Seret

Surely if you lost magnetic confinement the plasma would impinge on the walls of the vacuum vessel, which also presumably contains the coolant? That seems like it could be pretty explody. I presume they're designed with a containment building around them for just this reason, similar to a fission reactor. ITER's website simply mentions a "reinforced" tokamak building, but it's unclear whether that's to stop stuff getting in or out.

Tbh you need both. Demand definitely stimulates a quick response from commercial entities, but there's also a role for governments to play by forcing supply side changes through policy. You often get chicken and egg problems where there's a lack of demand due to a lack of supply. Individual suppliers also don't have a strategic viewpoint (and neither do consumers for that matter).

It will still generate a fair amount of low- and mid-level nuclear waste, due to the high neutron flux in the reactor. It won't generate the more troublesome high-level wastes you get in fission reactors, but it'll still need to be disposed of in specialised waste streams. A containment failure could be pretty frisky too, you'd want to make damn sure there was no chance of venting your nasty plasma to atmosphere.

That's because it's not realistic to think that we could stop using fossil fuels any time soon. It's too much of a fundamental shift in the infrastructure underpinning our civilisation. Change will come, but it will take time, and we'll need to use transition technologies before we reach a really low-carbon system. This won't be done in a decade, or in ten decades.

Well, since exploration has been proceeding exceedingly well since the end of manned exploration it looks like it'll live. The whole manned/unmanned thing is a bit of a pointless distinction IMO. If you take a systems view of an exploration project there are always humans in the loop. From a strictly technical point of view it's most efficient to have the humans sitting back here on Earth overseeing the actions of the remote part of the system. Our probes aren't sophisticated enough to be fully autonomous yet, they're essentially humans exploring by telepresence.

The problem with that is that the price going up also makes formerly uneconomically recoverable fossil fuels viable. Many of the figures you hear quoted for "X number of years of coal/oil/gas remaining" are just the economic reserves, which means the number can go up as the price rises. That's why we've been saying we've got 50 years of oil left for quite a while now. However, it's still a finite resource so we can't play that game forever. We don't have any choice about whether we wean ourselves off fossils fuels, just about when we choose to do it. I suspect we'll still be using them in a hundred years the way we still consume vast quantities of coal, which is a Victorian age energy source.

What was it about the device that was supposed to be special? What you've described there is just atomic fission, which is how every nuclear reactor works.

I don't think he was being entirely serious.

IIRC Frank Drake originally conceived it as a nice way of expressing the agenda for a convention. The fact it's endured shows what an inspirational thinker he was. What is interesting about the equation is the way we're now starting to fill in the blanks a bit. The last few years have been a bonanza of discovery about the abundance of exoplanets, and we're starting to get a rough idea about the distribution of environmental conditions like gravity and insolation. Still way more guesses than data though.

There's no one solution, we need to be doing a whole lot of stuff to transform our energy systems. Energy demands are going to increase substantially, we're going to need to be using everything we can get our hands on to satisfy it. Demand side Domestic and commercial Houses need to be superinsulated, including retrofitting existing stock as much as possible. We can already make homes that have very limited space heating demand even in cold climates, their main heat demand is hot water. Solar hot water and PV are proven technologies, these should be on every roof. In hot climates that require aircon roof PV should be mandatory. Reversible heat pumps and microCHP should be used, and smart grids to deal with all the microgeneration and eventually distributed storage. Energy pricing should be fluid and based on the instantaneous carbon intensity to incentivise demand smoothing technologies like storage, which could possibly use parked EV batteries if they can get the pricing right. In cold climates we need to see district heating using CHP, same fitted to every large building (possibly using absorption chillers for cooling demand). There's an opportunity there to use waste streams like cooking oil converted locally to run these on biodiesel. Industrial Lots of good stuff going on here about load shedding and smart motor drives, just keep pushing for better efficiency. Generally industry responds quite well to efficiency opportunities because they're well aware how waste affects their bottom line. Transport Short term keep pushing for high fuel efficiency through improved ICEs and weight reduction. Further electrification. Transport is a tricky one though, it may be an appropriate place for a hydrogen economy, but this is only worth doing if there's an abundance of clean generation (I have little faith in CCS being a realistic way to produce reformed hydrogen). Supply side Stop burning coal ASAP. Replace coal base load with nuclear (including ramping up the thorium cycle as an alternative to uranium) and top up with gas. Keep stacking in renewables as fast as possible, of all sorts (wind, solar, wave, tidal, large hydro, small hydro, etc). Increase interconnectors and set up regional supergrids for areas that lack them. Build smart grids, including grid-scale and localised storage. Give grid operators control over potentially everything that's plugged into the grid so that they can manage demand by shedding or dumping loads (eg: refrigeration/HVAC, EVs). Honestly, there are about a zillion things we need to do to build a more sustainable and cleaner energy system. Some of it we're doing already, some we're seriously dragging the chain on. Some of the answers are technical (Hello fusion boffins! How you getting on?) some are economic, some political, some cultural. Change isn't going to come easy, but it's worth doing. It's only going to get harder the longer we leave it.

