizons team celebrate an early high-resolution image of Ultima Thule that appeared on Earth late on flyby day, January 1st.
JHUAPL / SwRI / NASA / Henry Throop

The flyby past Ultima Thule was far more challenging than our 2015 New Horizons exploration of the Pluto system. The event culminated on January 1st in complete and utter success. Every planned observation, by every one of the scientific instruments aboard New Horizons, performed according to plan. The spacecraft did, too. Our navigators delivered us to a flyby so on target that we came within 1% of our closest approach aim point — 3,500 km from Ultima Thule — and we missed our precise arrival time by just 19 seconds. All this, after a nearly 3.5-year journey spanning a billion miles! The flyby simply could not have gone better.

Ultima & Thule
Once Ultima Thule was revealed to be bilobate, the New Horizons team nicknamed its two lobes “Ultima” (the larger lobe) and “Thule.”

The flyby’s immediate results were history making: the farthest exploration of any world in history, the first close-up examination of a Kuiper Belt object, and a smashing hit for public engagement in space exploration that spanned more than a thousand articles, five TV documentaries, and hundreds of front-page headlines, not to mention a dedicated ode to exploration called “New Horizons” by Queen front man Brian May.

Now, with new datasets arriving at Earth every week, the scientific returns are also beginning. We are already working on a first paper, which I am leading as the mission principal investigator, for the leading journal Science. And even before that, on January 11th, just 10 days after the flyby, the New Horizons team submitted 40 separate abstracts for talks chock full of observational and modeling results to be given at the Lunar and Planetary Science Conference in Houston this March.

Ultima Thule is providing an amazing glimpse into the primordial solar system, when planetesimals came together to form the planets. As summarized in my invited review abstract for the March meeting perhaps the most important initial finding is that Ultima Thule is the first contact binary object ever observed “in the wild,” where it formed, and in a largely unmodified state since its birth. The object consists of two similarly red, similarly reflective lobes measuring approximately 14 and 19 km across, respectively. Their similar nature points to their individual accretion in a swarm of like objects, followed by a gentle merger. Nothing like this has ever been seen up close and personal before.

Ultima Thule in color
Ultima Thule as seen in color by New Horizons during its flyby.
NASA / JHUAPL / SWRI / Roman Tkachenko

In addition to discovering that Ultima Thule is a contact binary, we also found that it apparently has no satellite, rings, or atmosphere. These weren’t big surprises but pieces of the larger puzzle of understanding this ancient and primitive body. Other puzzle pieces include distinct albedo (i.e., reflectivity) markings on the surfaces of Ultima and Thule, including both bright and dark markings, various kinds of quasi-linear albedo features, and a bright collar encircling the joint, or “neck”, at the interface between the two merged lobes. And strangely, the Ultima Thule binary is rotating much more slowly (with a period of about 16 hours) than would be expected from a simple merger event. Who ordered that?

At this writing, only about 1% of all the data that New Horizons collected on Ultima Thule and its environment have been radioed back to Earth. The entire downlink of these many precious observations will take until August or September of 2020. But some of the most important datasets, including the very highest resolution images and the first comprehensive compositional spectroscopy will reach Earth before the end of February.

As a result, many more scientific results will no doubt soon be forthcoming. As for New Horizons itself, even as it downlinks its treasure trove of data about Ultima Thule, it is already collecting new observations of the radiation and dust environment farther and farther out in the Kuiper Belt. It’s also taking new telescopic imagery to explore other objects, a project that will continue until the spring of 2021.

Next year, our mission team will propose a second, even deeper exploration of the Kuiper Belt to NASA. The prospects for that future mission of New Horizons became brighter in recent weeks with the success of the Ultima Thule flyby, the amazing results being obtained, and the news from our engineering team that the flyby left more propellant in the tank for future exploration than they had predicted!

This blog is part of a four-part series covering the Ultima Thule flyby. Click on the links below for the other installments:

Part One: New Horizons on Approach to the First Exploration of a Kuiper Belt Object
Part Two: Planning the New Horizons Exploration of Ultima Thule
Part Three: Ultima Thule is Dead Ahead


Image of stargate


January 23, 2019 at 10:18 pm

The photos certainly look like an impact pair but is it also possible that it is a single body that was caught in the act of fission or splitting apart? Perhaps a single larger object, heated during the condensation of the solar nebula from which it formed was also subjected to a transient non-uniform gravitational field which started the fission process? Later this "aborted binary" was frozen in as the environment cooled - could explain the similar compositions and lack of impact features!

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Image of Jim-Baughman


January 25, 2019 at 7:18 pm

The slower-than-expected rotation rate could be explained at least a couple of ways—first, that both “Ultima” and “Thule” were, in their lonely solo wanderings, rotating in opposite directions; thus if they combined in a slow collision most of this momentum was cancelled out. Sunlight also might have played a role. It does for asteroids closer to the sun, either speeding their rotation up or slowing it down based on even slight differences in albedo on opposite hemispheres. Of course at MU69’s vast distance from the sun this force would be weak, but since it has been operating for 4.5 billion years it might have had a measurable effect.

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