<p>This graphic in Wired.com's <a href="https://www.wired.com/story/new-horizons-first-photos-ultima-thule/" rel="nofollow noreferrer">Finally, New Horizons' First Photos of Ultima Thule</a> suggest the contact process could have been extremely slow. Rather than two bodies on independent but similar heliocentric orbits approach slowly, touch, and stick, it proposes that they were gravitationally bound and very slowly approached each other as angular momentum was lost somehow.</p>

<p><a href="https://i.sstatic.net/LVLUg.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/LVLUg.jpg" alt="enter image description here"></a></p>

<blockquote>
  <p>NASA/JHUAPL/SWRI/JAMES TUTTLE KEANE</p>
</blockquote>

<p>There are no fast processes for two bound bodies so far from the Sun to loose angular momentum quickly, so it would have had to be very slow. </p>

<p><a href="https://astrobob.areavoices.com/2019/01/02/ultima-thule-more-clearly-now-two-spheres-touching/" rel="nofollow noreferrer">Astro Bob says</a></p>

<blockquote>
  <p>In this step-by-step view we see how Ultima Thule probably formed when smaller chunks stuck together to form two objects. The twin-lobed structure of Ultima Thule vindicates accepted theory — called the accretion process — of how the solar system built itself up. NASA / JHUAPL / SwRI</p>
  
  <p>The reason the two objects still appear so neatly intact is that they approached each other at about the same speed you back into a parking spot — 1 to 2 miles an hour — and slowly fused together. Take a look again at color photo and focus on the “neck” of the asteroid. The rest of Ultima Thule is reddish-brown, probably from methane or other organic compounds exposed to sunlight, but the neck is pale, almost white and indicates lots of fine-grained material. During today’s press conference, mission scientists suggested it could be material that rolled downhill from the steep slopes where the two bodies make contact. Much more detailed photos are on the way along with compositional measurements that will help answer that question in more detail.</p>
  
  <p>Ultima Thule formed at the beginning of it all from small, icy chunks that coalesced into two planetesimals. The two spiraled into toward one another and joined to form what astronomers call a “contact binary.” If you want to know what things looked like shortly after the first solid materials coalesced from the solar nebula, Ultima Thule appears to be as close as we’ve come so far. Like taking a ride back in a time machine.</p>
</blockquote>

<p><a href="https://i.sstatic.net/hov6w.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/hov6w.jpg" alt="enter image description here"></a></p>

<blockquote>
  <p>Here you can see the difference in the neck area vs. the rest of Ultima Thule. NASA / JHUAPL / SwRI</p>
</blockquote>

<hr>

<p><a href="https://i.sstatic.net/6whcU.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/6whcU.jpg" alt="enter image description here"></a></p>

<p>Let's calculate the gravitational attraction between a 10 km and a 20 km diameter snowball. </p>

<p>I'll assume the density is 1. The could have both voids (many "cosmic snowballs" do) which would lower it, but rocks as well which would raise it, but this is just an order of magnitude calculation.</p>

<p>The distance between centers is the sum of the radii, or 15,000 meters, and we can approximate them as spheres, then use Newton's <a href="https://en.wikipedia.org/wiki/Shell_theorem" rel="nofollow noreferrer">shell theorem</a> to treat each one as a point at their center. </p>

<p><span class="math-container">$$ F=\frac{G m_1 m_2}{r^2}$$</span></p>

<p><span class="math-container">$$m = \rho \frac{4 \pi}{3} r^3$$</span></p>

<p>With rho of 1 g/cm^3 or 1000 kg/m^3 I get m1, m2 = 5.2E+11, 4.2E+12 kilograms, the force pushing each body into the other (using G=6.7E-11 N m^2 kg^-2) is about 650,000 Newtons, or <strong>roughly 66 tons</strong>. That's 2/3 of the thrust of a <a href="https://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)#Merlin_1D" rel="nofollow noreferrer">Merlin-1D engine</a> (~900 kN) on each asteroid pushing it towards the other 24/7!</p>

<p>After a while, the touching similar ices will start to fuse, adding some mechanical strength as well. That's over a long time, but things this far out have plenty of undisturbed time on their hands. </p>

<p><a href="https://i.sstatic.net/ZCnDC.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ZCnDC.jpg" alt="Merlin 1D engine"></a></p>

<p><a href="https://en.wikipedia.org/wiki/File:SpaceX_Testing_Merlin_1D_Engine_In_Texas.jpg" rel="nofollow noreferrer">Source</a></p>

