Or feeding it directly to a plasma thruster (basically a 200 kW microwave oven with a nozzle). Much higher exhaust velocity = higher fuel efficiency, but lower thrust.
> There's nothing for us out there, unless you are unusually attracted to radiation-blasted vacuum.
One could have said something similar about the American west, or Australian outback. Not vacuum, but a hostile environment. In fact, 80% of the Earth is inhospitable without the help of technology (the oceans, deserts, and ice caps). Slightly better technology will allow us to live anywhere in the Solar System.
Perhaps you see nothing out there, but I've done some real estate development in the past, and all I see are opportunities.
> If you can't click your fingers and have a fleet of starships so vast that they block out the stars, you don't live in a post-scarcity society.
Once upon a time fire was high tech, and fire-makers and fire-bringers could trade their fire skills for other stuff. Nowadays fire-making is absurdly easy and cheap - turn a knob on a gas stove, or flick a disposable lighter, and you have fire. It's not a skill you can trade for other stuff any more, and fire is no longer a scarce commodity.
What most people mean by "post scarcity" is not that everything imaginable is absurdly easy and cheap (your flick of fingers for starships), but that the necessities of life are easy and cheap. These are things like shelter, food, clean water, etc. A society where robots and automation supply those things for people, without them having to work themselves, is in that sense post-scarcity.
Some things, like beachfront property, Manhattan penthouses, and gold, are scarce for physical reasons, and advanced tech isn't going to change those physical reasons. But those items are not necessities, either. People don't get harmed by a gold shortage the way they get harmed by a food shortage.
Solar panels are not heat engines, so Carnot cycles are irrelevant. The best cells have now reached 46% in the lab ( http://www.nrel.gov/ncpv/image... ). The high efficiency cells use multiple types of semiconductor stacked up. Each type is optimized for a different wavelength. Note that cell is not the same as panel, because less than 100% of the panel area is cells. Multi-layered cells currently are used on spacecraft and in concentrator modules, because they cost more than single-layer cells to produce. The efficiency gain only makes sense for those applications.
1% is too high, 100 ppm is the correct number for a "good" metals-rich asteroid. However, most of the value in an asteroid is the bulk materials. A ton of metallic asteroid is worth at least $5 million if turned into something useful in high orbit, because that how much it costs to deliver *anything* to that orbit today. the PM content is 100 grams, worth ~$3,200. Anyone who thinks PM is the reason to mine asteroids hasn't talked to a mining geologist about ore values.
Right now, the best cost for getting stuff to GEO transfer orbit is ~$5000/kg. That includes propellant to complete getting to GEO, which is the desired destination. Hauling loads of asteroid rock to Earth orbit, and processing them for propellant could generate about 100 tons a year, thus worth $500 million/year, with 40-100 tons of starter equipment, and an additional 100 tons/year for each 20 tons more equipment (mainly more ore tugs to fetch rock faster). The trick is to do it at low enough cost to make a profit, but it's not orders of magnitude away, it's roughly profitable with reasonable development costs.
I was one of the contributors to this plan, and one of the big misconceptions is that NASA is the only player in space. In reality, worldwide space industry was $323 billion in 2014, and NASA's $17.6 billion only represented 5.45%. Most of the 1250 active satellites in Earth orbit are commercial ones, and a lot of innovation is happening in that arena. For example, ion thrusters for boosting to synchronous orbit are standard procedure these days, using solar arrays 2.5 times as efficient as the ones powering the Space Station. SpaceX is working on recovering their first stage so it can be used again, while NASA is going backwards on the SLS solid boosters. In the Shuttle era the boosters were 2/3 re-used, on SLS they will be thrown away.
As long as the developers bring in enough fill dirt and build higher up, that's fine. They could even dredge up some of the local dirt and use it to make marinas and lakes, then pile it up where the buildings go.
Your question assumes a static economic situation, which is not what will happen. Launching whole factories to process the raw asteroid rock will cost too much at first. What you want to do instead is launch a starter kit of production machines. You use those machines to make parts for *more* machines out of some of the asteroid, plus some parts you bring from Earth. As your collection of machines in space grows, you can make a wider range of items, and need less from Earth.
In addition to growing the factory, you devote part of the output to products for sale. You start with the easiest products, like water and carbon compounds for fuel, because most anything in space requires some fuel, and they can be extracted by simple heating. You convert H2O + C --> O2 + Hydrocarbons, which is common rocket fuel mix. As you are able, you can start to make higher value items, like spacecraft parts.
