Gravity and its Implications on Human Space Colonization

Co-written by Gaurav Dubey and Jonathan Maloney where Jonathan focuses on the physics of gravity and Gaurav expounds on the health implications.

Perhaps since the dawn of the human race, people have looked to the stars and theorized colonizing our moon and other planets. The rapid technological advancements of recent decades, especially as it relates to space travel, have begun to make this interstellar proposition a genuine possibility in the not so distant future. Considering the rapid decline of our current planet due to climate change, among other serious concerns, many people are actually looking to innovative mogul billionaire’s like Jeff Bezos and Elon Musk to pioneer our way to what will essentially be “contingency Earth 2.0”.

Surely, if anyone could help us colonize the Moon or Mars, it would be these two and other dynamic, resourceful billionaires like them (it certainly goes without saying this would likely be the most expensive expedition in human history and many billions, if not trillions of dollars would be necessary). But what are some of the major limitations humans currently face in trying to colonize space? It turns out, the differing gravitational fields humans would experience is one of the biggest barriers to space colonization today. This article will explore the physiological impacts of differing gravitational fields on the human body and how these translate to our current challenges in the context of space colonization. But first, before exploring the health implications of extended microgravity, let's discuss what exactly gravity is.

What is Gravity?

Gravity, as you may already be aware, is one of the fundamental forces of nature. In addition to the strong, weak, and electromagnetic forces, there is gravity. While it is the weakest of the fundamental forces of nature, it is the most intuitive as we regularly encounter it in our daily lives. If you jump up, you come down. Try dropping something off of a bridge and it will most certainly fall downwards toward the ground. You may have, as do I, remnants from your childhood in the form of scars as we constantly battled with gravity when we played. We envy the birds as they soar through the air effortlessly as we have no way of leaving the ground ourselves unless we use one of our many flying machines. Gravity is everywhere. It is a force that we are all intimately aware of.

Gravity is this omnipresent entity that’s answer to the question “why is it there?” continues to evade us to this day. This question is similar in spirit to the existentially pointed “why do we exist?” All we know is that it does exist and it is distinct and worthy of the title “fundamental force.” What is more, even though we cannot answer the question of why it exists, we can describe it with great precision thanks to the work of both Newton and Einstein.

How Do We Describe Gravity?

The idea of gravity was first proposed by Sir Isaac Newton who, supposedly, was inspired by an apple falling from a tree that he happened to be sitting under at the time. He discovered that the same force that made this apple fall to the ground is the same force that causes the moon to “fall” around the Earth (i.e., orbit). Gravity is further responsible for causing all the planets to orbit the Sun and is the sole reason the entire solar system stays together instead of zipping off in all directions. In fact, extrapolating further to the grandest scales, it is gravity that holds our entire galaxy together.

Beyond recognizing gravity as a fundamental force, Newton was able to describe it mathematically, which is encapsulated in the following equation:

Following the work of Newton, Einstein later modified Newton's gravitational theory to that of general relativity. Centuries before Einstein, astronomers had observed a small flaw in Mercury's orbit as predicted by Newton's laws, but no one new exactly why. At one point, members of the scientific community even entertained the possibility of a new planet in between the Sun and Mercury in order to “fix” this anomaly instead of trying to replace it with a new theory. Eventually, one young bright physicist decided to try and craft a new theory in order to explain not only this anomaly, but would also have to accurately describe all of Newton's predictions as well.

In 1915, Einstein published the “Einstein field equations,” which are the cornerstone of his theory of general relativity [1]. Beyond describing the anomaly in Mercury's orbit, phenomena such as gravitational lensing, black holes, and gravitational waves were predicted. The first experimental evidence in support of Einstein's theory came in 1919 during a solar eclipse where gravitational lensing was observed [2]. This evidence lead to overnight fame for Einstein as the scientific community was forced to accept his theory signaling a paradigm shift in gravitational science. More recently, other phenomena predicted by general relativity have been experimentally verified. In 2015, gravitational waves were observed for the very first time and just last year, the world saw the first ever image of a black hole [3,4].

The Einstein field equation is:

Qualitatively, the left-hand-side of this equation describes how matter/energy curves spacetime whereas the right-hand-side describes how matter/energy moves through curved spacetime. Out of everything that Einstein contributed to the field of physics, this equation is the nexus for what is arguably his magnum opus, the general theory of relativity.

Gravity and Life

As far as we know, gravity has been a part of the universe since its inception. Even the formation of our solar system would never have happened if it wasn't for this force. The Earth itself congealed out of a primordial distribution of rock and gas and these individual pieces would have no interest in ever coming together to form a larger body if gravity didn’t exist.

