In 2013, I walked through roughly 24 grand challenges to space settlement and set about breaking it down as a scientific, engineering, and business problem simultaneously. The key was to find a way to flatten the investment and innovation curves required (reducing time and saving money) while making things modular and keeping consistent standards for the first major phase of space settlement. This series will break down each challenge, the current state of the solution set, and proposed paths to move forward efficiently.

Part 3:  Microgravity Health Effects

Microgravity causes a number of health issues for astronauts. Some are serious, long term issues that are not immediately reversed upon returning to earth. Others fall into simple convenience and curiosity issues. We are unsure if they have a long term impact, but they have not been observed to have one. As for radiation, that is a complex enough topic to warrant a separate article. Also, there are issues with bacterial and viral infections that are amplified by the space environment. Again, that’s a big enough topic for a separate article.

Bone and Muscle Loss


The Problem

Astronauts lose 1 to 1.5 percent of bone density for every month spent in orbit. The technical term for this is spaceflight osteopenia. There are two types of cells associated with bone maintenance in the body. Osteoblasts lay down new mineral along the surface of existing bone. Osteoclasts are large cells that break down old bone and release the calcium in the bloodstream. The process is nearly identical for both systems, so the skeleton remains the same shape and mass over time on Earth.

Under stress, the bone becomes thicker as the process of breaking down old bone is slowed, or the process of adding new bone is accelerated, or both. In the absence of stress, such as from gravity, the process of breaking bone down accelerates and the process of adding new bone slows or stops. Upon return to earth, the process is reversed. The astronaut will regain almost all, but not all, bone mass after a few months.

Under stress, the bone becomes thicker as the process of breaking down old bone is slowed, or the process of adding new bone is accelerated, or both. In the absence of stress, such as from gravity, the process of breaking bone down accelerates and the process of adding new bone slows or stops. Upon return to earth, the process is reversed. The astronaut will regain almost all, but not all, bone mass after a few months.

Exercise Solutions

The main solutions to this problem are exercise, diet, and medication.

Astronauts and cosmonauts work out two hours per day. Up until 2008, the exercise equipment was not adequate to completely offset the bone loss. It was able to slow it enough to make six month visits to the ISS fairly safe, but stays exceeding one year would have been considered too risky for long term health.

In 2008, The ISS installed a new exercise system called the Advanced Resistive Exercise Device, or ARED. Between this device and refinement of medication and diet, the problem of bone loss on ISS was finally halted. This is the culmination of decades of research going back to flights of the NASA Gemini capsules, when bone loss was first observed. This enabled the new program of one year stays on board the ISS, which will allow astronauts and cosmonauts to observe the long term benefits of these systems. For reference, a flight to Mars is 6 to 9 months.

Additionally, the Japanese module of ISS is doing bone density studies of fish and roundworms to quantify the genetic expression within osteoclasts. In microgravity, these osteoclasts tear down bone faster than they would on Earth. The medaka fish study indicates that the problem is rooted in the expression of several genes in these cells (epigenetics), and accelerated activity from cellular mitochondria. A separate study found eleven genes associated with various organs (brain, eyes, ovary, testis, liver, and intestine, with ovary being the most impacted.

While the fish study worked with fish that had actually been hatched on the ISS, the roundworm study will cover four generations of the worms in both a small centrifuge on the ISS and microgravity. This study will also examine genes associated with muscle loss. Since the worms have an invertebrate cytoskeleton, that part of the study will be mostly focused on genetic expression and not direct translation to human health issues. The muscle loss issue is more directly translatable to human patients.

There are also radiation issues associated with bone loss. The issue here is what role cell damage as a result of cosmic radiation and solar flares would have on the performance, epigenetic pressure, and health of osteoblasts and osteoclasts. This is one reason the centrifuge on board ISS for small invertebrate studies is so critical. It allows for the radiation and microgravity issues to be resolved separately when compared to control groups on the ground.

Open Issues

With the new exercise/diet/medication regimen for space travelers, is the quality of bone mass over the course of the flight as good as the bone mass before flight?

Open Issues


Further Research

Is the bone created on orbit with additional exercise, diet, and medication as good as the bone that was lost?

This is a key research point of the extended one year crew missions on ISS.  We should see research papers published on this subject in the coming few years.

This issue can be largely resolved on ISS before the system is retired.  

What is the role of radiation in bone loss versus microgravity?

Radiation also damages the cells that make and break down bone.  Even if we patch the damage from microgravity itself, would the combination of radiation and microgravity present a complex problem?

