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Everything posted by sevenperforce

  1. Well so actually that’s one of the reasons the definition is what it is. We wanted to be able to decide whether exoplanets should be characterized as planets or not, and so the concept of “big enough to be round” and “has the right location and size to clear its orbit” were chosen because those things could be measured from far away. Planetary astronomers are divided to some degree between the study of body composition and the study of orbital mechanics and system evolution. The current definition largely reflects consideration of the latter before the former.
  2. The vertices are above you in absolute altitude but they are "below" you in the sense that they are beneath the plane tangent to the surface at your feet. So regardless of how it came to be, or if it once started out as a natural planet, once you make/magic this thing, it is not technically a planet anymore... Well, technically, this definition merely says "big enough" to have enough gravity to force it into a spherical shape. It doesn't have to be spherical.
  3. Stationkeeping for NRHO is minimal, and that's what the PPE is for. Stationkeeping for NRHO is the result of gravitational perturbations and nothing else. There's no atmosphere up there. In comparison, the ISS is so close to Earth that it is basically scraping the surface of the atmosphere. That's why it needs periodic reboosting.
  4. Then you'll hate how it looks from Earth's inertial reference frame. Funny that it makes a retrograde re-entry to Earth. I wonder why.
  5. That is the X-band high-gain steerable antenna used for most downlinks to transmit data back to Earth.
  6. This tweet evidently is only giving the distance of Apollo 13's record, not stating the distance which Orion will achieve.
  7. This part I'm gonna have to wrestle with a bit. Because what you're describing is a lot like what I've thought I understood about light. You use the word molecule above - and I figured a hydrogen atom in a gas (skipping over the plasma part for now) would behave a lot more like regular matter than something like a photon. Just as a rock can have a temperature, I figured the constituent atoms would have a fraction of that... Well, for one thing, it's correct to think of individual atoms behaving more like photons than like clumps of matter. That's particle-wave duality for you. It might seem counter-intuitive, but individual atoms zipping around freely in a gas can't have a "temperature" at all. They're just particles with some velocity relative to their surroundings. You can think of it in terms of information, if you want: if an atom could have a "temperature" then where would the information about its temperature be stored? It's literally just an atom. That's not quite right. An atom's kinetic energy comes from its velocity, which of course is relative to its surroundings. However, energy stored in an excitation state is intrinsic to the atom. The electrons of an atom have orbitals, discrete locations relative to the nucleus at which they can be located. When an atom absorbs energy (either from absorbing a photon of light or from an electromagnetic interaction with another atom during a collision), that "pushes" the electron up into a higher orbital. The electron will then "fall" back down to the ground-state orbit, releasing a photon with energy equivalent to the difference in potential energies between the two orbitals. A single hydrogen atom with a single electron has 6 different orbitals (n1 to n6). When the electron "falls" to n1 from n2, n3, or n4, it releases an ultraviolet photon, when it falls to n2 from n3, n4, n5, or n6 it releases a visible-light photon, and when it falls to n3 from n4, n5, or n6 it releases an infrared photon: So an atom can store potential energy in excited states, and it can release that energy in the form of photons, but that's distinct from concepts of "temperature" which depend only on relative velocity. Exactly right. Plasma isn't not-atoms. An oxygen plasma is made of oxygen atoms, a hydrogen plasma is made of hydrogen atoms, and a nitrogen plasma is made of nitrogen atoms. Plasma is still very much composed of atoms, they are just atoms which are missing one or more electrons. Remember hydrogen, up above? I showed what happens when you add energy to an atom up to the n6 orbital. However, if you give that electron enough energy, it will eventually break free of the atomic nucleus altogether, zipping off as a free electron. However, the static electrical charge of the electron and the atomic nucleus want to pull them back together, and so this new state of matter remains electrostatically neutral. Plasma is a state of matter in which at least some of the electrons which would normally be bound to their atomic nuclei have so much energy that they are bouncing around freely between atoms. There's one other issue, though: to be a plasma, you don't have to have complete electron disassociation. An oxygen atom may only lose a single atom and it would still be part of a plasma. And while @Terwin is correct that you couldn't have a "water plasma" because you would first need to break the covalent electron-bonds before stripping off any electrons, that's not true of all molecules. Diatomic nitrogen's triple bond is extremely strong, and so you can absolutely have a N*2 ion in which the covalent bonds remain but a valence electron has been stripped away. It should be noted that in a plasma-gas cloud that is 90 million degrees, you're going to have complete disassociation. Yep! All electrons are identical, so there's no difference if they get swapped around from atom to atom.
