A six-month stay in space induces physiological changes to the human body and a trip to Mars will be thrice that duration
In the first 58 years as a spacefaring species, humans have mastered lower Earth orbit and missions to the moon. We are now in an age where not only government agencies such as NASA and Roscosmos have access to space, we now have commercial entities pushing the boundaries and ambitions for missions to Mars.
It took three days for the crew of the Apollo missions to reach the moon, a mere 0.38 million km away. On current technology, a mission to Mars will take around seven months to travel 55 million km, and this is if the planets align (literally) to make the journey shorter. A successful manned mission to Mars would arguably be the greatest achievement in human history. After decades of research on spaceflight, one crucial question remains: is it possible to send humans to Mars and return them in a healthy condition?
A mission to Mars presents significant physiological and psychological challenges. The body has evolved in a 1G environment on Earth, so our skeletal (movement) muscles, bones, balance (vestibular) system and our engine (the cardiovascular system) are all adapted to work effectively here on Earth.
Once in space, the body will begin to adapt to an environment without gravity, where physical and physiological capabilities are surplus to requirement. These changes take place on six-month missions to the International Space Station but it is a whole magnitude greater on a potential 18-month mission to Mars, where there is greater exposure to space radiation, limited space to exercise, and of course, three times the duration.
Skeletal muscles spend all waking hours resisting gravity and stopping us from crumpling to the floor. In space, muscle of the size and strength we are familiar with are excessive, so the body begins to get rid of them.
Bones are adapted to resist reaction forces from the ground as we take our several thousand steps per day. In gravity, there are no ground reaction forces and a distinct lack of steps for around 22 hours per day. Loss of a trabecular bone found in the pelvic bones and vertebrae in the spinal column will likely degrade to such an extent during a Mars mission that it is impossible to recover. This could render astronaut’s frail and prone to fractures upon return to Earth.
Our bodies use the heart and lungs as an engine to drive everything that we do. Our bodies take in oxygen, which is delivered around the body to produce ATP [adenosine triphosphate, or energy-carrying molecule found in the cells of all living things]—the universal energy currency that powers everything from digestion, to brain activity, and movement. On Earth, the heart constantly works against gravity. When someone faints, we lie them flat so that this effect of gravity is removed taking the stress off the heart. In space, this stress is completely absent as we have no weight (which is a product of mass and gravity, the latter of which is absent). In the days following arrival to space, blood and other fluids shift towards the head causing the "puffy face" that is often seen in astronauts. After a few weeks, the body adapts by reducing the volume of blood in the body. The movement muscles are less active, so demand for energy and oxygen are reduced; and since demand for the supply of oxygen is reduced, the transport vessels (the blood) are reduced.
PSYCHOLOGICAL CHALLENGES are just as profound as physiological challenges. Astronauts are sent on a nine-month journey, in a small spacecraft with finite resources, separated from the most extreme environment known to man, by just a few millimetres of metal. If things go wrong on the Martian surface, the SOS of: “Houston, we have a problem”, will not receive a basic reply for over 30 minutes. The psychological impact of these challenges cannot be underestimated and are arguably as important as the effect of spaceflight on the whole body physiology.
We need to do more to find counter-measures for these declines. This could involve traditional exercise-based countermeasures (which we know at best only slow the rate of decline), drugs, or even modern health technologies, such as gene therapy. For any countermeasures to be effective, we first need to understand the mechanisms through which we lose muscle in space. This can be achieved using model organisms such as C elegans, recently sent to the International Space Station to study muscle loss. We could send these microscopic roundworms to Mars over several generations to see how these simple organisms adapt over such a long period in space. Other candidate interstellar species include eight-legged "water bears" known as tardigrades, which are found in extreme environments around the world and are known for their resilience. These organisms survive extreme temperatures, pressure, and even boiling. If these organisms can’t survive a round trip to Mars, humans have no chance.
AFTER PROLONGED spaceflight, the human body is suited to spaceflight and is no longer compatible with life in a 1G environment. This creates significant challenges for the proposed return mission from Mars. Spaceflight is a model of accelerated ageing. If we sent a 40-year-old astronaut to Mars, after 18 months of spaceflight s/he could return with the physiology of an 80-year-old. We know from International Space Station missions that some of these losses can be recovered, but this is after six months in space and in an environment you can move around in and exercise effectively in. What if the astronaut lives in a capsule such as the NASA Orion space-craft proposed for a Mars mission? This is the size of a car—how much can you realistically do living, eating, sleeping in a car for 18 months floating through space?
One potential solution has been identified by the Dutch Space exploration company, Mars One. A one-way mission mitigates several of the deconditioning problems of spaceflight. If you become adapted to an environment without gravity or with significantly less gravity (1/3G on Mars), you simply don’t come back to Earth. But is this really a solution? To not come back?
THE SURVIVAL of any species is determined by its ability to reproduce and sustain life. Can our species sustain itself on Mars? Would embryonic development produce viable offspring on Mars, and either way, would it even be ethical to try? Research in simulated microgravity (using equipment called a clinostat) has shown that fertilisation can take place in microgravity, but birth rates are negatively affected as placental cells do not develop properly. It is important to note that these experiments do not simulate the effects of space radiation. Radiation (of a sufficient dose) can kill embryos but in surviving offspring, major malformations are unlikely. The expected effects of space radiation might include an increased incidence of cancer or impaired cognitive function. Little research has been done in the area of mammalian reproduction in space, but it is an area we need to explore if colonising Mars is to become a reality.
In 2013 there were over 0.2 million applicants for a one-way mission to Mars with the company Mars One. The aim is to create the first settlement on Mars by 2031. One thing is for sure, we are entering an era of the second space race, and one where more than two players are in the game.
(Christopher Gaffney is a lecturer in sports science at the UK's Lancaster Medical School and a researcher on spaceflight-induced muscle loss)
(This article was first published in Down To Earth's print edition dated May 1-15, 2019)
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