A future Space Settlement Lab could solve for gravity and radiation at various levels of each.  This is not currently planned by NASA or New Space companies, but is something I would recommend.  

Can the genetic expression of cells that are sensitive to microgravity be temporarily altered to make them less sensitive?

This is one possible goal of isolating the responsible genes, and applying gene therapies.  However this opens a lot of questions on reversibility, side effects, and use with other species. 

Animal and plant research here should continue in an effort to further isolate the cause and effects.  This should also extend to settlement of lower gravity worlds such as the moon and Mars. 

What role would artificial gravity on a spun spacecraft have in reducing bone and muscle loss? What level of gravity is necessary to completely offset bone and muscle loss?

There has very little practical research on either spinning a large spacecraft or spinning two spacecraft around each other via a tether.  There is an issue with possible vertigo if the spin is too fast over too short a distance. 

Future deep space vehicles should spin against their departure stages.  Even vehicles to Low Earth Orbit can spin against the stage that brought them to orbit.  A low-cost, early version could be a Falcon second stage and DragonLab.

How much exercise is required in lunar or Martian gravity to offset bone and muscle loss?

We only have data points for microgravity and Earth gravity, with some centrifuge work on Earth for extended times in higher gravity.  As any economist will tell you, two points does not a trend make.  

This needs to be expanded with a spinning lab or surface missions, or ideally both.

Since genetic expression is part of the problem, would permanent space colonists require genetic modification to adapt more quickly to Mars or other worlds without ill effects? Would this have any downside, or would it be like curing genetic diseases on earth?

We see a convergence of isolating the parts of the body and genome that are sensitive to microgravity, and gene therapies that remove genetic illness.  This could have space settlement implications. 

We will see near-term work on ISS with invertebrates.  In the long term, we should have a space lab with the ability to use in-house created gene splicing elements for plant and animal testing at various gravity levels. 

Vision Problems

Fluids in the spine tend to be pulled downward by gravity during waking hours. In microgravity, they can pool behind the eyes and pressure the shape of the eye, causing blurry vision. This effect will be reduced, but not necessarily totally offset, after the person returns to Earth.

A new experiment will be tried that uses pants that lower the pressure in the lower body relative to the upper body. The goal is to effectively drain this fluid from the head so that it doesn’t cause any distortions or damage. The pants would be worn several hours a day. This could also help with some of the minor issues discussed below.

Again, a spinning spacecraft would in theory offset this issue by forcing the fluids to pool in the lower body as happens on Earth.

Minor Problems

The following are other impacts of microgravity on the human body. These are relatively minor and either are resolved on orbit or resolved very quickly upon return to Earth.

Height - Astronauts get slightly taller in space, due to the lack of pressure on their spinal columns.

Fluid Shifts - Because fluids travel freely in the body, rather than being pulled down by gravity, astronauts tend to have puffy faces and skinny legs compared to their appearance on Earth. After several weeks in orbit, the body learns how to deal with the new environment and the effect is lessened.

Restroom Issues - When the space shuttle would arrive in orbit, there would tend to be a “long line for the restroom”. Without gravity pulling fluids into the legs, the kidneys act as if a person is overhydrated when they are not. Similarly, space travelers returning to Earth need a lot of fluids to avoid dehydration after they are safe on the ground. The lack of gravity also tends to induce constipation.

Congestion - Again, due to fluid balances, most space travelers tend to have stuffy noses and other symptoms that resemble a cold or allergies. Decongestants help suppress the symptoms.

Motion Sickness - For the first few days on orbit, many space travelers can go through motion sickness symptoms.

Sleep - Astronaut sleep tends to be disrupted. On the negative side, in Low Earth Orbit, there is 45 minutes of sunlight followed by 45 minutes of night, every 90 minutes. Spacecraft life support systems tend to be loud. Like, “sitting next to the engines on an airliner for six months straight” loud. Any environment with constant, constant noise will induce stress.

On the positive side, sleeping strapped into a floating sleeping bag in microgravity is absolutely ideal in terms of being the absolute most perfect mattress imaginable. Amusingly, some astronauts are so used to sleeping with a pillow that they will physically strap their head to the pillow to keep the pillow and their head in contact all night. The body also tends to let the arms float free sticking out in front of them, a bit like the fetal position. Some astronauts have woken and been startled at the sight of their own hands in front of their face, looking a bit like someone about to strangle them.

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