Ultima Thule is Weirder than we Thought

2  Months

It’s been two months since New Horizon’s new year flyby of Ultima Thule, proper name (486958) 2014 MU69. Data from the flyby is still trickling in and being poured over by planetary scientist, but the results so far show Ultima Thule to both be a good fit for data taken from the ground, as well as much weirder than we first thought. In this post we’ll go over some of the preliminary findings of the New Horizions team all in one place.

Note: If you want to read more about New Horizons path to space, I highly recommend Alan Stern’s Chasing New Horizons

UltimaThule or Ultima Thule?

When the New Horizons team was searching for a target after the flyby of Pluto, they were looking really far out into our solar system. At those distances, the light reflected from the sun wouldn’t be enough to detect for most earth-based telescopes. Instead, the team turned to a different technique, stellar occultation. This is when light from a star is blocked by an intervening body. It’s like seeing the silhouette of a person against a bright sky. Only, because the stars are points of light, the following was all they could collect

Each white band was the starlight, and each gray break was where something had gotten between the star we were looking at and us. Overlayed are two spheres’s that would mostly explain the shape causing light being blocked out, but we weren’t sure if Ultima Thule was a single oblong cigar-shaped body, two sphere’s touching like in the visual, or something different like a cloud of debris.

When we got there, we finally got our answer. It was a contact binary, where two bodies were stacked on top of each other like a snowman

Note: how well did the occultations do? This is the overlay of Ultima Thule over those original occultations.

Surprise # 1: Light Curve

This first surprise actually came before the flyby. As New Horizons was approaching Ultima Thule (MU69), it noticed that MU69’s light-curve wasn’t changing with time. If MU69 was a contact binary, we would expect it to be spinning. There were a few main explanations for why we weren’t seeing any spinning

• MU69 was not spinning (unlikely because of how contact binaries form)
• MU69 axis of rotation was almost perfectly aligned with New Horizons
• A cloud of debris was obscuring the light curve

It turned out to be the second option, that New Horizons was almost perfectly in line with MU69’s axis of rotation!

Here’s what it looked like in action as New Horizons got closer

How Do contact Binaries Form?

You’ve heard me call Ultima Thule a contact binary, but what is a contact binary? It’s when two bodies end up colliding at a slow enough speed that instead of one, or both, being destroyed into smaller particles, they just stick together from gravitational attraction. I’ve placed one of the slides from NASA’s New Horizons press conference below because it provides a good visual.

Surprise #2: What a Shiny Neck?

While I have been using black and white photos to show Ultima Thule in this post it’s not actually black and white, its red-ish. This is quite a common color in the Kuiper belt and outer solar system. If you remember the photos of Pluto from the first New Horizons post, it has the same reddish color. Surprise number two was not the color of the body, but of its neck. It’s whiter, and therefore more reflective than we expected.

Why is it so shiny? I don’t believe we have an answer to that yet, but the neck region does correspond to the area with the greatest slope, meaning it might have to do with how material “rolls” down the slope.

Surprise #3: It’s a Pancake, not a Snowman

In this post, I have referred to Ultima Thule’s snowman-esque shape. NASA even referred to Ultima Thule as a snowman in their press conferences. When New Horizons finally flew behind Ultima Thule, it found that it’s shape was not of a snowman with spheres on top of each other, but instead, two pancakes stacked on their edges as in the figure below. What especially blows my mind about this configuration is how unstable it looks.

Is this all we’ll find out?

It’s expected to take about 18 months for the rest of the data from the flyby to be transmitted back to Earth. This treasure trove of new information is bound to bring up more surprises about Ultima Thule than just the ones we covered here. Additionally, New Horizons will continue to observe other Kuiper Belt objects. Its location in the Kuiper belt provides it better imaging of Kuiper belt objects than Hubble and some on the team want to use New Horizons remaining fuel for additional flybys of other Kuiper belt objects. Knowing Alan Stern’s tenacity I wouldn’t bet against another flyby. 

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