Along this path, you may find products and materials you can deliver to Earth for less than the ground-produced version. A first such item would be "pristine meteorite samples". Collectors, jewelers, and scientists will pay large amounts for them, and they don't require any processing, just delivery. Precious metals are often mentioned, but they occur in ratios of 30-100 parts per million in asteroids, meaning you have to do a *lot* of processing to get them out. It may not be worth the trouble until much later in the industrial development. But whenever your cost gets below the competing Earth cost, then go ahead and do it.
> I disagree mostly because solar really didn't get cheap until the Chinese began to flood the market with panels, around 2010-2011 or so.
It wasn't the Chinese so much as solar grade silicon production. Prior to about 2009, demand for silicon for solar cells was smaller than for electronics. So solar piggy-backed on existing silicon foundries. But electronics-grade silicon is expensive (~$400/kg) because even one defect can ruin a chip. Eventually solar cell production got big enough that solar-grade silicon was worth it's own foundry lines. Defects in a solar cell just degrade the output a little bit, they still function just fine. The lower quality product was much cheaper to make ($18/kg last time I looked). Since the raw silicon was a major component of final panel cost, you had dramatic cost reductions for a few years.
Now we are back to more incremental cost reductions, but the panels are now so cheap that the "balance of system" (panel mounts, labor, wiring, inverters or transformers, permits, etc.) is the majority of the cost, and that's where work is being done to reduce them more.
> The real question is, is she bound for Prison or will Obama pardon her.
I'm sure Ed Snowden can give her some relocation advice:-).
The hypocrisy would be particularly pungent if she gets a pardon for mishandling classified data, and Snowden doesn't. The law only applies to peons, right?
> The thing is, we know for pretty high probability that (for example) Ceres has huge deposits of water.
The Carbonaceous Chondrite type asteroids contain up to 20% carbon compounds and water, which can be converted to hydrocarbons and oxygen, which is high-thrust rocket fuel. There are 13,000 known "Near Earth Asteroids", and we are finding 1500 more a year. NEA's are a lot easier and faster to return to Earth orbit, since we can use a Lunar gravity assist in both directions for our mining tug. Yeah, sure, mine Ceres eventually, but for starters the NEA's are the easiest to get to.
A properly designed space elevator (see my class notes for details: https://en.wikibooks.org/wiki/... and slides: http://imgur.com/a/cCTY5 ) carries on-board propulsion for orbit makeup. It doesn't look anything like the pictures you usually see in the media, though. The continuous ground-to-GEO concept can't be built, even with carbon nanotube cables. It would be inefficient even if you could build it. More modern designs based on much shorter *rotating* cable systems are more efficient. Even an efficient modern design needs more traffic than we have today to justify the large construction cost.
> Unless you are able to use that material in space
That's the intent for early asteroid mining. Space industry is already $323 billion a year total, and a major consumable for all space missions (mostly satellites in Earth orbit) is fuel. Any future Lunar or Mars missions would add large demands for fuel to the existing traffic. But even just existing commercial satellites need fuel to get to their operating orbit and maintain position. If they run out of fuel, or parts break, the satellite has to be written off and replaced. A fuel depot and repair station would save billions a year. To run the depot you need a fuel supply, plus supplies for the repair crew. That's the first market for asteroid mining. Anything else will develop over time.
> Earth is where the money is.
Right. Communications, satellite TV and radio, GPS, weather, ground mapping. The money is down here, but the hardware is in orbit.
> but materials like iron
There is no shortage of iron on Earth. There's a shortage of iron in orbit, where it costs at least 3 times it's weight in silver to deliver. And so does anything else you want to put in orbit. Mining in space to use in space can retrieve 350 times the initial fuel load back to Earth orbit. As long as you find a use for a reasonable percentage of that returned mass, you win over launching it from Earth.
Well, as a space systems engineer who does do cost calculations, the math goes like this:
Some asteroids, the Carbonaceous Chondrites, are up to 20% carbon compounds and water. These can be reformed to hydrocarbons and Oxygen, providing high thrust rocket fuel. An asteroid tug consumes about 2% of the returned mass as propellant. So the "return on fuel" is 10:1. It takes 2-3 years for the tug to do the return to cislunar space (near the Moon's orbit). 3 years gives a 115% rate of return. A tug is typically good for 5-8 trips before you have to replace the main parts (solar arrays and electric thrusters), so you can amortize the mass of the tug itself over that many trips.
