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DRT

New Horizons zooming in...

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Posted (edited)

It’s a snowman....

My next door neighbour just asked me if my observatory scope could pick it up.  He seemed a bit disappointed when I said “no”...

Edited by Hallingskies

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1 hour ago, lenscap said:

Since the electric forces that hold atoms & molecules apart are many orders of magnitude stronger than gravity, say doubling the gravity is unlikely to make much difference to the shape or density  of the combined lump. Now if one piece was massively bigger than th other, different story.

That explains why the two big lumps remain separate, but not why the smaller lumps became round. It seems most unlikely they formed entirely of very small pieces.

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This BBC news article is a good overview https://www.bbc.co.uk/news/science-environment-46742298

It suggests that the two lobes probably came together during the formation of the solar system. I guess the only change has been painfully slow sublimation and irradiation of the surface.

Nothing much happens out there in the Kupier belt, at least not until some maniac drives through the neighbourhood at 17km/s :)

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Just to belabour the point, I raise the diagram is conjectural, but it shows quite large spheres combining to form the larger spheres Ultima and Thule, which themselves refuse to fully fuse under apparently identical conditions.

Clearly something changes, but I am struggle to really understand what this would be.

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The two lobes formed from much smaller objects - by randomly sticking together small objects you you will end up with a sphere-ish shape, so this is a different process from the hydrostatic equilibrium that causes much larger objects like the Earth to be round. To put it differently, gravity is not forcing the two lobes to be rounded, this is merely the statistically most likely shape. 

Eventually the two lobes became gravitationally bound and drifted together to form a contact binary. As the gravity of the total system is way to little to influence the shape of the object, the two lobes have not flowed together.

Another factor that is important is the rotation of the body. I imagine (but don't know) that the axis of rotation divides the bodies and they are also actively separated by centrifugal force.

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Posted (edited)

With reference to the live science update from the New Horizons team  here on Thursday 3rd, Chris Lintott (BBC Sky at Night) asked the same question regarding the fact that two spherical objects have come together but have not formed a larger spherical object. The response  is supplied by Mark Showalter (SETI Institute) at roughly 44:30 minutes. Interestingly he points towards the fact that there could be a moon or moons that have carried off the angular momentum of the two parts of Ultima Thule which enabled it to join together slowly. So far there hasn't been any moons spotted yet but he is waiting to investigate further images as and when they get downloaded.

Looking forward to The Sky at Night which is broadcast on 13th January BBC4 22:40 entitled `Beyond Pluto` and takes a look at the latest from the New Horizons mission. 

Edited by Phil Fargaze

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I believe that Brian Cox discusses the size objects need to be such that they will become spherical as a result of their own gravity in one of his documentaries (and briefly shows the maths that backs up the assertion).  I may be misremembering, but I think it's somewhere in the region of 250km in diameter; somewhat larger than Ultima Thule.

James

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Interesting,but the answer to Lintott's question dopesn't really address my point  - why is this final collision different?

On 05/01/2019 at 02:00, JamesF said:

I believe that Brian Cox discusses the size objects need to be such that they will become spherical as a result of their own gravity in one of his documentaries (and briefly shows the maths that backs up the assertion).  I may be misremembering, but I think it's somewhere in the region of 250km in diameter; somewhat larger than Ultima Thule.

In which case the question is 'why are the smaller parts spherical?'

It seems hugely unlikely for the two parts to assemble only from very small pieces, leaving two large bits to drift together.

To me, there has to be some sort of 'phase  change' e.g. was the system vastly more energetic before the collision, or was the material hot?

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I find myself wondering if the apparently unlikely formation of these two objects being joined into a larger one might well be due to them once being part of a larger group that robbed them of sufficient momentum that they could bump into each other with sufficient energy to melt some of the surface ice and stick together as it re-froze rather than drift apart again.

James

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32 minutes ago, Phil Fargaze said:

Amazing new evidence regarding the shape of Ultima Thule! Latest release

Wow - thanks for posting :thumbsup:

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That is incredible. I guess it helps address the question asked earlier about why two spherical objects did not become one ... they aren't (even vaguely) spherical themselves.

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Yep, very interesting and clearly demonstrates the danger of single data point use and analysis!

Extra data, it’s not spherical, yep, makes far more sense.

Edited by Alan White

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9 hours ago, Phil Fargaze said:

Amazing new evidence regarding the shape of Ultima Thule! Latest release

huh, didn't see that one coming! That's quite a surreal gif of the flyby in the article aswell 😮 

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The NH headquarters have been a bit quiet since the `pancake and dented walnut` revelation. I suppose all is ok, they must be hanging around because of that slow download. 

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