Extra-orbital ejection via electric rail gun - If you can accelerate an object to a speed of about 40,000 miles per hour (plus a bit more to overcome initial atmospheric drag), you can fling it into interstellar space. The idea has great appeal as a solution to high-level nuclear wastes. I can envision a nuclear industry based on closed cycle, inherently safe, reprocessing, fast-breeder reactors designed to generate copious energy for some decades during which they "burn" the worlds accumulated warhead stockpiles of plutonium and enriched uranium, reprocessing their way to a separation of dangers from intensely radioactive wastes with short half-lives that can be allowed to burn themselves out on-site, components that can be transmuted to less dangerous forms, and ultimately a vastly reduced core of material that must be stored for a long time or gotten off the planet, by which time transition to the next era during which another sustainable source replaces the outgoing nuclear industry. Anyway, getting the really bad stuff off the planet is best if it can be done. Can it?
The reason I gravitate toward a rail gun or similar to get hot toxics off the planet is because I envision it as something that can be used continuously, forever shooting a stream of projectiles into infinite interplanetary space, never to harm anyone, until there is nothing left to rid ourselves of. With a coil gun, if you receive a package to dispose of, you put it in the gun and eject it from the planet. In contrast, if you were to generate hydrogen, say, and use it to power rockets to eject the hot toxics, it would be a big deal each time. What if the rocket blows up, etc.
Some thinking on how to most cheaply and reliably get hot toxics off the planet might be interesting. A rail or coil gun might not be a good answer. Surely people have thought about this before; I'll see if I can find their thoughts.
Hydroponics - I played with it once just a little bit, and think it might be fun to do it on a backyard garden scale. Dunno...
Heliostats - I've made two little proof-of-concept models, but nothing useful like this guy is trying to do. http://www.redrok.com/concept.htm
Passive heliostat - this idea, which has been bouncing around between my ears for years, is a passive heliostat that would be composed of an array of monuments with reflective facets oriented individually and carefully, but without any tracking functions (stationary), in a terrain near a concentrated solar energy collector of some sort.
Each site would be evaluated and designed individually. The idea is that each reflective facet will project the reflection in a trajectory throughout the day such that it starts on one side of the target at the start of the day, and then ends on the other side at the end of the day. The target will only receive energy from that facet for a short time, so as the sun's motion moves the facet's reflection off of the target, another facet's reflection moves in. And so on through the day. And the year. Every moment that the sun shines, a different set of facets reflects sunlight onto the target, while the balance of the facets reflect theirs to positions on either side of the target as they have passed through or await their turn to pass through the target area. Thus, the target receives concentrated solar energy all day long.
But what a waste of solar reflections not to put tracking mechanisms on the pedestals! Most of the reflected energy will NOT be reflected onto the target.
Yes, but the idea of "passive" is so that no maintenance is ever required (save maybe cleaning the reflective facets).
Plus, just because the majority of the reflection is NOT to the target does not mean you can't do something about that. What can be done to recoup some of that reflected energy?
To recoup some of the reflected energy, one envisions not one, but a series of concentrating solar energy collecting points arranged wherever it makes sense to place them based on the unique design for the terrain of interest. For example, it may be that by arranging the facets in a just-so way, areas can be defined for secondary curved reflectors such that, as the spots from many facets passing through or alongside the main collection point pass through the area, they are secondarily reflected at the target area, adding to the primary reflections' energy.
As the facets' reflections passed through and near the main collection area, their concentration would diminish rapidly with distance from the main point (because some would be intended for other seasons as well as nearby times). A point would be reached where yeah it's hot but not enough to put in collectors. Concentration will diminish as the reflections fan out. Variations on this idea would occur with each specific case.
Genetic algorithms come to mind. I am no expert on genetic algorithms, but the basic idea is that you have a computer program that can try a gazillion simple things in an environment where the combinations that tend to work get rewarded and get a competitive advantage over the other arrangements that were tried. Eventually, properties emerge from the whole, and these properties can be manipulated by how you set up the competitive advantage in the software.
A computer would be programmed with a model of each facet (where it is and how it is oriented on its supporting monument). If each facet's orientation were randomized, only a slight number of them would point at the target at any given moment. Most facets would not be rewarded and would be allowed to go on to the next go-around. How the rewarding is done depends on what kind of genetic programming you are doing and is outside the scope here, but I'll try to find some links for further reading.
Anyway, the computer model can orient the facet of each monument in the model any way it likes. After all, it's only software and data. Over and over the software changes the orientation of the facets so as to meet specific criteria of maximizing energy reflected by the array of facets onto the main target area over the course of the day and year. Specification of the criteria to be met is also method-specific and outside the scope here.
This is not a simple Monte-Carlo series of iterations with random inputs from defined distributions. Nor is it any sort of least-squares fitting scheme where model is matched to measurement. Rather, the genetic algorithm I'm talking about "learns" from "experience" as it tries different things. It's weird. I'll see if I can find links to video of some robots that were assembled from basic blocks that could, say, turn a notch, or bend some degrees back and forth, and connected to other modules at the ends. The success criteria programmed into the thing was that it learn to move about. After a while, the thing had learned to do that and it is bizarre.
Anyway, the fantasy becomes one where a patch of land is considered for this sort of solar energy collection. The location of each pedestal, in addition to the orientation, is an input parameter for each facet of the array on the terrain of interest. This could result in the discovery of interesting arrangements of primary facets and secondary facets (secondary facets reflect sunlight not from the sun but from other facets). Who knows, you might wind up with a complex arrangement of multiple reflections, where a downside might be that dirty reflectors radically diminish power. Reflectance (or dirtyance, if you like) would be factored into the model as, say, a function of time since last cleaning or rain or whatever.
In the end and in effective perpetuity, provided the facets remained reflective, energy would be concentrated from the sun onto the target, available for harvesting. Nothing to do but make sure nothing obstructs the facets' ability to reflect. The individual monuments supporting the reflective facets, plus the target location, would make up a functional monument that would go on as long as the pyramids because of the value of the solar energy the monument makes available.
Of course, if humanity converts the atmosphere into a perpetual cloud, the passive heliostat above would be useless. People who had never seen the sun would come to wonder what in the world it was for. Aliens would be thought by some to be responsible. Some would think it was a communications relay station among alien outposts, pointing to the various confluences that would emerge, like pattern out of chaos, as evidence of purpose. So let's hope humanity can avoid ruining the atmosphere and can make the transition to a time when most energy used by humanity will be like that involved in the monument of reflective faceted pedestals.
OK, enough daydreaming for now. One of these days maybe I'll actually look into these things, but experience at work tells me it'll be difficult unless there's a team of diverse talents available.