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Comment Re:Not to be a wet blanket... (Score 1) 197

Yes, wake me up when you've recreated Earth's vast diversity of industrial infrastructure on the moon.

Spacecraft are incredibly complex thing, and you're proposing to build them on a place where you're starting with absolutely nothing. And why? To save launch costs? Yes, launch costs are expensive relative to peoples' everyday experience, but they're only a (ever-diminishing) fraction of the cost of a whole mission.

If you're planning to wait until you can outright build entire spacecraft on the moon, you're planning on pushing Mars missions off by many generations. Even the concept that simple raw, bulk sheet metal of even comparable quality (and thus mass) to that available on Earth will be produced on the moon after two decades of high budget dedicated effort straddles the line between "crazy ambitious" and "crazy". Let alone being able to build it into something of relevance with sufficient reliability, and let alone being able to produce it at a rate that, after factoring in consumables that you have to ship from Earth to keep workers alive and all industrial processes running (consumable feedstocks, maintenance, etc) isn't vastly higher than on Earth.

There is absolutely nothing "cost saving" about operating on the moon; it is a huge money sink, and will continue to be so for generations. The same with Mars. You don't go there to save money, you go there as a very long-term investment in the future.

Comment Re:No Dragon 2 Soft Landing Yet (Score 1) 197

Well, they did produce that huge carbon fiber tank. Which appears to have failed during one of their pressure tanks. Really, building such a huge rocket out of composites is crazy ambitious (if not just crazy), but my hat goes off to them if they can succeed.

They've also made a mini-Raptor that they've started putting through tests. The fact that they've apparently managed those chamber pressures without corrosion problems so far is very impressive.

It occurred to me the other day that they have an interesting potential "halfway" route to ITS, which is that since they clearly plan to have different variants of the spaceship (cargo, crew, tanker), they could start off with the cargo variant and instead of a cargo fairing, have an interstage and use that to boost an elongated Falcon 9 (like the Falcon Heavy central core). So the spaceship would function as a first stage until it got its own booster so that it could function as a second stage. It'd be a perfect testbed for their new technologies (same construction style and engines as the booster, just smaller), while at the same time boosting SpaceX's launch capabilities into the super-heavy range. They'd want to use more atmosphere-optimized nozzles, but apart from that... it's already designed to handle much greater heat loads as well as full propulsive landings.

Comment Re: No Dragon 2 Soft Landing Yet (Score 1) 197

He's first going to have to learn how to launch that fast. That's one area where SpaceX hasn't had much success - getting its launch turnaround times down. Hopefully they will in the future. Also, since an explosion takes them out for half a year or more (regardless of turnaround times), they better up their reliability by an order of magnitude or more, since each increase in launch rate means more possible rockets that can fail. And of course they want the ITS booster to have a service life of 1000x launches, which means an immensely high reliability.

Anyway, SpaceX's big goal is to have their satellite service give them a nearly unlimited demand for launches in the coming decade, as well as a correspondingly huge income from global sales of satellite net / communications services - and to funnel those profits into ITS. Time will tell... but there's certainly no shortage of ambition.

Comment Re:Not Happening Anytime Soon (Score 1) 196

That's the biggest concern I have. People tire of ongoing expenses. ISS seemed neat at first; now everyone hates it. Why would a moon base fare differently?

Long-term presences in space need to very quickly cut ties with earth, on order of greatest resource dependencies down to smallest resource dependencies. Aka, first things like oxygen, propellant, etc, then to industrial chemicals, of increasingly smaller quantities, with increasingly diversified manufacturing facilities, with very complex/low volume chemical feedstocks and manufacturing processes coming last. Cutting all ties is a process that would take centuries. But you can start with the low hanging fruit, bit by bit, and keep stockpiles of everything needed for maintenance that you can't produce locally.

Unfortunately, running counter to this is expansion. Because if you double the size of your operations, you also double your resource demands. So you need to improve resource independence at a faster rate than you grow.

Part of the problem with the moon is that it's just not a great place for ISRU. Volatiles are rare. We've never even sampled any moon that aren't depleted in volatiles, although there's some data to suggest that various volatiles might be scattered in permanently shaded areas (all of them, in the same place? That's a good question). Surface mineral diversity is limited - primarily light, non-volatile elements. Oxygen is at least widely abundant, but locked up tightly. And while the moon offers short transit times, it's surprisingly not that advantageous concerning delta-V. You can't aerocapture there, landing is fully powered (no parachute deceleration), and to get there you have to already be on such a high apogee orbit that it's not much more energy to go into a Mars transfer. Gravity is less and night is two days long. There are a couple "maybe" peaks of eternal light, but that doesn't mean that they're colocated with volatiles; the last I looked into it it looked like the closest suggested find of water was dozens of kilometers away from the nearest such peak, which would be quite the commute (and thus low throughput / high wear).

The moon is certainly the "cautious" option; emergency returns / resupplies are easy there, and communication fast. Its main value appears to be a testing ground for systems while minimizing risk. But it's not a very appealing place from a settlement perspective.

