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Comment Re:a very large planet, 15 times the Earth (Score 1) 134

And something basically to that effect was for a period at the IAU conference the definition being haggled over. A lot of people went home at that point thinking that either that would get voted in as the definition, or there would be no definition, and were fine with either outcome. The committee however changed the proposal before the vote came up.

Comment Re:The universe. (Score 1) 134

These are what the IAU came up with, in a vote that was very controversial among its membership. An association dominated by astronomers, not planetary scientists, who were by and large against the decision. And a set of terminology that you can often find flatly ignored in scientific papers. Example. In short, the only group that the IAU is able to bludgeon into using their term is the general public (using the "We're scientists, if you don't use our term you're wrong and ignorant" gambit), not the scientific community itself.

Comment Re:If the singularity doesn't happen... (Score 2) 134

Stop feeding the troll ;) If a person can't handle an argument without name calling, they're not worth your time.

For what anyone not trolling :) There is nothing magical about existing on Earth that allows a nuclear reactor to run. Earth does provide a few conveniences, mind you - your mass budgets are unlimited, and cooling is easier. But nothing about either bulk nor mass prevents nuclear reactors from operating in space, by any stretch, and the two main things limiting their use have been a lack of need and NIMBY (the former being little applicable in the former USSR, they used them quite a bit, although they still lacked a need for high powers and so generally kept them fairly small; in the US, NIMBY limited the US to just one launch, although the US developed a number of other systems, some to flight-ready status, on the ground).

The typical mass balance for a in-solar system fission fragment rocket (measured simply by MWt, not MWe, since thrust is direct) is about 20% payload, 20% structural, 35% reactor, and most of the rest toward various aspects of cooling. The nuclear fuel makes up only about 2% of the total mass (figures from the Callisto baseline). For an interstellar mission, however, the fuel would make up the a large minority or the majority of the mass, trading significantly reduced acceleration for significantly longer acceleration times. On an in-solar-system version, power density is about 6kWt per kilogram of reactor mass (that 35% figure above). This is actually quite low by large-space-reactor standards; many modern multi-megawatt reactor research projects for NEP and defense purposes (example) often deal with density figures of 50-100 kWe per kilogram, including cooling. But a fission fragment reactor has a sparse core and has to rely extensively on moderation / reflection to keep up a sufficient neutron flux; higher core density is prohibited because then the fragments would thermalize.

One thing that's neat about a fission fragment reactor is that, like systems like VASIMR, it can operate in various output modes, trading ISP for thrust as needed. In pure fission fragment mode it's ISP is is ridiculously high, nearly 1m sec; your thrust is purely the relativistic fission fragments from each reaction, carrying the majority of the reaction's energy away. However, you can inject gas into the stream as reaction mass, limited only by the density to which your magnetic nozzle can keep the stream confined. So where higher thrust maneuvers are needed, you can use the same engine (up to the aforementioned extent, of course; you're not going to take off from a planet with a FFRE!)

Comment Re:Taxonomy and location (Score 1) 134

So it's up to the planetary scientists to do something about it if they think it makes little sense

Do what? They make up less than 20% of the membership of the IAU. It's a bunch of astronomers. What do you want them to do, file a lawsuit?

They're doing the main thing that they can, which is complain about the "definition" foisted upon them, as Stern was doing above. Something you apparently find fault with.

All I hear is a bunch of bitching about it but no serious counter proposals.

That's your fault if you don't pay attention to the debate, because there have been tons of alternate proposals.

If the IAU decision wasn't scientifically useful then it will be ignored anyway.

And hence a giant stink that lowered the discourse for nothing.

How do you see this as even remotely similar? If you take a shrew from Ohio and you place it in Nepal, does it cease being a shrew and become a dwarf shrew that no longer counts as a shrew?

Actually biologists do stuff like that all the time.

No, they don't.

There are species that are considered different based almost entirely based on location

No, there aren't.

but it does happen and it's not irrational.

No, it doesn't, and yes, it is.

Seriously, you're going to cast doubt on the guy who came up with the Stern-Levison parameter that's used to make that distinction?

When he says something igorant, yes I am.

Right. Got it. The guy who co-invented the Stern-Levison parameter doesn't know how to calculate a Stern-Levison parameter. But you do. Thank you! I take it your name is Harold Levison?

