> Earth is actually quite an inhospitable planet as well.
Perhaps in some light this may be true, but compared to Mars, Earth has a natural atmosphere that we can breathe, water we can drink, and produces plant life that we can eat. I'd call that downright hospitable.
Where do you get your water and food? Do you subsist on a foraging diet and drink water only from nearby streams.
Technically speaking, the carrying capacity of the Earth under such conditions is low. Prior to the advent of agriculture and modern sanitation the human population was naturally limited. First by the availability of food, then by the problems of disease inherent in city living. The technology to live the way humans do today is only about a century old. Our food and water typically are the products of industry and technology. Mars would up the ante a bit but in essence would be little different. The norm on Earth is not free roaming foragers living naked out of doors.
Actually, most of the water we drink does come from relatively nearby (particularly relative to hauling ice from the poles). There's a reason the Sahara is not densely populated. Likewise for our food.
Most of Mars' water is at its south pole, which is so cold that much of the water ice is actually buried under a layer of "dry ice" — frozen carbon dioxide.
Basically, anywhere you are on Earth, you're closer to nutrition and fuel than you would be anywhere on Mars. And the reason why can have diets that don't involve foraging is because we've grown so numerous. That's also something that isn't true on Mars.
My point is that the norm for folks living in the developed world on Earth is for their food, water, and housing to be obtained via a very technologically advanced and typically world spanning industrial supply chain. The amount of effort and technology that goes into something as simple as buying a single hammer at the hardware store or turning on the faucet and getting fresh drinking water is stunning. The difference between what's required to support a Martian civilization and an Earth civilization is not so much a difference in fundamentals as it is merely a difference in degree. And possibly it's less of a difference between, say, living in New York city today and living on Mars in 2030 than NYC today vs. NYC in 1800.
As far as water, the largest deposits of H2O on Mars are near the poles but it exists in enormous quantities at nearly every location on Mars a few meters below the surface.
Atmosphere itself is plant technology. It wasn't here before life and it won't be here after. Earth WAS pretty inhospitable, life just overcame those hurdles. Chances are we can do it again. It won't be easy, cheap or quick though.
I don't think you quite understand. Earth's atmosphere existed all along; its composition was just different before plants came into the picture. But even plants need certain atmospheric conditions, such as the existence of an atmosphere and certain elements in it. Now, it's true that the composition of Mars' atmosphere is all wrong even for plants (for example, very little nitrogen), but more fundamental than that, its atmosphere is literally barely there — the gas around the planet is exceptionally thin. Venus, Earth and Mars are like the three bears — Venus has way too much atmosphere to support life as we know it, Mars has way too little, while Earth's is just right.
Venus is hot enough to sterilize anything that even approaches its surface. Earth's most radical extremophiles don't even come close to being able to withstand that. Even life made out of many common metals wouldn't be able to exist because it would melt them.
Since we don't know the limits of what might be considered "life," I suppose we can't 100% say it's impossible, but that's why I said "life as we know it."
There are many materials that melt above 500C, so it gives theoretical possibility for existence of life on Venus.
I couldn't be carbon-based life though.
BTW, Venus civilization would have similar reasons of not going to Earth as our reasons are of not going to Mars: way too cold and not sufficient atmospheric pressure.
"Earth is actually quite an inhospitable planet as well. We survive on it because of our technology - Mars may be no different."
Seriously? That's not entirely accurate. It's true the humans is able to defend against predators and poor environments with our tools, but that's not something unique to our species. Mars is not capable of sustaining life as we know it, and that's the basic problem. The amount of tools needed to convert Mars to such a place may be prohibitive.
The trouble isn't surviving on Mars. The trouble is building infrastructure on Mars that will allow people to survive there without continuous resupply from Earth, at a cost measured in (at a minimum) tens of thousands of dollars per kilogram.
"at a cost measured in (at a minimum) tens of thousands of dollars per kilogram"
[citation (or math) needed]
The minimum cost for sending cargo to Mars even with only current levels of technology is far, far lower than tens of thousands of dollars per kilogram. A reusable Falcon Heavy (which is being researched currently) could potentially launch payload to Mars for circa $100/kg. To put it another way, that's 10,000 tonnes per $1 bil.
That's the expendable Falcon Heavy. SpaceX is also working on a fully reusable Falcon 9, which would naturally allow for a fully reusable Falcon Heavy. The fuel costs for a Falcon Heavy are around half a million dollars, if we posit that overall operational costs per flight could be kept to around 1.5 million dollars then costs of delivering cargo to Mars would be around $100/kg. That's a pretty straightforward extrapolation of current trends. If we imagine some ways to make the process more efficient we could imagine batching up cargo in LEO and pushing it to Mars via electric propulsion (a proven technology), for example.
These are pretty decent conceivable practical minimums for cost of shipping cargo to Mars within the next 20 years, though any estimate (minimum or otherwise) beyond that time frame would be entirely speculative due to the potential for technological advances.
You claim that the figures for Falcon Heavy cost of launching to Mars are "optimistically low", but that is not the case. Firstly, Mars launch is a well understood problem on the scale of launch to GEO. Second, the figures SpaceX are quoting are launch prices which include a substantial profit margin. The actual costs (especially after development costs have been amortized) are lower. And as I pointed out this doesn't represent a proper reasonable estimate of the floor for launch costs to Mars even over the next 20 years, let alone over the next, say, 50.
Earth is not all that inhospitable even if you lack technology. There are something like 10 quintillion insects inhabiting earth. The number of bacteria is several orders of magnitude greater. These things all get by just fine thanks to Earth's sweet conditions.
But if you dropped a colony of one of these natural survivors on Mars, it would simply die. Even what we consider to be harsh environments on Earth (e.g. Antarctica, the bottom of the sea, the top of Mt. Everest) are quite hospitable relative to the average on Mars.
Earth is actually quite an inhospitable planet as well. We survive on it because of our technology - Mars may be no different.
I think there are thousands on this planet who would be willing to give it a shot if they thought there was even just a slim possibility of success.