Mars is a difficult target for humans.

It’s a long trip. At closest approach Mars is over forty million miles from Earth. Current technology limits how fast we can travel, The faster we go, the sooner we get there, but the more fuel we need, both to accelerate to cruise speed and then to decelerate to get into Martian orbit. Realistically, the trip will take months to years. Difficulties with providing life support and psychological pressures of a small crew in a small space will be exacerbated by the length of the journey.

In addition to life support and crew interactions, extended exposure to zero gravity is known to be deleterious to human health. Even with remedial measures like exercise, astronauts returning from extended visits to the ISS can barely walk. Spinning a spacecraft can provide a kind of pseudo-gravity that can counteract these effects, at the cost of increased complexity. Creating Earth-normal gravity is a massive challenge in spacecraft design, but Mars level gravity, just 38% of Earth gravity, might be doable.

One last danger is the high radiation exposure in deep space. In Low Earth Orbit, Earth’s magnetosphere protects the ISS and its crew to a great degree. Outside the magnetosphere the radiation environment is intense.

Radiation hazards come in two flavors: solar wind and galactic cosmic radiation (GCR). The solar wind is a gale of energetic charged particles emitted by the Sun and streaming outward. It mostly consists of ionized Hydrogen nuclei—protons. Galactic cosmic radiation consists of extremely energetic, totally ionized nuclei like He⁺⁴, N⁺⁷,or Ar⁺¹⁸. GCR is much more energetic than the solar wind, but the solar wind as it streams into extrasolar space pushes back against the flow of GCR. The radiation fluence experienced by a spacecraft between Earth and Mars is a combination of both.

Shielding against the solar wind and GCR is possible, but it carried a weight penalty. The good news is that, for charged particles, the best shielding is low atomic number material. For the solar wind, hydrogen-rich materials like water or plastic are optimal. For GCR, it’s more complicated, but lower Z materials are still better than high Z materials like lead. If a spaceship were built of plastic and water storage tanks were wrapped around the crew spaces, it would go a long way toward shielding a spacecraft on its way to Mars.

One potential solution to the problems of gravity and radiation is to use a asteroid as a spacecraft. Assuming the asteroid is consolidated, i.e., held together by cohesion of its components as well as gravity. If the asteroid is a rock instead of a loose conglomeration of pebbles and dust, it can be hollowed out and spun without coming apart.

Whatever the spacecraft, there is a particular kind of trajectory known as a cycler that may well be optimal. A vehicle on a Mars cycler trajectory passes by both Earth and Mars on a regular basis. The most famous of these is the Aldrin Cycler, first postulated by the Apollo astronaut Buzz Aldrin. If it starts within the open launch window, the cycler takes about five months to get from Earth to Mars and another eighteen months to get back. A cycler that starts at Mars (in a different launch window) takes five months to get to Earth and another eighteen months to get back. A pair of cyclers could provide relatively rapid transit between Earth and Mars.

So far the Mars cycler notion has been envisioned as a way to send bulk cargo to Mars on a recurring basis. But there might be a better way. The Aldrin cycles takes 5 months to get from Earth to Mars, then goes out into the Asteroid Belt and back to Earth in about nineteen months. Suppose the first spaceship to follow the Aldrin cycles, during its time in the asteroid belt, found an asteroid that it could inject into the cycler trajectory. Then we’d have an asteroid going from Earth to Mars and back. So what?

Suppose further that the crew that finds the asteroid hollows out a space for crew deep enough beneath the surface that the bulk of the asteroid itself protects the crew against cosmic radiation. If the crew sets the asteroid spinning it would create spin gravity in the crew compartment. The crew space needs a controlled environment but cargo spaces may not need them. Creating a hive of spaces inside an asteroid sculpted like this makes it vastly more useful than any spacecraft we could build. And, it takes relatively little fuel to keep the cycler in its orbit.

The problem here is that many asteroids are rubble piles—held together by gravity and little more. Any push on one part just rearranges the other parts. Push hard enough and the pieces fly apart. They may come back together, they may not. So you can’t use a rubble pile as a cycler, or any kind of ship.

What you need is an asteroid that is consolidated. Big enough asteroids (like Ceres or Vesta) can consolidate from the force of gravity alone. Those huge asteroids are too massive to move onto a cycler trajectory. Smaller ones, say 100 meters or so in diameter, may be consolidated enough to (a) hollow out for interior storage and (b) spin fast enough (1 RPM or so would be enough) to give Mars level gravity. Keep the rooms far enough beneath the surface (a few meters is enough) and the asteroid provides radiation shielding to crew and cargo.

The tricky part is that spin gravity is just centripetal force. Up would be toward the center of the asteroid, down would be the inner side of the surface. Since the asteroid would be spinning faster than most natural asteroids, ingress and egress would have to be at the poles.

In honor of this concept I’ve written a short story about asteroid pirates (yes, really) attacking such an asteroid on an Aldrin Cycler trajectory. It should be available as a Kindle short in time for the holidays.