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Everything posted by sevenperforce

  1. I did not hear the term being used, at least not regularly, until SX started dumping multiple satellites per mission. I know that GPS and Iridium can be called 'constellations' - but my impression was that each of those were large satellites that were (effectively) individually placed by different rocket launches. So - is there a different term for that? Or does 'constellation' simply refer to a group of related satellites? The Iridium constellation launch campaign delivered between two and seven satellites to LEO per launch, depending on the launch vehicle capabilities. They used a variety of launch vehicles, including Delta II, Proton-K, and Chang Zheng 2C. Iridium's replacement constellation, Iridium-NEXT, began launching on Falcon 9 rockets in 2017, with approximately 10 satellites being delivered per launch. The Globalstar constellation started launching in 1998 and had about a dozen satellites go up in each launch. I was actually incorrect about the first GPS satellites; the original GPS launch was in 1978. Of course those particular satellites have long since been retired. The GPS sat launch campaign only sent up a single satellite per launch until the USA-66 mission in 1990, which launched two at the same time. Yeah, you could do this easily enough. A simple relay like Quequiao needs a mass of no more than half a ton with enough propellant to brake itself into a lunar orbit, and so a Falcon 9 could definitely send four of them to TLI with enough margin for first-stage recovery.
  2. A real object does have a real trajectory but you can have a real object with a real trajectory that is on an escape trajectory such that it will never again form a closed orbit relative to any object, due to the expansion of the universe.
  3. Constellations have been around for quite a while now. The GPS constellation was launched beginning in 1993 and the Iridium communications constellation was launched beginning in 1997. We haven't had much in the way of a need for a lunar relay constellation. All crewed landings took place on the near side of the moon, as well as all robotic landers up until Chang'e 4 (which had a dedicated communications link to Earth via a separately-launched relay, Queqiao, at L2). The orbiters we have (like the LRO) are out of communications with Earth half the time, but that's fine since they can just uplink whenever they come back around. You wouldn't need six smallsats; you can get away with three. That's because you're on the "Earth" side of the moon half the time anyway, so as long as one of the satellites has line-of-sight to a satellite that has line-of-sight to Earth, you're fine. One problem with constellations around the moon, at least in low lunar orbit, is the distribution of mascons that make it hard to maintain orbit. There are a handful of frozen orbits, though, or you could put the constellation in a more distant orbit. For Artemis II, the crew in Orion will be isolated from Earth briefly during the flyby, but not for very long. It's a free-return after all. Artemis III will place Orion in NRHO which has constant communications with Earth, and the landing will be on the south lunar pole with line-of-sight to Earth. In subsequent missions, Gateway will be in NRHO, and it will have relay capability if NASA wants to do landings on the far side of the moon.
  4. With the same success we can say that they are orbiting Crab Nebula, or any other reference point, because there is infinite number of open trajectories and reference points. The closed trajectory differs from them principally, as you can have only one unique reference point (letting alone the reference point own motion and orbital perturbations, let the Universe be considered static for this purpose). The eccentricity of an orbit can be greater than 1. As long as your trajectory is still along a path which can be defined as an orbit with a periapsis and an eccentricity, it's still an orbit. That condition remains until your object gets close enough to another massive body to cause its trajectory to deviate, at which point the periapsis and eccentricity relative to the original reference point are lost. There's no static "bulky mass of matter" in a globular cluster; everything is just orbiting the stars which happen to be closer to the center than they are at any given point. The same is true of galaxies. Our own cluster, the Local Group, has essentially nothing in the center; it's basically dumbbell-shaped, with the Milky Way and a number of smaller galaxies on one end and Andromeda and a number of smaller galaxies on the other end. At one level up from the Local Group you have the Virgo Supercluster, which includes the Local Group. The clusters in the Virgo Supercluster orbit the Virgo Cluster, which is the heaviest cluster in the Virgo Supercluster. Unlike the Local Group, the Virgo Cluster does have a single giant galaxy in the center, the supergiant elliptical galaxy Messier 87: The supermassive black hole at the center of Messier 87, M87*, produces the relativistic jet shooting out of the galactic core. It was the first black hole ever imaged by the Event Horizon Telescope. The black hole at the center of Messier 87 is probably the "most-orbited" discrete object that we orbit. Although the Virgo Supercluster is considered to be a lobe of the Laniakea Supercluster, the latter is no longer gravitationally bound and has already begun to disperse due to the dark-energy expansion of the universe. So we are not in any closed orbits above the level of the Virgo Supercluster (to answer @kerbiloid's question). The largest gravitationally-bound object in our local universe is the Shapley Supercluster, a collection of dozens of major galaxy clusters with a total mass more than 10,000 times that of the Local Group. We are close enough to it that its gravity is still tugging the Virgo Supercluster in its general direction, but not so strongly that it will be able to overcome dark energy expansion.
