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[wierd]

Well, to be specific, "heat" is "diffuse kinetic energy", as opposed to "Directed kinetic energy" (such as found in a net velocity, or inertial quanta of a group mass.)

Again, IR photon is not a form of this energy, it is a byproduct of energy release from matter in such states trying to find a lower ground state through photon emission, the emissivity curve of which is well defined.

Now, It could be that there is asymmetrical radiation of this energy if say, a planet is tidally locked with a star. The sun-ward side will be much more illuminated, and will thus emit more and higher energy black body photons than will the dark side. There will be mixing from convection currents that transports that energy to the dark side of the planet, but that typically would not be sufficient to completely resolve the asymmetry.

For a normally rotating body that is not tidally locked with its star, the rotation of the body will assure a more homogeneous distribution of thermal energy on the surface of the planet, assuring that blackbody emission does not contribute much to the object seeking a new orbit via emission.

That's my understanding anyway. 

[Maximum Spin]

Again, IR photon is not a form of this energy, it is a byproduct of energy release from matter in such states trying to find a lower ground state through photon emission, the emissivity curve of which is well defined.

Wait, were you trying to tell me that? I was wondering who you were explaining that to, but assumed it probably wasn't me because there was no reason in the world for you to think I didn't know that. If it was me you were trying to explain that to, yes, I am aware of that. That's why I explicitly separated "heat radiated as photons" from "heat energy" earlier.

Now, It could be that there is asymmetrical radiation of this energy if say, a planet is tidally locked with a star.

We're actually talking about heat generated due to collisions between particles within the system, not heat radiated in from the sun.

[LordBaal]

So we already discarded the planetary rodeo? What I'm gonna do with all the rope I just ordered you guys?!

[McTraveller]

Oh my... so many things....

A) The major reason Earth rotation speed is getting smaller every year "due to the tides" is not friction.  It's gravitational energy exchange with the moon. The tidal bulge gives the moon a slight tangential (with respect to its orbit around Earth) component to gravity, which kicks it into a slightly higher orbit.  It's on the order of single-digit centimeters per year. The reaction force to this slows earth's rotation.

B) Conservation of momentum always applies, even if kinetic energy is lost.  The bulk Earth does exchange some angular momentum with its atmosphere, but this is, in general, not something that causes meaningful slowing. It is a measurable effect, but sometimes it is Earth-slower, wind-faster and sometimes it is Earth-faster, wind-slower.  I think somewhere computed that global warming, with higher average wind speeds, will be something like a few milliseconds slower rotation per day per year kind of thing.

C) The Earth can "lose" angular momentum to space, if atmospheric molecules with high angular momentum also have escape velocity, because those particles will never be able to crash back to earth and then give the momentum back.  But this is a small effect.

D) If Venus is slowing by 24 seconds a year, it's due to gravitational effects or perhaps differences in the core and crust rotation, not because of atmospheric "drag".  Atmospheric wind speeds are not increasing that much per year, and Venus is probably not losing that much atmospheric mass per year (see C above).

E) A system of objects has two "components" to angular momentum: the angular momentum of the particles spinning about their own axes and rxp - the cross product of the displacement vector of the object from the system center of mass and the linear momentum of each object.  This is why a collision of two non-spinning objects in space generally ends up causing those objects to start spinning.

F) Asymmetric thermal radiation is an effect, but is really small.  We saw this on ... whatever probe that was a couple years ago.  But for a planet....yes it's a real effect, but decimal dust.  And in general unless it's an outer planet, the effect is going to be dwarfed by radiation pressure from the star anyway.

G) "To be specific"... at least in the academia from which I came, there is no such thing as "heat energy".  There is heat transfer (transfer of energy between two objects due to the difference in their temperature", but there is only kinetic and potential energy.  The energy that gives rise to temperature is kinetic energy.

