A human mission to Mars will take anywhere from two and a half to three years. That is NASA’s best estimate, with each leg of the trip taking six months and including an 18 to 20 month stay on the Red Planet. That does not sound like an extremely long-term prospect until one considers the fact that the world record for the longest single stay in Earth orbit belongs to Soviet cosmonaut and physician Valeri Poliakov at 437 days and 18 hours aboard the Mir space station in 1994-1995. That is less than half the time it would take to complete a mission to Mars.  

Our collective experience in space falls short of what is necessary for interplanetary travel. Although Poliakov administered tests on himself during the flight and submitted to further studies after, the data were limited to that of a single human being. It was impossible to know if his results were unique or if they would be common to others traveling in space for a year or more. Valeri Poliakov’s mission left the question open: “Would another human do better or worse given the same conditions?”

When we think about occupational damage to the body, we often think in terms of normal wear and tear through impact, repetitive motion, stress, and strain. In space there is very little impact.  We often imagine this to be a benign environment, but that is not the case. Astronauts and cosmonauts come back to Earth physically changed. The longer they stay in space, the more profound the changes. This observation has left scientists asking, “Why is long-duration spaceflight so hard on the human body? And how can we ameliorate these changes?”

Almost 60 years ago, when the USSR and the US were first preparing to send humans into space, no one knew exactly what to expect. The US spacecraft, Pioneer, confirmed the existence of radiation belts that help to protect the Earth against solar radiation. There was a fear that spacecraft and spacesuits could not adequately protect humans against solar radiation. There was also the concern that a pilot might not behave rationally while orbiting the Earth without the normal effects of gravity. These basic concerns over radiation and weightlessness were laid to rest between the years 1961 and 1963. Humans could not only survive, but thrive and do meaningful work in Earth orbit.

With the exception of the Apollo missions to the Moon, all human spaceflight activities have been in low Earth orbit. This is anywhere between 160 to 2,000 kilometers (99 to 1,200 miles) above the Earth, but most missions occur at an altitude range of 322 to 442 kilometers (200 to 275 miles). While that might not seem very far, humans on the International Space Station (ISS) are traveling at approximately 28,163 kph (17,500 mph), completing a rotation around our planet every 90 minutes. In orbit, they are in a continuous state of free fall, experienced as weightlessness or microgravity. This is not a normal environment for the human body, born and bred in gravity.

Scientists have known for decades that this environment has an impact on human physiology beyond the challenges of shielding against radiation. Humans lose bone mass through loss of calcium as a consequence of living in microgravity. Blood tests have indicated this fact since the earliest years of human spaceflight. During Project Gemini in the mid-1960s, physicians noted changes in vision among astronauts. Even after a decade of requiring stringent exercise regimes for Soviet and Russian cosmonauts, physiologists and physical therapists have noted changes in their health and fitness that do not recover after flight.

Humans evolved on Earth with its gravitational pull. What scientists need to know before sending humans to Mars is what happens to life in a different gravitational environment.

The ISS presented opportunities for more systematic, multinational studies on the effects of long-term spaceflight on human physiology. In a collaboration between America and the Russians, a study was launched to begin to quantify the specific changes in human physiology that occur in orbit. From March 2015 to March 2016 Mikhail Kornienko and Scott Kelly became orbiting test subjects. They underwent very thorough pre- and post-flight examinations to establish both base-line health and the long-term effects of spaceflight. And, as luck would have it, Kelly has an identical twin brother, Mark, who has also been an astronaut. Thanks to the twins’ willingness to volunteer for a truly unique experience in human spaceflight, there was an ideal control specimen here on earth.

NASA and Roscosmos selected a number of investigations for the mission that focused on key health issues. Each of the US investigations was grouped into one of seven categories: functional, behavioral health, visual impairment, metabolic, physical performance, microbial, and human factors. Each category included investigations into symptoms that must be better understood in order to send humans on that three-year mission to Mars.

1. Functional (the ability to perform integrated tasks)—Tasks such as sighting an object and moving to pick it up requires fine coordination of reflexes, senses, and nerve responses. This coordination changes in space.
2. Behavioral Health—The mental fitness of travelers to Mars is essential to a successful mission. The isolation and monotony of long-distance space travel may challenge good mental hygiene and require mitigation.
3. Visual Impairment—Changes in ocular pressure during spaceflight may not return to normal after flight. Scientists wanted to understand better when and how these changes occur.
4. Metabolic—Every organ and system in the human body has its own response to microgravity. Metabolic studies examine the effects of long-duration spaceflight on body chemistry, the heart, and immune systems.
5. Physical Performance—Without the tug of gravity that we experience here on Earth, muscles atrophy and bones weaken. Compensating for the loss of round-the-clock Earth gravity with only a few hours of daily cardio and resistance exercises in space provides only a modest solution.
6. Microbial—It is often overlooked that humans depend on billions of microbes to maintain our health. Scientists want to know what happens when microbial species live in space for long periods. Do they also change, and if so, what effect does that have on space travelers?
7. Human Factors—These include issues of repetitive motion injuries and deterioration of fine motor skills. The goal is to assess how astronauts interact with equipment and tasks, and how that affects both their productivity and wellbeing.  

During the “Year Study,” Kelly and Kornienko traveled more than 241,401,600 kilometers (150,000,000 miles) during their 340 days aboard the ISS. That is the equivalent to the shortest possible path to Mars. Work continues as scientists and doctors analyze inflight results and look for any lasting effects that spaceflight has had on their bodies. Now that there is a sample of three year-long space travelers—Poliakov, Kelly, and Kornienko—it may be possible to refine investigations using other test subjects and come closer to understanding what toll long-duration spaceflight really takes on humans.