sevenperforce

Well, no one really knows the answer to that. But we do know that most of the mass we deal with on a daily basis is relativistic mass, not rest mass. One kilogram of water has the "rest mass" of approximately 0.015 milligrams...roughly 1/6 the mass of a human eyelash.

Photons do have mass, just not rest mass.

@Dr. Kerbal For any rocket engine, you need two things: an energy source and a working mass (also known as reaction mass or remass). The simplest rocket engine is a cold-gas thruster. You've got a compressed tank full of nitrogen gas. The energy was put into the nitrogen gas when it was pressurized; open the valve and the pressure pushes the gas out. The momentum of the escaping reaction mass confers equal and opposite momentum on the vehicle. A monopropellant thruster (like HTP or hydrazine) comes next. Here the energy is not contained in pressurization, but in the chemical energy of the propellant. When hydrazine decomposes into ammonia and nitrogen gas, it releases heat which in turn accelerates that ammonia and nitrogen gas to produce thrust. It's important to note that even though the exhaust and the reaction mass are the same thing, here, that doesn't necessarily need to be the case. You could have a hydrazine thruster that mixed the exhaust with water in order to produce a cooler exhaust with more thrust, even though the water would make the entire affair less efficient. You could even have a hydrazine thruster for a jet aircraft which used a turbofan to compress air and mix it into the exhaust, which would actually be more efficient. The energy source and the remass can be separate. One example where remass and energy source are partially separate is actually none other than the Space Shuttle Main Engine. When burning liquid hydrogen and liquid oxygen together, you achieve the maximum amount of energy with two pounds of hydrogen for every one pound of oxygen. However, the SSME actually mixed in four extra pounds of hydrogen to help cool the engine and provide additional remass. For a nuclear thermal rocket like NERVA, the energy source and the reaction mass are completely separate. The remass is liquid hydrogen and the energy source is the nuclear reactor. You could use any remass -- liquid hydrogen, liquid methane, even water -- and get a different performance level, but the energy source remains the same. A hypothetical antimatter engine would work the same way. Antimatter is a way of storing energy. So you would inject a small amount of antimatter into a large propellant flow (like liquid hydrogen or liquid methane or even water). The antimatter would annihilate with a small amount of the propellant, which would heat up the rest of the propellant and push it out as reaction mass.

Words are descriptive, so we should describe things. Not try to prescribe concepts.

Lower power requirements, too, I believe. You can save a lot of energy when you are pointing your signal directly at a receiver, rather than vomiting it into the void. The square-cube law is a harsh mistress.

It just occurred to me that one of the complaints about using the barycentric definition is that it will change over time. But the tug-of-war definition changes over time too. It is not dependent on the mass ratio of the primary and the secondary, but it is dependent on the distance between the two. When the moon formed, just outside the Roche limit around four Earth radii away, its tug-of-war score would have been 103.3, making it solidly a moon and definitely not a double planet. The score for the most distant moon of Jupiter, Carpo, is just 2 if you use its semi-major axis. But it has a highly eccentric orbit, so if you use its Jovian apoapsis of 2.43e7 km, it scores 0.98. So Carpo, at just 3 km, is sometimes a moon of Jupiter but sometimes a binary planet with Jupiter. That doesn't make sense.

Short-period comets don't even necessarily have an orbital path into the Kuiper belt, let alone the Oort cloud. Jupiter-family comets have an aphelion below the orbit of Saturn, and Encke-type comets don't even reach the orbit of Jupiter at aphelion. I was under the impression that that coma of a comet typically only becomes visible when it is well within the orbit of Mercury, but it looks like I had assumed wrongly...there are short-period comets with detectable coma which never even dip below the orbit of Mars. It looks like there are even a class of "active asteroids" which are main-belt asteroids that outgas periodically, giving them cometary properties. I suppose that "comet" is less about position and more about intrinsic qualities...I'd be happy to lump comets and centaurs together with "irregular planets" in the nomenclature above. The revised nomenclature is actually simpler: Major Planets. Large, spherical bodies in low-eccentricity orbits near the system's invariable plane which comprise more than one half of the total mass of all objects which cross their orbit. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Moons. Bodies which orbit at all times within the Hill Sphere of a larger body. Minor Planets. Objects which are neither major planets nor moons. Bound Planets. Bodies which are in resonant orbits with major planets, such as Pluto, Gonggong, 2002 TC302, and the various trojans. Irregular Planets. Bodies which are not bound planets and either cross the orbit of a major planet, have high eccentricity, or have a highly inclined orbit. Eris, Sedna, centaurs, and most comets. Quasi-Planets. Bodies which are neither bound planets nor irregular planets. Ceres, Makemake, most asteroids, and some comets. I've been doing plaintiff litigation mostly. Once I graduate I'll see what my options are.