Or maybe I'm misunderstanding what was meant by trying to distinguish between "heat generated due to collisions between particles within the system" and "heat radiated from the sun."  Internal collisions between particles in an object do not "generate" heat - they are actually conservative (unless you are talking about collisions that result in chemical or nuclear reactions, which convert potential energy into kinetic?).  The key is that energy transferred from an unbalanced external source (like the sun) might change the momentum of the center of mass of the object while the energy exchanges internal to the object cannot change the momentum of the center of mass of the object.

Spoiler (click to show/hide)

A neat - and not intuitive - exercise is to consider a system of two masses connected by an ideal spring.  Start initial conditions where both masses have zero velocity.  Apply an impulse to the system exactly in line with the spring.  Say both masses are mass 1kg (for a total mass of 2kg) and the impulse applied is 2 N-s.  This will give the velocity of the center of mass of the system 1 m/s.  BUT - what is the kinetic energy of the system?  Do you think it's only 1 joule?  You can show that it is at least one joule, but it might be much more!  It actually depends on how that impulse is applied - the only way to get 1J of kinetic energy is to apply the impulse equally and simultaneously to both masses.  ANY other application will require more than 1J input energy, and will result in oscillation between the two
masses as well as translation of the center of mass.

Also note: the velocity of the center of mass will be constant in all cases you pick an inertial reference frame - it won't oscillate!*

*I'm pretty sure on this one - I haven't checked. I think if you pick one of the masses, you might get an apparent oscillating CG velocity, but that's because the masses are an accelerating frame.

[Culise]

As McTraveller and Maximum Spin seem to have that side of the discussion well in hand, I'll simply touch on this quick. 

However, it's notable that Venus has an extremely slow rotational period, along with a really dense atmosphere. that took billions of years however. But not 10^18 years.

Major issue with that: Venus is *extremely* weird.  Not only does it have a rotational period of 243 days, its rotation is also retrograde.

Is its rotation truly retrograde? I would class it as 'insufficiently prograde', given its orbit.

Sidereally, does it not still spin as expected of most significant bodies in the system? (Uranus being the notable exception.) Just slower than the apparent observable back-rotation of the Sun around it? And Mercury seemingly has a stretch in its orbit where the slow 'absolute' rotation (3:2 ratio) is outpaced by the 'sun falling behind it'.

Or maybe I'm wrong, I have a spinning head trying to visualise the various levels of relative spinning from the various observers one might involve.

Unlike six of the eight planets, Venus rotates clockwise on its axis when viewed from above the ecliptic sorry, the solar system's invariant plane and hence is defined to move in a retrograde direction.  Given your mention of Mercury, you may be confusing apparent retrograde motion of its orbit as viewed from Earth with Venus' retrograde rotation about its own axis.  This apparent retrograde motion in its orbit is an artifact of the fact that the Earth itself is moving and thus vanishes when its orbit is viewed from a reference frame fixed with respect to the sun.  Its normal orbital motion around the Sun is irrelevant to its rotation on its own axis, as any observation of its orbit as viewed from Earth.  This is also true on Earth; the length of the Earth's day has no coupling to the length of its year, to the best of my limited knowledge.  By the bye, Uranus also has perfectly ordinary orbital motion, orbiting the Sun in a counterclockwise direction when viewed form above the ecliptic.  Again, its rotational motion is what is unusual, rotating about its own axis in a retrograde direction. 

Visualization can be helped by picking a single observation point at a time.  When observing a planet's rotation, sit above its north pole and watch it spin: counterclockwise is normal, clockwise is retrograde.  When you observe a planet's orbit around the sun, sit above the Sun's north pole and watch its revolutions around the Sun: counterclockwise is again normal, and clockwise is retrograde.  Disregarding matters like barycenters, we can decompose planetary motion into two parts relevant to this explanation.  Rotation is solely about the planet's own axis, while its orbital motion is about the external body it...err, orbits (the sun, in the case of all planets).  No known planets in the solar system exhibit retrograde motion in their orbits, even if their rotation is retrograde.  Retrograde orbits are solely the domain of less than a hundred of the several hundred-thousand known asteroids, not to mention comets. 

