Starship Asterisk*

Chris Peterson wrote: ↑Mon Feb 11, 2019 11:02 pm

Ann wrote: ↑Mon Feb 11, 2019 10:56 pm

neufer wrote: ↑Mon Feb 11, 2019 8:00 pm
Victor Weisskopf (MIT) calculated from basic physics that the highest
land mountains on Earth (from their base) must be less than ~4.9 km

http://www.hk-phy.org/articles/mount_hi ... igh_e.html

The height of Mount Everest is ~8.8 km.

So according to basic physics, Mount Everest is like a bumblebee that shouldn't be able to fly, but it flies anyway?

Depending on how you define the base of Mount Everest, it's between 4.2 and 5.2 km from base to top. You get to 8.8 km by building Mount Everest on top of a basement that is well above sea level.

A better example of a mountain that seems to break the above calculation is Mauna Kea, which is about 10 km high. But more than half of that is below the ocean, which might be part of the explanation.

The mountains in the calculation are essentially free standing cones.

Actually the mountains in Weisskopf's calculation are free standing cylinders.

More realistically shaped mountains could be twice as high as Weisskopf's formula:

• H_Mars < 26 km as compared with Olympus Mons at 25 km.

There are many theories about how these bodies formed.

I take it back, having the spin axis at the point of connection would not minimize the tension at the connection, because tension would seem to be at a maximum at the axis. So perhaps we are indeed looking at an object which is pulling itself apart, or has frozen at a point in the process of pulling itself apart.

However, if we imagine a non-spherical object spinning in a dusty environment, if one end happens to have higher velocity than the other with respect to the axis of spin, that end will encounter more dust than the other, and if the dust sticks, the greater accumulation of mass at the more rapidly moving end would cause the axis of spin to migrate closer to the middle of the object.

The derivation by Weisskopf "only" purports to give limits within an order of magnitude. (At least that's how I read it.)

Nitpicker wrote: ↑Tue Feb 12, 2019 1:15 am
The derivation by Weisskopf "only" purports to give limits within an order of magnitude. (At least that's how I read it.)

It's certainly a much better estimate than "order of magnitude."

"Consider the problem in terms of energy to give a rough estimate on the maximum height that the mountains on earth can attain."

No sensible answer to my above Q.
I repeat; is UT a snapshot of a body coming together or flying apart?
J.

Interplanetary fidget spinner.

JohnD wrote: ↑Tue Feb 12, 2019 8:05 am No sensible answer to my above Q.
I repeat; is UT a snapshot of a body coming together or flying apart?
J.

Nobody knows. But if you extend the time frame back enough and forward enough, it would probably be true.

JohnD wrote: ↑Tue Feb 12, 2019 8:05 am No sensible answer to my above Q.
I repeat; is UT a snapshot of a body coming together or flying apart?
J.

Why either? It's almost certainly a body that formed from the collision between two bodies. But now? Whatever separating force it is experiencing from its rotation is balanced by self gravity and the material strength of the bond between them. It is a stable body. In a billion years it will be substantially unchanged.

JohnD wrote: ↑Tue Feb 12, 2019 8:05 am
I repeat; is UT a snapshot of a body coming together or flying apart?

UT a snapshot of two bodies that have:

• 1) come together by first flinging off "moons" to lose angular momentum [J]
  
  2) and currently are losing rotational energy [= J²/(2 I)] by bending at the
  neck region until the combined object achieves maximal moment of inertia I

In order to fly apart they must receive an impact from the outside
in order to regain both angular momentum & rotational energy.

neufer wrote: ↑Tue Feb 12, 2019 2:03 pm

JohnD wrote: ↑Tue Feb 12, 2019 8:05 am
I repeat; is UT a snapshot of a body coming together or flying apart?

UT a snapshot of two bodies that have:

• 1) come together by first flinging off "moons" to lose angular momentum [J]
  
  2) and currently are losing rotational energy [= J²/(2 I)] by bending at the
  neck region until the combined object achieves maximal moment of inertia I

In order to fly apart they must receive an impact from the outside
in order to regain both angular momentum & rotational energy.

A collision isn't the only possible way to alter the rotational angular momentum. The YORP effect can result in either spinning up or spinning down, and that's caused by nothing more than sunlight being absorbed on one side and reradiated on the other as it rotates. If this object were closer to the Sun, that effect might have significantly altered its rotation over a billion years.

In this case, the effects of YORP are extremely low (but not zero) given the size of the body, its distance from the Sun, and the fact that it's spin axis points generally toward the Sun. So practically, a collision is likely to be the only thing that matters over the next few billion years.

Chris wrote:...it's spin axis points generally toward the Sun.

But isn't that true for only a fraction of it's orbit? Wouldn't this object present it's thin sides as often as it's rotund aspects?

