This is one of the places that in-situ materials change the game. Having a metallic (or even iceball) asteroid in earth orbit really changes the formula on the availability of shielding and fuel for a mission anywhere. Honestly I am in the "skip Mars" camp and I think we should go straight to the asteroids.
Shielding mass = inertial mass. How do we give it enough delta-V?
Bringing from Earth a few tons of nuclear-powered jet engine to use water as propellant would take building such an engine first. We're not even close.
Chemical engines are lightweight and well-understood. Lifting kilotons of chemical fuel is not feasible, though. So we need to build an orbital chemical / cryo facility to produce at least something as simple as liquid hydrogen and liquid oxygen, again by the kiloton.
This is on top of building an orbital shipyard to construct the actual spaceship, with all that heavy shielding around it, and the skeleton to carry it, the fuel, and the habitable compartment. It's going to be a spectacular feat of space-borne metallurgy, mechanical processing, including precise machining, quite some chemistry to support that all and produce necessary plastic parts, saying nothing about the power plant to feed this all.
Here we assume that we have an asteroid that has enough metals, carbon, and water neatly parked on Earth orbit. How do we achieve that, and how many decades would it take to bring it here, is not even considered.
This all would be great to have! But getting straight to Mars seems simple in comparison.
> Bringing from Earth a few tons of nuclear-powered jet engine to use water as propellant would take building such an engine first. We're not even close.
That's not true. NASA built and tested designs for a nuclear engine in the 80s, human rated even, and the results were spectacular.
> Bringing from Earth a few tons of nuclear-powered jet engine to use water as propellant would take building such an engine first. We're not even close.
True, but small scale solar thermal propulsion systems have been built and require minimal mass. Isp is a largely function of collector area (mylar film is lightweight) and reaction mass. If the water is cracked and the hydrogen used for propulsion, an Isp of 1000 is achievable, far surpassing chemical rockets.
Either way, we need much more efficient, much lighter shielding than we currently have. It’s just one of the. Any significant breakthroughs required to make trips to Mars (for humans) remotely practical.
Far more likely, if less romantic to some, is going to be a future of robotic exploration and mining. Once that’s well underway, sending humans on long journeys becomes far more feasible.
The scoop: radiation damage during space travel is similar to aging damage. Known anti-aging drugs help. More is needed to make interplanetary travel feasible.
As we move into space we're going to notice just how sensitive we are to our environment. It's like the higher rate of skin cancer among people with less melanin - not noticed until people moved far away from where their ancestors lived.
Gravity is more pervasive. It affects everything in biology and why shouldn't it? It's been a constant in the background that every direction evolution took transitively depends on.
