About a decade ago, we discovered several new forms of carbon molecules, a spherical shape called buckyballs and a long, thin form called carbon nanotubes (CNT). Carbon nanotubes are strong the way diamonds are hard (and for much the same reason). In fact, they're so strong that there's a possibility that they could be used to build an amazing thing called a space elevator, a cable anchored on the Equator and stretching so far into space (60,000 miles) that it's held up and taut by centrifugal force from the Earth's rotation. Truck-sized vehicles called climbers would be able to pull themselves up that cable and deliver cargo to geosynchronous orbit for one thousand times less than the current cost using rockets.

We don't know yet if we can make a CNT cable strong enough;  theory says that the individual molecules are about five times as strong as we need, but we don't know if we can make them into a cable that retains enough of that strength. The way to create a cable might never be found, or it might be discovered tomorrow.

If we do find it, a fairly detailed engineering study done with NASA funding shows that we'll be able to build an SE in about five years for about $6 billion. (8% of NASA’s budget for that period; Bill Gates or Paul Allen could do it by writing a check.) With space elevators in place (you need more than one because there are a number of dangers that could sever them; the second one would cost $2B), a substantial manned mission to Mars could be mounted for less than a billion dollars. Universities could send unmanned probes to Jupiter. Boy Scout troupes could send up satellites the size of the original Sputnik. Electricity beamed down from solar power satellites could make the the "hydrogen economy" a viable replacement for fossil fuels. In the longer term, millions of people could migrate to lunar colonies or space habitats at places like the Lagrange points.

I've only been reading TOD for a short time, but there's one possible energy source that I haven't seen mentioned.

Solar-Power Satellites

Wait before you scoff! I know, they were a big deal in the 70's until further analysis and the economic realities of our space program showed they couldn't compete with terrestrial PV.  That's still true, but there's a looming possibility that could change the economics and make them both more probable and nearer term than nuclear fusion.

A new form of carbon molecule, long hollow cylinders called carbon nanotubes (CNT), was discovered in 1991. Carbon nanotubes are strong the way diamonds are hard (and for much the same reason). In fact, they're so strong that there's a possibility that they could be used to build an amazing thing called a Space Elevator (SE), a cable anchored on the Equator and stretching so far into space (100,000 km) that it's held up and taut by centrifugal force from the Earth's rotation. Truck-sized vehicles called climbers would be able to pull themselves up that cable and deliver cargo to geosynchronous orbit for at least 100 and possibly 1000 times less than the current cost using rockets.

We don't know yet if we can make a CNT cable strong enough; theory says that the individual molecules are about five times as strong as we need, but we don't know if we can make them into a cable that retains enough of that strength. The way to create a cable might never be found, or it might be discovered tomorrow.

If we do find it, a fairly detailed engineering study done with NASA funding shows that we'll be able to build an SE in about five years for about $6-10 billion. (8% of NASA’s budget for that period; Bill Gates or Paul Allen could do it by writing a check.) With space elevators in place (you need more than one because there are a number of dangers that could sever them; the second one would cost $2-4B), a substantial manned mission to Mars could be mounted for less than a billion dollars. Universities could send unmanned probes to Jupiter. Boy Scout troupes could send up satellites the size of the original Sputnik.

More important for this forum,  SEs would bring down the cost of SPS; the original proposals were dominated by the cost of getting all that mass into space.  It's not outside the realm of possibility that we could have the first SE up in ten years, and the first SPS beaming down gigawatts five years after that.

Or maybe never.  The big unknown is our ability to make CNT cables strong and light enough.  (We need a strength of at least 63 gigapascals at a weight of 1330 kg/m³).  A graduate student could discover how to do that tomorrow, or a billion-dollar research program could take two decades and fail.  I'd take bets at 2:1 that we'll succeed within ten years, but maybe not at 3:1.

Another possibility with the SE: there's a lot of hydrogen up there, most of it not combined with other elements like oxygen.  An SE in the shape of a tube might make it possible to pipe H2 down to Earth. This H2 would be an energy source, not just a transport mechanism, plus we could probably recover a fair amount of the kinetic energy of that gas falling 36,000 km.

Pointers:
http://www.mill-creek-systems.com/HighLift/intro.html
http://www.liftport.com/files/521Edwards.pdf (2.7MB)
http://liftwatch.org
http://liftwatch.org/tiki-directory_redirect.php?siteId=26

Hydrogen? What hydrogen?

how are you going to make it come down against atmospheric pressure?

Hydrogen is lighter than the other gasses, that is why it is not found on Earth. It tends to escape into space. If you want to bring it back, you have to pump it.

Another way of seeing this is by noting that the thermal velocity of hydrogen molecules at room temperature is comparable to Earth escape velocity. A significant fraction of the molecules therefore will be fast enough to simply leave and join the solar wind.

Finally, as a purely logical argument, if hydrogen would fall to Earth inside the SE, why not outside? All that hydrogen would already be here, then, would it not?

Neither of these seem really feasible, ... To "dip it out of Jupiter", you need far more difficult space infrastructure than the SE itself. The transport from Jupiter, whether liquid or pressurized, will not exactly be a picnic, either. It will make oil supertankers look like reed rafts.

All this for a substance that is actually fairly cheap right here on Earth.

Not as an energy source, it's not. H2 is cheap because it can be produced by using energy to break it out of molecules like water and hydrocarbons. Water, hydrocarbons, and energy are cheap, therefore H2 is cheap. The latter two of those, actually pretty much the same commodity, are soon going to cease to be cheap. Hydrogen coming down the SE tube is an energy source, not just a transport mechanism.

Look, let's face it, energy itself is quite plentiful on Earth. There is all this sun shining down on us ... nuclear ... hydrocarbons that are just a little bit more expensive to get at than oil...