Of course, I prefer Venus to Mars, but that's neither here nor there ;) I'd like to see a parallel program for both, as the same sort of booster and transfer stage can be used for both, so it's only habitat / ascent stage development costs that are doubled. And once you get past the differences in feedstock sources, production industrial processes converge (Venus advantaged by the higher power availability and easier ability to get rid of heat - excepting in the case of cryogenics, where Mars holds the advantage)

Comment Re:Younger astronauts (Score 1) 196

One, there would be howls of protest. Two, you're not taking that argument to its logical end. You should only send pygmy women by that logic.

Women do consume less resources (by a good margin on average) and take up less space, but if I recall correctly are more vulnerable to radiation-related disease. So it's a tossup depending on what factors are constraining your mission architecture.

Comment Re:Rockets are too expensive (Score 1) 196

I have read the book, and it's an absurd degree of wishful thinking. Just ignoring the huge number of things that they just gloss over or omit outright, the materials technology they're talking about is about two orders of magnitude away from what we actually have, and might even be physically impossible. Measurements of individual carbon nanotubes (let alone bundles, let alone bulk fibres) don't approach the strengths being talked about there. Colossal carbon tube does better on an individual tube basis, but again, we're nowhere even close to the materials tech required. And for what? For a massive, very low throughput, tiny safety margin, most-failure-modes-unaccounted-for, low-power-efficiency means of access to space? Colour me unimpressed.

If you want something better, I recommend looking into Lofstrom loops (launch loops). Current materials tech, high efficiency, high throughput per unit mass, no orbit restrictions, and works even on tidally locked bodies.

Comment Re:Rockets are too expensive (Score 4, Interesting) 196

Quite true. The materials technology required is about two orders of magnitude away from actual materials technology, for starters. And among the countless other problems with space elevators, they're not actually all that efficient. Laser power beaming over those distances works out to single-digit transfer efficiencies, and microwave power beaming even less (microwave power beaming to space can be efficient, but only if the receiving antenna is huge). And no, you can't regularly hang things or run power wires up a space elevator - the mass of the cable has to be vanishingly small.

Active-suspended structures, such as Lofstrom loops, are a much better choice. Power transfer efficiency can be greater than 50% and current materials technology should be sufficient. They can also be designed to shoot payloads into any orbit (unlike space elevators), and work independent of the properties of the body in question, as well as having far greater throughput per unit mass. There's really no reason to choose a space elevator over a Lofstrom loop.

Comment Re:Too good to be true. (Score 2) 187

It doesn't work like that. Radiative heating/cooling works via exchange of IR. You're not just giving it up; everything you're radiating at is proportionally radiating back at you. So you cool the most when you're radiatively exchanging with something that's very cold. Aka, you want to be radiatively exchanging with the cosmic microwave background, not with low-altitude clouds. That's the whole point of radiating at low absorption frequencies in the atmosphere: so that you're exchanging with space, not with atmospheric air.

Comment Re: Fake News (Score 1) 273

1. That was just an old theory, and not a widely accepted one.

2. Given what we've just seen, it demonstrably isn't.

That doesn't mean that there aren't compounds formed at great pressure that can remain stable at moderate pressures and represent very dense energy sources - there surely are. Metastability is a very real thing. But apparently not in the case of metallic hydrogen at ~STP.

Assuming that this actually even was metallic hydrogen; even that is somewhat in dispute.

Comment Re:Maybe (Score 1) 206

Indeed, on both counts. And in particular I like the word "rogue planet". Again you have an adjective imparting additional information about another object ("Rogue X"), "rogue" can be readily quantified ("Not in a stable orbit around any particular star or cluster of stars"), and it's a very evocative term. And rogue planets are absolutely expected according to our current models. They'll be incredibly difficult to find, but they're out there.

We're also coming to the realization that there's a lot of objects, potentially including large ones, that are only tenuously bound to our solar system. And it's likely that we readily exchange this mass with other nearby stars over cosmologic timescales; parts of our solar system (primarily distant ones) likely formed by other stars, and things that condensed during the formation of our star system are likely now orbiting other stars.

Comment Re:Maybe (Score 1) 206

The short of it, Jupiter moves things around; it's very good at scattering other bodies, even large ones. First it dragged outer populations into the inner solar system, then scattered inner solar system material out, and then on its retreat pulled outer solar system material back in. It's actually a very big deal that it did that, as it brought ice into the inner solar system.

Comment Re:Maybe (Score 2) 206

1. "Adjective nouns" need to have similarity to "noun" but aren't necessarily a subset. Gummy bears aren't a subset of bears either.

Gummy bears are not a scientific term. Besides, the IAU itself already uses the word dwarf in this manner. Dwarf stars, dwarf galaxies... but carved out an inexplicable exception for dwarf planets.

