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What are the risks of space for humans?

Pierre Henriquet
Pierre Henriquet
Doctor in Nuclear Physics and Columnist at Polytechnique Insights
Key takeaways
  • Humans are not naturally equipped to live in space so many precautions must be taken when leaving the surface of Earth.
  • Technical and technological improvements have made it possible to avoid the inconveniences caused by old space equipment.
  • However, the human body undergoes many changes: congestion, loss of taste, loss of muscle mass or even weakening of the bones.
  • Exposure to cosmic rays can cause changes ranging from cataracts to increased risk of cancer to infertility.
  • Much work remains to be done to create a viable space environment for humans in the long term.

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?

An eventful history

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.

20th June 2021, Thomas Pes­quet per­forms a space­walk to install new solar pan­els on the ISS. Cred­it: NASA.

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 16th 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.

Influence(s) on the human body

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.

Thomas Pes­quet’s face changed sig­nif­i­cant­ly between the offi­cial pho­to on the ground (left) and his stay in micro­grav­i­ty on the ISS (right). Cred­it: ESA.

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.

What are the effects of space radiation?

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.).

Study car­ried out on the twin astro­nauts Scott & Mark Kel­ly for 25 months, includ­ing 12 months in orbit around the Earth for Scott. Cred­it: Gar­rett-Bak­le­man et al.

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.

1Incom­plete recov­ery of bone strength and tra­bec­u­lar microar­chi­tec­ture at the dis­tal tib­ia 1 year after return from long dura­tion space­flight » – Sci­en­tif­ic Reports vol­ume 12, Arti­cle num­ber: 9446 (2022) – https://www.nature.com/articles/s41598-022–13461‑1
2« The NASA Twins Study: A mul­ti­di­men­sion­al analy­sis of a year-long human space­flight” – SCIENCE – 12 Apr 2019 Vol 364, Issue 6436 – https://​www​.sci​ence​.org/​d​o​i​/​1​0​.​1​1​2​6​/​s​c​i​e​n​c​e​.​a​a​u8650

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