BDanielMayfield wrote: ↑Tue Feb 12, 2019 3:08 pm

Chris wrote:
...it's spin axis points generally toward the Sun.

But isn't that true for only a fraction of it's orbit? Wouldn't this object present it's thin sides as often as it's rotund aspects?

It's a gyroscope like the Earth pointing to fixed celestial spot
(and currently points... AWAY FROM THE SUN).

However, its southern hemisphere will enjoy a very long summer.

Chris Peterson wrote: ↑Tue Feb 12, 2019 2:14 pm

neufer wrote: ↑Tue Feb 12, 2019 2:03 pm
In order to fly apart they must receive an impact from the outside
in order to regain both angular momentum & rotational energy.

A collision isn't the only possible way to alter the rotational angular momentum. The YORP effect can result in either spinning up or spinning down, and that's caused by nothing more than sunlight being absorbed on one side and reradiated on the other as it rotates. If this object were closer to the Sun, that effect might have significantly altered its rotation over a billion years.

In this case, the effects of YORP are extremely low (but not zero) given the size of the body, its distance from the Sun, and the fact that it's spin axis points generally toward the Sun. So practically, a collision is likely to be the only thing that matters over the next few billion years.

"The total radiative repulsive force exerted on an 800 by 800 meter (reflective) solar sail is about 5 newtons at Earth's distance from the Sun."

The total (solar plus thermal) radiative repulsive force exerted on UT at 43.4 AU is about 2 newtons.

A small fraction of those 2 newtons is currently pushing on Thule's trailing leeside to spin UT up.

However, much of that spin up will probably be undone when UT reaches the other side of its orbit.

Chris Peterson wrote: ↑Mon Feb 11, 2019 2:12 pm

Odysseus wrote: ↑Mon Feb 11, 2019 2:05 pm Why does that seem like the wrong axis for the spin it's describing? Shouldn't the arrow be pointing left (or right) if UT is rolling "towards" us in the image?

What in the image makes you think it's rolling towards us?

I can only explain what I thought was happening with a cruddy GIF. So please excuse this cruddy GIF:

Odysseus wrote: ↑Tue Feb 12, 2019 9:22 pm

Chris Peterson wrote: ↑Mon Feb 11, 2019 2:12 pm
What in the image makes you think it's rolling towards us?

I can only explain what I thought was happening with a cruddy GIF. So please excuse this cruddy GIF:

https://www.etymonline.com/word/waffle#etymonline_v_4778 wrote:
WAFFLE (v.) 1690s, "to yelp, bark," frequentative of provincial waff "to yelp, to bark like a puppy" (1610); possibly of imitative origin. Figurative sense of "talk foolishly" (c. 1700) led to that of "vacillate, equivocate" (1803), originally a Scottish and northern English usage. Late 17c. Scottish also had waff "act of waving," variant of waft, which might have influenced the sense.

WAFFLE (n.) "kind of batter-cake, baked crisp in irons and served hot," 1744, from Dutch wafel "waffle," from Middle Dutch or Middle Low German wafel, from Proto-Germanic *wabila- "web, honeycomb" (source also of Old High German waba "honeycomb," German WABE), related to Old High German weban, Old English wefan "to weave". Sense of "honeycomb" is preserved in some combinations referring to a weave of cloth. Waffle iron is from 1794.

• -------------------------------------------
  `Twas brillig, and the slithy toves
  Did gyre and gimble in the WABE;
  All mimsy were the borogoves,
  And the mome raths outgrabe.

`That’s enough to begin with,’ Humpty Dumpty interrupted: `there are plenty of hard words there. “Brillig” means four o’clock in the afternoon — the time when you begin broiling things for dinner.’

`That’ll do very well,’ said Alice: and “slithy”?’

`Well, “slithy” means “lithe and slimy.” “Lithe” is the same as “active.” You see it’s like a portmanteau — there are two meanings packed up into one word.’

`I see it now,’ Alice remarked thoughtfully: `and what are “toves”?’

`Well, “toves” are something like badgers — they’re something like lizards — and they’re something like corkscrews.’

`They must be very curious looking creatures.’

`They are that,’ said Humpty Dumpty: `also they make their nests under sun-dials — also they live on cheese.’

`Andy what’s the “gyre” and to “gimble”?’

`To “gyre” is to go round and round like a gyroscope. To “gimble” is to make holes like a gimblet.’

`And “the WABE” is the grass-plot round a sun-dial, I suppose?’ said Alice, surprised at her own ingenuity.

`Of course it is. It’s called “WABE,” you know, because it goes a long WAY BEfore it, and a long WAY BEhind it — ‘

`And a long way beyond it on each side,’ Alice added.
-------------------------------------------------

neufer wrote: ↑Tue Feb 12, 2019 9:55 pm

Odysseus wrote: ↑Tue Feb 12, 2019 9:22 pm

Chris Peterson wrote: ↑Mon Feb 11, 2019 2:12 pm
What in the image makes you think it's rolling towards us?