Precious metals are a side effect to this, because they only occur at 15 parts per million, even in metallic asteroids. The main value is in the bulk components. Fuel is the easiest thing to extract, because that only takes heating, and concentrated sunlight can provide the heat. But you can find uses for the other 80% of asteroid material, bulk shielding against radiation if nothing else.
> The appeal of asteroid mining is that they appear to be conglomerations of relatively pure ores
For Platinum-group metals, relatively pure means ~15 ppm in asteroids. On Earth, the vast majority of these metals sank to the core, because they are "iron-loving" (mix well with Iron), and that's where the Iron went. Metallic asteroids are the result of protoplanets large enough to *also* develop iron cores, but later collisions broke them up and exposed the core bits, where you can reach them.
Nonetheless, when you do the math, 15 parts per million is frosting on the value of asteroid rock. Most of it is in the bulk elements you can use in space directly. Space is already a $323 billion industry, so there is a lot of value in not having to launch stuff at great expense.
Indeed. Portraits of George Washington show him carrying a sword. Thus short-range non-lethal weapons certainly existed at the time of the Bill of Rights.
Besides, there is a M-26 Taser version used by the military, so it is definitely a military weapon.
Transactions are already verified in seconds. Each node in the network looks at an incoming transaction, and performs a number of tests on it (is the digital signature valid, did they have sufficient funds to make the transaction, etc.). If it fails any test, the transaction is not relayed to other nodes. Verified transactions eventually reach miners, who attempt to find a checksum (hash) for a block of transactions. Finding a valid hash is computationally difficult, on purpose. That makes it hard to edit past transactions, since you also have to find a corresponding hash for the edited data.
So: verified = seconds, included in a block = 10 minutes on average. How long you wait depends on the nature of the transaction and how sure you want to be. You run a convenience store, and a customer is buying a snack? Seeing the transaction appear on a distant network node is enough, since it had to travel through 3-4 nodes to get there, and each one verified it. There is less than a 1 in 10,000 chance of the transaction being invalid. If you are using a payment processor, most of them guarantee the validity of the transaction, so there is no reason to wait once they OK it.
If you are selling a house, it would be wise to wait an hour or two for multiple blocks to appear *after* the one with your sale. Each block is chained to the one before it by including the previous block hash as part of the current block data. So they are time-ordered, and each one requires intense computation to create. Any attempt to change an old transaction requires re-doing all the computation for all following blocks, because all the hash values change. Since the vast majority of chips capable of doing the computation are working on *new* blocks, to win the bitcoin reward that comes with a block, there are not enough chips to *redo* the work, and your payment is secure. In the case of a house sale, the rest of the paperwork takes hours, and the escrow agent typically demands the funds be delivered the previous day anyway, so waiting a few hours for maximum security isn't a problem either.
> I haven't studied to know if the larger block size significantly addresses the speed of verification issue
Larger blocks don't change the network verification time. What they do is increase the number of transactions that can fit in a block afterwards. If transactions are not yet in a block, they are held in a "memory pool" of recently arrived transactions. When a block shows up, the included transactions are deleted from the memory pool. The pool prevents spending the same funds twice. As soon as one transaction spends it, any later transaction, even a second later, will be invalid. If there are too many transactions to fit into blocks, the memory pool would grow without bound, and eventually exceed the memory capacity of the node. Block data (older transactions) are typically stored on hard drives, which are much much larger capacity.
More transactions per second might eventually exceed the ability of a node to verify them as they arrive, or network bandwidth for the node, but the 8 MB block size has been tested and found not to do that yet. If bitcoin gets a lot more popular, and the blocks get much bigger, eventually nodes would need to be server-grade machines, rather than home hardware, but that's a long way off. A Raspberry Pi can handle current traffic.
Standing inside the fence of a rectenna array makes as much sense as going inside the furnace of a coal power plant. In other words, no sense at all.
300W/m^2 is the average intensity on the receiving antenna elements. The intensity outside the fence is much much lower. There's a buffer zone between the edge of the antenna and the fence.