CC BY-SA 4.0
2019-01-04T01:55:37.493Z
Something I don't get is how they glued together ? Obviously not by a fusion that a hit could have caused nor some paper glue, if you have a clue that'd be great !
aybe
CC BY-SA 4.0
2019-01-04T02:00:27.410Z
@Abye IIRC this was discussed in the 2019-01-02 press conference.  I recall them saying that the impact speed was below escape velocity so gravity alone was enough to hold the two lobes together.  I'll see if I can track down a reference for that.
Alex Hajnal
CC BY-SA 4.0
2019-01-04T02:17:03.967Z
@Aybe I've added some information and a simple calculation.
uhoh
CC BY-SA 4.0
2019-01-04T02:31:35.260Z
@Aybe The bit from the press conference talking about approach speed [starts here](https://youtu.be/5ZnFiN1cXS0?t=626) (specific bit is [~80 seconds later](https://youtu.be/5ZnFiN1cXS0?t=708)).  The speed of collision is given as "one or a few miles per hour".  [This source](https://news.ycombinator.com/item?id=18811511&goto=item%3Fid%3D18809772) computes UT escape velocity as approximately 16 miles per hour.  The bit discussing how strongly bound the lobes are [starts here](https://youtu.be/5ZnFiN1cXS0?t=971).
Alex Hajnal
CC BY-SA 4.0
2019-01-04T02:36:45.413Z
@AlexHajnal your comments [1](https://astronomy.stackexchange.com/questions/28994/what-process-shaped-ultima-thule/29018?noredirect=1#comment51832_29018), [2](https://astronomy.stackexchange.com/questions/28994/what-process-shaped-ultima-thule/29018?noredirect=1#comment51835_29018) are longer, more informative, and better sourced than [your answer](https://astronomy.stackexchange.com/a/28997/7982) is!
uhoh
CC BY-SA 4.0
2019-01-04T02:45:01.320Z
@uhoh I read the question to be asking whether Ultima and Thule are believed to have been liquid or solid when they came together.
Alex Hajnal
CC BY-SA 4.0
2019-01-04T02:47:51.263Z
@AlexHajnal maybe just throw all those goodies in your answer where they will last longer, and more importantly, over the next few days many more readers will see them? Comments are considered temporary and can be deleted at any time, and people don't bother wading through comments.
uhoh
CC BY-SA 4.0
2019-01-04T01:49:55.113Z
2019-01-04T02:23:24.480Z
2019-01-04T02:23:24.480Z
uhoh
<p>Ultima Thule is a contact binary comprised of two formerly separate solid objects that gently<sup>1</sup> (probably &lt;5 km per hour) came together.  If the objects had been molten when they came together they would have formed a spherical object.  Since the lobes are distinct the objects must have been solid.</p>

<hr>

<p>As for "understand[ing] how these two elements merged together", Ultima and Thule are believed to be have been two objects that formed separately through accretion.  At some point the two (solid) objects collided at a very low velocity of "one or a few miles per hour"<sup>1</sup>.  Since the escape velocity at point of impact is <a href="https://news.ycombinator.com/item?id=18811511&amp;goto=item%3Fid%3D18809772" rel="nofollow noreferrer">about 16 miles per hour</a><sup>2</sup>, gravity alone would be sufficient to keep the two lobes together<sup>1</sup>.</p>

<p><sup>1</sup> This topic was discussed in the 2019-01-02 New Horizons press conference.
The presentation talking about formation mechanism <a href="https://youtu.be/5ZnFiN1cXS0?t=626" rel="nofollow noreferrer">starts here</a> (the portion discussing approach speed is <a href="https://youtu.be/5ZnFiN1cXS0?t=708" rel="nofollow noreferrer">~80 seconds later</a>).  A discussion of how strongly bound the lobes are <a href="https://youtu.be/5ZnFiN1cXS0?t=971" rel="nofollow noreferrer">starts here</a>.</p>

<p>A note about the video links above: The official JHU APL video linked to above is missing the first few minutes.  If the video is updated to include the complete press conference then the links above will point to the wrong times.  The first two links are to Jeff Moore's presentation and the third is him fielding a question from Bill Horowitz (CBS News).</p>

<p><sup>2</sup> To quote <a href="https://news.ycombinator.com/item?id=18811511&amp;goto=item%3Fid%3D18809772" rel="nofollow noreferrer">the source</a>:</p>

<blockquote>
  <p>Modeling it as two spheres, one 16 km across the other 12 km, with a density the same as water, ... [a]t a distance of 8 km from the center of mass, the escape velocity ... would be about ... 16 miles/hour.</p>
</blockquote>