I can only explain what I thought was happening with a cruddy GIF. So please excuse this cruddy GIF:

Click to play embedded YouTube video.

I love me some Parker Squares Circles!

Odysseus wrote: ↑Tue Feb 12, 2019 9:22 pm

Chris Peterson wrote: ↑Mon Feb 11, 2019 2:12 pm

Odysseus wrote: ↑Mon Feb 11, 2019 2:05 pm Why does that seem like the wrong axis for the spin it's describing? Shouldn't the arrow be pointing left (or right) if UT is rolling "towards" us in the image?

What in the image makes you think it's rolling towards us?

I can only explain what I thought was happening with a cruddy GIF. So please excuse this cruddy GIF:

The object is rotating around the spin axis like a propeller. The change in viewpoint is the product of the probe moving rapidly past it.

The shape model diagram here is nice. But I'd like to see an annotated picture of the raw image(s) that brought about this surprising shape conclusion.

There are currently 157 images posted at: http://pluto.jhuapl.edu/soc/UltimaThule-Encounter/

There is a "crescent phase" shot that does make one edge of the UltimaThule aggregate appear more like a pair of coins than a pair of marbles, but I am not clever enough to see any one of those images and realize that I am seeing a star that should have been occulted by a spherical Ultima or Thule lobe.

MarkBour wrote: ↑Wed Feb 13, 2019 6:00 pm The shape model diagram here is nice. But I'd like to see an annotated picture of the raw image(s) that brought about this surprising shape conclusion.

There are currently 157 images posted at: http://pluto.jhuapl.edu/soc/UltimaThule-Encounter/

There is a "crescent phase" shot that does make one edge of the UltimaThule aggregate appear more like a pair of coins than a pair of marbles, but I am not clever enough to see any one of those images and realize that I am seeing a star that should have been occulted by a spherical Ultima or Thule lobe.

Images of high enough quality to easily pull out such detail are most likely embargoed for about another year, in order to provide exclusivity to the researchers who are publishing papers on such subjects as the shape of the object.

Chris Peterson wrote: ↑Wed Feb 13, 2019 6:10 pm

MarkBour wrote: ↑Wed Feb 13, 2019 6:00 pm The shape model diagram here is nice. But I'd like to see an annotated picture of the raw image(s) that brought about this surprising shape conclusion.

There are currently 157 images posted at: http://pluto.jhuapl.edu/soc/UltimaThule-Encounter/

There is a "crescent phase" shot that does make one edge of the UltimaThule aggregate appear more like a pair of coins than a pair of marbles, but I am not clever enough to see any one of those images and realize that I am seeing a star that should have been occulted by a spherical Ultima or Thule lobe.

Images of high enough quality to easily pull out such detail are most likely embargoed for about another year, in order to provide exclusivity to the researchers who are publishing papers on such subjects as the shape of the object.

Wow, that's surprising.

A new image beamed back in the New Horizons data
shows the unexpected physical characteristics of
a typical resident of Ultima Thule:

MarkBour wrote: ↑Thu Feb 14, 2019 5:18 pm

Chris Peterson wrote: ↑Wed Feb 13, 2019 6:10 pm

MarkBour wrote: ↑Wed Feb 13, 2019 6:00 pm The shape model diagram here is nice. But I'd like to see an annotated picture of the raw image(s) that brought about this surprising shape conclusion.

There are currently 157 images posted at: http://pluto.jhuapl.edu/soc/UltimaThule-Encounter/

There is a "crescent phase" shot that does make one edge of the UltimaThule aggregate appear more like a pair of coins than a pair of marbles, but I am not clever enough to see any one of those images and realize that I am seeing a star that should have been occulted by a spherical Ultima or Thule lobe.

Images of high enough quality to easily pull out such detail are most likely embargoed for about another year, in order to provide exclusivity to the researchers who are publishing papers on such subjects as the shape of the object.

Wow, that's surprising.

Why? It's pretty common practice. Most Hubble images are embargoed. The idea is that the professionals who are part of the program get first crack at publishing. In many cases their grants are part of the funding.

Chris Peterson wrote: ↑Thu Feb 14, 2019 5:54 pm

MarkBour wrote: ↑Thu Feb 14, 2019 5:18 pm

Chris Peterson wrote: ↑Wed Feb 13, 2019 6:10 pm

Images of high enough quality to easily pull out such detail are most likely embargoed for about another year, in order to provide exclusivity to the researchers who are publishing papers on such subjects as the shape of the object.

Wow, that's surprising.

Why? It's pretty common practice. Most Hubble images are embargoed. The idea is that the professionals who are part of the program get first crack at publishing. In many cases their grants are part of the funding.

I guess just surprising to me. The more I think about it, the more I realize it should not have been surprising. You're quite right.