Deriders of the new NASA direction have latched on to the announced human asteroid mission in the 2025 timeframe as something they "can't imagine" and therefore is not worth doing. Of course, the administration is talking up the "science" that can be done on an asteroid, and how this could better inform us should the need arise to divert or destroy one that threatens Earth. This is good politics as nothing motivates like fear, but for those of us who think the human spaceflight program is really about preparing us to live at the future homes of humanity, asteroids would seem to be just a stop on the way - I disagree.
As I've written previously, the new NASA direction isn't about asteroids - it isn't about destinations - it's about going and specifically, it's about going to Mars. I'm not sure NASA knows yet why they're going to Mars, but they're focusing on the technology to get there and get back safely, and some of the stepping stones along the way are asteroids. As such, although I will often advocate that I think asteroids are a much better future home for humanity, I recognize that in terms of the battle lines of this debate, asteroids are neutral or worse, disposable.
So how does one live on an asteroid? I've regularly heard this question asked by intelligent people. They point out the low gravity and how with just a misplaced step an astronaut could be hurtled into escape velocity and lost forever! NASA's mission to an asteroid will most likely be conducted on the surface, so this is a real risk, just as it is for astronauts conducting spacewalks on the International Space Station. However, the settlement of an asteroid would have little use for the surface, except perhaps as a place to lay solar panels, as all the interesting stuff happens below the surface.
The primary reason is radiation. Just like on the Moon or Mars, humans will need to live underground to provide passive protection from galactic cosmic rays and solar storms. On Earth (and Venus) the predominate protection from radiation is provided by the atmosphere, miles and miles of it. To achieve the same level of protection only a dozen feet or so of regolith is required.
Robotic probes will be sent ahead of NASA's human mission to an asteroid. More than likely, only an orbiter, but a much more capable robotic lander makes a lot of sense. For the long term settlement of an asteroid, it will carry essential drilling equipment which it will use to drill straight down. After digging down for a while, the robotic drill will turn some significant angle and keep drilling. The hole it produces need only be big enough to maneuver a crew module into without bumping the sides - once they arrive, weeks or months later. The right-hand-turn the drill makes is sufficient to protect the crew from radiation, which can only move in straight lines. If mirrors are installed on the turn the crew can enjoy natural sunlight and a view of the stars.
Having secured the safety of the crew from ionizing radiation, they are now free to get to work. Using drilling tools the astronauts can prospect deep into the core in search of the richest metals, or collect volatiles which can be purified into drinking water or oxygen for breathing.
Soon, they'll dig a long circular tunnel with a radius of at least 894 meters. The outside edge of the tunnel is lined with metal track. A simple electric train runs the length of it, completing a full circuit in just one minute. On a parallel track the astronauts enter an open carriage which accelerates them up to rendezvous with the ever moving train. As they speed up the astronauts feel the gentle pull of centripetal force as it builds to a full Earth-standard gravity.
As the astronauts step onto the train they cease being astronauts and become settlers. They now have access to resources, protection from radiation and a full Earth-standard gravity. The colony can now grow. The train can be extended compartment by compartment and deck by deck to accommodate the growing population. Excess metals and other materials can be exported to other settlements in the solar system.
Another one of the things the settlers may wish to do is to place airlocks on some of the tunnels that lead to the surface and place transparent plastic material or even glass over the ones dedicated to bringing in natural sunlight. That way the entire internal space of the asteroid can be pressurized and the settlers will be free to work and play in zero-g without spacesuits.
Here's an animated gif of humanity's future home inside the asteroids.
Imagination is a precious part of space advocacy. Yes, we must guard it with scientific skepticism but not so much that we're afraid to dream.