Pluto is absolutely not "much like" "big rocks", and the fact that you'd make this claim is a profound expression of ignorance on the topic.

You are seriously arguing that Pluto is nothing like other "dwarf planets" or other large rocky/icy objects in our solar system?

Pluto is more like Mars than it is Ceres, at the very least. As for other dwarf planets... we have no idea, we've never even been there. Going by things that would be counted as dwarf planets if they were free orbiting, there's a massive range of properties. What's the universal property (apart from size / hydrostatic equilibrium / general terrestrial nature) between Pluto, Luna, Ceres, Ganymede, Callisto, Europa, Io, Titan and Triton? Answer: not a damn thing. They're all radically different environments. Some are more similar to each other than others, but they're anything but a logical "group" distinct from the terrestrial planets.

Versus "big rocks", however, the comparison is even more ridiculous. There is literally nothing beyond "they're both made of solid matter" in common between Pluto and a typical large asteroid. Including, for starters, Pluto isn't made of rock. It has some unknown percentage of rock in its interior, but it's overall made of ices, with a thin gas atmosphere (not all that different, structurally, from the ice giants Neptune and Uranus, although the latter two are obviously on a much larger scale and reach much higher pressures in the gaseous state before transitioning to the ice states).

Comment Re:If the singularity doesn't happen... (Score 1) 134

I'm sorry, I must be a nutter. I was under the mistaken view that we live in a world where there are many dozens of different designs for fission reactors that have been developed, with new designs being developed and prototyped every year, and full scale reactors being produced on scales orders of magnitude larger than is required for spacecraft propulsion. Little did I know! Thank you for correcting me for my sinful error.

Comment Re:What the hell is the big deal with "planet"? (Score 1) 134

The most ridiculous thing about the "cleared its orbit" standard is... MOST planets didn't clear their orbit. Jupiter, and to a lesser extent Saturn, did. Particularly in the case of Mars. Mars does not dominate it's neighborhood, a fact clearly reflected by how low of a percentage of asteroids are in a Mars resonance vs. Jupiter. Mars has a significantly lower Stern-Levison parameter than Neptune, and yet Neptune has freaking Pluto in its neighborhood. And even if one wants to argue that Pluto is too small versus Neptune to count as "not cleared", it certainly isn't too small compared to Mars to count. The reason Mars does not have things even bigger than Pluto in its neighborhood comes down to one word: Jupiter.

And I know some people will say, "but the Stern-Levison parameter says Mars would have". It says no such thing. The Stern-Levison parameter is about a body's ability to relatively clear its orbit of asteroids, not protoplanets. It's based around the size and orbital distribution of our current asteroid belt.

But of course, this was not a scientific reality seeking a definition. They had a definition they wanted (that Pluto wouldn't be a planet) and were trying to come up with some sort of scientific reasoning, any reasoning, as to why. This is quite clear from their statements on the topic, they already had the result they wanted and were playing around with different reasonings to get it. And this mangled, self-contradictory definition is what they came up with and passed at the last minute (when most people had left thinking that there either wasn't going to be a new definition or that it would be based around hydrostatic equilibrium, based on what had been discussed previously, and were fine with either outcome). And so now we have a situation where a "dwarf X" isn't an "X" from a body that otherwise declares dwarf things to be smaller versions of the same thing, where exoplanets aren't planets, based on a lie that all planets have "cleared their own neighborhoods", without any sort of clear definition as to what a "neighborhood" or "clear" is.

Heck, if I wanted to be pedantic I could point out that not even Jupiter would meet their definition because - again, to be pedantic - it does not orbit the sun. The point that Jupiter orbits (the Sun-Jupiter barycentre) is almost always outside the sun.

Comment Re:Improving taxonomy (Score 1) 134

While far be it from me to defend the IAU, that is just nonsense. If anything we need better definitions and more categories and the IAU got the ball rolling on this.

What the IAU "got the ball rolling on" was chaos. They had a bunch of astronomers telling planetary scientists to use a definition that they disagree with. Many have taken to just ignoring it, in the peer reviewed research. To give two examples of how absurd the definition is: 1) the definition states that something is only a planet if it revolves around the sun, not other stars - and yet the IAU has an exoplanets working group. Exoplanets aren't planets! 2) The concept that "a dwarf X isn't an X" is not only linguistically absurd, it's a view not even shared by the IAU itself, which is more than happy to consider, for example, dwarf stars to be stars.

