What are the risks of space for humans?

It is no sur­prise that humans are not nat­ur­ally equipped to live in space. Explor­ing it requires a great deal of tech­nic­al adapt­a­tion, years of train­ing, not to men­tion high mor­ale and phys­ic­al fit­ness. The first human space­flight took place on 12 April 1961, when Yuri Gagar­in made his only trip around the Earth in the Soviet Vos­tok capsule.

Although human pres­ence in space was once rare, there have been people in space con­tinu­ously for the past two dec­ades mainly thanks to the fam­ous Inter­na­tion­al Space Sta­tion (ISS), where astro­nauts from dif­fer­ent coun­tries take turns work­ing in and out­side the station.

Space is often referred to as being very dan­ger­ous for humans, but what are the risks of ven­tur­ing into this extreme envir­on­ment? How can they be min­im­ised? Is a space acci­dent pos­sible? And what influ­ence does space travel have on a human being?

The main prob­lem in space is that there is not just one haz­ard to watch out for, but a mul­ti­tude of factors that if not prop­erly con­sidered can all lead to crit­ic­al situ­ations. And we learned this from the very begin­ning of the space age.

One of the first incid­ents occurred just four years after Gagarin’s first flight. He remained in the pres­sur­ised cab­in of his space­craft through­out his jour­ney whilst his coun­ter­part Alexei Leonov attemp­ted the first space­walk in a space­suit. After ten minutes out­side the space­craft, he decided to return but real­ised that the air pres­sure inside his suit had inflated so much that he could no longer fit into the air­lock. As a res­ult, he had no choice but to risk let­ting the air escape from his suit, redu­cing the pres­sure to 1/3 of atmo­spher­ic pres­sure (at the risk of a gas embol­ism) to finally be able to return to the safety of his ship. Today, there is no longer any risk of such an event hap­pen­ing. Firstly, because space­suits are much less flex­ible and elast­ic than Leonov’s and, secondly, because mod­ern space­suits oper­ate under a pure oxy­gen atmo­sphere, which means that the interi­or can be sub­jec­ted to much less pres­sure than Leonov experienced.

But a space­suit (called the EMU or Extra­ve­hicu­lar Mobil­ity Unit for the Amer­ic­an mod­el and the Orlan for the Rus­si­an mod­el) is not only used to main­tain a breath­able atmo­sphere and bear­able atmo­spher­ic pres­sure for the astro­naut. It also pro­tects against anoth­er extreme envir­on­ment­al con­straint in space: temperatures.

In fact, in a vacu­um, with no warm air to “stir up” the tem­per­at­ure around the astro­naut, the tem­per­at­ure dif­fer­ences between the lighted and dark sides are gigant­ic. The illu­min­ated parts, which are dir­ectly exposed to the Sun­’s rays, can rise to 120°C, while the tem­per­at­ure of the shaded parts can drop to ‑100°C. As such, water-cool­ing cir­cuits are integ­rated into one of the lay­ers of the suit to redis­trib­ute the heat from the hot parts to the cold parts and main­tain a bear­able interi­or tem­per­at­ure for the astro­naut. And everything is fine… so long as this cool­ing sys­tem does not leak!

On 16^th July 2011, while out­side the Space Sta­tion, Itali­an astro­naut Luca Par­m­it­ano of the European Space Agency felt water on the back of his neck. In weight­less­ness, water behaves in a pecu­li­ar way: it curls up and floats in front of the amused astro­nauts. But if it touches a human’s skin, it sticks to it, held in place by a force called “sur­face ten­sion”… which is fine when you have a cloth to wipe it off, but can be much more ser­i­ous when you are alone in your suit, unable to touch your own face, and the water builds up more and more, threat­en­ing to gradu­ally cov­er your eyes, nos­trils or suit visor.

For­tu­nately for Luca, the space­walk is imme­di­ately abor­ted and, with the help of his part­ner, astro­naut Chris­toph­er Cas­sidy, he man­ages to re-enter the Sta­tion with his eyes closed, the micro­phone and then the head­phones gradu­ally turned off by the advan­cing water. Once the pres­sure was restored in the air­lock, the crew on board entered in a hurry, unscrewed the hel­met, and finally sponged off the water which, after exam­in­a­tion, was indeed com­ing from the cool­ing system.