Earth is approximately 4.5 billion years old and in all of that time, gravity has worked to shape the landscapes as well as anything that might find itself on its surface. About 3.5 billion years ago, something truly remarkable happened. The components of matter came together in just the right combination to give rise to life; something that, as far we know, has occurred nowhere else in the universe. Life, just like anything else that might find itself on Earth's surface, is subject to the force of gravity due to the planet. As a result, as life evolved to occupy the different corners of the biosphere, it had to obey the rules imposed by its environment. One of those rules is the force of gravity; that is, life is uniquely designed to survive in Earth's gravitational field.

As you know from earlier, the force that an object feels due to gravity is directly related to the masses of the objects involved. For life here, the masses involved will be the mass of the object (i.e., you, me, or any other biological organism) and the mass of the Earth. If there are changes in the mass of the object other than that of the organism, it can lead to deleterious consequences. This shouldn't be too surprising given our evolutionary history and how natural selection has fine-tuned our biological machines (i.e., our bodies) to survive in only Earth's gravitational field. How our bodies and life in general deteriorate outside of Earth's gravity is an intense field of study. To date, there have been a number of interesting experimental results that indicate a significant barrier for our species if we ever hope to travel to the stars. Now that we know what gravity is and how life is uniquely designed for Earth’s gravitational field, let’s explore some of these experiments and their implications for human space colonization.

Extrapolating the “One-Year Mission” To Model Long-Term Space Travel

The journey to Mars, by current space travel standards, is a whopping three years. Before we can colonize a planet like Mars, we need to figure out how to get there first. In an expedition to Mars, humans would have to adapt to “weightless” gravity conditions during the trip there and approximately one third of Earth’s gravity while on the red planet [5]. Thanks to an experiment where an astronaut spent one year in space (dubbed the “one-year mission”) we can better predict the repercussions of a three-year Mars expedition and it’s impact on our biology [6]. The effects of microgravity on the human body are perhaps some of the most dramatic and the central focus of the rest of this article.

Physiological Consequences of Microgravity on the Human Body

It is has been clearly established in clinical literature that microgravity influences different biological systems like bone and muscle as well as the heart and brain [7].Furthermore, it is known to enhance cancer risk as well [7]. Microgravity-induced alterations in the autonomic nervous system (ANS) contribute to derangements in both the mechanical and electrophysiological function of the cardiovascular system, leading to severe consequences in humans following extended space travel and of course, potential interstellar habitation. According to NASA, microgravity conditions affect your “spatial orientation, head-eye and hand-eye coordination, balance, locomotion, and you’re likely to experience motion sickness” [5]. Perhaps even more disturbing is the effect microgravity conditions have on bone density.

Microgravity Conditions Degrades Bone 12 Times Faster Rate Than On Earth

In the absence of Earth’s gravity and in the microgravity conditions of space, NASA scientists have discovered your bones lose minerals at an alarmingly faster rate. In fact, instead of losing 1% bone density over a year in the elderly, astronauts were losing over 1% density per month! A 2014 scientific review investigating the impact of microgravity on bone cells and mesenchymal stem cells discusses two studies that demonstrated “astronauts and cosmonauts did show a distinct loss of bone mineral density in the lumbar spine, the pelvis, and the proximal femur” [8] and “the extent of bone loss varied up to 20%” [9,10]. NASA scientists have discovered that “Bisphosphonate drugs have shown to be effective in preventing bone loss” and thus, utilizing pharmacotherapies to prevent rapid bone degradation may indeed be the avenue researchers and clinicians take to cultivate adequate conditions for long-term space travel/inhabitation.

Simulated Microgravity Impairs Heart Function

In 2015, I was briefly a PhD student at the University of Miami Miller School of Medicine working on a NASA sponsored project dubbed project CASIS (Center for Advancement for Science in Space). While I was only a student for a few months before deciding not to pursue my doctoral degree at the time, I did have the opportunity and privilege to work on a study involving spinning cardiac stem cells in a rotating incubator to simulate microgravity conditions to assess how gravity affected their differentation. In 2018, acclaimed stem cell scientist Dr. Joshua Hare and his team published their findings, which indeed demonstrate “simulated microgravity directly impacted the in vitro development of cardiac NC (Neural Crest) progenitors and their contribution to the sympathetic and parasympathetic innervation of the iPSC-derived myocardium” [11]. Put more simply, the microgravity conditions created by spinning the incubator housing these cells resulted in abnormal cellular development and differentation. These findings have stark implications on the sustainability of extended-space travel and potential interstellar colonization.

Closing Thoughts on Reconciling the Microgravity Dilemma

The aforementioned barriers to space colonization and extended-space travel, as they relate to microgravity conditions, only addresses some of the most significant and physiologically pressing concerns. The biological implications of extended exposure to microgravity conditions is still being studied and potential solutions to combat the known adverse effects are currently being hypothesized and deliberated. If humanity ever has hopes of moving beyond this planet, overcoming these barriers will be paramount to success. Musk and/or Bezos may provide humanity with the technology that is capable of propelling us to distant worlds, but if we don’t figure out how to overcome the microgravity challenges, we may be confined to our blue and green world for the foreseeable future.