For Questions That Don't Merit Their Own Thread

[Wizard Kerbal]

What are hyperbolic escape trajectories?

[kerbiloid]

When your orbital speed exceeds the escape velocity (sqrt(2) * circular orbit velocity), you leave the orbit following a trajectory hyperbolic in shape, and aren't rotating around the same celestial body.

For the Earth the escape speed is ~11.2 km/s.

If your velocity is very high, the hyberbolic trajectory becomes an almost straight line.

[Dr. Kerbal]

Is the MEGATHREAD a MEGATHREAD yet? 

[Admiral Fluffy]

6 hours ago, Dr. Kerbal said:

Is the MEGATHREAD a MEGATHREAD yet? 

Yes.

What is the difference between an SSTO and a Spaceplane?

[kerbiloid]

SSTO can be a vertical rocket and use no wings.

[DDE]

Here we go again

https://earthsky.org/space/methane-on-enceladus-methanogens/

[kerbiloid]

The article picture says everything.

Spoiler

The ice and water proportions are a little different. The ice is at least 30..40 km thick.

[JoeSchmuckatelli]

6 hours ago, DDE said:

Here we go again

https://earthsky.org/space/methane-on-enceladus-methanogens/

Methane on Titan isn't 

[kerbiloid]

Titan is a cow.

[K^2]

On 7/7/2021 at 6:14 AM, JoeSchmuckatelli said:

What does the theory of cosmology that the universe is largely homogeneous at large scales get us? 

This recent article discusses it, but does not explain why it is a fundamental part of the accepted understanding of the distribution of matter 

https://www.google.com/amp/s/www.sciencenews.org/article/galaxy-giant-arc-3-billion-light-years-long-cosmology-space/amp

(confusing b/c when you read about things like the Lanakai Supercluster you get the impression that gravity does impart some sense of organization to the distribution of galaxies / matter, which seems inimical to the concept of homogeneous distribution) 

Edit: @K^2 are you willing to take a stab at helping me understand this? 

I'm not aware of any consequences to fundamental theory. We expect universe to be homogenous on large enough scale, but what exactly is "large enough" is not fixed by anything specific as this is an emergent behavior. I don't think we learn anything about fundamental properties of gravity from this. However, cosmological models usually assume that observable universe is homogenous. 3bly structure challenges this assumption. If that assumption is wrong, we can't rely on simple models of universe expansion, meaning their predictions might be wrong. Again, I don't think there are qualitative impacts, but we might be wrong about age of the universe by a few billion years, or something like that.

That's my read on this, but I'd definitely run it by somebody who has more expertise on cosmology to be sure. I could easily be missing something.

[JoeSchmuckatelli]

4 hours ago, K^2 said:

I'm not aware of any consequences to fundamental theory. We expect universe to be homogenous on large enough scale, but what exactly is "large enough" is not fixed by anything specific as this is an emergent behavior. I don't think we learn anything about fundamental properties of gravity from this. However, cosmological models usually assume that observable universe is homogenous. 3bly structure challenges this assumption. If that assumption is wrong, we can't rely on simple models of universe expansion, meaning their predictions might be wrong. Again, I don't think there are qualitative impacts, but we might be wrong about age of the universe by a few billion years, or something like that.

That's my read on this, but I'd definitely run it by somebody who has more expertise on cosmology to be sure. I could easily be missing something.

So - if 'large enough scale' is truly 'large enough' then structures like the Great Attractor, and Lanaikea etc become just part of the mix and homogeneity is assumed once more?  

(kind of like how a sponge is a sponge, but when you zoom in on it you see the foam of matter and trapped bubbles, but zoom out again and its just a sponge?  Is that a decent analogy?)

 

I find cosmology fascinating - but my education is spotty at best.  Can you recommend something a non-physicist might read to get a better understanding of why homogeneity is assumed and how it shapes our understanding of the universe?

[kerbiloid]

1 hour ago, JoeSchmuckatelli said:

if 'large enough scale' is truly 'large enough'

A Popper-happy criterion of the homogenous Universe scale:
"The scale of the Universe is large enough if and only if the Universe structure is homogenous at that scale."