[Rose]

Then there's Mercury's apparent motion from the pov of earth, which sometimes reverses, which is also referred to as being in retrograde.I had a coworker that blamed any tech trouble he had on that.

[Starver]

Unlike six of the eight planets, Venus rotates clockwise on its axis when viewed from above the ecliptic sorry, the solar system's invariant plane and hence is defined to move in a retrograde direction.  Given your mention of Mercury, you may be confusing apparent retrograde motion of its orbit as viewed from Earth with Venus' retrograde rotation about its own axis.

No, I'm not mixing it up with Mercury (that kind of retrograde ('astrological' retrograde, shall we call it?) is also visible in Jupiter/etc), and my mention of Mercury was to link it to (venerio/mercurio)centric solar-retrgradiness, regardless of orbit-to-orbit observations that produce the parallax-induced illusions of counter-orbiting. Or whatever the actual terms may be. But, i.e., at perihelion, the close 3:2 relationship between Mercury's year and day is foiled by the planet swing past the Sun quicker than its steady(/ier) rotation and the Sun retrogrades (can rise back up from a sunset!) from a static Mercurial observer's POV.

After jotting the prior note, I spent a few moments entirely failing to confirm or deny my impressions that against the firmament Venus still spun the same way as its orbit, but slower.

The best I could do was confirmation that a Venusian year was 1.92 Venusian solar days. There's two solutions to that, before you go looking at actual figures (forward and backwards w.r.t. the tidally-locked spin-state), and (in my head) the one that matched the 240-odd Earth days (spin) vs 220-odd Earth days (orbit) was to be rolling around in the same direction. i.e., after 1 year, the Sun (by dint of being orbited) rotates around Venus once, and (by mental arithmetic) 330-odd degrees of spin-against-the-background happens, which seemed to me to give the slightly less than two effective (solar) days.

But I might have flipped a sign, somewhere.

By the bye, Uranus also has perfectly ordinary orbital motion, orbiting the Sun in a counterclockwise direction when viewed form above the ecliptic.  Again, its rotational motion is what is unusual, rotating about its own axis in a retrograde direction.

Not what I was getting at, with Uranus. The inclination is sideways (to the normal near-perpendicular-to-the-ecliptic angle seen elsewhere). Either slightly less than 90° and retrograde, or prograde but, because >90°, effectively retrograde. But, either way, totally swamped by the weird super-precessing/spiralling path the (standing-'on'-Neptune) observer would observe the Sun taking above their head.  Celestial mechanics would either be more obvious to a Uranian, or much harder to derive than the way we managed to reimagine the prior epicyclic attempts to model things.

That was a headache, as well.

[Teneb]

Then there's Mercury's apparent motion from the pov of earth, which sometimes reverses, which is also referred to as being in retrograde.I had a coworker that blamed any tech trouble he had on that.

As I have found in my time as tech support, technology is literally magic and anyone who claims otherwise hasn't worked as tech support. Your coworker is perfectly justified.

[Culise]

No, I'm not mixing it up with Mercury (that kind of retrograde ('astrological' retrograde, shall we call it?) is also visible in Jupiter/etc), and my mention of Mercury was to link it to (venerio/mercurio)centric solar-retrgradiness, regardless of orbit-to-orbit observations that produce the parallax-induced illusions of counter-orbiting. Or whatever the actual terms may be. But, i.e., at perihelion, the close 3:2 relationship between Mercury's year and day is foiled by the planet swing past the Sun quicker than its steady(/ier) rotation and the Sun retrogrades (can rise back up from a sunset!) from a static Mercurial observer's POV.

After jotting the prior note, I spent a few moments entirely failing to confirm or deny my impressions that against the firmament Venus still spun the same way as its orbit, but slower.

