JoeSchmuckatelli

Do we even really need a similar size? Haven't we been using Delta 2s for Mars probes?

Dang - never knew that. Nor that they'd been thought about as g-force moderation. I thought it was only good for meeting aliens on the sea floor! There (also) goes my solution to the manned giant battle robot Commander's Module

Grin - politicians wanting the coolest new toys. @magnemoe - yes, our local infantry support / recon stuff is working and handy, and the Predators (et.al.) have been wildly successful, and the drone swarm stuff looks deadly and promising... TBH - I'm not surprised at industry or political interest in the technology... But that the professionals are not disuading the ground vehicle adventures is surprising. @magnemoe But as someone who has been in the 'keep mechanized fighting vehicles in the fight' game (read: service, supply, recover, repair and reissue taking up far more time than sending rounds downrange) - investment in heavy ground combat vehicles (at this time) is a fools errand. The Russians have always been good at putting simple hard to break easy to fix stuff in the hands of the troops - which isn't a bad idea. They're also really good at artillery and rapidly expanding their emw capabilities - you would think that their collective experience would ameliorate any interest in overly technical solutions to simple problems

I'm highly leery of the Western fascination with the use of drones / unmanned vehicles. Two things come out of Russia's adventures in Syria: their unmanned AFV experience was dismal and their ability to interfere with NATO signals is increasing. Absent the ability to have unrestricted use of the em spectrum for these drones - especially in peer to peer or peer to near-peer conflicts - the risks do not outweigh the benefits.

Theia is a proposal to explain the similarity of earth and lunar rocks... And from what I have seen it makes sense for it to cool into a tidally locked body I'm not familiar with capture and tear - but two questions: * if the moon was captured wouldn't it have retained some of the original pre-capture spin until the denser 'side' settled towards the earth? * why do we need a capture scenario at all? (whether impact or capture) - isn't it possible for two co-orbiting bodies to coalesce from same primordial dust with the larger retaining its angular momentum and spin while the smaller becomes tidally locked?

Once the bombardment slows down and the system is a little less chaotic, it becomes a bit easier to understand. I've always kind of thought that the Moon was the Earth's shieldmaiden - but it looks like the favor isn't returned; in fact it looks like the blows the earth slips can be 'lensed' into the moon Of course looks can be deceiving - so I ask folks smarter than I

Grin - hard to insult a guy who's painfully aware of his own limitations! But tbh - colorblindness isn't one of my many faults. So - back to the image; if you enlarge it, you can see approximately the same number of large yellow-shaded craters on both hemispheres... but quite a bit more small to mid-sized yellow craters on the near side. Certainly they're easier to spot against the basaltic plains (Mares) of the near side, but again it looks like there's 3x as many. *Note: Fully willing to be shown that my perception is inaccurate - which in itself would be an answer to my OP question!

Huh - my rough count showed a 3-1 difference.

Stumbled on this today: https://gizmodo.com/incredible-new-map-of-moon-shows-its-every-nook-and-cra-1843029458 Apparently the Copernican "from a billion years ago to today" Craters are all colored yellow in the images. The thing that's immediately interesting to me is that there are more Copernican craters on the face of the moon we see rather than the 'dark side.' Can anyone explain why this is? i.e. is there a process involved that makes it more likely for a meteor to strike the inner face of the moon than the far side?

Thanks for the link! (And LOL: "as a result, the APS is somewhat inaccurate when fired out of water" )

A nice technological solution to an extremely rare problem. It doesn't look like a great solution for what SF guys need to do once they're feet dry. Given the bullet length, they're getting at best a pistol-length barrel - so I'm guessing (don't speak RU) that the effective range of this is about 25m. Also, while I can't quite tell by the video, it looks like the leading edge is ridged; cavitation possibly? So again, a nice technological 'solution' to a problem that won't / doesn't come up often in the real world

Fibonacci Sequence Question: Is there something weird going on between the Zero and the second One? -- As you probably know, the Fibonacci Sequence starts out: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, etc. It's been so long since I was in school, and my kids are starting to get into this stuff... With a weak maths background, I don't have the answers for them; but my daughter asked me a question about the zero, one, one in the sequence. (Full disclosure: she's Freaking Brilliant - and I just want to try to stay abreast / encourage her - so this is either something brilliant, or something simple that I haven't grasped) Everything from the second 'one' makes sense (1+2=3, 2+3=5, etc.). But is there something funny going on in the first three steps? (My google-fu isn't finding anything, so I figured I'd ask here. Thanks in advance!)