A corollary:
"If you can see Laniakea, Great Attractor, Great Wall, or Dark Flow, your scale is not large enough, weak human bugs."

[JoeSchmuckatelli]

23 minutes ago, kerbiloid said:

The scale of the Universe is large enough if and only if the Universe structure is homogenous at that scale.

Won't lie: I'm struggling with this.  Hence my 'sponge' analogy.  One thought: if we zoom out far enough for homogeneity, and have a theory of gravity that works at that scale - are we losing something in understanding gravity at scales like the structures we see?  Can a general ' homogeneous' gravity theory actually predict what is going on (w/r/t evolution, speed and distance) at short to mid-range galaxies and clusters?  

 

I don't have an answer or even a quibble - but I do find it very interesting.

[kerbiloid]

53 minutes ago, JoeSchmuckatelli said:

Hence my 'sponge' analogy. 

Sometimes they call it hologram.

53 minutes ago, JoeSchmuckatelli said:

if we zoom out far enough for homogeneity, and have a theory of gravity that works at that scale - are we losing something in understanding gravity at scales like the structures we see?  Can a general ' homogeneous' gravity theory actually predict what is going on (w/r/t evolution, speed and distance) at short to mid-range galaxies and clusters?  

There are four known forces in the observable Universe (except the Yoda's one): gravitational, electromagnetic, weak, and strong interactions.

They believe that all four unite at some measurement scale, so the Universe stands on all four.
The electromagnetic with the weak, then the electroweak with the strong, and finally all of them should unite with the gravitational one.

So, basically they are the same iteraction at different scales of detalization.

But what about formulas?

Both electromagnetic and gravitational forces are described as a brutal and simple (m₁*m₂) / r².
There are quantum detalisation of that, but they are just trying to explain the resulting (m₁*m₂) / r².

Both weak and strong don't have even nearly such simple approximation, their calculation is anything but just (m₁*m₂) / r².

So, there is something rather incomplete in the United Force understanding, when the same interaction is sometimes a potential force (1/r²), sometimes not.

On the other hand, there is a voluntary mix between the physical i.e. empirical electromagnetism and gravitation and the mathematical potential force.

The only reason to think that the electromagnetism and the gravitation are 1/r² is that our observations show them being 1/r² at the available measurement accuracy, so the theory just arbitrarily declares them to be potential, because anyway there is no experimental data to expand the theoretical model beyond 1/r².

But they can actually be as complicated as weak, strong, and maybe some other cases of the united interactions, yet unknown, and work as potential forces only at our available scale of measurements.

Don't forget, the human direct experiments are limited with 100 AU distance and 100 years duration. That's by orders of magnitude less than the interstellar scale, let alone the intergalactic ones.
And even the galactic spinning requires a "dark matter" which "can't be observed in other way but gravitational", and at the same time is used to describe the gravity in terms of the existing gravity theory.
(Btw doesn't any material body emit heat (i.e photons) just from pure statistical physics, lol? A rather strange "matter" which doesn't).
I.e. the whole "dark matter" hypothesis looks like a typical 

Spoiler

self-pulling from the swamp by hair.

Popper should applause.

We can just compare what we can see in the telescopes at the scales comparable to ours, and ensure that at same scales the matter acts similarly.
The stars and planetary systems are rather same.
The galaxies are same but to the same degree unexplainable without a mystery "dark matter".
But it's a pure guessing, what happens at greater scales, between the galaxies. 

There is a "red shift", but it explains the observable from positions of the local physics, i.e. like a crystal sky theory was perfectly explaining what's above the flat earth.
There is an "expanding universe", but any infinite expansion is a perpetuum mobile, let alone what's the "expansion" of the thing which by definition includes both time and space.
It's expanding on what clock? External one? But the Universe by definition includes all possible clocks. So, if it's expanding, it's not a Universe, the Universe can be only static, because any dynamics can be measured only relative to some external coordinate axis.
So, the Universe is by definition a holographic (actually, just fractal) sponge, tiny parts of which are observable.