CC BY-SA 4.0
2019-01-03T09:51:58.870Z
The "neck" is a low point in the local gravity, so loose material would tend to gather there. This may mean that the two lobes are even less strongly joined than they appear -- basically just touching one another, with the point of contact hidden under an accumuilation of dust, gravel and rocks (all possibly made of ices, rather than rock).
Steve Linton
CC BY-SA 4.0
2019-01-03T09:55:11.577Z
@SteveLinton That's certain possible.  Can you think of any ways that that hypothesis could be tested using the (yet-to-come) NH data? I can think of one but it's only statistical.
Alex Hajnal
CC BY-SA 4.0
2019-01-03T10:43:26.523Z
It may be possible to tell whether the surface material at the neck is more finely divided based on its brightness, spectrum, etc. I don't see much chance of working out exactly what's happening deep inside the neck, but I am not an astronomer.
Steve Linton
CC BY-SA 4.0
2019-01-03T16:38:24.423Z
Also, I note the brighter coloration of the material around the neck region, suggesting that there is a difference with material comprising the lobes themselves.
The_Sympathizer
CC BY-SA 4.0
2019-01-04T10:42:56.780Z
@SteveLinton The method I was thinking of is: Find the best-fit spheres for Ultima and Thule (ignoring the bits axial to and as large as the point of contact) where 50% of the actual surface is outside the sphere (more data is needed to do this with any accuracy).  Assume that the terrain at point-of-contact was typical and determine for each lobe what terrain profile would be needed to keep the average elevation that of its best-fit-sphere.  This should provide a guess at what terrain might have existed at the point of contact (hill, valley, flat) and how much compaction may have occurred.
Alex Hajnal
CC BY-SA 4.0
2019-01-03T08:46:11.077Z
2019-01-04T06:18:22.067Z
2019-01-04T06:18:22.067Z
Alex Hajnal
<p>It was a slow collision between two lumps of ices, but at these temperatures, ice is a kind of rock. Energy would be released in such a collision which resulted in the partial fusion of the two bodies, but not enough energy to significantly disrupt either one. </p>

<p>The resulting body does not have sufficient mass for its gravity to pull it into a spherical shape, and hence after the two bodies had joined, they remained in the "snowman" shape.</p>

CC BY-SA 4.0
2019-01-04T20:10:56.953Z
There certainly looks to be no evidence of one rock rolling around on the other immediately after contact.
Wayfaring Stranger
CC BY-SA 4.0
2019-01-03T08:46:03.720Z
2019-01-03T08:46:03.720Z
James K
<p>When I see this picture, I'm trying to understand how these two elements merged together and have two hypothesis :</p>

<ul>
<li>two balls of lava in fusion together </li>
<li>two solid rocks fused at their contact point </li>
</ul>

<p>But it appears that the collision speed was <a href="http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20190102" rel="nofollow noreferrer">not particularly fast</a> so I'm not sure in the end. </p>

<p><img src="https://i.sstatic.net/rsDVW.png" alt="enter image description here"></p>

<p>What process likely formed Ultima Thule? </p>

CC BY-SA 4.0
2019-01-04T15:12:07.840Z
FWIW, Dr Brian May has [stereo pairs of Ultima Thule](https://brianmay.com/brian/brianssb/brianssbjan19a.html#03) on his site, in both wide eye & cross eye formats.
PM 2Ring
<planetary-formation>
What process shaped Ultima Thule?
aybe
# What process shaped Ultima Thule?

(Asked by: username_0 on 2019-01-03)

When I see this picture, I'm trying to understand how these two elements merged together and have two hypothesis :

* two balls of lava in fusion together
* two solid rocks fused at their contact point

But it appears that the collision speed was [not particularly fast](http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20190102) so I'm not sure in the end.

![enter image description here](https://i.sstatic.net/rsDVW.png)

What process likely formed Ultima Thule?

**Tags:** planetary-formation
> **Comments:**
> **username_1**: FWIW, Dr Brian May has [stereo pairs of Ultima Thule](https://brianmay.com/brian/brianssb/brianssbjan19a.html#03) on his site, in both wide eye & cross eye formats.  

---

## 3 answers

### Answer by username_2 on 2019-01-04. Score: 2 | Accepted answer

This graphic in Wired.com's [Finally, New Horizons' First Photos of Ultima Thule](https://www.wired.com/story/new-horizons-first-photos-ultima-thule/) suggest the contact process could have been extremely slow. Rather than two bodies on independent but similar heliocentric orbits approach slowly, touch, and stick, it proposes that they were gravitationally bound and very slowly approached each other as angular momentum was lost somehow.

