Over the next few years human exploration of our nearby solar system neighborhood may go from what was science fiction just a handful of years ago, to a new reality. A permanent human presence on the Moon, complete with an orbiting spaceship that shuttles astronauts to and from the lunar surface, will eventually become a launch pad for human missions to Mars. A new model of public-private-academic partnerships between NASA, well known companies such as SpaceX, but also many other ambitious companies, as well as research universities, will need to push the current limits of science and technology. This will be demanded by the rigors and challenges of long-term long-distance human space travel.
Under the Artemis program, NASA is planning to return humans to the Moon by 2024. If all goes to plan, the first woman and the next man on the Moon will begin a process of scientific exploration and technological development and testing that will lead to a sustainable human presence on the lunar surface. A large network of U.S. companies and international partners will work with NASA to contribute the mission-critical technologies and capabilities that will be required to establish and maintain such an operation. NASA’s Space Launch System rocket and Orion spacecraft will move astronauts and equipment to and from Earth. And an orbiting lunar outpost, the Gateway, will be built to provide the infrastructure necessary to sustain a human presence on the Moon.
But there is also a greater purpose to these missions. It is a stepping stone, both figuratively and literally. All the science and testing of advanced technologies, and everything that is learned about how humans survive in microgravity environments for prolonged periods, will go towards the push to take humans to the red planet — to Mars.
The challenges involved in achieving this are significant. According to NASA, missions to the moon are over 1000 times farther than to the International Space Station. But getting to Mars is entirely different. That is a 34 million mile one way trip (when the orbits of Mars and the Earth are at their closest).
At these distances astronauts will be on their own, and will have to survive and thrive in extremely inhospitable environments with what they carry with them. The tolerance for technological failure will be very low. Computers — supported by machine learning and artificial intelligence algorithms that do not yet exist — will need to be able to take in data on-the-fly, and make inference risk calculations and present options for decisions in the face of possibly limited and incomplete information. They will not have the luxury of having been pre-trained on all possible scenarios they may encounter back on Earth prior to deployment.
And before future astronauts embark on such a long journey, scientists need to better understand how the physiology of the body will be affected by the travel conditions of space over prolonged periods. This includes the brain. The brain is particularly important because survival will depend on the astronuts’ abilities to rationalize and engineer solutions to problems that may be difficult to predict, and they will have to solve them under constrained and less than ideal conditions. So understanding how the brain and mind will react and cope is critical.
What Happens to the Brain Under Microgravity
The brain, like all the organs and physiological systems in the body, evolved to support life under an incredibly narrow set of conditions. Animals, including humans, interact, survive and thrive in environments with variables defined by narrow ranges. Temperature, pressure, and oxygen levels must all be within the right levels. Too hot or too cold, too much or too little pressure on the body, or too little or too much oxygen can all lead to conditions that are unable to support life.
Similarly, the body has evolved to work properly only under the right amount of gravity. The cells that make up your body, including all the cells in your brain, are made up of organelles, which literally means ‘tiny organ’. Different organelles have different functions inside a cell. Mitochondria produce a cell’s energy. The nucleus contains the genetic information of a cell, which gets translated and produced into proteins in the rough endoplasmic reticulum and Golgi apparatus, two other organelles. And so on.
Another important organelle is the cytoskeleton. The cytoskeleton consists of a vast network of different kinds of fibers — microtubules, actin filaments, and intermediate filaments — that are continuously extending and retracting. Think of it like an internal Lego set inside a cell where pieces are added and taken away to form long interconnected networks of Legos as needed. It is the cytoskeleton that pushes out from inside a cell and gives it its shape. It is what counteracts the forces of gravity pushing down on the cells of your body. In neurons, the main class of brain cell, the cytoskeleton also acts as a conveyor belt, transporting small packages of signaling molecules critical to the electrical communication between connected neurons between different parts of the cell.
Too much or too little gravity and the functions of the organelles, proteins, and other molecules that make up cells start to change in ways scientists do not fully understand.
For example, neurons in ‘brain organoids’ — tiny spontaneously forming brain-like structures derived from transformed stem cells that can be cultured in a Petri dish — have gone up into space in an effort to begin to understand how neurons are affected by microgravity. Brain organoids sent to the International Space Station were compared to controls organoids that spent an equal amount of time in a lab here on Earth. While much still needs to be investigated, one seeming difference are changes in the shapes and sizes of organoids that spent periods of time under microgravity conditions compared to controls. In space, the force of gravity is much less, which means that the counter-forces exerted by the cytoskeleton pushing out on the cells may be excessive, causing them to grow differently. The physiological consequences of this mismatch — what effects it might have on the functioning of neurons and the brain — remains an unknown and requires additional research. (Disclaimer: While the author was not involved with this work, he currently collaborates with these labs on related research.)
On-Going Studies in Humans
With an anticipated long-term human presence on the Moon expected soon, and human missions to Mars in the not to distant future, research is beginning to systematically address the effects prolonged periods in space has on the brain.
Using magnetic resonance imaging (MRI), which can non-invasively reconstruct the anatomical structure of the brain in three dimensions, a number of studies have looked at the effects of the duration of microgravity on the brain. Changes in the brain’s position, mechanical properties and tissue volume have been reported. The distribution of cerebrospinal fluid (CSF), which is the chemically unique fluid in which the brain and spinal cord are bathed in, can be affected by changes in gravity. In particular, it can affect the recirculation and turn-over of CSF. This is important because the brain and spinal cord don’t just float in a fixed volume of CSF, as if they were floating in a jar that contained a fixed amount of fluid. But rather, CSF is constantly being produced and resorbed by different parts of the brain. The degree of turn over between CSF production and resorption is critical. It has to be perfectly balanced because if it is not, and production exceeds resorption, the amount of fluid increases. Because the volume inside the skull is fixed, any increase in CSF can lead to increased hydrodynamic pressure on the brain. CSF is mostly water and water is incompressible — so the squishy brain gets compressed instead. Left unchecked, this can lead to neurological and cognitive deficits, and even death.
Microgravity changes to the microstructure of the brain have also been reported, including changes to the structural connectivity of the networks of neurons. Such changes are not isolated to a particular region of the brain, but seem to occur throughout — from cerebellum, which is critical to motor function and balance, to the visual cortex which processes visual information, to regions of the brain involved in producing cognitive functions.
What isn’t yet clear is which changes reflect positive adaptations in the brain to microgravity environments that will allow humans to better perform under such conditions, versus which changes reflect pathological dysfunctions that might inhibit performance or produce unacceptable risk. What is clear however, is that much more research will need to be done before humans can safely travel long distances out in space for long periods.