Yep, I see now that we were not so far apart as I thought. We could base our distinction on whether TNOs are classical or resonant. I think the best reason for ruling out Pluto, Orcus, Otrera, Clete, and Ixion as planets is that their orbits are controlled by the influence of Neptune. But that leaves us struggling over cubewanos like Makemake as well as scattered objects like Eris and Sedna. Well, I'm not actually so sure about that. Mercury is physically smaller than Ganymede and Titan, but more massive than both combined. Apart from Earth, it is the densest planet in our solar system. Mercury almost certainly lost significant portions of its mass as the result of its location so close to the Sun, either via a planetesimal impact or due to extreme conditions during the formation of the solar system. If Pluto, Eris, or Makemake had formed at just 0.3 AU from the sun, they would be very different worlds than they are. If we want to describe objects by their intrinsic qualities, we can do so with fairly high specificity (that is, without overlap) based mostly on formation, history, and size: Small solar system bodies: Planetesimals. Monolithic, undifferentiated primary solar system objects formed by gravitational accretion of cosmic dust. Examples include Arrokoth, Themisto, Ananke, Caliban, 3552 Don Quixote, and the like. These are unlikely to be larger than a few dozen kilometers. Rubble piles. Secondary solar system objects which coalesced from shattered fragments of either planetesimals or protoplanets. Many comets and most asteroids are rubble piles, including Itokawa, Antiope, Malthide, Bennu, Churyumov–Gerasimenko and Ryugu, as well as small planetary moons like Phobos, Deimos, Janus, and Saturn's ring shepherd moonlets. Rubble piles can be very small, but can grow very large when formed from the breakup of protoplanets, such as with Proteus and Hyperion. Protoplanets. Objects which grew larger than 100-200 km early in the solar system's history had sufficient internal gravity and radioisotope content to cause melting, resulting in at least partial internal differentiation but without sufficient self-gravitation to become ellipsoidal. These include the largest asteroids and many of the solar system's moons. Examples are Vesta, Pallas, Eunomia, Interamnia, Hebe, Phoebe, and Psyche. Worlds: Planemos. Protoplanets larger than around 400 km have sufficient gravitation to have fully differentiated interiors and reach hydrostatic equilibrium. They are ellipsoidal in shape and can experience internal geodynamic events like diapirs and volcanism. The smallest are the size of Miranda and Mimas and the largest are the size of Pluto. Dwarf worlds. At around 1.5e22 kg, planemos become large enough to retain gaseous molecular atmospheres, although the heat of the inner solar system will strip away the atmosphere from dwarf worlds located there. Most of these are moons, and they range in size from Eris and Triton to Titan and Ganymede. This includes our moon. Terrestrial worlds. At around 2.7% the mass of Earth, worlds become large enough to retain liquid water on their surfaces, provided they are in the habitable zone. Mercury, Mars, Earth, Venus, and a few known exoplanets. Gas giants. At around 5-6 Earth masses, gravitation becomes great enough to retain hydrogen, and so the body no longer has a solid surface. These can go up to about 13 Jupiter masses before their internal gravitation becomes great enough to fuse deuterium and become a brown dwarf or protostar. Uranus, Neptune, Saturn, Jupiter, and most known exoplanets. If we want to describe objects by their role or position in a solar system, we have a completely different set of descriptors: Major Planets. Large, spherical bodies in low-eccentricity orbits near the system's invariable plane which dominate the orbits of small bodies within the system. These generally represent much, more than one half of the total mass of all objects which cross their orbit. These are the "big eight". Minor Planets: Bound Planets. Smaller bodies which are in resonant orbits with major planets, such as Pluto and the various trojans. Quasi-Planets. Bodies which lie near the invariable plane in low-eccentricity orbits, do not cross the orbit of major planets, and do not compose more than half of the total mass of all objects which cross their orbit. Ceres, Makemake, and garden-variety asteroids. Irregular Planets. Bodies which have distant, high-eccentricity orbits or which have orbits inclined at more than 45 degrees from the invariable plane: Eris, Sedna, and so forth. Moons. Bodies which follow closed orbits around other bodies. Comets. Bodies which cross the orbits of at least one major planet with a periapsis nearer the sun than the nearest major planet. Centaurs. Bodies which are not comets or bound planets but which cross the orbits of multiple major planets. If you want to combine nomenclature from both descriptor sets, great. Just make sure it's consistent. I think we can probably do that math (and get excited about it) without needing to have a special name for it, though...right? I've been working in law firms for half my life so at this point I'm kinda stuck with it. Still love it.