[![enter image description here](https://i.sstatic.net/LVLUg.jpg)](https://i.sstatic.net/LVLUg.jpg)

> NASA/JHUAPL/SWRI/JAMES TUTTLE KEANE

There are no fast processes for two bound bodies so far from the Sun to loose angular momentum quickly, so it would have had to be very slow.

[Astro Bob says](https://astrobob.areavoices.com/2019/01/02/ultima-thule-more-clearly-now-two-spheres-touching/)

> In this step-by-step view we see how Ultima Thule probably formed when smaller chunks stuck together to form two objects. The twin-lobed structure of Ultima Thule vindicates accepted theory — called the accretion process — of how the solar system built itself up. NASA / JHUAPL / SwRI
>
> The reason the two objects still appear so neatly intact is that they approached each other at about the same speed you back into a parking spot — 1 to 2 miles an hour — and slowly fused together. Take a look again at color photo and focus on the “neck” of the asteroid. The rest of Ultima Thule is reddish-brown, probably from methane or other organic compounds exposed to sunlight, but the neck is pale, almost white and indicates lots of fine-grained material. During today’s press conference, mission scientists suggested it could be material that rolled downhill from the steep slopes where the two bodies make contact. Much more detailed photos are on the way along with compositional measurements that will help answer that question in more detail.
>
> Ultima Thule formed at the beginning of it all from small, icy chunks that coalesced into two planetesimals. The two spiraled into toward one another and joined to form what astronomers call a “contact binary.” If you want to know what things looked like shortly after the first solid materials coalesced from the solar nebula, Ultima Thule appears to be as close as we’ve come so far. Like taking a ride back in a time machine.

[![enter image description here](https://i.sstatic.net/hov6w.jpg)](https://i.sstatic.net/hov6w.jpg)

> Here you can see the difference in the neck area vs. the rest of Ultima Thule. NASA / JHUAPL / SwRI

---

[![enter image description here](https://i.sstatic.net/6whcU.jpg)](https://i.sstatic.net/6whcU.jpg)

Let's calculate the gravitational attraction between a 10 km and a 20 km diameter snowball.

I'll assume the density is 1. The could have both voids (many "cosmic snowballs" do) which would lower it, but rocks as well which would raise it, but this is just an order of magnitude calculation.

The distance between centers is the sum of the radii, or 15,000 meters, and we can approximate them as spheres, then use Newton's [shell theorem](https://en.wikipedia.org/wiki/Shell_theorem) to treat each one as a point at their center.

$$ F=\frac{G m\_1 m\_2}{r^2}$$

$$m = \rho \frac{4 \pi}{3} r^3$$

With rho of 1 g/cm^3 or 1000 kg/m^3 I get m1, m2 = 5.2E+11, 4.2E+12 kilograms, the force pushing each body into the other (using G=6.7E-11 N m^2 kg^-2) is about 650,000 Newtons, or **roughly 66 tons**. That's 2/3 of the thrust of a [Merlin-1D engine](https://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)#Merlin_1D) (~900 kN) on each asteroid pushing it towards the other 24/7!

After a while, the touching similar ices will start to fuse, adding some mechanical strength as well. That's over a long time, but things this far out have plenty of undisturbed time on their hands.

[![Merlin 1D engine](https://i.sstatic.net/ZCnDC.jpg)](https://i.sstatic.net/ZCnDC.jpg)

[Source](https://en.wikipedia.org/wiki/File:SpaceX_Testing_Merlin_1D_Engine_In_Texas.jpg)

