Humans are simply not adapted to space. We evolved in 1G, the amount of gravity near the surface of the Earth, and are well suited to that environment. Spending a lot of time in microgravity, such as aboard the ISS, has a number of physiological effects. Now that some astronauts have been spending long periods of time aboard the ISS, researchers have been better able to understand these effects.

One recent study involved astronaut Scott Kelly, who spent 340 days in the microgravity of ISS. This study involved heart function – the hypothesis was that spending lots of time in orbit would reduce the strain on the heart, because it would no longer have to pump blood against gravity. Over time this would weaken the heart. That is, in fact, what they found. Kelly’s heart lost about 27% of its mass over the 340 days. However, it then regained that mass after months back in normal gravity, readapting to 1G.

What remains to be seen is if there are any long term consequences of losing and then regaining so much heart mass. One concern is that this may change the overall shape and ratio of the atria to the ventricles, and may predispose to atrial fibrillation, an abnormal conduction in the heart.

As an interesting aside, the same study also looked at swimmer Benoît Lecomte who spent 159 days swimming an average of 5.8 hours per day in the Pacific. He also experienced a 25% loss of heart mass. This was due to spending so much time horizontal and floating. This shows that swimming can be a good physiological marker for microgravity, at least for heart function. The study also shows that in both men exercise did not prevent this effect. Astronauts aboard the ISS have a rigorous exercise program to reduce bone and muscle loss, but apparently this did not prevent heart muscle mass loss.

The motivation for this research is to test the plausibility of prolonged human missions off the Earth, either in orbit, on the moon, or on deep-space missions to Mars or beyond. How will humans respond to spending years outside 1G? We know from previous studies that the effects of chronic microgravity include bone loss, muscle atrophy, heart deconditioning, vestibular disorientation, adverse effects on vision, anemia, and immunodeficiency. Exercise mitigates the bone and muscle effects, but not the others. This is partly why some argue that space is not meant for people, we should just send or robots there instead. Certainly, our robots will do the heavy lifting in terms of exploring and working in space for these and other reasons, but it does not seem we are going to abandon human spaceflight anytime soon.

Space also carries other hazards for humans, including extreme cold and near vacuum. These can be handled by having an enclosed pressurized and heated environment, such as aboard a spaceship, a space station, or inside a spacesuit. A far greater problem is radiation. The atmosphere and magnetic field of the Earth protect us from cosmic rays and the solar wind. But without them, exposed in space, we would receive enough radiation even over a few months to significantly increase our risk for cancer.

On the Moon and Mars the solution is to build our habitats underground, shielded by meters of rock. Otherwise significant shielding would be necessary. For ships the problem is more difficult. We can build shielding, the problem is balancing the need for shielding with the added weight. There are potential solutions here as well. One is to store the supply of water for a long journey in a layer in the hull. Water makes a good radiation shield. Or, as in the show Avenue 5, you could store human waste in a hull layer to form a “poop shield”. This may not be very appealing, but it could be effective.

There are also composite metal foams that have great radiation shielding properties and are relatively light weight. I suspect that some combination of metal foam and water storage will be a primary radiation shielding for any deep space human missions. But there is another option – if you have the energy to spare you can generate a local magnetic field, and this will deflect any charged radiation, as does Earth’s magnetic field. Although living in a strong localized magnetic field may have some unintended consequences.

What about the microgravity problem? There have long been two proposed solutions to this problem for spaceships, neither of which are gravity plating which will not be an option for the foreseeable future if ever. One is simply acceleration. For at least part of a long journey you can accelerate at (ideally) 1G, and once you get half-way to your destination then turn around an decelerate at 1G. Conventional rockets would not have the fuel for this, but some kind of nuclear engine might. Even if you could accelerate at a fraction of 1G (30% or so) that would be better than nothing. This also would make life aboard ship perhaps much easier.

For now the most practical way to simulate gravity is through rotation. For a space station this would be relatively easy – just build a big ring and once you get it spinning you can simulate 1G. You can even have inner rings at lower gravity for functions that would be better in reduced gravity. And they can be attached in the middle to zero-G segments. There is already a private company with plans to build such a station. A space ship would be a little more complicated, because it would need to have one configuration optimized for acceleration, and then once coasting through space would then change to a rotating configuration. One proposed design would simply split the ship into two parts connected by a long cable, and then rotate them around each other. They can then be rejoined for landing, take off, and docking maneuvers.

These are all solvable problems with today’s technology – they just have to be engineered, designed, and built. Simulating Earth gravity would be harder on the Moon and Mars because you can’t simply spin something as easily as you can in orbit or drifting through space. One proposal, however, is for a train that will travel on a tilted circular track at high speed. This seems rather tricky, but not impossible.

The show, Expanse, deals well with the problems of gravity in a future where we are colonizing the solar system. It is a dominant factor at play in almost every situation, both too much and too little. They even build a space station inside Ceres and “spun it up” to increase the gravity from centripetal force, although only to 0.2G. Further research will likely show how much gravity is necessary to sufficiently minimize the long term negative effects of living in low gravity. Although, there are inherent negative effects and then there are physiological changes that are only a problem if you ever want to visit or return to Earth. If you spend your life on Mars, the 0.376G may be just fine.