Hi Geoff. Dani Eder here. I would say the most important thing we have learned is "there are a whole lot of Near Earth Asteroids". In 1980 there were 52 known NEA's ( http://neo.jpl.nasa.gov/stats/ ). Today we are rapidly approaching 13,000 at a rate of about 1,500 new ones a year. This has completely changed the accessibility of raw materials. Given that we now have well developed electric propulsion, 90% of NEA's take less fuel to reach than the surface of the Moon.
Even if you want to go to the Moon, the math says to mine asteroids for propellant to get there. Some asteroids are up to 20% water and carbon. This can be reformed to Oxygen and Hydrocarbons, which makes good high-thrust chemical propellant for landers.
Next I would say the advances in computers, automation, robotics, and high bandwidth communication are important. O'Neill assumed 10,000 people in a colony, because that's how many people it would take to build solar power satellites and all of the supporting tasks to keep them alive. Today we can think about automating it or controlling a lot of that from the ground. That means we can bootstrap construction with a small team of actual humans in space (some tasks still have to be hands-on).
Lastly, I would mention that the "O'Neill Cylinder" design, while pretty, is a nightmare from an engineering safety standpoint. One meteorite or out of control cargo ship hitting those giant windows, and all your air leaks out. You want stuff like multi-layered Whipple shields to break up incoming objects, and a layered and compartmentalized pressure hull. If you want natural sunlight, bring it in through protected openings.
The "classical" space elevator (ground to GEO) can't be built, even with carbon nanotube cable. There are more modern versions that can be built. Realistic engineering designs have to consider a lot of factors that artist's illustrations you most likely have seen don't.
Competitive with ground power, or it would never be built in the first place.
In space you get ~7 times as much sunlight as the average place on Earth. That's due to absorption, night, and weather that happens down here, but not in space. The logic is then you can spend up to 7 times as much building your space solar power system and be competitive with Earth solar power. If it costs you more than that, just build ground solar.
Slashdot Mirror
Or feeding it directly to a plasma thruster (basically a 200 kW microwave oven with a nozzle). Much higher exhaust velocity = higher fuel efficiency, but lower thrust.
> There's nothing for us out there, unless you are unusually attracted to radiation-blasted vacuum.
One could have said something similar about the American west, or Australian outback. Not vacuum, but a hostile environment. In fact, 80% of the Earth is inhospitable without the help of technology (the oceans, deserts, and ice caps). Slightly better technology will allow us to live anywhere in the Solar System.
Perhaps you see nothing out there, but I've done some real estate development in the past, and all I see are opportunities.
> If you can't click your fingers and have a fleet of starships so vast that they block out the stars, you don't live in a post-scarcity society.
Once upon a time fire was high tech, and fire-makers and fire-bringers could trade their fire skills for other stuff. Nowadays fire-making is absurdly easy and cheap - turn a knob on a gas stove, or flick a disposable lighter, and you have fire. It's not a skill you can trade for other stuff any more, and fire is no longer a scarce commodity.
What most people mean by "post scarcity" is not that everything imaginable is absurdly easy and cheap (your flick of fingers for starships), but that the necessities of life are easy and cheap. These are things like shelter, food, clean water, etc. A society where robots and automation supply those things for people, without them having to work themselves, is in that sense post-scarcity.
Some things, like beachfront property, Manhattan penthouses, and gold, are scarce for physical reasons, and advanced tech isn't going to change those physical reasons. But those items are not necessities, either. People don't get harmed by a gold shortage the way they get harmed by a food shortage.
Solar panels are not heat engines, so Carnot cycles are irrelevant. The best cells have now reached 46% in the lab ( http://www.nrel.gov/ncpv/image... ). The high efficiency cells use multiple types of semiconductor stacked up. Each type is optimized for a different wavelength. Note that cell is not the same as panel, because less than 100% of the panel area is cells. Multi-layered cells currently are used on spacecraft and in concentrator modules, because they cost more than single-layer cells to produce. The efficiency gain only makes sense for those applications.
1% is too high, 100 ppm is the correct number for a "good" metals-rich asteroid. However, most of the value in an asteroid is the bulk materials. A ton of metallic asteroid is worth at least $5 million if turned into something useful in high orbit, because that how much it costs to deliver *anything* to that orbit today. the PM content is 100 grams, worth ~$3,200. Anyone who thinks PM is the reason to mine asteroids hasn't talked to a mining geologist about ore values.