> **Comments:**
> **username_0**: Something I don't get is how they glued together ? Obviously not by a fusion that a hit could have caused nor some paper glue, if you have a clue that'd be great !  
> **username_3**: @Abye IIRC this was discussed in the 2019-01-02 press conference.  I recall them saying that the impact speed was below escape velocity so gravity alone was enough to hold the two lobes together.  I'll see if I can track down a reference for that.  
> **username_2**: @Aybe I've added some information and a simple calculation.  
> **username_3**: @Aybe The bit from the press conference talking about approach speed [starts here](https://youtu.be/5ZnFiN1cXS0?t=626) (specific bit is [~80 seconds later](https://youtu.be/5ZnFiN1cXS0?t=708)).  The speed of collision is given as "one or a few miles per hour".  [This source](https://news.ycombinator.com/item?id=18811511&goto=item%3Fid%3D18809772) computes UT escape velocity as approximately 16 miles per hour.  The bit discussing how strongly bound the lobes are [starts here](https://youtu.be/5ZnFiN1cXS0?t=971).  
> **username_2**: @AlexHajnal your comments [1](https://astronomy.stackexchange.com/questions/28994/what-process-shaped-ultima-thule/29018?noredirect=1#comment51832_29018), [2](https://astronomy.stackexchange.com/questions/28994/what-process-shaped-ultima-thule/29018?noredirect=1#comment51835_29018) are longer, more informative, and better sourced than [your answer](https://astronomy.stackexchange.com/a/28997/7982) is!  
> **username_3**: @uhoh I read the question to be asking whether Ultima and Thule are believed to have been liquid or solid when they came together.  
> **username_2**: @AlexHajnal maybe just throw all those goodies in your answer where they will last longer, and more importantly, over the next few days many more readers will see them? Comments are considered temporary and can be deleted at any time, and people don't bother wading through comments.  

---

### Answer by username_3 on 2019-01-03. Score: 4

Ultima Thule is a contact binary comprised of two formerly separate solid objects that gently1 (probably <5 km per hour) came together. If the objects had been molten when they came together they would have formed a spherical object. Since the lobes are distinct the objects must have been solid.

---

As for "understand[ing] how these two elements merged together", Ultima and Thule are believed to be have been two objects that formed separately through accretion. At some point the two (solid) objects collided at a very low velocity of "one or a few miles per hour"1. Since the escape velocity at point of impact is [about 16 miles per hour](https://news.ycombinator.com/item?id=18811511&goto=item%3Fid%3D18809772)2, gravity alone would be sufficient to keep the two lobes together1.

1 This topic was discussed in the 2019-01-02 New Horizons press conference.
The presentation talking about formation mechanism [starts here](https://youtu.be/5ZnFiN1cXS0?t=626) (the portion discussing approach speed is [~80 seconds later](https://youtu.be/5ZnFiN1cXS0?t=708)). A discussion of how strongly bound the lobes are [starts here](https://youtu.be/5ZnFiN1cXS0?t=971).

A note about the video links above: The official JHU APL video linked to above is missing the first few minutes. If the video is updated to include the complete press conference then the links above will point to the wrong times. The first two links are to Jeff Moore's presentation and the third is him fielding a question from Bill Horowitz (CBS News).

2 To quote [the source](https://news.ycombinator.com/item?id=18811511&goto=item%3Fid%3D18809772):

> Modeling it as two spheres, one 16 km across the other 12 km, with a density the same as water, ... [a]t a distance of 8 km from the center of mass, the escape velocity ... would be about ... 16 miles/hour.

> **Comments:**
> **username_4**: The "neck" is a low point in the local gravity, so loose material would tend to gather there. This may mean that the two lobes are even less strongly joined than they appear -- basically just touching one another, with the point of contact hidden under an accumuilation of dust, gravel and rocks (all possibly made of ices, rather than rock).  
> **username_3**: @SteveLinton That's certain possible.  Can you think of any ways that that hypothesis could be tested using the (yet-to-come) NH data? I can think of one but it's only statistical.  
> **username_4**: It may be possible to tell whether the surface material at the neck is more finely divided based on its brightness, spectrum, etc. I don't see much chance of working out exactly what's happening deep inside the neck, but I am not an astronomer.  
> **username_5**: Also, I note the brighter coloration of the material around the neck region, suggesting that there is a difference with material comprising the lobes themselves.  
> **username_3**: @SteveLinton The method I was thinking of is: Find the best-fit spheres for Ultima and Thule (ignoring the bits axial to and as large as the point of contact) where 50% of the actual surface is outside the sphere (more data is needed to do this with any accuracy).  Assume that the terrain at point-of-contact was typical and determine for each lobe what terrain profile would be needed to keep the average elevation that of its best-fit-sphere.  This should provide a guess at what terrain might have existed at the point of contact (hill, valley, flat) and how much compaction may have occurred.  

---

### Answer by username_6 on 2019-01-03. Score: 3

It was a slow collision between two lumps of ices, but at these temperatures, ice is a kind of rock. Energy would be released in such a collision which resulted in the partial fusion of the two bodies, but not enough energy to significantly disrupt either one.

The resulting body does not have sufficient mass for its gravity to pull it into a spherical shape, and hence after the two bodies had joined, they remained in the "snowman" shape.

> **Comments:**
> **username_7**: There certainly looks to be no evidence of one rock rolling around on the other immediately after contact.  

---