  8. And of course there's a website that lists all of this. The moon was at apogee two days before Orion launched, on November 14, with a distance (center to center) of 404,924 km. It will be at perigee this Saturday, November 26, with a distance of 362,826 km, and it will be back at apogee on December 12, with a distance of 405,869 km. Orion will be Distant Retrograde Orbit from November 25-30, so its "greatest distance" will be achieved with the moon closest to Earth, as shown by this mission track in the reference frame rotating with the moon (source is NASA): Here blue is the Earth and the green dot and line show the moon's location relative to Earth, moving between apogee and perigee. Orion only completes one half-orbit in DRO before performing the burn to take it back down to the moon, and NASA states here that the DRO is about 70,000 km away from the moon. So at its greatest distance from Earth, it will be 432,826 km from the center of the Earth or 426,448 km in absolute altitude. Apollo 13's record was 400,171 km in absolute altitude, set not because the distance from the moon was particularly great (although it was about 100 km greater than in past missions) but because the moon was near apogee during that flyby. So it would appear that Orion WILL break Apollo 13's record, not because of the Earth-Moon distance, but because of its greater distance from the moon.
  9. However, Snoopy was not intended to carry humans to the distance it achieved, whereas Orion was. "Orion will be entering a distant retrograde orbit beyond the moon, breaking the record set by Apollo 13 for the [greatest distance from Earth achieved] by a [pressurized spacecraft with sufficient internal volume to hold multiple humans and an outer mold line suitable for controlled atmospheric re-entry] at 248,655 miles from Earth." Although I wonder if even that is true. It seems to me that there's a pretty substantial difference between the distance to the Moon at perigee and at apogee, which might need to be taken into consideration. At perigee, the moon can be as close to Earth as 356,400 km (221,500 miles), while at apogee it can be as far as 406,700 km (252,700 miles). So if any of the Apollo missions took place close to apogee, their command module parking orbits may well have been farther from Earth's surface than Orion. It's also unclear whether the number provided by NASA above is actually a measurement of distance from Earth's surface or if it is based on distance from the center of the Earth.
  10. An object will only lose energy via cooling if it can transfer its thermal energy to something cooler than itself. It can transfer thermal energy kinetically, through elastic collisions, or it can do so electromagnetically through blackbody radiation. For intergalactic gases in a dense galaxy cluster, the gas cloud is doing both of these things. The reason we can see the gas clouds at all with the Chandra x-ray telescope is that the gas clouds are so hot that they're radiating in the x-ray spectrum. We are quite literally the electromagnetic heat sink for distant galaxies. If you had a dense hot gas cloud in otherwise-empty space, then the gas cloud would expand. If it was heavy enough to have meaningful self-gravitation, this expansion would also cool it, because the kinetic energy of its particles would be converted into gravitational potential energy.* But the gas between galaxies can't expand because it doesn't have anywhere to go; it is being squeezed together by the incredible inertia of multiple galaxies. *Counter-intuitively, expansion does NOT produce cooling unless the expansion performs some work. That work can be pushing a piston, displacing the gases in a lower-pressure environment, expanding through a de Laval nozzle to produce thrust, or in the case of a giant gas cloud, converting kinetic energy into gravitational potential energy. On the other hand, if I was to spacewalk outside of the ISS and pop a balloon filled with high-temperature, high-pressure gas, all those gas molecules would fly off in every direction with the same average velocity and thus maintain their "temperature" indefinitely (although they would of course rapidly be lost among all the other diffuse molecules in outer space). On the other hand, compression does always cause heating, because you cannot compress a gas without applying a force to it and thus performing work, which adds energy. Well, what's heating up the gas particles in this instance is the fact that they are being compressed by the inertia of the galaxies that are being dragged together by gravity. There are four different galaxy clusters (or subclusters) presently colliding with each other in this image. Subcluster B is smashing into subclusters A, C, and D at a speed of approximately 3,000 kilometers per second. That is the incredible pressure which squoze (as far as I'm concerned, that's the technical term) the gas between the galaxies to a temperature of 90 millionºC. Thus the gas clouds in the cluster are much hotter and thus much greater emitters of light than the stars around them. Here's where it gets a little complicated. Individual molecules can't "cool" by thermal radiation; their temperature is simply their velocity, and all velocities are relative, so they have no way of knowing how "fast" they are going in the first place. In addition, because energy is quantized, they wouldn't be able to "cool" gradually at all; atoms and molecules can only emit or absorb specific lights within specific wavelengths corresponding to their energy states. Thermal radiation, more properly known as blackbody radiation, occurs on the order of macroscopic objects comprising many particles such that there are many