2. Since the notion of the asteroid space colony is essential to one of your characters, then YOU HAVE TO GO WITH THAT SETTING or else you're going to have a different story.

It seems odd to me that if there were an asteroid large enough to host a colony (of the order of Eros, for example), that it hasn't been found yet. Which leads me to believe that the points aren't stable enough for a large asteroid. Any large asteroid that was at those points have long since been ejected out of the Moon-Earth system, or have crashed into either the Moon or the Earth long ago.

It seems to me that these people are just looking for space rocks that are constantly at those points, which would imply the existence of true equilibrium points around the Earth -- but wouldn't necessarily support large asteroids.

That said, I think a hollowed out asteroid is a lot closer than a terraformed mars, even if the asteroid has to be moved to earth orbit from a trans-martian solar orbit (though sizeable asteroids frequently come near earth fairly often, making some people nervous but offering opportunities for this sort of engineering).

In order to produce a hollow asteroid, we'd need something better than modern rockets and a better power source than solar or fission. I'd say this is at least 10 years away. In those years, other technologies will be developed too -- be careful.

In order to terraform mars, we'd need massive power sources (at least controlled fusion) and genetic engineering with far more flexibility than we have now. Note that if we can produce plants capable of using the martian soil, and whatever earlier organisms actually did the bulk of the terraforming, we should also be able to do some pretty cool things with ourselves. Also, if we can predict climate well enough to terraform, we won't have any more honest disputes about environmental threats to the Earth. We should also be good at predicting other complex systems (stock markets?).

I have trouble picturing this for less than 50 years. In that time, I'd expect some really major and fundamental changes, like telepathy (wire a cell-phone straight to my brain). Either be prepared to deal with this, or to explain why we don't have it, or just make the unrelated parts of the story interesting enough that this can be ignored.

If the setting isn't central to your story, there are still some factors worth considering. Mars is big. The population can be whatever you want. An asteroid is small. The population is capped at around (I'm guessing) a hundred thousand people, and they know it too. Mars would most likely have large unsettled areas that someone adequatly skilled could hide in. An asteroid is concave and artificial -- heaven for a fascist government. Mars may (or may not) have large areas of frozen or dry wasteland. An asteroid would have areas of zero-gravity and (optionally) zero-atmosphere.

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First of all, decide what you're interested in. If it's your setting, then be very careful with it. Otherwise, throw up whatever is convinient and then shut up about it. Your readers will forgive absurdities if and only if they can ignore them.

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If an asteroid isn't big enough make it a hollowed-out-comet. Come up with some reasonable technology that allows comets to be steered into place (really elegant AI might use a few nukes to move a comet on the right vector for reverse-slingshot effect to slow it down, for example).

If you want Mars terraformed in less than a century, come up with a believable nanotechnology. If your nanotech has a believable power source, you'll be lightyears more credible than 90% of the stuff out there.

If you want to explain the lack of telepathy, teleportation, or antigravity, that's pretty easy: dazzle your reader with more believable technology. Computers with artificial consciousness and physical fitness in a bottle come to mind.

Also remember that a few tons of metal smashed into a flat sheet can encompass a very large volume. So a modest nickel-iron asteroid might make a pretty big space colony if you just melt it and blow air into it.

In Enders game, the bugger's had also done this. Ender's final training occurred while living in a hollowed asteroid.

Eros, which I even used as an example, is not in a geocentric orbit.

That's all.

Asteroids may not be all they're generally thought to be, after all. Comets are fairly solid, because they're mostly made of water, which packs together (partly by sublimating and refreezing). Current thinking about asteroids is that the larger ones are loose balls of gravel with some boulder-like chunks, held together by their own (very small) gravity. It would be impossible to hollow something like that out. You could mine them pretty easily, though.

I would vote for a lunar colony, though. A moonbase would probably be a necessary step in serious exploitation of the inner system. It isn't too far from Earth, there's enough gravity for convenience (keeping things stuck to the ground) but not so much as to be inconvenient (you can rail launch from the ground). No atmosphere, but it would be easier to build stuff than assembling an equivalent sized space station (you would need a space station too, I think, just not a colonty sized one). There are a lot of minerals on the moon itself, and so on and so forth.