Right now, the best cost for getting stuff to GEO transfer orbit is ~$5000/kg. That includes propellant to complete getting to GEO, which is the desired destination. Hauling loads of asteroid rock to Earth orbit, and processing them for propellant could generate about 100 tons a year, thus worth $500 million/year, with 40-100 tons of starter equipment, and an additional 100 tons/year for each 20 tons more equipment (mainly more ore tugs to fetch rock faster). The trick is to do it at low enough cost to make a profit, but it's not orders of magnitude away, it's roughly profitable with reasonable development costs.
I was one of the contributors to this plan, and one of the big misconceptions is that NASA is the only player in space. In reality, worldwide space industry was $323 billion in 2014, and NASA's $17.6 billion only represented 5.45%. Most of the 1250 active satellites in Earth orbit are commercial ones, and a lot of innovation is happening in that arena. For example, ion thrusters for boosting to synchronous orbit are standard procedure these days, using solar arrays 2.5 times as efficient as the ones powering the Space Station. SpaceX is working on recovering their first stage so it can be used again, while NASA is going backwards on the SLS solid boosters. In the Shuttle era the boosters were 2/3 re-used, on SLS they will be thrown away.
As long as the developers bring in enough fill dirt and build higher up, that's fine. They could even dredge up some of the local dirt and use it to make marinas and lakes, then pile it up where the buildings go.
It would be easier to bring in fill dirt and jack up the buildings than trying to inject mud underground.
http://www.arctic.noaa.gov/rep...
(Data from Grace satellite measurements, by NOAA)
> It's inevitable to reduce CO2 emissions, because fossil fuels will simply be exhausted.
We will not be able to breathe before we run out of fossil fuels. At 5000 ppm CO2 (0.5%), people start having trouble breathing.
Your question assumes a static economic situation, which is not what will happen. Launching whole factories to process the raw asteroid rock will cost too much at first. What you want to do instead is launch a starter kit of production machines. You use those machines to make parts for *more* machines out of some of the asteroid, plus some parts you bring from Earth. As your collection of machines in space grows, you can make a wider range of items, and need less from Earth.
In addition to growing the factory, you devote part of the output to products for sale. You start with the easiest products, like water and carbon compounds for fuel, because most anything in space requires some fuel, and they can be extracted by simple heating. You convert H2O + C --> O2 + Hydrocarbons, which is common rocket fuel mix. As you are able, you can start to make higher value items, like spacecraft parts.
Along this path, you may find products and materials you can deliver to Earth for less than the ground-produced version. A first such item would be "pristine meteorite samples". Collectors, jewelers, and scientists will pay large amounts for them, and they don't require any processing, just delivery. Precious metals are often mentioned, but they occur in ratios of 30-100 parts per million in asteroids, meaning you have to do a *lot* of processing to get them out. It may not be worth the trouble until much later in the industrial development. But whenever your cost gets below the competing Earth cost, then go ahead and do it.
> I disagree mostly because solar really didn't get cheap until the Chinese began to flood the market with panels, around 2010-2011 or so.
It wasn't the Chinese so much as solar grade silicon production. Prior to about 2009, demand for silicon for solar cells was smaller than for electronics. So solar piggy-backed on existing silicon foundries. But electronics-grade silicon is expensive (~$400/kg) because even one defect can ruin a chip. Eventually solar cell production got big enough that solar-grade silicon was worth it's own foundry lines. Defects in a solar cell just degrade the output a little bit, they still function just fine. The lower quality product was much cheaper to make ($18/kg last time I looked). Since the raw silicon was a major component of final panel cost, you had dramatic cost reductions for a few years.
Now we are back to more incremental cost reductions, but the panels are now so cheap that the "balance of system" (panel mounts, labor, wiring, inverters or transformers, permits, etc.) is the majority of the cost, and that's where work is being done to reduce them more.
> The real question is, is she bound for Prison or will Obama pardon her.
I'm sure Ed Snowden can give her some relocation advice :-).
The hypocrisy would be particularly pungent if she gets a pardon for mishandling classified data, and Snowden doesn't. The law only applies to peons, right?
> The thing is, we know for pretty high probability that (for example) Ceres has huge deposits of water.