Would the orbit of a captured satellite (planet) around a stellar mass black hole be any different from the orbit of a planet around a star of the same mass? i.e. if you were to magically replace our sun with a black hole of the same mass, would that change Earth's orbit in any meaningful way? * I used the phrase "captured planet" as I assume the formation of a black hole would destroy the original satellites of the star collapsing into a BH... but I guess the real question is what I typed into the 'i.e.' - wondering if things like the distance matter, etc. (I know that the sun's mass determines earth's orbital velocity at our current distance from the sun... just wondering if it matters whether the central mass is a star or BH) (shrug/head scratch)

Grin I love this stuff. Thanks for the long reply! I really appreciate people taking the time to write responses to my questions, and I learn something new every time. e.g. I've read about frame dragging, but never heard of the Kerr metric until today (now I have more reading ahead of me!). Back with more questions after I've had a chance to read more and scratch my head a bit!

https://www.goodreads.com/book/show/939740.The_Integral_Trees (still reading the replies but thought it might throw this out there) Interesting to see people thinking about the satellite orbiting the circumference of the cylinder... I was trying to visualize an orbit around the ends (going from end to end) of the cylinder - which I know would not happen naturally or be stable... More of a thought experiment than anything Have you seen Niven's story linked above?

The question: what would an orbit look like for a satellite around a non-spherical planetary or stellar mass object? i.e. if instead of a spherical planetary mass object - we had a cylindrical mass, the length of which was equal to the diameter of a planet or star? would every orbital path around such an object be elliptical? If a circular orbit were possible, how? Caveats: I know why objects over a certain mass tend to be spherical (c.f. hydrostatic equilibrium) I also know that with a spherical body we can / do think of it as a point mass, because regardless of where the satellite is in relation to the planet or star, a line drawn from the satellite through the center of mass will measure the same diameter of the sphere in any direction I suspect that we cannot think of a cylindrical object as a point mass, because if you drew a line from the orbiting object through the center of mass of the cylindrical object, you get different amounts of 'stuff' depending on where the satellite is in relation to the cylinder... ----- I thank anyone willing to chit-chat with me about this stuff; stuck at home with the kids and this kind of thinking is my 'brain-break' from 6th grade social studies discussions with my 12 y.o.! also, missing the 'good ol' days' when I could meet up with college friends and babble idly about science over a beer.

Grin - as I typed my response above, his words kept niggling the back of my mind; clearly I have a few days of thinking ahead to try to parse out why I've been so pigheaded about this! -- and thanks for the arc-second bit; I've read around that several times but never had it put so succinctly.

I'll try. Again, I'm acknowledging profound ignorance of the maths and trying to picture what is going on. FWIW - I'm a reader of physics and cosmology, but not a student or scholar... in fact 'frozen caveman lawyer' would be closer to the truth! My curiosity is driven by several seemingly inconsistent things I've read about (in no particular order): Photons are presumed to be massless, and thus unaffected by gravity; except we've photographs of gravitational lensing - used as one of many proofs of Einstein's theories, ergo gravity is an effect of mass curving spacetime - but here, where an outside observer might think the path of the photon is actually being turned... the answer is that from its frame of reference it's still going straight, but spacetime is curved near the lensing mass; thus no 'gravitational field' acted upon the photon The search for the Higgs (& etc.) is a quest to find a particle to help rectify the Standard Model with GR... but if they succeed, won't that mean accepting gravity has field like properties; and if so... how does a massless photon get turned by a mass? The folks who launched New Horizons past Pluto used Newtonian physics to perform this incredible feat - they did not have to 'get down into the weeds' with Einstein; meaning Newton's physics is a pretty damn good approximation - and while it doesn't demand that gravity be a field... it doesn't refute this possibility either. The precession of Mercury isn't well described using only Newtonian physics, but is described well by Einstein's theories... so at some point Newton's approximation isn't sufficient and Einstein's is better... but why? Our planet experiences seasons: the axial tilt keeps our Northern Hemisphere toward the sun during summer and away during winter... but why does it behave differently than a photon? I.e. if a fast moving photon's trajectory compared to the background stars can be bent by curved spacetime near a large enough mass... why doesn't the earth's path follow the same 'straight line' idea (I.e. moving in a straight line but through space that has been curved by the mass of the sun to keep it in an orbit... which in this case might keep the Northern Hemisphere always toward the sun rather than generally toward Polaris)… which, I get that the spin stabilization is an enormous factor; but that seems inconsistent (to me) with curved spacetime - I.e the planet is affected differently than the photon (read; it's not 'still traveling in a straight line through curved space) -- and this, to me makes more sense if gravity is a field; the spin stabilization keeping the axis aligned with the fixed stars 'just works' better referencing a field produced by the mass of the sun attracting the mass of the earth (or each, the other) than does curved spacetime Tidally locked bodies could be due to either a 1 rotation to 1 orbit, or because they're not uniformly balanced and thus the denser part will be closer to the mass it orbits (like a Weeble will wobble but not fall down)... but then a tidally locked body also looks like one behaving like a photon; from its frame of reference it's following a straight line, but the outside observer sees it as travelling a circular path defined by the mass it orbits warping spacetime. All of which probably helps you understand my confusion less than before I tried to explain. (Insert rueful grin) But it's like this; the coin dropped into that funnel at the zoo starts out in a straight line, but is then captured by the curve of the funnel, which spins it down to the donation bin. The coin, in effect, acts like the photon; its path would otherwise be straight except for the fact that the surface it is traveling on is curved - and the axis of its rotation is all over the place. IF that is a decent analogy to the curvature of spacetime defining the path of an object around another massive object... why isn't the earth's axis of rotation all over the place as well?