I'd like to see a citation on this. I highly doubt that you can simulate the formation of a solar system where multiple Mars analogues can coexist in the same orbit

False equivalency. There's a difference between "two Mars sized planets existing in the same orbit" and "Mars' orbit having been cleared". And more to the point, the biggest problem with the concept of Mars clearing its orbit is that its orbit was already largely cleared when it formed. According to our best models, Jupiter reached all the way in to around where Mars' orbit is today, and had cleared almost everything to around 1 AU. Earth and Venus accreted from planetesimals between each other. Mars accreted from planetary embryos ejected to the space in-between Earth and Jupiter. Without Jupiter's migration, simulations produce an Earth-sized Mars and several planetary embryos in the asteroid belt on eccentric / high inclination orbits, something akin to the situation between Neptune and Pluto - except with the embryos nearly Mars-sized.

3. In a geological sense yes. But the current definition of planets is based on orbital mechanics, after which Earth is a lot closer to Jupiter than to Ceres/Pluto.

Huh? By what aspect of orbital mechanics? By semimajor axis and velocity, Earth is much closer to Ceres than Jupiter. Are you talking inclination and eccentricity? Then we should boot Mars in favour of low inclination / eccentricity asteroids.

4. Hydro-static equilibrium as a dividing line is way worse. There are roughly 100 TNOs where we don't really know whether they are elliptical.

Hydrostatic equilibrium can be very easily estimated based on mass, which can be approximately deduced within a range of feasible albedos and densities, and very accurately deduced if the body has a moon. By contrast, it's almost impossible to estimate neighborhood clearing to any distance beyond Neptune, or at all in the case of extrasolar planets. Which, to reiterate, the IAU definition says aren't planets, even though they have an extrasolar planet working group.

We'd have to visit each and every one of them with a probe just to put them in the proper category.

This is utter nonsense.

Meanwhile, it's completely clear which bodies qualify for the "clearing its orbit" rule.

No, it's not. We have virtually no clue what lies in the outer reach of our solar system. As we speak there's a search for a new planet that could be as big as an ice giant. It's a huge open question as to whether it would have cleared its neighborhood, and it will be very difficult to ascertain.

All currently qualifying planets have roughly 99% or more of the mass in their orbit in themselves. Ceres has 30%.

You seem to have some weird concept going on that "semimajor axis = orbit". Ceres has nothing of significance in its orbit. The asteroids are not all in the same orbit. They're certainly more likely to cross each others orbits, but that's not the same thing.

And again, since you apparently missed it: the reason that the inner solar system is largely cleared except for the asteroid belt (and the reason that the latter exists) is Jupiter. Mars did not clear its own neighborhood.

5. The definition should be mutable. Why should a planet that gets ejected keep counting as a planet?

You seriously have to ask why something that hasn't changed but is in a different location shouldn't suddenly be declared to be something entirely different? If you take a rabbit to Canada does it suddenly become a dwarf rabbit?

6. I highly doubt life could form in a non-cleared orbit.

Once again, you're stuck on this misconception that the only orbital parameter that exists is the semimajor axis. And also apparently a notion that stable orbital resonances don't exist.

Orbits can come in a wide range of forms. If you want to see how crazy they get, check out Epimetheus and Janus ;)

As for a life bearing celestial in orbit around another (gas giant) planet: I don't think anybody feels bad about calling that one a moon? As in "Yavin 4".

The funny point with your example being, that whenever you illustrate a large round (hydrostatic equilibrium) moon in sci-fi - Star Wars, Star Trek, Avatar, whatever - people invariably keep calling it a planet and having to correct themselves. We inherently recognize "large, round object with relevant gravity = planet", and have to shoehorn our minds into not using that term.

7. "Within each other's periapsis and apoapsis" seems like a reasonable enough definition that neither Ceres nor Pluto qualify for.

Once again, you ignore most orbital elements (seriously, stop right now and go read the Wikipedia article on orbital elements). We don't live in a 2D solar system. And your notion is oversimplified even for 2D.

All of this, let alone other aspects such as mass ratios, resonance, metastability, etc. And it gets even more complicated when you view the solar system not as a 2-body problem but a multi-body problem. Then things like horseshoe orbits, Lagrangian points, etc come into play.

8. Yes that's silly but that'll probably be changed easily enough and has no effect on Pluto.

1) It's over a decade later. Where's the fix?
2) It's just a symptom of how horribly hasty and ill-thought-out their action was.

9. How are you planning to ascertain hydro-static equilibrium for an exoplanet if we can't even do it for Varuna.

What are you talking about? Varuna is the size of Ceres. The fact that it hasn't been declared a dwarf planet by the IAU is again a symptom of the IAU's dysfunction on this issue. See #18. By contrast, we'd have no snowball's chance in hell of identifying all potential orbit crossers for it.

The fact that you bring up Varuna makes me think that you feel it shouldn't be a planet because it's an oblate spheroid. If so, that just reveals yet another problem with your understanding: you need to go look up the definition of hydrostatic equilibrium. Hint: if Varuna wasn't an oblate spheroid, then it wouldn't be in hydrostatic equilibrium.

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