The main reason stated by most astronomers who backed the decision is almost invariably (seriously, read interviews with them), "I don't want my daugther having to memorize the names of sixty different planets". As if that's even slightly a valid reason for making a scientific decision.

Jupiter and Earth bear almost no resemblance to each other and yet they both are planets.

Exactly! And yet rather than kick the gas giants out, they kicked out another solid body that has far more in common with Earth (including, I should add, active geology and weather) rather than the bodies that have almost nothing to do with Earth.

In reality they should probably be different categories of entities. We used to consider Ceres a planet a long time ago

And we should. Believe it or not, because some scientists in the 1800s changed their mind about something doesn't mean that this is some sort of eternally correct decision. They had no clue about the concept of what bodies would end up in hydrostatic equilibrium and the consequences thereof.

Let's go to biology. We label species all the time based on location and proximity to other similar animals rather than the much simpler "can they mate" question.

How do you see this as even remotely similar? If you take a shrew from Ohio and you place it in Nepal, does it cease being a shrew and become a dwarf shrew that no longer counts as a shrew?

Or geography. We label mountains and bodies of water precisely based on what they are next to. You could reasonably consider the Mediterranean Sea as a part of the Atlantic Ocean if you really wanted to.

So because it hasn't "cleared its neighborhood" does it suddenly become the Mediterranean Pond despite being a size that we traditionally call a sea? Do we arbitrarily declare that there's only 8 mountains in the world and all others are "dwarf mountains" that aren't really mountains because we think there's too many mountain names for kids to memorize?

Umm, ok. Presuming that is true...

Seriously, you're going to cast doubt on the guy who came up with the Stern-Levison parameter that's used to make that distinction?

It would be equally true to say that Earth wouldn't be a planet if it wasn't orbiting the Sun but equally irrelevant as well because it manifestly does.

Right. Because it totally makes sense to have an perfect copy of Earth orbiting in a larger star's habitable zone (and thus have a lower Stern-Levison parameter) not be a planet while its perfect copy is.

Pluto is very much like Ceres and other big rocks

Pluto is absolutely not "much like" "big rocks", and the fact that you'd make this claim is a profound expression of ignorance on the topic. And should I add, one of my greatest peeves about the IAU's decision. Since their discoveries long, long ago both Pluto and Ceres had been nothing more than specks, dots of light. We knew next to nothing about either of them. Then finally, at long last, right before we were about to get real data on both of them, rather than even waiting for the data to arrive, they decided to make declarations about them. The most unreasonable time they could have picked in a century to do so, that's when they did it. And don't even get me started on how the vote went down...

Pluto is about as different from a "big rock" as you can get. Pluto has an atmosphere. With apparently complex nitrogen photochemistry and clouds going on in it. With snowfall and frosts, and a chemically diverse surface. And glaciers, carving out canyons. And signs of what appear to be flowing liquids of an unknown nature in the past. And cryovolcanoes. And tectonics. And types of terrain we don't even have a clue what they are. And the incredible Sputnik Planum, something never before seen in our solar system: effectively a planetary mantle exposed to the atmosphere, an area devoid of crust where the underlying eutectic-ices slowly roil on the surface in supermassive convection cells, with icebergs the size of mountains riding around on top of them and collecting on the shores.

This is what you call a "big rock"? It's a heck of a lot more geologically interesting than half the "planets". And absolutely nothing like the sort of primitive, unaltered, inactive bodies that it's now lumped in with.

Comment Re:Wow, and I thought the existing Sednoids were n (Score 2) 134

Another thing I think argues for a universe full of planets: star frequency is proportional to size. The largest are rarest while the smallest are the most common. This continues all the way down: M class stars (red and brown dwarfs) make up 75% of the stars in the universe. We have more trouble estimating brown dwarf counts than red because they're not easy to observe, but they appear very abundant. But once you get below the cutoff for D-D fusion... we just can't see them. Why should we assume that the distribution just stops at brown dwarfs?

Comment Re:There is a 9th planet (Score 5, Interesting) 134

You mean 10th. You forgot about Ceres.