The best I could do was confirmation that a Venusian year was 1.92 Venusian solar days. There's two solutions to that, before you go looking at actual figures (forward and backwards w.r.t. the tidally-locked spin-state), and (in my head) the one that matched the 240-odd Earth days (spin) vs 220-odd Earth days (orbit) was to be rolling around in the same direction. i.e., after 1 year, the Sun (by dint of being orbited) rotates around Venus once, and (by mental arithmetic) 330-odd degrees of spin-against-the-background happens, which seemed to me to give the slightly less than two effective (solar) days.

But I might have flipped a sign, somewhere.

I think that may be possible.  For instance, I can take a guess at the particular error you made by comparing it to, say, the problem of Earth.  Now, you agree that our planet has both a prograde orbit and rotation, correct?  Given that the sidereal rotation period of Earth is 23.9345 hours and the sidereal orbital period is 365.256 days (8766.14 hours), this results in a rotational angular velocity of 0.262516 radians per hour and an orbital angular velocity of 0.000716756 radians per hour.  Now, if you just add these straight (since they're both prograde, after all), you get a total angular velocity of 0.263233 radians per hour, resulting in a total period of 23.86913 hours that very much does not match our actual solar day of 24 hours.  On the flip side, by instead subtracting the orbital angular velocity from the rotational angular velocity, you do get the correct number of 24.0000 hours in an Earth solar day once you trim down to the appropriate significant figures.

You may find this a bit more helpful as well; its derivation of a useful equation to this is much easier.  I'd walk through the derivation here step by step, but I'm not sure it's actually completely necessary when it's outlined more clearly there.  Even better, it doesn't rely on moving angular velocities through distinct rotating versus fixed reference frames, which was my first instinct and is what tangled me up as well the first time I tried to write this post. 

EDIT: Minor units glitch.

[Starver]

What would have been useful to cement this oft-given fact would have been a visual depiction, but (when I took time to go looking) it seemed that only Mercury deserved such illustration.

Finally found one "retrograde"-as-in-spin in amongst all the myriad* of "retrograde"-as-in-apparent-motion depictions. And nowhere does there seem to be unambiguated terms consistently coined like, say, retrorevolving vs retro-orbital (and most of the latter, Triton excepted, are actually more "retrotracking", anyway, for the aforementioned viewpoint-induced illusion of temporary backward movement).



* - maybe literally! There could possibly be 10,000 of the other type for every one of the one I wanted.

[Max™]

Vaguely relevant but from Uranus you could occasionally add your rotational velocity to the motion of the solar system I think: http://calgary.rasc.ca/howfast.htm

Also regarding the suggestion of heating up Mars to increase the pressure? That would work with a container of some sort, but atmospheres can and do expand and contract due to their day/night heating/cooling cycles already, simply adding more energy into the Martian atmosphere without raising the mass is going to result in more convective transport upwards which isn't going to help anyone trying to breathe something which already makes the wispy shit on top of Everest look like soup.

As for the rotation bit on Venus, the upper atmosphere is superrotating and the lower atmosphere is essentially motionless relative to the surface so the shiny upper layers are getting inflated several times a "day" and sinking back down on the other side multiple times each "night" which I'm sure is worth considering in the issue of atmospheric drag and whatnot.

[LordBaal]

What about the interstellar asteroid? Arachnids? Tyranids? The pools are open gentleman.

[smjjames]

Would have been cool to land on it and get samples, though it would have required seeing it long before we did and a ship capable of reaching it.

[martinuzz]

It's God's middle finger

[smjjames]

It'd probably be funny to name it that, but nope, it's name is basically Hawaiian for 'first visitor from the stars' (that we've recorded that is). Literal translation though is 'a messenger from afar arriving first'. https://www.axios.com/the-first-asteroid-seen-from-outside-earths-solar-system-takes-bizarre-shape-2511132598.html

It's also got this long spindle shape, which is pretty unusual for an asteroid. The image is an artist conception of it.