Meteoric Nickel-Iron

At various times, it has been suggested that melting a nickel-iron asteroid and then ballooning it with water from the inside might make a viable world.  Would the same apply for building an interstellar spaceship?  To my surprise meteoric nickel-iron is stronger than steel.  Tensile strength of meteoric nickel-iron is not readily available but compressive strength is 430 megapascals.  For stainless steel the range is 170-310 megapascals.  Melting and ballooning a nickel-iron asteroid would not only reduce the enormous launch costs of a large quantity of steel into Earth orbit, but likely provide a stronger vessel.  With that compressive strength, even at high speeds, micrometeors would likely have more of a pitting effect than penetration.

A ten meter thick wall would also provide substantial radiation shielding, although there are secondary radio activation effects when incoming neutrons hit the iron.  (I read this somewhere yesterday, and now can't find it.)   As a first layer of radiation shielding (with water tanks inside, and then a thin layer of lead) it would likely be pretty effective against neutrons and galactic cosmic rays.

In case you are wondering how you melt an asteroid: same answer as how you eat an elephant: one bite at a time, or close.  Big parabolic mirrors focusing sunlight.  Asteroids are generally dark on the outside (albedoes usually .06 or below); so rapid absorption of heat, and being metal, the heat spreads rapidly.  Reaction thrusters on the surface to get it spinning.  Once spinning, inertia keeps it spinning as the heat destroys and vaporizes the reaction thrusters.