Don't they have to use bladders for that, too?

That's a step in the right direction. I don't remember WDRs before the single-engine static fires. I guess with three Raptors bolted on there, they are a little more cautious.

Wow. Uhm -- so, metabolic gas is produced as a byproduct of digestion by bacteria which are unable to fully metabolize certain food chemistry. The human gut does not produce any metabolic gas by itself, apart from the action of gut bacteria. I burst out laughing.

No, the argument is that deliberately choosing a compound lexeme like “dwarf planet” to exclude something from being a planet is contrary to how language typically works and is therefore prone to causing confusion.

Using the Harvard liquid-water definition is a touch arbitrary, but it would make definitions pretty simple. Just five intrinsically-defined classes: Star. Any body with a mass greater than 1.5e29 kg. Protostar. Any body with a mass greater than 2.5e28 kg but less than 1.5e29 kg. Planet. Any body with a mass greater than 1.6e23 kg but less than 2.5e28 kg. Protoplanet. Any body with a mass greater than 3.7e19 kg but less than 1.6e23 kg. Asteroid. Any body with a mass less than 3.7e19 kg. A satellite is a body which orbits another body in a higher class than itself. Pairs of orbiting bodies in the same class are called binaries. The satellites of planets and protoplanets are called moons. Super simple.

The graduate English electives I took while getting my physics undergrad were some of my favorite classes ever. Linguistics was the best. Doesn't hold a candle to how much fun law school is, though. Well, the distance between the bodies does change, sure. And we won't always be able to determine separation between exoplanet multiples. But that's not really a problem. Things change. "Triton used to be a dwarf planet until it was captured by the giant planet Neptune and became a moon." "Our moon will continue to move away from us until it is no longer our moon but a planet of its own, forming a binary with us." "Pluto and Charon are a dwarf planet binary with such a close mass ratio that they are both tidally locked to each other." "We have discovered that there are two different worlds orbiting Gliese 790! We can't yet tell whether they are a binary planet or if one is a moon of the other, but they're large enough to both be potentially habitable." No problems arise from this usage. There's another solution, though, if we want to define planetary bodies by their intrinsic properties first, then by their position in a system. In 2019, Harvard astrophysicists determined that worlds less than 2.7% the mass of Earth would never be able to have liquid water on their surfaces because their gravity was too low to prevent runaway water loss even at the edge of their star's habitable zone, regardless of the size of that habitable zone. The largest moons in our solar system are less than 2.7% the mass of Earth. The smallest planet (Mercury) is more than 2.7% the mass of Earth. Seems like a good cutoff to me.

this isn't how loveing language works. This is the worst argument and everybody who makes it should feel embarassed While I take no offense, I'm also unembarrassed. This is exactly how language works. Not trying to pull rank or anything, but in English common parlance, lexemes formed by open compounding, using a modifying dependent along with a noun, strongly imply that the thing being identified is a subset of the group represented by that noun. Fighter jet, writing desk, compact car, pickup truck, potted plant, garage door. These are all endocentric compounds, where the adjectival dependent modifies and restricts (but does not alter) the meaning of the obligatory head. Endocentric compound lexemes should be distinguished from ordinary noun phrases, like "the brown desk" or "a small horse" where the adjective denotes a characteristic without defining a subclass. It is true that we have lexemes in which an open compound (with the adjective+noun structure) is used to denote a new class rather than a subclass. These are exocentric constructions. A red head is not a particular type of head, for example, and a bird house is not a subclass of (the common usage of) houses. But such constructs are the result of evolution and fixation of the lexeme within a dialect, not prescription. If one of our goals in sharing science is to make science accessible to the public, we shouldn't deliberately choose a nomenclature where a modifier+noun combination denotes a new class rather than a subclass. It's inherently confusing. To borrow one of the examples above, it would be like saying, "Oh, we don't call any land masses islands anymore. We call them dwarf continents. But a dwarf continent is not a type or subclass of continent." That would be ridiculously confusing. First, you're overspecifying for effect. Second, the point is to identify a binary. "Binary" should be applicable beyond the moon-planet division, and it should be applicable without reference to an external attractor. What about rogue planets? There's no star to deal with, there. A rogue planet is a binary system if the barycentre is ever in free space and it is a satellite system if the barycentre is not ever in free space. You don't need to reference any third body. It doesn't matter what orbit you're in. Or you can define it the other way, if you prefer: a system is a binary system if the barycenter is always in free space and it is a satellite system if the barycenter is ever under the surface of either body. That way you avoid questions about whether Sol+Jupiter is a binary system. This definition applies to asteroid binaries and planetary binaries and stellar binaries and black hole binaries (and even galactic binaries, with a little nudging). It's universally applicable. Because Ceres is a world. It is gravitationally rounded. It's also large enough to have significant influence on objects which share its orbit. It has approximately fifty trojan asteroids. And if Pluto was where Ceres was, it wouldn't be a planet either. It's not large enough to clear the asteroid belt. Evidence, yes, though not sufficient evidence. Ceres has trojans too. That's why I would prefer a definition like "the mass it exerts resonant influence on is greater than half the total mass that crosses its orbit." But yeah obviously a gas giant is in hydrostatic equilibrium. It doesn't have a surface. This was my initial impulse but on reflection I think the rule should be "all of the time". Otherwise you run into the question of whether Jupiter and the Sun are a binary. It's easy to make it precise. If the barycentre ever dips beneath the surface of the larger body, it is a satellite system. If it never does, it's a binary system. Applies to stars too. This is where solar system formation tends to help us out, I think. Very large objects with very inclined orbits are very likely to get themselves ejected. But it also signals that we need to think about separating intrinsic descriptors from extrinsic descriptors. Calling something a "giant planet" or a "world" or a "planetoid" describes its intrinsic qualities. Calling something a "moon" or a "major planet" or an "irregular planetoid" describes its extrinsic qualities. You can cross-combine these descriptors without difficulty. For example, you could have an irregular giant planet.

