Piscator

Well, one of the few things Gateway's proposed orbit is actually good for is providing an uninterrupted line of sight to Earth, so this would probably work quite well even without relay satellites.

The storage of genetic information is actually an area in which I would see a lot of room for diversity. You would likely need some kind of polymer made of pretty much the same chemical elements as DNA (as those are the most available und useful for this purpose) but after that, all bets are off in my opinion. After all, even earth life commonly uses two types of macromolecule with slightly different backbones and bases (DNA and RNA) and apparently it's not so rare to find bases other than the five regular ones in nature. (It seems even artificial ones have been successfully created and introduced into gene sequences.) If you consider that there are about 30 variations of the genetic code as well, it seems highly unlikely that life of extraterrestrial origin would use a system that would closely resemble ours. There seems to be too much room for variation.

Well, being able to reposition your sensory organs quickly and energy-efficiently has its advantages, especially for a species which relies on vision heavily but also has a rather narrow field of view.

In addition to what kerbiloid said, the appendix also seems to serve as a reservoir for gut microbes in case of ... severe discharge events. As for the main point of the discussion, I wouldn't say that life as we know it is the only option (in reality as well as in fiction) but as long as we're talking about chemical life some similarities are probably inavoidable. The number of elements suitable for building macromolecules is severely limited, so I would expect other forms of life to use them roughly in the same way and for the same purposes we do. On the other hand, even among known life there is a huge amount of metabolical pathways that could be used as a basis for speculation. There's - as has been rightly pointed out - quite a lot of exotic stuff going on right under our noses. In regard to the atmosphere first approach, it would probably have been a good first step to check what would make sense planetologically. A methane/oxygen atmosphere would be chemically unstable (possibly even explosively so). Same would be true for ammonia, hydrogen sulfide, hydrogen(!) and carbon monoxide in combination with oxygen. Basically any highly reduced and highly oxidized compounds would tend to react with each other over time, making the atmosphere more or less unstable in the long run. Large amount of noble gasses would also be tricky, since they are either quite rare in the universe or tend to get lost from the atmospheres of planets the size and position of earth due to their low molecular mass. For the same reason, hydrogen compounds would likely be quite rare on an earth-like world, even in the absence of free oxygen in the atmosphere. Hydrogen - freed by photochemical reactions - would steadily get lost to space. For a world comparable to our rocky planets, this leaves us basically with the classics: nitrogen, carbon dioxide and oxygen (if sufficiently replenished). It might get interesting again though, if we increase mass and/or the distance to the star.

Getting argon to react is indeed very difficult. More importantly though, it's completely pointless in the first place. Argon has no known role in "life as we know it", because forming those exotic flourine compounds is basically all argon can chemically do. It's not that earth life is somehow uncreative or not trying hard enough, it's the fundamental properties of argon that make it useless in biochemistry.

The "life as we know it" argument tends to be a bit overplayed in my opinion. With exactly one data point (and a still somewhat rudimentary knowledge of it), we simply can't tell if our kind of life is one of hundreds of wildly different arrangements or in fact the only type possible in this universe. While I don't consider the latter hugely likely, I would also guard against the "anything goes" approach. For every "hey, why not?" there's usually an "oh, that's why" and many of the ideas we consider plausible are simply a result of our lack of knowledge. (I'm looking at you, argon-fixating bacteria.) That said, it seems like a safe bet to assume that nitrogen would play an important role in the biochemistry of extraterrestrial life (the number of elements that can "combine with some reasonable amount of complexity" is surprisingly limited). I disagree though, that this would require huge amounts of diatomic nitrogen in the atmosphere, which is an exceedingly hard molecule to utilize biologically. Since molecular nitrogen is a rather convenient end product of metabolic processes involving nitrogen species, I would expect at least some build-up in the atmosphere (which would in turn give organisms an incentive to develop nitrogen fixation), but I'm not sure this would need to result in more than trace gas levels. I might be pushing the analogy too far, but after all, carbon-fixating organisms get by with very little carbon dioxide in the atmosphere too.

I think, the point was that the static fire is part of the quality contol scheme. Like, check your engines and your heat shield at the same time. This obviously only works if the tiles that came loose were actually faultily attached in the first place. If we repeat the procedure and a whole different set of tiles comes down, the attachment method definitely needs improvement.

I assume the atmosphere as a whole survives because it is constantly replenished, rather than because of the individual molecules staying around for any serious amount of time. There might be a gas torus effect though, in which escaping gas molecules remain in orbit around Saturn and occasionally return to the moon.

From context, I would assume that the tweet was made with more of a "megapixel" kind of resolution in mind, rather than the strict linear definition. That is, the idea seems to be to use a mirror with ten times the area of Hubble's which would fit nicely into a Starship hull.

Considering the much-discussed problem of perchlorates in the Martian soil, I find the claim that "there's no chlorine on Mars" a little surprising. (Sodium is present too, by the way.)

You wouldn't launch your rocket "straight up" though. Inclination doesn't mean the inclination of your launch trajectory in regard to the ground, but the inclination of the resulting orbital plane to the equator.

The problem seems to be that the non-magnetic cargo and/or passengers would be squashed flat.

You shouldn't forget that each use of your cannon imparts an impulse on it (opposite to the impulse imparted on the payload). That means that you'd either have to use propellant after all or cleverly time your shots in such a manner that your orbit remains stable, for example by firing in opposite directions or firing half an orbit later. PS The latter method (launching the payload in the same direction on opposite sides of your orbit) wouldn't work, as it would decircularize the orbit rather quickly.

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?