We've already developed and test flown our next manned spaceflight system, it's called the Orion Capsule. the ONLY reason nasa doesn't have the ability now is because they didn't feel like human-rating the delta IV heavy.
Reminder both the Delta IV heavy and the Altas V would be able to handle Orion, and both have near-perfect launch records (a single partial failure for each, improper destination orbit).
however, orion was designed as a deep-space research vessel. It has much more radiation protection that other spacecraft and was designed explicitly with long-term, beyond LEO missions to lunar orbit and to mars.
And interesting caveat to space radiation, its not like radiation we know and love here on earth. Space radiation, specifically solar and interstellar wind, are actually highly energized ionized atoms moving at near-light-speed (we're talking 50%+ lightspeed). Our sun emits every atom on the periodic chart up to iron, meaning you can expect a single iron atom moving at near-lightspeed to tear apart any molecules in its way until all that energy is dispersed.
Apollo astronauts litterally saw this radiation, as small but bright flashes randomly appearing in sight. They didn't have special solar wind protection partly because they were only exposed for 2 weeks tops and partly because the technology simply wasn't there yet.
The reason other spacecraft don't need this armor, and the reason the ISS is in a 400km orbit, is the Earth molten iron-nickel core, which generates our unusually strong magnetic field, diverting these highly charged, energetic ions away from earth. This also means there are areas of extreme radiation surround earth, where the diverted solar wind is concentrated, called the Van Allen belts (pic related). The background radiation anywhere else in local space isn't actually that high, its just the risk of a high-energy iron ion giving you cancer someday. the ISS orbits just inside the lower Van Allen Belt, meaning no other spacecraft has needed solar wind armor.
Also, orion was simply not designed to run crew and resupply missions to the ISS. That was the shuttle's job.
Instead of using Orion, Obama told nasa (at Elon Musk and a few other huge nerds' request) to offer those missions to commercial companies, whic so far has actually worked out pretty well, and we're about to see Spacex launch their first people into space later this year(or so they claim). My favorite proposal was the dream chaser, but unfourtunately during its landing test one of the langing gear didn't deploy. They later fixed the issue, but missed too many other deadlines and goals for the crew program and had to drop down to only sending cargo aboard it, but they have been contracted for six flights already. Picrel is the scale drop test article that actually failed.
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