Well - that's kinda the crux, isn't it? Viewing as a point mass is good for the maths... But I'm not trying to plot it out mathematically... I'm trying to understand what the math is saying about how gravity works. And at the risk of exposing my great ignorance, I'll expand: If we take away all rotation or spin or any active control of the object / satellite wouldn't it behave differently if one of the current descriptors were correct? If gravity is best described as merely an effect of curved spacetime, the object should follow Fig 1... but if its a field should it not follow Fig 2?

I love this part of your response - thanks for the reminder! So - if we're looking at something big, or something fast - or both like the gravitational lensing of light, the better approximation is Einstein? That I should see his work as mathematicaly descriptive rather than physically descriptive? Some of my reading has led me to understand his argument about lensing is that the light is not curved by the gravity of the lensing object - but rather it travels in a straight line through spacetime that has been curved. Trying to grasp curved spacetime as a reason for orbits /gravity as opposed to a field (~gravity being akin to magnetism) makes sense when you consider the feather and hammer drop at the same rate on the moon... This in turn led me to ask if this is a true description of gravity - why isn't fig 1 curving space to keep the satellite facing the planet? I. E. The satellite isn't pulled by some gravitational field originating from the planet... But is rather trying to go straight in spacetime that has been curved by the planetary mass. But then I've also seen that orbits actually work like fig 2... And from what I can see - fig 2 is at odds with curved space but would work if gravity were a field. Which is why I struggle to balance the two concepts. (side note - if anyone can suggest some good books explaining this, I'd love to see them!) EDIT - I just finished Ron Cowen's Gravity's Century which is a great read... but it hasn't answered the question I'm struggling with

Thanks! So... this might sound weird, or be too esoteric for this forum... but why is the orbit behaving in a Newtonian fashion? (Feeds into a larger thing I'm struggling with: why we can use Newtonian physics to sling a probe past Pluto, but we need Einstein to describe the precession of Mercury) In other words, if gravity is merely a reflection of curved space-time, why isn't Fig 1 correct?

Presuming no atmospheric drag or active control / guidance / gyroscopes / spin, etc... how would a 'dead' satellite in orbit behave? Does either Fig 1 or Fig 2 show the correct way a dead or otherwise unguided, unstabilized object in orbit would behave? If neither... can someone explain what would really happen and why? (For now... I'm guessing the answer is Fig 2, and willing to be corrected / educated! )

Heh my edit crossed your post. I'll read more about this and return better informed. Again, thank you - finding this very informative

Hmmmm. Gonna have to think about this for a bit. So... Are we using Newtonian physics for even far away missions like New Horizons? I remember reading a Scientific American article years ago about utilizing the geodesic for ultra low energy transfers between planets... I figured that meant we were using Einstein's theories to figure out orbital mechanics - but what you write also jibes with something else I've read about Newton's work being accurate enough for everyday physics problems. (by which I thought the writer meant terrestrial ballistics - but not interplanetary space flight). So what makes Mercury a special circumstance? EDIT: wait... I remember reading something about the transit of mercury being a big issue: I'll start there. Thanks for the discussion /answers this far!

Hmmmm. Okay... I can accept that. The bold part of your response has my attention: Why does orbital mechanics have nothing to do with curved spacetime?