  5. Well, interstellar rogue objects are orbiting Sagittarius A* and the rest of the galactic core directly, but I don't think that answers your question. Objects which are ejected from their galaxy of origin are still technically orbiting that galaxy, just on the outgoing leg of a hyperbolic trajectory. Until they get close enough to something for their orbit to deviate so that they're now on a hyperbolic trajectory around THAT thing, this condition remains.
  6. How about "in orbit around one or more stars"?
  7. Could have been an early shutdown or maybe Elon just got it wrong or the plans changed. I believe he previously said that the second static fire would be 16 engines but they ended up doing 14 instead.
  8. Yep, that's what I get when posting at 4 AM We've tossed this about before, but if I had my druthers, I'd use something like the following nomenclature: Star. Any body which currently or in the past sustained nuclear fusion in its core. Main sequence star. Any star in hydrostatic equilibrium which is currently fusing hydrogen in its core. Giant star. Any star in hydrostatic equilibrium which was previously a main sequence star but has exhausted the hydrogen in its core. Red giant. Any giant star with an inert core which is fusing hydrogen or helium in shells. Supergiant. Any giant star large enough to fuse helium into heavier elements via the alpha process. Failed star. Any star which was unable to sustain sufficient nuclear fusion to reach hydrostatic equilibrium. Stellar remnant. Any star which formerly reached hydrostatic equilibrium but no longer sustains nuclear fusion. Degenerate star. Any stellar remnant which is supported by quantum degeneracy pressure. White dwarf. A degenerate star supported by electron degeneracy pressure. Neutron star. A degenerate star supported by neutron degeneracy pressure. Black hole. Any stellar remnant which has collapsed to be smaller than its Schwarzschild radius. Stellar nebula. Any stellar remnant or portion of a stellar remnant which has been ejected into a diffuse cloud. World. Any gravitationally-rounded body which is not a star. Rogue world. Any world which is not in orbit around a star. Giant world. Any world too large to have a solid surface or crust, in which the transition between gas and liquid occurs above the critical point. Gas giant. A giant world comprising primarily hydrogen and helium. Ice giant. A giant world comprising primarily elements heavier than helium. Terrestrial world. Any world with a solid surface. Ethereal world. A terrestrial world with a persistent troposphere. Ice world. A terrestrial world with a surface composing of icy volatiles and no persistent troposphere. Rocky world. A terrestrial world which is neither an ethereal world nor an ice world. Planet. Any world in orbit around a star which is not a natural satellite. Major planet. Any planet which, together with its natural satellites, makes up the vast majority of the mass in its orbital neighborhood. Planet binary. Any pair of planets in orbital resonance, neither of which are a major planet, but which together with their natural satellites make up the vast majority of the mass in their orbital neighborhood. Minor planet. Any planet which is neither a major planet nor a member of a planet binary. Natural satellite. Any body in orbit around a star which is in orbital resonance with a larger body, other than a planet binary. Moon. Any natural satellite which stays within the Hill Sphere of a larger body. Planetary moon. Any moon which is also a world. Dwarf moon. Any moon which is not a world. Trojan satellite. Any natural satellite, other than a moon, which is in a 1:1 resonance with a larger body. Leading trojan. A trojan satellite at the L4 point of a larger body. Trailing trojan. A trojan satellite at the L5 point of a larger body. Resonant satellite. Any natural satellite with a resonance other than 1:1 with a larger body. Orbital neighborhood. The orbital neighborhood of a body is the set of objects orbiting the same star as that body which crosses the orbit of the body or of one of its natural satellites. Comet. A body with a sufficiently eccentric orbit that it experiences cycles of visible mass loss near periastron. Asteroid. Any body in orbit around a star other than stars, comets, planets, and the natural satellites of planets. Belt asteroid. Any asteroid which orbits entirely between the orbits of adjacent major planets. Centaur. Any asteroid which crosses the orbit of a major planet. Distant asteroid. Any asteroid which orbits a star at a greater average distance than any major planet.