Some kid they had.

Was there a static fire last night?

I know it defines a world beyond Neptune. It doesn't include Ceres.

Technically they do orbit a star. What about "planets that would be moons if they were placed in orbit around the largest planet in their system"? Nothing in the asteroid belt is larger than Pluto. Pluto masses more than everything in the asteroid belt put together. But more importantly, it's not "out there" by itself. It's out there with at least 892 objects in the same region of space. 482 of those actually share its exact same orbital period, and several of those are also "round" in the commonly-understood sense. That seems like saying "I see no benefit to calling all miniature horses, horses." Miniature horses are horses. It's right there in the name.

Yes, I agree. I just think their "discriminant" is ad hoc. Again, if you apply the discriminant to Ganymede or Titan then they become planets too. Well, actually Uranus ends up in a weird category. Jupiter and Saturn work together to influence the orbits of all the inner planets and Neptune controls the orbits of a bunch of TNOs but Uranus exerts very little influence despite being essentially just as big as Neptune. There are a lot of ways to draw arbitrary lines. Apart from the sun, Jupiter is twice as heavy as every other object in the solar system put together. On that basis, you could say that our solar system is actually a quasi-stellar binary with one star, one protostar, and three giant planets. This would make Ganymede and Mercury both terrestrial planets orbiting close to stellar primaries. I maintain that we should keep our terminology as close to common usage as possible, while also striving for consistency. The whole hubbub about Pluto really stems from challenges in public usage. Laypersons have a pretty clear idea of what "planet" and "moon" mean, and so changing the definition just seems weird. We shouldn't pick definitions that confuse the public even further. Think about dropping your terminology into lay conversations and evaluate whether it would make things less confusing or more confusing. For example, my terminology: "It's been 48 years since man walked on the surface of another world." "So far, we have only been able to land spacecraft on two other planets and two moons." "The year I was born, we completed flybys of every major planet in our solar system." "With the arrival of Dawn at Ceres, we have now placed orbiters around every planet out to Saturn and every world within the inner solar system." "Look how close Jupiter and Saturn are in the sky. You might think you're only staring at two planets, but you're also staring at 161 moons, including 11 worlds humans could one day set foot on." "Earth is the only major terrestrial planet orbited by another world. The moons of the planet Mars are the size of asteroids, and the rest of the moons in our solar system orbit gas giants or distant dwarf planets." You could make any of those statements to a layperson and they'd be able to follow along without a bunch of explanations or yammering about "planetary discriminants" or "clearing the orbit". Yet every usage is consistent.

See also: I, too, can invent maths that include or exclude portions of a dataset. You could change up the variables such that Pluto is definitely a planet and Mercury is definitely not. It's all ad hoc. But if you talk about "dominating the orbit" then you are getting somewhere. There are a number of ways to define domination of an orbit. For example, you could put it this way: A solar system body dominates its orbit if the total mass of smaller bodies with which it is in resonance is greater than the total mass of non-resonating bodies which cross its orbit. This means that if a solar system body has a large number of trojans and resonant bodies, it "dominates its orbit" even if there is a lot of other crap that crosses its orbit. Mercury dominates its orbit because it exerts resonant influence over Venus trojans, for example.

