> Putting aside the offsite-backup-for-humanity angle, if it becomes practical to send a significant population to Mars, wouldn't it consequently also be at least as practical to keep sending them stuff?

Not really, not with the expense that would entail. The only viable Mars colonization model assumes the creation of most needed commodities out of local resources.

> I have always assumed that any Martian colony would be receiving regular supply dumps from Earth for decades or centuries before becoming self-sustaining.

It such a colony required substantial supply dumps from Earth, and if this were anticipated in the planning stages, the colony wouldn't be funded in the first place. Transferring anything massive from Earth to Mars is extraordinarily expensive and will continue to be so.

Musk believes the cost can be US$500,000/immigrant. Assuming 100 kg/immigrant plus an absurdly low 1kg/day supplies and 150 days of travel gives a maximum transport estimate of $2,000/kg.

This is extraordinarily expensive, yes, but not out of the realm of question. Saffron is $1,100–11,000/kg, for example.

A colony of 1 million is also very unlikely to have the ability to produce pharmaceuticals and high-end chips which are cost competitive with Earth. Assuming a core weighs 20 grams, the shipping cost is $40.

Of course, this is as public-private partnership. Assuming the real cost is $2 million / immigrant gives a shipping cost of $200 - which is still going to beat the cost of starting and maintaining a set of fabs on Mars.

If Musk is right about the cost, then there will be goods exported from Earth to Mars. If Musk is wrong about the cost, then it's not practical to send a significant population to Mars.

> A colony of 1 million is also very unlikely to have the ability to produce pharmaceuticals and high-end chips which are cost competitive with Earth.

Well, cost-competitive with products imported from Earth, which is an easier standard to meet. Maybe Mars colonization will finally bring 3D printing into its own. People will discover they need an exotic part, so they radio earth for -- not the part itself -- but a diagram of the part, delivered by radio. Then they print it using local raw materials.

As to high-end technology, I think at least at first, the colonists will have to do without a ready supply of integrated circuits. They will have to live in a pre-iPod society for a while, more's the pity. :)

I can imagine an interview room twenty years form now -- a candidate says, "I'm willing to put up with many discomforts, even danger, to be among the first to colonize Mars." A few facts are delivered, then, "What? No cell phones? Are you serious?"

> If Musk is right about the cost, then there will be goods exported from Earth to Mars. If Musk is wrong about the cost, then it's not practical to send a significant population to Mars.

With a degree of resourcefulness by the colonists, I think there will be local exploitation of raw materials and production of needed goods, relatively quickly. Especially if every idea for small-scale local production, like 3D printers, is taken advantage of.

I see no reason for why Mars colonization is needed to "finally bring 3D printing into its own."

Anything appropriate for a Mars colony would be as appropriate for overwintering in Antarctica, or for the residents of various islands from the Cook Islands to Saint Helena, or similarly isolated place.

Presumably there's a cost-benefit matrix. Denser processors mean less needs to be shipped to Mars, but the harder it is to make locally. You mention "pre-iPod society" as if digital entertainment was the main loss. Pre-iPod also a pre-GPS-everywhere society, and pre-Internet-of-things society. Wifi embedded in every device may make a colony more likely to succeed. I would like a faulty CO2 scrubber to be able to notify the colony network, and wifi is cheaper than building cables. With that network in place in order to survive, a mobile device with phone-like communications is likely not only trivial but essential.

Regarding 'resourcefulness' - that's besides the point. A self-sustaining colony requires a huge number of people. (Who builds the fab units? Who are the doctors? Who repairs the sewage systems? Who takes care of food and O2 production? Who spends the 20 years to educate the next generation to the PhD level needed to support a high-tech frontier?) http://www.sciencedirect.com/science/article/pii/S0094576513... estimates 40,000 people, which is a shot in the dark for the minimum size. Musk proposes 1 million; his cost numbers don't work out with only 40,000 people.

Any rocket technology which is cheap enough to deliver 40,000 people for a self-sustaining colony on Mars is one which can support trade between the planets. Someone will want saffron. Someone will want white truffles. (Both are already in the $1000s/kg range.) Maybe not the first generation, who all volunteered, but surely the second.

Most nations on Earth engage in trade. I'm sure Mars would do so as well. Because of the expense involved, soon the only thing Mars would ship in would be computer components that require a large fab.

There's a difference between "can be self-sustaining" and "is self-sustaining." A successful Mars colony would engage in trade with Earth.