  9. I'm not here to say I told you so. But I did bloody tell you.
  10. Back to the OP, we had a new engine test over Thanksgiving: Lovely Mach diamonds. I'm guessing that this is still a low-throttle test. Those plumes are extremely orange, which suggests low combustion temperatures and a lot of molecular hydrogen burning in the ambient air.
  11. Oh, it’s terribly hackneyed. FWIW, it’s strictly the Pluto/Neptune problem, not the Pluto/Uranus problem. Virtually all of the Plutoids and other bodies in the Kuiper belt are in resonance with Neptune. By all rights, Pluto should be considered a satellite of Neptune, just like the trojans of Jupiter should be considered satellites of Jupiter. It feels very icky to think that Pluto would be a planet if it was close to the orbit of Mercury, and Earth would be a dwarf planet if it was out there near the orbit of Pluto. I’m all for categorizing things based on the role they play in stellar system evolution, but you’ve gotta have a limit somewhere.
  12. One of the problems with the OP (and with science fiction in general) is that there’s a disconnect between what ecosystems could exist and how ecosystems form. Even the deserts of Earth (the Sahara, Antarctica, etc.) weren’t always deserts. By the way, @Beamer, are you new? You seem uncommonly smart and I’m not sure why I haven’t interacted with you before. The best working hypothesis is that they formed close and got themselves ejected into more distant orbits by any number of interactions, but there are other possibilities.
  13. Titan has lakes and rivers and rainclouds. Of course these lakes and rivers and rain clouds are all full of liquid methane, not liquid water, but that’s beside the point. Jupiter, Saturn, Uranus, and Neptune are gas giants. Gas giants do not have oceans anywhere; they are so heavy that the gases which compose them transition gradually through supercriticality, so there is never any sharp surface transition (like the transition between air and liquid/solid like on Earth, Mars, Venus, and Titan). It should be noted that Uranus and Neptune are also commonly referred to as ice giants. Ice giants are still gas giants and there is nothing particularly icy about them since they are extremely hot. But unlike the “classic” hot gas giants, they just are composed mostly of heavier elements. It is believed that during the formation of stellar systems, the largest accretors suck up the majority of the hydrogen and helium in the protoplanetary disc, achieving a substantial amount of internal heat production through Kelvin-Hemholtz contraction. These form the gas giants, which rob the rest of the protoplanetary accretors the opportunity to grow in mass. Slightly smaller accretors are able to capture heavier elements in the hot disc and grow to significant size, but cannot hang onto hot hydrogen and helium and thus stop growing out at much lower masses. These smaller giants will be either completely ejected from the system or will be flung into more distant orbits, where they cool relative to the gas giants.
  14. 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.
  15. 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.
  16. 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.
  17. 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.
  18. That is the X-band high-gain steerable antenna used for most downlinks to transmit data back to Earth.
  19. This tweet evidently is only giving the distance of Apollo 13's record, not stating the distance which Orion will achieve.
  20. 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.
  21. 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.
  22. 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.
  23. 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.
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