The Carbonaceous Chondrite type asteroids contain up to 20% carbon compounds and water, which can be converted to hydrocarbons and oxygen, which is high-thrust rocket fuel. There are 13,000 known "Near Earth Asteroids", and we are finding 1500 more a year. NEA's are a lot easier and faster to return to Earth orbit, since we can use a Lunar gravity assist in both directions for our mining tug. Yeah, sure, mine Ceres eventually, but for starters the NEA's are the easiest to get to.
A properly designed space elevator (see my class notes for details: https://en.wikibooks.org/wiki/... and slides: http://imgur.com/a/cCTY5 ) carries on-board propulsion for orbit makeup. It doesn't look anything like the pictures you usually see in the media, though. The continuous ground-to-GEO concept can't be built, even with carbon nanotube cables. It would be inefficient even if you could build it. More modern designs based on much shorter *rotating* cable systems are more efficient. Even an efficient modern design needs more traffic than we have today to justify the large construction cost.
> Unless you are able to use that material in space
That's the intent for early asteroid mining. Space industry is already $323 billion a year total, and a major consumable for all space missions (mostly satellites in Earth orbit) is fuel. Any future Lunar or Mars missions would add large demands for fuel to the existing traffic. But even just existing commercial satellites need fuel to get to their operating orbit and maintain position. If they run out of fuel, or parts break, the satellite has to be written off and replaced. A fuel depot and repair station would save billions a year. To run the depot you need a fuel supply, plus supplies for the repair crew. That's the first market for asteroid mining. Anything else will develop over time.
> Earth is where the money is.
Right. Communications, satellite TV and radio, GPS, weather, ground mapping. The money is down here, but the hardware is in orbit.
> but materials like iron
There is no shortage of iron on Earth. There's a shortage of iron in orbit, where it costs at least 3 times it's weight in silver to deliver. And so does anything else you want to put in orbit. Mining in space to use in space can retrieve 350 times the initial fuel load back to Earth orbit. As long as you find a use for a reasonable percentage of that returned mass, you win over launching it from Earth.
Well, as a space systems engineer who does do cost calculations, the math goes like this:
Some asteroids, the Carbonaceous Chondrites, are up to 20% carbon compounds and water. These can be reformed to hydrocarbons and Oxygen, providing high thrust rocket fuel. An asteroid tug consumes about 2% of the returned mass as propellant. So the "return on fuel" is 10:1. It takes 2-3 years for the tug to do the return to cislunar space (near the Moon's orbit). 3 years gives a 115% rate of return. A tug is typically good for 5-8 trips before you have to replace the main parts (solar arrays and electric thrusters), so you can amortize the mass of the tug itself over that many trips.
Precious metals are a side effect to this, because they only occur at 15 parts per million, even in metallic asteroids. The main value is in the bulk components. Fuel is the easiest thing to extract, because that only takes heating, and concentrated sunlight can provide the heat. But you can find uses for the other 80% of asteroid material, bulk shielding against radiation if nothing else.
> The appeal of asteroid mining is that they appear to be conglomerations of relatively pure ores
For Platinum-group metals, relatively pure means ~15 ppm in asteroids. On Earth, the vast majority of these metals sank to the core, because they are "iron-loving" (mix well with Iron), and that's where the Iron went. Metallic asteroids are the result of protoplanets large enough to *also* develop iron cores, but later collisions broke them up and exposed the core bits, where you can reach them.
Nonetheless, when you do the math, 15 parts per million is frosting on the value of asteroid rock. Most of it is in the bulk elements you can use in space directly. Space is already a $323 billion industry, so there is a lot of value in not having to launch stuff at great expense.
Indeed. Portraits of George Washington show him carrying a sword. Thus short-range non-lethal weapons certainly existed at the time of the Bill of Rights.
Besides, there is a M-26 Taser version used by the military, so it is definitely a military weapon.
Transactions are already verified in seconds. Each node in the network looks at an incoming transaction, and performs a number of tests on it (is the digital signature valid, did they have sufficient funds to make the transaction, etc.). If it fails any test, the transaction is not relayed to other nodes. Verified transactions eventually reach miners, who attempt to find a checksum (hash) for a block of transactions. Finding a valid hash is computationally difficult, on purpose. That makes it hard to edit past transactions, since you also have to find a corresponding hash for the edited data.