That's why making "world" a term in planetary science rather than in astronomy solves the problem. When you talk about visiting new worlds, landing on new worlds, building colonies on new worlds, etc., you're talking about gravitationally-rounded bodies in hydrostatic equilibrium with solid surfaces. Doesn't matter where they are. Then planet can describe a body's position in the solar system. As the term was originally intended -- planete, describing the wandering motion of the planets known to the Greeks. "Double planet" or "planet binary" are equally okay, I suppose. In the lexicon I proposed, the Pluto-Charon system is a dwarf planet binary. Yes, I agree. I think they wanted to define MVEMJSUN as "planets" when they should have made them a sub-class of planet. It makes much more sense to say "We have major planets and dwarf planets" than to say "We have planets and then we have dwarf planets that are not planets but could be planets if they were in a different place." Timing is fine; we can only describe things as we observe them. Conversely, I don't think a question of whether Moon is a planet or a moon should be a matter of the system's orbital distance. So Jupiter and Sol are a binary planet? Easily dealt with by saying that if the barycentre is ever in free space, it is a binary. I don't love saying that planets can be moons. Moons can be worlds but moons should not be planets. Thank you. This is my view. Language is descriptive, not prescriptive. We should choose language which is as consistent as possible but which also describes common usage as closely as possible. I think "planet" and "moon" should describe the role that bodies play in orbital dynamics, because that's what aligns most closely with common usage. When you say "Astronomers have discovered a new moon!" a layperson's first question is usually, "Oh, what planet is it around?" Similarly, if you say "Astronomers have discovered a new planet!" then common lay questions would be "Oh, where is it?" or "Does it have any moons?" The lay usage of "moon" and "planet" align well with treating these terms as part of a hierarchy. So we should use the term planet, but planets should not be moons. The proposed planetary discriminants feel like an exercise in extreme ad hoc filtering. "Let's come up with an equation which DEFINITELY excludes Ceres and Pluto but DEFINITELY includes Mars and Mercury." Because reasons. That's why I prefer "dominates its orbit". Virtually everything that orbits closer to Earth than to Mars or Venus does so in some degree of orbital resonance with Earth. Virtually everything that orbits between Mars and Saturn does so in orbital resonance with Jupiter. What we know about solar system formation suggests that stable solar systems depend on resonance structures, so it makes sense to use this (not some ad hoc planetary discriminant) as the measure.

That's....inventive.

But you have really bad heat flux on the fixed overlapping zone. Stress test, yes. But it's not a good heat shield test. You need horizontal velocity for that. Eh, I doubt it. From t0 to t1, Starship does a boost-back to a landing zone at Boca Chica. From t1 to t2, Starship does either boost-forward or boost-back to a landing zone at the Cape. From t2 to t3, Starship does a boost-forward to a landing zone in Africa. After t3, Starship does an abort-once-around (AOA) to Boca Chica. The t2..t3 period may not be necessary; Starship may have enough dV to AOA after crossing t2. IT'S HAPPENING Interesting that the squid fins do NOT appear to have a lateral edge parallel to the rest of the vehicle, as in Neopork's fan renders.

I'd call them worlds but not planets. Star. Any body large enough to sustain nuclear fusion. Stellar Remnant. Any body which was formerly a star. World. Any gravitationally-rounded body with a chemically solid surface. Planet. Any gravitationally-rounded body which is not a star or stellar remnant, provided it does not orbit a barycentre inside another planet. Rogue Planet. A planet which does not orbit a star or stellar remnant. Giant Planet. A planet too large to have a solid surface. Planet Binary. Two planets which orbit a common barycentre. Major Planet. A planet or a planet binary which is not rogue and dominates all objects which share its stellar orbit (sharing its orbit means crossing its orbit but not the orbit of another major planet). Irregular Planet. A planet or a planet binary which is not rogue and has an eccentricity greater than 1/3 or an inclination more than 45 degrees from the invariable plane of the system. Dwarf Planet. A planet or a planet binary which is neither a rogue planet, nor a major planet, nor an irregular planet. Moon. Any body which orbits a barycentre inside a planet. Major moon. A moon that is also a world. Dwarf moon. A moon that is not a world. Comet. Any body which passes close enough to a star to undergo an outgassing cycle. Asteroid. Any body, other than a comet or a moon, which is too small to be a planet. Trojan asteroid. An asteroid which is gravitationally bound to a Lagrange point of a planet. Irregular asteroid. An asteroid which crosses the orbit of a major planet. Rogue asteroid. An asteroid which does not orbit a star or stellar remnant. Regular asteroid. An asteroid which is not a trojan, irregular, or rogue asteroid. That should just about do it. Also the word "planet" no longer feels real.