many energy levels and atomic transitions and so the radiation they emit follows a certain distribution. The gas cloud as a whole is of course emitting a bunch of blackbody radiation, as noted above, but that blackbody radiation is a distribution of photons produced by innumerable collisions between individual gas molecules and atoms. It should be noted that in this sort of a situation, there are no molecules to speak of. The temperature is far, far too high for covalent bonds; it's all a diffuse plasma. At lower temperatures, blackbody radiation can be produced by atomic transitions; two atoms smack into each other, raising the energy state of the electrons in both atoms, and as those electrons collapse back to their lowest energy, they release photons in frequencies corresponding to those different energy states. However, in this sort of extreme situation, the distribution of photons is dominated by the Bremsstrahlung process, where a charged particle moving at high speed loses energy and releases a photon when it passes through an electric field. The dominant luminous component in a galaxy cluster is thermal bremsstrahlung. No matter how fast it is going, a particle cannot produce bremmstrahlung radiation by itself. It has to interact with another particle. Considering that this region is producing thermal x-rays visible at a distance of 5.4 billion light years, the "temperature" of the cloud is not going to be the primary problem for your hypothetical spacecraft. The thermal radiation transferred to the spaceship would melt it six times faster than if it was at the center of the sun.
  11. So in a sense you're correct, but probably not for the reason you're thinking. The North Atlantic is not much above freezing, but a difference of even one degree spread through 146 cubic kilometers of ocean water is 1.46e17 joules: almost as much as the blast yield of the Tsar Bomba hydrogen bomb. So that's not quite right. Intergalactic gas clouds in a galaxy cluster are at an extraordinarily high temperature. They also contain an extraordinary amount of heat energy. However, that energy is not concentrated because the clouds are very diffuse; 1 cubic meter of space may only contain about 1000 particles. A cubic meter of air at the surface of Earth contains 2.46e25 particles. So the heat energy per unit volume is low, but the temperature is extremely high and the total heat energy is extremely high. Note that just because the density is very low does not mean that the "pressure" is low. Because of their high molecular velocity, the intergalactic gas clouds in a galaxy cluster exert quite a bit of pressure on anything they hit. Not much by everyday standards here on Earth, but still significant.
  12. Well, temperature and pressure are funny things. The temperature of a gas is really just a way of measuring the average kinetic energies of the molecules in the gas, and pressure is simply the average impulse per unit time produced by molecular collisions against a given internal area. Let's suppose the temperature in your room is 25°C. The individual air molecules you're breathing in and out right now are zipping around the room at around 500 m/s . . . some a little faster, some a little slower. For an average adult male, you're experiencing about 3.68e27 of these collisions every second. You feel these collisions and their kinetic energy as a combination of pressure and temperature. We could, however, increase the temperature in the room while decreasing the pressure. When we do that, you'll have fewer collisions (so you feel less pressure), but each collision will deposit more thermal energy in you. Because there will be fewer collisions, you'll gain heat much more slowly, but you'll definitely feel yourself getting hotter. Eventually, the oxygen partial pressure would be too low for you to maintain blood oxygenation, and you'd suffocate (and roast) simultaneously. Not fun. If I have a box of gas with an internal heat source (let's say a thermostatic electrical resistance heater) and I begin to add thermal energy to it, those molecules gain kinetic energy and begin zipping around the inside of the box faster and faster. As long as that box remains closed, the molecules will pick up more and more speed as I add more and more energy to the box. As those molecules bounce around the inside of the box, inelastic collisions with the box transfer some of their kinetic energy to thermal energy in increasing the temperature of the box. Eventually, the temperature of the outside of the box will reach the same temperature as the resistance heater, and the system will be in equilibrium.* Note that this process is independent of the AMOUNT of gas inside the box. It will still work whether I only have 1 milligram of air molecules inside the box (about 2e19 physical molecules) or if I've pumped 10 grams of air molecules inside the box (2e22 physical molecules). However, it won't work at the same rate. If there are more air molecules, there will be more frequent collisions, and so the thermal energy will propagate much faster. But you can see here, again, that temperature is independent of pressure. The box with just 1 milligram of air molecules may have the exact same temperature (average molecular kinetic energy) as the box with 10 grams of air molecules, but the latter box will have a much higher internal pressure (because, again, pressure is simply the average impulse per unit time produced on a given internal area of the container, and there are 1000 times as many air molecules in the latter box, resulting in 1000 times as much pressure). What happens if we open those boxes? Well, assuming that the temperature inside both boxes is much higher than