So: verified = seconds, included in a block = 10 minutes on average. How long you wait depends on the nature of the transaction and how sure you want to be. You run a convenience store, and a customer is buying a snack? Seeing the transaction appear on a distant network node is enough, since it had to travel through 3-4 nodes to get there, and each one verified it. There is less than a 1 in 10,000 chance of the transaction being invalid. If you are using a payment processor, most of them guarantee the validity of the transaction, so there is no reason to wait once they OK it.
If you are selling a house, it would be wise to wait an hour or two for multiple blocks to appear *after* the one with your sale. Each block is chained to the one before it by including the previous block hash as part of the current block data. So they are time-ordered, and each one requires intense computation to create. Any attempt to change an old transaction requires re-doing all the computation for all following blocks, because all the hash values change. Since the vast majority of chips capable of doing the computation are working on *new* blocks, to win the bitcoin reward that comes with a block, there are not enough chips to *redo* the work, and your payment is secure. In the case of a house sale, the rest of the paperwork takes hours, and the escrow agent typically demands the funds be delivered the previous day anyway, so waiting a few hours for maximum security isn't a problem either.
> I haven't studied to know if the larger block size significantly addresses the speed of verification issue
Larger blocks don't change the network verification time. What they do is increase the number of transactions that can fit in a block afterwards. If transactions are not yet in a block, they are held in a "memory pool" of recently arrived transactions. When a block shows up, the included transactions are deleted from the memory pool. The pool prevents spending the same funds twice. As soon as one transaction spends it, any later transaction, even a second later, will be invalid. If there are too many transactions to fit into blocks, the memory pool would grow without bound, and eventually exceed the memory capacity of the node. Block data (older transactions) are typically stored on hard drives, which are much much larger capacity.
More transactions per second might eventually exceed the ability of a node to verify them as they arrive, or network bandwidth for the node, but the 8 MB block size has been tested and found not to do that yet. If bitcoin gets a lot more popular, and the blocks get much bigger, eventually nodes would need to be server-grade machines, rather than home hardware, but that's a long way off. A Raspberry Pi can handle current traffic.
Standing inside the fence of a rectenna array makes as much sense as going inside the furnace of a coal power plant. In other words, no sense at all.
300W/m^2 is the average intensity on the receiving antenna elements. The intensity outside the fence is much much lower. There's a buffer zone between the edge of the antenna and the fence.
Hi Geoff. Dani Eder here. I would say the most important thing we have learned is "there are a whole lot of Near Earth Asteroids". In 1980 there were 52 known NEA's ( http://neo.jpl.nasa.gov/stats/ ). Today we are rapidly approaching 13,000 at a rate of about 1,500 new ones a year. This has completely changed the accessibility of raw materials. Given that we now have well developed electric propulsion, 90% of NEA's take less fuel to reach than the surface of the Moon.
Even if you want to go to the Moon, the math says to mine asteroids for propellant to get there. Some asteroids are up to 20% water and carbon. This can be reformed to Oxygen and Hydrocarbons, which makes good high-thrust chemical propellant for landers.
Next I would say the advances in computers, automation, robotics, and high bandwidth communication are important. O'Neill assumed 10,000 people in a colony, because that's how many people it would take to build solar power satellites and all of the supporting tasks to keep them alive. Today we can think about automating it or controlling a lot of that from the ground. That means we can bootstrap construction with a small team of actual humans in space (some tasks still have to be hands-on).
Lastly, I would mention that the "O'Neill Cylinder" design, while pretty, is a nightmare from an engineering safety standpoint. One meteorite or out of control cargo ship hitting those giant windows, and all your air leaks out. You want stuff like multi-layered Whipple shields to break up incoming objects, and a layered and compartmentalized pressure hull. If you want natural sunlight, bring it in through protected openings.
I recently did a class on space elevator design:
- Class notes: https://en.wikibooks.org/wiki/...
- Slides: http://imgur.com/a/cCTY5
The "classical" space elevator (ground to GEO) can't be built, even with carbon nanotube cable. There are more modern versions that can be built. Realistic engineering designs have to consider a lot of factors that artist's illustrations you most likely have seen don't.
Competitive with ground power, or it would never be built in the first place.
In space you get ~7 times as much sunlight as the average place on Earth. That's due to absorption, night, and weather that happens down here, but not in space. The logic is then you can spend up to 7 times as much building your space solar power system and be competitive with Earth solar power. If it costs you more than that, just build ground solar.