So, the current physical theories are enough good to describe local things, but are a Bacon & DaVinci epoch speculations even at galactic scale, and probably will be squashed into wet spot once the humans have reached the interstellar distances.

Edited July 9, 2021 by kerbiloid

[JoeSchmuckatelli]

Grin - yeah... I'm getting that.

Still - fun to read what the 'smart guy's best guesses' are - hence why I continually post Cosmology questions.  

[K^2]

9 hours ago, JoeSchmuckatelli said:

So - if 'large enough scale' is truly 'large enough' then structures like the Great Attractor, and Lanaikea etc become just part of the mix and homogeneity is assumed once more?  

Presumably. The problem is that if this relevant size is larger than observable universe, it doesn't help us any. It also makes the question of whether it really is homogenous on grand scale untestable. Technically, fractal fractal distribution of matter is still on the table, and that can have some interesting consequences.

[JoeSchmuckatelli]

38 minutes ago, K^2 said:

ractal distribution of matter is still on the table, and that can have some interesting consequences.

I've seen mention of that - but IIRC there is some pushback on the idea?

Edited July 9, 2021 by JoeSchmuckatelli

[K^2]

10 hours ago, JoeSchmuckatelli said:

I've seen mention of that - but IIRC there is some pushback on the idea?

Fractal universe would imply that average density is a function of distance over which you are averaging. If you plug something like that into GR equations, there are consequences to how the universe expands. There might be some observations that are more consistent with expected behavior of homogenous universe vs fractal universe. But this goes beyond my expertise. We're firmly in territory where you need a real cosmologist to clarify things.

The only thing I'm fairly certain about is that none of this puts fundamental physics into question. Just our understanding of structure and history of the universe. Which might be completely earth-shattering discovery to cosmologists, but for anyone in adjacent fields it's, like, "Oh, cool. Doesn't change any of the physics I have to worry about." I've left academia long ago, but that attitude probably won't ever go away. XD

[JoeSchmuckatelli]

3 hours ago, K^2 said:

Which might be completely earth-shattering discovery to cosmologists,

Yeah - some of my favorite arguments are between cosmologists - starting back when Phil Plait had the audacity to start a blog with a forum 

DM got a couple of people fairly heated 

Edited July 10, 2021 by JoeSchmuckatelli

[munlander1]

Every Mars rover I can think of used solid and inflexible metal wheels. Cars here on earth use inflatable rubber tires. 
I assume one of the reasons why metal wheels are used on the rover is because those don’t pop. Are there any other advantages to the wheels used?

Is there a difference in performance between a solid wheel and an inflatable wheel when traveling on Martian or Lunar regolith? What about earth sand?

[kerbiloid]

They are lightweight, and speed is low.

They don't need to jump and fall.

[cubinator]

Rubbers would not handle the extreme temperature shifts on Moon and Mars. They would deteriorate and fall apart. The rocks and sand on those planets are not smoothed as much by rain and wind, so every rock and piece of sand is extremely jagged and sharp. Moon dust is like broken glass. You need something that can drive over that kind of material without tearing itself apart on the first turn.

[K^2]

On 7/9/2021 at 6:31 PM, JoeSchmuckatelli said:

Yeah - some of my favorite arguments are between cosmologists

Their arguments certainly involve the broadest topics, subjects of matter, and you simply cannot deny the gravity of it all.

[JoeSchmuckatelli]

When Apollo came back from the Moon, there wasn't much up in space to hit.  We're planning new Moon missions, prospective return missions from Mars and other bodies, and nowadays, our sky is chock full of satellites - with SX launching constellations (and other competitors likely to follow).  

Given the concept of 'Big Sky, Little Bullet' theory* I assume that there's not much chance of a collision - but am I wrong?   

 

 

 

* In aviation, the Big Sky Theory is that two randomly flying bodies are very unlikely to collide, as the three-dimensional space is so large relative to the bodies.

[kerbiloid]

They spend billions to hit a sat in the sky intentionally.