The simple fact of being safe in the ISS does not pre­vent the human body from under­go­ing a cer­tain num­ber of changes, at all levels (body, organs, cells, genet­ics). The dis­com­fort usu­ally starts when the astro­naut arrives on board. Used to pump­ing blood upwards to coun­ter­act grav­ity, the heart con­tin­ues to work even when the human in ques­tion is weight­less and no longer feels its own weight. The res­ult is a red, swollen head, char­ac­ter­ist­ic of these micro­grav­ity states.

This con­ges­tion of the head and, among oth­er things, of the nas­al mucous mem­branes, which are also swollen with blood, has a dir­ect impact on the taste of the food eaten there. In such a situ­ation, the air does not cir­cu­late well in the nose. As the sense of smell is a sig­ni­fic­ant part of the taste sen­sa­tion of food, it loses much of its fla­vour (this loss will be com­pensated for in part by send­ing spi­ci­er-than-aver­age dishes).

The loss of muscle mass, if not com­pensated for by two hours of sport a day, can have ser­i­ous consequences.

But the impact on the human body can be more prob­lem­at­ic. In weight­less­ness, a simple push against a wall is enough to pro­pel you to the oth­er side of the Space Sta­tion. In fact, you use your muscle struc­ture much less than on Earth. This res­ults in a loss of mass which, if not com­pensated for (or at least slowed down) by two hours of sports ses­sions per day, can have ser­i­ous con­sequences upon return to Earth.

In par­al­lel with this loss of muscle, the bones also become more fra­gile and brittle. This patho­logy, gen­er­ally reserved for eld­erly people on Earth, is called osteo­poros­is. Even if this bone decal­ci­fic­a­tion is revers­ible once back on the ground, a study¹ con­duc­ted on 14 men and 3 women – before and after their stay in space – showed that even after one year of rehab­il­it­a­tion, the resorp­tion of the astro­nauts’ tibia struc­ture was still incom­plete. And of course, the longer the stay in space, the longer the return to normal.

There are many med­ic­al effects on the human body dur­ing a pro­longed stay in weight­less­ness: dizzi­ness due to imbal­ances in the inner ear, changes in eye pres­sure that can lead to ret­in­al detach­ment, urin­ary reten­tion, kid­ney stones, etc. How­ever, there is one final danger that should not be under­es­tim­ated: the effect of radiation.

In space, cos­mic rays form a shower of so-called “ion­ising” particles. Pro­longed expos­ure to these rays can cause mac­ro­scop­ic changes (burns, catar­acts) and micro­scop­ic changes (genet­ic alter­a­tions, ster­il­ity or increased risk of devel­op­ing can­cer). These cos­mic rays are essen­tially com­posed of pro­tons, elec­trons, and atom­ic nuc­lei, pro­pelled into space by the Sun (for low-energy cos­mic rays) and oth­er much more viol­ent phe­nom­ena such as explo­sions of massive stars or black holes swal­low­ing mat­ter (caus­ing high-energy cos­mic rays).

Intens­ive research is being car­ried out to pro­tect astro­nauts from space radiation.

On Earth, we are well pro­tec­ted from these cos­mic rays thanks to the Earth’s mag­neto­sphere which deflects a sub­stan­tial part of this particle flux, and the atmo­sphere which phys­ic­ally stops the little that remains. In space, we can no longer rely on the pro­tec­tion of the atmo­sphere (which is at a lower alti­tude). And even though the mag­neto­sphere still plays a role for the ISS, which orbits at an alti­tude of only 450 km, the same is not true for when humans ven­ture fur­ther into the Uni­verse: the Moon in the near future and to Mars in the longer term.

This is why intens­ive research is cur­rently being car­ried out, both on ways to pro­tect astro­nauts from this space radi­ation. But also on tools to meas­ure the radi­ation dose received on a daily basis, and on the bio­lo­gic­al effects of this radiation.

In this respect, one of the “most com­pre­hens­ive assess­ments we have ever had of the human body’s response to space­flight” comes from a remark­able study² car­ried out in 2015 on two twin broth­ers (Mark and Scott Kelly), one of whom stayed in space for 340 days while the oth­er remained on Earth. It was then pos­sible to fol­low these two genet­ic­ally identic­al men and to observe pre­cisely the changes brought about by the space envir­on­ment at dif­fer­ent levels (bio­chem­ic­al, immune, genet­ic, physiolo­gic­al etc.).

The con­clu­sion is that space travel sig­ni­fic­antly alters the func­tions of the human body, and while the vast major­ity of these are restored once back on the ground, much work remains to be done to cre­ate a viable space envir­on­ment for humans in the long term.