Why is space hard?
When Captain Picard is ready to fly up from the surface of the Earth to the Enterprise, he just hops onto a shuttle and zips up into orbit. We've been bombarded with these images for so long, a lot of non-technical people might not fully appreciate just how hard it is to get into orbit with current technology. So, for the benefit of those who aren't technically inclined, this article presents a brief overview of why it is so difficult and expensive to get into space.
The Fundamental Problem is Energy
It takes a lot of energy to lift a spaceship up out of the Earth's atmosphere, and make it go fast enough to stay there. Low earth orbit is around 100 miles above the surface, and the spaceship needs to be going about 17,500 miles/hour to avoid falling back down to Earth. To completely escape the Earth's gravity (for example, to go to another planet) requires the spaceship to be going about 25,000 miles/hour.
That's not all, though. At liftoff, the spaceship not only has to carry itself, but all of its fuel, too. In the space shuttle, the weight of the fuel at liftoff is around six times the weight of the spaceship plus engines plus tanks and so forth. As a result, a large fraction of the fuel required to get to orbit is actually spent in lifting fuel. This is like having a 747 where everything except the first class section is filled with fuel. Airplanes don't have this problem because they don't fly nearly as high nor as fast as a spaceship, plus they don't have to carry oxygen or reaction mass (see below).
The Fundamental Problem is Fuel
Part of the reason spaceships need to carry so much fuel is that the chemicals in rocket fuel don't release enough energy when burned. Fuels with a higher ratio of energy to mass would require less fuel to get to orbit, which means less fuel needed to lift the fuel. A significant advance in rocket fuel technology would bring down the cost of launching a spaceship dramatically.
Unfortunately, the fuels we have today seem to be pretty close to the limit for the amount of energy you can extract from a chemical reaction. The next logical step is to look at nuclear reactions. Any kind of nuclear-powered rocket engine would likely provide orders of magnitude more energy per kilogram of fuel. Even including the weight of the engine itself, you could easily build a spaceship where the payload is bigger than the engines and fuel.
Fission, or splitting atoms, is well understood technology, but very dirty. This is the process used in all nuclear power plants, and it creates a lot of highly radioactive waste. Even given that cheap space launches provide new options for nuclear waste disposal (i.e. launch it into the Sun), the odds of nuclear disaster from a launch accident are very high. Just figuring out how to build the rocket engine without spewing radiation everywhere is a huge challenge.
Nuclear fusion, or combining atoms, promises to be much cleaner, and more powerful than fission. Any Star Trek-style shuttle would have to use a small fusion reactor for power, since that's the only way to provide enough power in a small enough space in a clean enough fashion. The only problem is that nobody has figured out how to build a controlled fusion reactor, much less one small enough to power a little shuttle.
The Fundamental Problem is Reaction Mass
If we can build a fusion reactor powerful enough to launch a spaceship, that presents a new problem: reaction mass. By Newton's Laws, for every action there is an equal and opposite reaction. In other words, if you want to go forwards really really fast, you have to push something else backwards, and there has to be a lot of it or it has to go really really really fast. The stuff you push backwards is called "reaction mass."
This isn't a problem for airplanes and cars, since the Earth and its atmosphere provide a ready supply of reaction mass. But once you get into space, where there's no atmosphere, reaction mass is hard to find. Modern rocket engines use the spent rocket fuel as reaction mass, but a fusion engine produces relatively little spent fuel. As a result, the spaceship will probably wind up carrying tanks of water (or something similar) to use as reaction mass....and we're stuck with the same problem as before, in that you need more reaction mass to accelerate all the reaction mass you're carrying.
Fortunately, if the fusion engine is powerful enough, we can get away with a lot less reaction mass. But this leads to another trade-off: the less reaction mass you use, the less efficient your rocket engine, because more of the energy is going into accelerating the reaction mass instead of the spaceship.
Clever Solutions
All this applies to the traditional launch of a spaceship from ground into orbit. There are some clever ideas for completely different ways to get into orbit which (if feasible) would be even cheaper than Captain Picard's fusion-powered shuttle.
The most practical is a space elevator. Imagine a giant rope attached to the Earth at the equator, and extending about 25,000 miles into space, with a counterweight at the end. A rope this long would remain suspended from the rotation of the Earth, just as you can spin a bucket of water over your head without spilling. Spaceships could be hoisted up the rope, and released straight into orbit, with only small engines needed. Energy could be transmitted up the rope from the ground, and the entire Earth would act as a reaction mass.
The biggest problem with the idea is that nobody knows how to build a rope strong and light enough. Most of the strength of the rope would be simply holding up the rope itself, and not the cargo. There are some who believe that advances in material technology over the next several decades are likely to lead us to a strong enough material, but that's hardly a sure thing.
Other Problems
The hardest thing about space right now is just getting there, but it doesn't take a big leap of faith to assume that in the not-too-distant future the cost of launching a spaceship into orbit will fall dramatically. Either the invention of a practical fusion reactor or a space elevator would do the trick nicely.
Once we're in space, we'll have to deal with the fact that outer space is a hostile, unwelcoming place. Solar radiation, no atmosphere, and extremes of hot and cold are the norm. In an orbital colony where people live their entire lives, providing adequate protection from solar radiation and meteor strikes will likely require outer walls several feet thick. Fortunately, in zero-g, building very large structures is easy, since the structure doesn't have to support its own weight (unless you want to spin it to provide artificial gravity). A Moon or Mars colony is simpler, since there's lots of rock (and on Mars, atmosphere) to provide protection, and gravity comes for free.
The Future?
I believe human colonization of space is inevitable, and it will be made possible by a substantial improvement in launch technology. I could be wrong, of course, but it doesn't take a big leap from where we are today to a technology which makes going into orbit as cheap as flying from New York to London. The only question is when.