the temperature of the surroundings, that means the velocity of the air molecules is much higher inside. Those air molecules will all immediately zip off in every direction and be replaced by the slower-moving air molecules in the room. Granted, those high-speed air molecules will still be in the room, at least until they bounce into other air molecules a couple billion times and lose speed, but because the temperature within a gas is based on the average molecular kinetic energy within a volume, the actual temperature of the room will only increase slightly. The gases between galaxies in dense galaxy clusters are very diffuse, but the molecules are moving extremely fast: on the order of 200 km/s or even higher. And while there's not much for them to bump into, everything else they're bumping into is ALSO moving at 200 km/s or higher, so it's not like they slow down. The gas cloud may be very diffuse, but it's still very, very hot. And inside a gravity cluster, local gravity keeps it from escaping, so. . .there you go. *Of course, it's not actually in equilibrium, because the box in turn will lose thermal energy to its surroundings. Even if the box is thermodynamically isolated from its surroundings, it will lose energy to blackbody radiation. But we're ignoring that for the time being.
  13. Certain metals like titanium, iron, beryllium, and zirconium can all burn hot enough (in the presence of a sufficiently aggressive oxidizer) to exceed 2500°C and reach the "blue-white" color temperature. And increasing the temperature isn't a problem; you're dumping enough exhaust gases that it's really quite impossible to melt your nozzle, if you're designing it right. But trying to get really really hot exhaust isn't really the point. You want your combustion chamber to be as hot and high-pressure as possible, but you want your throat and nozzle to convert that heat into exhaust velocity.
  14. I'm begging you, just learn the rocket equation. You will quickly be able to deduce many important things all on your own. Until then, please consult this handy guide. Guide To Fictional Spaceships. For a spaceship that can jaunt back and forth between the surfaces of different planets in a matter of days: You're going to need entirely new physics (hyperdrives, warp bubbles, etc.) at the very least. Whatever new physics your spaceship uses, it is going to obviate any need for ordinary engines, propellant tanks, or other features commonly seen on rockets. Your spaceship can be any shape you imagine and your engines can have any appearance you imagine, because you can make the rules for whatever new physics you want it to use. Such trite and unimportant details like "waste heat" and "propellant capacity" and "delta-v" are necessary only if they are important to your plot, because your new physics obviates the Carnot cycle and the rocket equation. Do you want a spaceship that can travel from the orbit of one planet to the orbit of another planet in a matter of weeks, or travel back and forth between the orbits of various planets on a single propellant tank? You're going to want a near-future high-energy low-thrust propulsion system like VASMIR or Mini-Mag or zeta-pinch, or a brute-force approach like orbital Orion or antimatter-thermal propulsion. Your spaceship will be restricted to space alone and its engine will probably produce radiation you'll have to deal with. Since you're stuck in space, radiators can handle waste heat, but they'll probably need to be quite large. It's possible that your spaceship can be constructed in-space with a single conventional rocket launch, but multiple launches will probably be necessary. Your propellant needs will be highly specialized, so you can't use ISRU, and your propellant will still probably make up half or more of your initial mass. Your engine exhaust nozzle will not look like a conventional engine nozzle. Do you want a spaceship that can go from the surface of Earth to the surface of another world, or that can take significant payload to LEO and return intact, all in a single stage? You're going to need a combination of chemical engines (for thrust) and nuclear thermal engines (for efficiency). Propellant will be 90% or more of your liftoff mass. If you're going to another world, the journey will use a Hohmann transfer, so it will take a long time. If you're going beyond LEO, you're certainly not coming back home without refueling somewhere. If your final destination has an atmosphere, you're going to have to figure out how to manage aerodynamic heating and maneuvering as well as descent and landing so that you can reach the ground intact. You won't have to worry about waste heat because you'll be dumping all the heat into your exhaust. Your exhaust nozzles will look reasonably normal. Do you want a spaceship that can fly around like an airplane in the atmosphere but can also be used as an orbital ferry? You're going to want some combination of jet engines, rocket-combined-cycle engines, chemical rocket engines, and/or nuclear thermal engines. Propellant will be at least 60-70% of your loaded weight, more if you're not using nuclear thermal propulsion. It will need to a full propellant refill, either on the surface or in-flight, any time it wants to go to orbit. You're not going to have margin for dual-axis thrust so it will either need to be a dedicated tailsitter or it will need to take off and land on a runway. Your exhaust nozzles will look reasonably normal, and waste heat is not a problem. Do you have some other set of requirements outside of what is discussed above? Learn the rocket equation and figure out what kind of performance you'll actually need, and go from there. Hopefully that settles it.
