Piscator

I guess instead of using a cryogenic procedure it would be easier to sequester carbon dioxide chemically as carbonates. It might be as simple as bubbling air through a large vat of dissolved sodium or ammonium hydroxide, then dissociate the carbonate by heating where and when you need it. Since the sabatier reaction is exothermic and needs to run pretty hot, there might be some nice synergies you might be able to use as well.

I'm not sure we're quite on the same page here. My comment was meant in reference to the suggestion of using liquid nitrogen/oxygen as reaction mass rather than hydrogen. My point was that water would probably be a better choice, since it's easier to store and has a lower molecular mass than both of those gasses, thus providing a better ISP. I was not suggesting to use it in any ISRU processes whatsoever. The ISP of a NTR using water would probably be somewhat lower than that of a hydrolox engine (since both produce the same exhaust, but a reactor needs to run cooler to keep it from melting), but it might still be decent enough to outweigh the hassle of having to work with molecular hydrogen. That said, I'm pretty sure you wouldn't want a chemical and a (separate) nuclear propulsion system on your spaceship, since the additional mass would likely negate any benefits the individual systems would have.

When you're at the point of considering using liquid nitrogen or oxygen, you might as well consider water. It's easier to store and a lighter molecule.

Since the exhaust of some solid rocket motors can be assumed to consist of sizeable amounts of alumina, producing silicon dioxide might actually not be that terrible. At least if the design is kept simple enough. That said, if you're trying to process lunar regolith into rocket fuel, you might want to think about some kind of metal + oxidizer solid propellant or even a metal + LOX hybrid design. The less hydrogen you need, the better.

I think he said "a dozen times a day", which seems more reasonable.

Glad someone else did the math, although I think your calculation is off by two magnitudes. Using your numbers, the amount of stellar material swept up would be only 0.3 mg. Also, the human frontal area is closer to 5000 square centimeters rather than 500. This would give you 1.35 kJ of kinetic energy which is close to the 4 kJ I got. Still, quite a significant amount compared to thermal energies involved.

I have to agree. Winning a game by not explaining the rules is ... let's say ... a bit childish. That said, being teleported (in the above sense) to the surface of the moon for 10 minutes might actually be quite survivable, depending on where you land exactly. In the centre of the trailing hemisphere for example you would be catapulted (more or less) straight upward which would easily give you twenty minutes before having to worry about the moon's surface again. Of course, the teleport back would probably kill you then. Regarding the teleport to the sun, it's probably less the difference in relative speeds that wouldn't kill you (which is far more severe in the sun's case) but the fact that the sun's surface is not exactly a surface. Still, being bombarded by photospheric plasma at 30km/s is probably not exactly healthy.

Well, why make a star when there are already literally billions of them? Travelling a couple of light years to find a suitable one seems not all that complicated compared to building one from scratch.

I don't know about strong, direct sunlight but (uncorroded) pyrite is pretty golden under regular lighting conditions too, not just by torchlight. Also as a quick side note, the traditional value of gold doesn't originate from its interesting coloration alone but also from its mechanical and chemical properties, that is, its workability and high resistance to corrosion, as well as its relative rarity. (After all copper looks as interesting as gold color-wise but is less highly regarded because it's a more common element and has a tendency to patinize.)

The answer to this question seems to mainly depend on the available technology. If you have closed-loop life support systems at your disposal (which seems like a reasonal thing to have if you're shipping people around between stars) the capacity of a planet is only limited by your capability to build autonomous habitats. This wouldn't leave the planet exactly national park worthy though. By the way, is there a reason why you're race can't use birth control, that is, only get enough children to replace accidential deaths? Managing population growth this way is always easier than sending people off to space colonies.

Are you sure? The other flap could be bent in the opposite direction to not offset the center of mass.

Wait, so the colonizers are not humans but another alien race? Why bring cattle and horses then? Or am I interpreting these terms to narrowly and you're talking about the respective equivalents in their home biosphere?

Well, even without self-replicating machines, you can still produce them. You just have to bring the right infrastructure along. As for why there are few large-bodied venomous species on earth, there's probably more to it than sheer luck. Maintaining venom glands is not without cost, so species will only have them if they absolutely need them. Since large species have other means of offense or defense due to their size, it's smaller species who most profit from venoms. On the other hand, since the fairly large Komodo dragon is likely to be venomous too, it's not exactly a hard rule (although the dragons are still small compared to their prey).