Anything as long as it doesn't invoke Rule #34.

Hmm, that's a lot of certainty to hang off a big bag of whatifs...

Needs more patterns. Try out some tiger stripes.

That's you sorted with monopropellant for the next hundred years then.

Possibly not a prefect analogue, but I think the point I was making about the ability of the target to provide enough resistance to force the projectile to transfer a lot of energy stands. Not that it really matters, because nobody is going to be firing rail guns at orbital debris anyway.

This is the bottom line really, and why we just don't send humans to do the job. We can get it done using a robot on the kind of budgets space agencies actually get, and at levels of risk that are palatable to everybody involved. If we tried to do the same using a human on that budget we'd be sending one mission every few decades, still with a dicey chance of success. We do exploration the way we do (ie: unmanned) because it actually works.

Indeed. The main role that humans in space can serve is learning how to put humans in space. The robots have got the long-range and long-duration exploration covered, but if we're ever going to get a foothold outside Earth then we need people up there doing the research. Exploration isn't something humans are well-optimised for, the machines have us beat in terms of endurance, support requirements, and tolerance of the conditions likely to be faced.

Couple of quick points: At the top of it's arc it's not going to have anywhere near the muzzle velocity, so it's would be bringing a lot less KE to the party. Spacecraft are built pretty lightweight, and a KE projectile of that type would struggle to transfer a lot of energy into it. Regarding that second point, tanks often find this when using their (extremely high velocity) guns on light armoured vehicles or softskins. High velocity small diameter penetrators are devastating against well-armoured targets, but against less tough opponents they simply overpenetrate. The target offers so little resistance that the projectile goes right through both sides without losing much speed (and therefore without delivering much energy into the target). There are numerous stories of lucky armoured personnel carriers being hit through the (unoccupied) troop compartment by sabots, with no real effect except a bit of additional ventilation. Many APCs are, like spacecraft, made from aluminium and it's telling that KE rounds aren't used against aircraft, which are constructed the same way. Tanks always carry an explosive round for the main gun in addition to their KE rounds, and they use this on light vehicles because it's much more likely to get a kill. Now, I'm not saying a rail gun hit won't completely mash a spacecraft that it scores a direct hit on, because it will, but the actual amount of energy dumped into the target could be well below what you expect. Hits to peripheral equipment such as solar arrays would do a negligible amount of damage. You'd just get a nice neat hole the same diameter as your railgun round.

There's no question unmanned missions generate a greater return of science output for the investment of resources. The fact we haven't sent any manned exploration missions for decades should be the first clue. It's not even a topic of discussion IMO, unmanned is the default choice. The analysis would have been done over and over, and we've never selected manned for any of our planetary exploration. Essentially it's a technology problem. If we sorted out cost to orbit, long distance propulsion and closed loop life support then that might swing the balance a bit more towards manned, but right now it's just not practical.

That's colonisation, which isn't on the cards. Exploration is about acquiring new knowledge, sounds a bit sciency to me. Science output is the goal of exploration missions, it's how their performance is measured. You personally may have different goals and expectations, but if they're not aligned with those running the space programmes you're probably just setting yourself up for disappointment.

Er, no we didn't. Not even a little bit. If anything the pace has accelerated as our capabilities have increased. We don't need to send humans to explore the solar system, we're doing a grand job without them.

That's about the only way it would work, but it would still be pretty risky and it'd involve positioning your gun at exactly the right point on the globe. Impacts are messy, getting the projectile to transfer all of it's energy directly along a retrograde axis would be pretty much impossible. Some of your debris would even have more energy after the impact than before.

To be practical the original spacecraft would have had to be designed with this in mind originally. In reality things like discarded stages are really only designed to operate for their expected useful life (which is very short). Re-use once in orbit would have to be designed in, the same way that ground maintenance activities like replenishment and inspection need to be designed in.

Sorting it would be an unbelievable nightmare too. What the hell do you do with composite materials? Pretty much everything used in a spacecraft is a composite or an alloy, breaking these things down to their constituent raw materials and reworking them into a form where you could even feed them into a manufacturing process is often uneconomical down here on Earth. Doing it in orbit is just a little bit crazy. We are such a long way from being able to carry out manufacturing processes in orbit, let alone doing it to aerospace standards. What the OP is proposing is such far-future tech that's it's difficult to say anything coherent about it.

The main vote for water is that you need to bring heaps of it with you anyway.