  15. Yes, because reusability is hard. Just emphasizing something here. The “bare dry mass” of the Shuttle system — the external tank and the engines — had a combined mass of ~36 tonnes. The orbiter, engines, and tank combined had an empty mass of 114 tonnes. So “reusability systems” for the Shuttle drove up weight to well over 3X the “bare dry mass” of the vehicle. Without even recovering the main tank.
  16. It's not proportional, though. The displacement LIGO detects is indeed infinitesimal, but that means that its effect on time dilation is correspondingly that many billions of billions of trillions of times smaller.
  17. There was no advantage to rotatable rear flaps on Starship in the first place, so the word "another" does not seem appropriate here. In which part of the descent envelope? The belly-flop, or the landing burn? During the belly-flop, the rear flaps are at maximum drag when they are fully deployed. Rotating them around the transverse axis of the vehicle would reduce their exposure to the airstream and thus reduce drag. During the landing burn, the rear flaps need to be as the lowest-drag position to allow the vehicle to pitch up and achieve a vertical landing. It would be like a drogue parachute in regards to ensuring that the vehicle slams straight into the ground nose-first like a lawn dart. I'm sure it will not be. Yes, because reusability is hard.
  18. By my math, Artemis 1 will be halfway to the moon in 12 minutes.
  19. So you know this analogy, right? We know why objects have inertial mass: it's a combination of relativistic mass from bond energies and invariant mass from the Higgs mechanism. That all fits very nicely within special relativity. And we know that bent space changes the way that objects move. The basic description of general relativity is quite concise: Space-time tells matter how to move; matter tells space-time how to curve. An object in a gravitational field isn't actually being acted on by an outside force; rather, it's simply following a straight line in curved space. That's not THAT counter-intuitive. What's a little more puzzling is WHY matter tells space-time how to curve. What is it about inertial mass which curves space? Why is it that space is always curved exactly the same amount for any specific amount of inertial mass? There's no apparent reason for this. You could imagine an object which has a lot of gravitational mass (curving space a lot) but little inertial mass, or an object which has a lot of inertial mass (high resistance to force) but very little gravitational mass, but that's never the case. Why not? What the OP has conjectured -- and what doesn't seem to be entirely impossible, as far as I can tell -- is that perhaps the answer lies back in special relativity. Special relativity tells us many things, but one thing it assures us of is that there is no way to tell the difference between acceleration of your reference frame and acceleration due to gravity. If you are in a box and you measure your local g to be 3.721 m/s2, you could be on the surface of Mars. But you could also be in a spaceship accelerating at 3.721 m/s2, or you could be on an elevator on Earth accelerating downward at 6.09 m/s2, or you could be on the inner surface of a 744-meter-wide ring rotating at approximately 1 rpm. You don't know, and there's literally no way for you to find out. Consider the rubber-sheet analogy above. The rubber sheet is a 2-dimensional analogue of 3-space. If a 2-dimensional sheet is placed in a gravitational field with a vector perpendicular to the plane, then mass will quite obviously stretch the 2-dimensional sheet into that third dimension, causing simulated gravity. But we also know that acceleration fields are source-invariant. So if you have a rubber sheet on a spaceship which is accelerating at 1 gee perpendicular to the plane of the sheet, then the same thing will happen. All we have to do, then, is add another spatial dimension. If 3-space is being accelerated through 4-space, then the resistance of inertial mass to acceleration will cause the 3-space to curve into the 4th dimension, causing gravity.
  20. This is the part that is wrong. Time dilation does happen as a result of a gravitational field, but this effect is very small and requires an extremely strong gravitational field to even be measurable. For example, you are 1/6 as heavy on the moon as you are on Earth, but time does not pass 6 times faster on the moon. So it is not a “precisely equal change” at all. The change in the rate of the passage of time associated by the small spacetime ripple that is a gravity wave is going to be incomprehensibly smaller than the ripple.
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