As mentioned in the conference, what's exciting about this is that this is a "primitive" contact binary, whereas bodies like 67P/Churyumov–Gerasimenko and 25143 Itokawa - if they are contact binaries - are evolved. Ultima Thule could prove key to understanding such bodies - whether they actually are originated as contact binaries, and if so, how they evolved from there to their current forms.

I would rather refer to hollows, bumps and wrinkles on such a primitive body like this one, if I may say...

Toma - In the press conference they said 35 m/px will be the highest resolution , so 4x today's image.

I've wondered if some of the apparent bumps are remnants of previously accreted smaller bodies. But I realize that topography may require low sun angle images.

My first analysis highlighted three possible craters...

I suspect that already happens.
Just wait until somebody calculates the insulation curves for the entire orbit.

Given the angular momentum of a binary, it should act like a gyroscope as it goes around the orbit.
In simpler terms, to understand Utima Thule, you have to think about meat on a barbecue...

UT spends half of its time rotating and roasting horizontal to the sun like a chicken on a spit.


UT spends the other half of its time rotating and roasting vertical to the sun like a donner kabob


When UT is roasting "vertically" the cryo-ice 'drippings' accumulate at the neck.
I suspec they exert quite a torque when they warm up one-quarter of an orbit later.

That is a very good point. Although the closure rate would be very low as pointed out in the presentation, if there was any residual rotation rate in one or both bodies then one could imagine an extended period of contacts and transfer of angular momentum until the two bodies settled down.

I haven't seen any explanation of how angular momentum is lost, allowing a co-rotating pair of objects to spiral together and touch.

I've wondered this too: once both bodies are tidally locked to one another (as Pluto and Charon were very quickly after formation), wouldn't they remain stable, barring a close encounter or collision with an interloper?

Eh, Sunlight? (Carl Sagan voice) Billions and billions of photons...over quadrillions and quadrillions of time intervals...

The side spinning towards the sun is blue-shifted, the side spinning away is red shifted.
More energy and thus more force, is experienced by the side hit by blue-shifted photons.

They said some material would be ejected resulting in the slow down.

I wonder how much of it is water ice. It looks like a dirty snowball - or rather a dirty snowman.

Yes, I saw those slides. But I took them to mean that first, the little bodies were ejected from the area, leaving the largest 2 circling each other. Then they spiraled down and touched. But how? Sunlight is mighty weak out there for photonic pressure to be the driving force.

Here are 3D images based on the highest resolution image and newly published images on the New Horizons "raw" images page.
My interpretation is that Ultima Thule is similar to comet 67P even at smaller than global scale with blocky regions, smoother plains and depressions.
Some of depressions could be of impact origin but I don't think that lower resolution images allows proper investigation.

Cross-eye version:


Anaglyph version:



Animated gif:



Credit: NASA / JHUAPL / SwRI / Daniel Machacek

Nice job - the denoising does help for perceiving depth. It still looks like the small ball is roughly spherical, while the larger ("Ultima") has a more flattish surface facing us.