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You cannot bet the lives of astronauts without a ground truth: you can suppose that in Elysium Planitia there is a huge buried glacier and send a minimalist mission, where astronauts/colonists are suppose to support themselves and synthesize return propellant with locally extracted water, but if they discover that water is now evaporated or is buried too deep for their drilling machines.

But to have a good prospection you have to send astronauts, so the firs mission has to relay on what you bring from Earth (even ISRU has to be based on imported LH2) and the firsts goals has to by science & exploration to select the best site for a permanent base.
I like very much GW's plans with an all-in-one exploration, prospection and colonization experiment.

As an SF writer I like cloud cities because they are cool. But in reality, before talking about the technical possibility of colonizing Venus atmosphere, we have to find a good reason to do it.
Why have we to go to Venus?

A human mission on surface, if technically possible with atmospheric pressure suits and Stirling coolers, will be only a very short flag and footprints.
If we think to build floating bases to harvest atmospheric CO2 and use it as a source of fuel and plastic materials, we can do the same things still in orbit, without coping with sulfuric acid clouds and risking to fall down in a 400°C/100 bar inferno.

It will be different if we will be able to develop surface mines and factories: in this case a cloud city can be e valuable connection between Venus surface and orbit. Surface mines and factories has to be full automatized, controlled by high temperature resistant machines.
In a future, it will be also conceivable to implant on Venus surface some sort of synthetic life, based on fluorocarbons, especially projected to grow-up at 400°C in a CO2 environment.

Cloud cities will be build by machines (or synthetic beans) using locally mined materials and will serve as a connection with orbital habitats.

I intended five people with two pilot engineers, two MD trained in molecular biology and biochemistry and a geologist. If we have to reduce the number to four, only one MD/biologist, two pilot engineers and a geologist. All the astronauts have to be cross trained in paramedics, piloting, geology and basic lab technics.

For the moment the best suited for a large colony is the Orion Drive, but in a future, investing on it, I think we can build a miniaturized version with magnetic nozzle and mini-pulse unit with a laser driven  hohlraum or a magnetic compression (mini mag Orion? http://en.wikipedia.org/wiki/Mini-Mag_Orion ) as a primary and fusion-fission hybrid as secondary and tertiary.

This will have less political problems, because the pulse units cannot be considered bombs, needing an external device to implode.

Actually laser are very inefficent, but there are some interesting research in high efficiency laser that reach 75% of optical power conversion.

http://phys.org/news/2011-05-scientists … laser.html

MAKS was also very interesting it would be propelled by RD-701 LOX-RP1-LH2 tripropellant rocket: a 300 atmosphere rocket that changing mixture ratio with altitude.

http://en.wikipedia.org/wiki/MAKS_%28spacecraft%29

http://en.wikipedia.org/wiki/RD-701

With better understanding of pusher plate ablation and jet collimation, and miniaturizing the pulse units using the status of the art of three stage nukes, it will be possible to build a quite efficient lightweight space corvette in the range of 1000 tons of dry mass, with 5000-6000 s of specific impulse?

The hull of an Orion-drive propelled spaceship was all made in steel?

Bussard's Polywell has the size of a washing machine and Lerner's DPF is even smaller, but first we have to see if they works.

We still don't know if fusion will be available and the hypothetical reactor will be economic and lightweight enough. Another possibility are the hybrid fission-fusion reactors: in this case sub break-even huge magnetic confinement reactor are coupled with thorium-uranium power plant. Hybrid plant are fixed and lightweight fission rocket use U233 produced by plants.
I surely hope Bussard's Polywell eventually will work and we will have fantastic water propelled QED rocket with high thrust and 2000-3000 s of specific impulse, but how can we know?

It's not so simple: everything traveling at a relative velocity more than 3 km/s has a kinetic energy greater than its weight in TNT. So a spaceship can launch multiple rubbish object, that may be difficult to localize and intercept.

I doubt that magnetic confinement fusion reactors will be lightweight enough to be placed inside spaceships. They are very massive and needs huge waste heat radiators to cope with neutron heating, so a fusion propelled spaceship will have at the best a thrust just a little better than electric propulsion, unless we will be able to make pure fusion mini-bombs, like Project Dedalus pellets. But at the moment we don't have fusion, so hard fiction writers are forced to use fission, chemicals of solar electric propulsion.

Military nuclear warheads make a lot of damage if detonated in a planet atmosphere, but, if detonated in space, are not so dangerous like a 500-1000 tons spaceship impacting at 10-20 km/s. So in an Earth centered scenario you are perfectly right in not giving Orion Pulse Units to civilians.
But in my scenario, Earth is declined, poor and polluted and most part of mankind live in rich and beautiful space habitats, in Earth Moon Lagrange points, on Moon, Mars and Mercury (poles) surface, in Venus and Mars orbit, around the Main Belt's asteroids, and the moons of gas giants. So these guys, born and grown-up in an artificial space habitat and never been on Earth, will probably fear a kinetic kill more than a nuke strike.
The real problem is that a spaceship is a WMD, even without nuke, by only its kinetic energy, so if private civilians can buy spaceship, the risk for the habitats remains.

So the question in my opinion is not "private Orion or not private Orion", but "private spaceship or not private spaceship".

Ark may be a solution, but in this scenario it would be better to nuke them before they grow-up.