Under no reasonable standard is Pluto 9th. 10th, fine. I'd go out on a limb and argue at least 17th, also adding in the "planetary moons" that meet a hydrostatic definition even better than Pluto: Luna, Ganymede, Callisto, Europa, Io, Titan, and Triton. Using "planet" on the basis of of "body large enough to assume hydrostatic equilibrium but not undergo fusion" and moon as "body that orbits a planet".

Hydrostatic equilibrium is a very meaningful definition. A body not in hydrostatic equilibrium is made of primitive materials; it's the sort of place you'd go to learn about the formation of our solar system. A body in hydrostatic equilibrium has experienced internal heating, movement of fluids, chemical reactions, etc. It's the sort of place you go to learn about geology and search for life.

Or if you'd rather, you can apply the Captain Kirk test. Put it up alone by itself on a viewscreen. Would Captain Kirk say "Beam me down to that planet" or "Beam me down to that asteroid?" It's silly, but it's basically another way to say, "is the word functioning as normal people would use the word?" Of course, if there was another, bigger body in the background, they might say "beam me down to that moon". But we've all seen sci-fi where we're told that a body is a moon but people keep accidentally referring to it as a planet. The Forest Moon of Endor, for example. If it's a gigantic round thing, a part of us wants to call it a planet, even if we also know it's a moon. No reason not to just have them both as descriptive terms: a "planetary moon".

(And IMHO, if there's anything we should be kicking out of the "planet" club, it should be the gas giants... followed next by the ice giants. Seriously, how much is Jupiter like Mars?)

As for Stern, he once presented a rather interesting classification scheme (ironically, in the same paper as the Stern-Levison parameter was proposed). Basically, forget about all of these nouns, just have a good list of adjectives. You can have various things from sub-dwarf planet to super-giant planet to indicate the mass; prefixes like "gas" or "ice" or "rocky" to indicate the character: other adjectives to indicate its orbital parameters (including its "neighborhood" if you prefer), etc. Why limit yourself? Cite as many adjectives to describe it as are appropriate to the situation.

Comment Re:If the singularity doesn't happen... (Score 4, Informative) 134

First off, there is no "theoretical maximum speed" to how fast a given propulsion mechanism can get you. You can get to 0,999c by shooting tennis balls out the back of a spacecraft with a slingshot - if you're willing to build a spacecraft comparable in size to the universe ;)

Secondly, nuclear pulse drives really are an antiquated idea, I don't know why people obsess over it. Their minimum sizes are way too large and they're inefficient, with low ISP compared to more modern ideas. Longshot, BTW, is technically NPP, although a more modern variety. Still inefficient and very heavy, and nowhere close to a technology that could be achieved in a reasonable timeframe from where we are today.

Of the many, many concepts now available, I'd personally go for fission fragment propulsion. It's so straightforward: get the most out of fission by having individual fission reactions propel your spacecraft directly. And from a design perspective, it's pretty straightforward particle physics / fission reactor design, just in an unusual (suspended) configuration - the suspension already demoed in the lab. But that's, again, just one of many possibilities.

Comment Re:If the singularity doesn't happen... (Score 1) 134

Unfortunately, no. Their gravity is far too weak for them to provide a significant "slingshot" effect.

Also, the fact that there are many of them isn't really the big help that it might sound. One, there's many in a very large volume of space. Two, they have very different orbits. Even if two are physically "close" to each other in a given location at a given point in time, you still have a lot of delta-V to overcome.

Comment Re:Wow, and I thought the existing Sednoids were n (Score 3, Interesting) 134

I don't think the process of exchange can be fast - if those bodies had galactic escape velocity, after all, they wouldn't stay here for long

They don't have escape velocity; they're stuck with us until something perturbs them. But the key point is that when something is that far out, it's very easy to perturb. And our stellar neighborhood is not static. Indeed, one of the alternative theories to explain the sednoids is that rather than a planet X, the orbits are due to one or more stellar passes nearby our solar system.

So far we're still not seeing very far out, we're just barely spotting these things, and only when they're near perihelion. There's much more out there yet to discover, and so far all signs point to that our solar system doesn't just "stop" anywhere, it just keeps on going. Heck, we only know about the Oort cloud because comets have such distant aphelions.

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