Seems a bit of a stretch to postulate an alien biosphere biochemically similar enough for their venoms to work on earth creatures, but working under this assumption, I don't really see the need to introduce our fauna at all. On the one hand - as has already been pointed out - you could try to work with what is already there and adapted to the ecosystem, on the other hand, why would you even need cattle and horses at all? If your technology is advanced enough to make an interstellar trip, it should be safe to assume you have more advanced modes of transportation or nutrition at your disposal. If you're set on your space cowboy scenario because yee-haw, here's something you might want to try: - Put your lifestock in protective suits - Pick an island, sterilize it, start from scratch (shoot any returning fauna on sight) - Ask the Australians how they do it

Would it in fact be greener, though? According to the numbers I could find, the fuel efficiency per passenger seems to be roughly the same as that of a Boeing 747 (12l per 100km for 6 passengers vs. 12l per 1km for ~ 600 passengers).

I wonder if the recent study took into account the role phosphonium compounds might play in the Venusian phosphine cycle. Phosphine is a weaker base than ammonia, but it should react quite readily with concentrated sulphuric acid nevertheless. This could mean that phosphine is regularly captured in the clouds and transported with the acid rain into the lower layers of the atmosphere where it would be less susceptible to photochemical reactions. On the other hand, I don't know if phosphine below the cloud layer would have even been detected by the methods employed in the study in the first place.

Existing in the sense of being a by-product of food sterilization or other applications of cold plasma?

[snip] It's literally impossible to make any kind of alcohol from AN unless you involve some kind of nuclear reaction to create the carbon.

Sounds like they might be working on a dedicated mission to Uranus . . .

Could you perhaps give (or link to) a short summary on how an orbital airship would work (especially how you would get it to orbital velocities) and why it would be more cost-effective than conventional alternatives? I get the impression we're not all on the same page here.

Actually, you probably wouldn't get an airship to work on Mars at all. As you mentioned, the Martian air is really thin. Since the lift depends on the mass of the displaced air, your lift/volume ratio would be a hundred times lower than on Earth. (I also don't think that the lower gravity would actually help, since the lift more accurately depends on the weight of the displaced air, rather than its mass.)

Not quite sure it has been explicitly mentioned yet, but using the two-burn strategy of going into solar orbit first, then choosing your intercept orbit is also always less effective than burning for intercept orbit directly because you don't make the best use of the Oberth effect of your origin's gravity well. Since this can amount to quite a noticeable saving in delta-V, it makes sense to wait until your planet of origin is in the optimal position and only then make a single transfer burn.

I love biological solutions to this kind of problem, but I see one or two problems with it. The biggest one is that the sequestered carbon likely wouldn't stay afloat indefinitely. When the organisms die, their remains would likely loose their buoyancy, sink into the hotter parts of the atmosphere and react with the oxygen they released earlier. Also, since there's no way to create chains of pure carbon as far as I know (and the biological generation of other modifications of pure carbon seems also somewhat unlikely), you would probably need more hydrogen than the Venusian atmosphere can currently offer to bind a sufficient amount of carbon.

Alternative plan: Produce 300 megatons of space turtle bait and hitch a ride. I'm not a 100% sure, but I think instead of a simple substraction, you would need to calculate the hyperbolic excess velocity, which would come out somewhat more favourable. The faster your exhaust leaves the gravity field, the less affected it would be by it.

Another interesting complication would be the gravity of the pushed object. For rockets this can obviously be ignored, but on a planetary scale it becomes a factor. If an engine's exhaust velocity is below the body's escape velocity, the exhaust would eventually fall back onto the body and achieve nothing. For the moon, chemical rockets would be sufficient (~4km/s vs. ~2.4km/s) but with a somewhat reduced effective Isp. (Not quite sure by how much though. I guess we would have to calculate the excess velocity for that.) For a body the mass of earth, we would need something with a bit more punch Isp-wise. In the realm of antimatter-propulsion, all of this can probably be safely ignored though.