We've already got the technology to fuse light atoms into heavier atoms in the H bomb. Suppose it takes 50-100 years to develop a high-tech fusion process (super H bombs, fusion drive, etc.) that can move the mass into Earth orbit. Given that, I think it's a credible leap to say you might fuse light atoms like hydrogen and oxygen into titanium, aluminum, or whatever you like. Start with rock and ice, end up with metal. Heck, given 200 years we might be able to start with sunlight and make metal.

There are probably thousands of 1 kilometer asteroids in Earth crossing orbits. One of these would give you well over a billion tons of atoms. To give you perspective, the Edmund Fitzgerald weighed 13,632 tons empty. Thus, if you have the power to fuse atoms into heavier atoms, an ordinary asteroid could become a colony almost a million times bigger than a really big freighter.

If you don't have that technology, you can still find enough metal to make the Edmund Fitzgerald in an asteroid that's made 0.0001% of metal and 99.9999% something else.

Breccia is called that because you can break it up with your hands. It would be easy to bore through, but it doesn't have much strength and it is pretty completely permeable (so all your air would leak out). You would be better off mining the asteroids for nickle, iron, and other metals, silicates and other rocky stuffs, and getting carbon, oxygen, hydrogen and nitrogen from comets.

Right now NASA is looking at carbon nanotubes--also called buckytubes--bonded in somekind of highly durable resin to create a super polymer. It should be possible to achieve a strength higher than 50 GPA at a density of about a gram/cc--easily strong and light enough to build a space elevator (we have other materials--like Zylon--which are theoretically strong enough but would require a pretty massive structure, make that unreasonably massive ). Materials scientists have already demonstrated a technique for "growing" buckytubes from buckyballs (C60 molecules that resemble a soccerball and can now be produced in substantial quantity by burning hydrocarbons under controlled conditions) using a nickle-cadmium (or some such thing) alloy as a catalitic element.

Don't underestimate the difficulty of building things in micro-gravity and free-fall environments either. The low but perceptable gravity of the moon, combined with its geological stability (menological?) could make construction of a colony fairly easy...the moon might eventually serve as a sort of "dry dock" for constructing future generations of space craft.

As for elemental transmutation, we can already transmute gold into lead by bombarding it with alpha particles and neutron radiation. In fact, we can transmute most elements into slightly heavier elements, and break pretty much any element into lighter elements...but it isn't exactly cheap, and never will be, because the quantities of energy involved are substantial (and even if you manage to reduce the energy cost to the theoretical minimum, it is still really energy intensive). Mining asteroids and using the elements as you find them will always be cheaper than transmuting them into other stable elements.

Besides which, asteroids contain a much higher percentage of metals (particularly heavier metals) than you find in the Earth's crust. Most of our heavy metal is found in the core of the Earth, not at the surface (think about some basic principles discovered by some naked Greek a couple of millenia ago for a moment and you'll see why that is so). You'll find all the metal you want in an asteroid, the real need is for lighter elements. Comets will supply some of that, but not all, and not without some effort (most comets have much higher delta-V relative to Earth than is common with asteroids, which can make catching them more of a challenge).

A word about space travel (and combat). Never have anything less powerful than nuclear power in space, it just isn't realistic. We actually already have a nuclear powered hybrid ramjet/rocket engine on the shelf (in ramjet mode, a fission reactor superheats air for a jet reaction--in rocket mode, the atmospheric intakes are closed and a hydrogen supply is superheated by the reactor). It was developed in the seventies and never used because of political concerns (not valid environmental concerns). And we have fusion weapons and are developing non-fissioning fusion initiation technology (in other words, "controllable" fusion). Our spacecraft all use nuclear powered "batteries" (basically a lump of fissionable material that acts as the heat source of a heat exchanger, the heat sink being provided by space).