Re: Why a heat shield?
What it seems to amount to is that this is the only way down, while we're launching stuff into space from Earth. One of the best reasons to go grab an asteroid and mine it, would be to get consumables like water/amonia/whatever. In orbit you've got virtually free electricity from your solar panels, so if you've got chemicals then you can get oxygen to breathe, water to drink (and grow plants) and nitrogen for atmosphere and fertilizer. If you've got water and electricity you've also got hydrogen and oxygen - i.e. rocket fuel.
If we could just get that sort of bulky stuff in space, rather than boosting it up from earth, then all sorts of things in space become much more feasible. As well as a lot less expensive, and a bit less dangerous. At that point we could have re-usable spaceplanes that refuel in orbit and can then slow down from orbital speeds before entering the atmosphere. Some horrific percentage of the shuttle's weight was heat-shield, and an even more horrific percentage was fuel.
We have these fundamental design problems that dog everything that we do. All the weight you carry up from earth must be accelerated to orbital speeds (17,500 mph ish). And the more you carry, the more fuel you need to launch it, and the more fuel you carry, the more fuel you need to lift that fuel. Which is why we mostly throw away bits of our rockets on the way up. And of course, to come back down, you have to lose that 17,500 mph somehow. Currently that's by aerobraking.
Now you could do like aeroplanes, and use lift to help get you up to a good height, before heading for space, but even then you need rockets for once you run out of atmosphere. And then you have to carry these, plus your jets, plus fuel for both, and a heat shield, as you can't carry enough fuel to slow down.
So Virgin's (Scaled Composite's really) design is to use well understood jet technology to carry their space plane to 50,000 feet. That saves loads of weight, and hassle. Then the spaceplane does the rest. Currently they're just after sub-orbital joyrides - but I presume they can also carry a smaller rocket to boost a satellite, instead of passengers with another design. I don't know if you can make a big enough carrier plane to carry a space vehicle with the weight of fuel and shielding to get a useful payload to orbit. But I'd be surprised if that's not possible.
Option 2 is what SpaceX are doing. Make rockets cheaper. No-one's seriously done re-designs on this stuff since the 60s/70s. So they dumped horrible chemicals or hard to handle liquid hydrogen. Instead they're using liquid oxygen and kerosene. Nice money saving. Then they're planning to land the first stage of their rockets instead of dumping them. So you carry a little more fuel, then land them and re-use.
Third is Reaction Engines. They're the old HOTOL lot, still going, but now Skylon. Use a SABRE engine that works like a jet at low speeds, then gets up to supersonic RAMJET speeds, and then will have to use stored oxygen once the atmosphere's too thin.
Oh, and there's an XKCD for everything apparently: linky