On 27 March 2015, astronaut Scott Kelly voluntarily voyaged into space on a unique one-year mission for NASA, which is due to come to an end on 1 March 2016. His identical twin brother Mark, a retired astronaut, has also been integral to the study, although his feet have remained firmly on planet Earth for the duration of it.

This unprecedented experiment has offered scientists a rare opportunity to study the effects long-term spaceflight has on the human body, which is vital if NASA hopes to one day send astronauts on a mission to Mars.

The fact that the brothers are identical twins is crucial to the investigation, as they share virtually the same DNA. This means scientists will be able to closely compare any physiological and mental changes that have occurred between them during the experiment.

Prior to Scott’s arrival at the International Space Station, NASA researchers collected genomic, physiological, molecular and other data from each twin, something that has continued to be reviewed and compared during the mission and will be studied further after Scott has returned to Earth.

Biological samples including blood and saliva are expected to uncover more evidence on the physiological effects of spaceflight, as the investigation will look closely at how environmental stressors, such as microgravity, radiation and confinement, affect the muscles, heart and brain. Behavioural changes will also be compared and documented, in order to better understand how the likes of reasoning, perception and decision-making are also affected by long-term space missions.

The physiological effects of space

Space travel can have a dramatic physiological effect on the human body. Reduced gravity, even over a short period of time, can be detrimental. On Earth, our bodies are constantly working against gravity and this helps to keep our muscles and bones strong, so without it they will effectively start to weaken. Bone and muscle loss is a common side effect of microgravity, as it makes physical activities less demanding. The reduced workload on your muscles and bones makes your body think those cells are no longer required, so over time they will waste away.

Astronauts also grow a few centimetres in height while in space because the spine stretches and lengthens without gravity pushing it down. This in turn can lead to back aches. Astronauts typically suffer from headaches, nausea and swelling while their bodies adapt to their new environment.

The science of identical twins

There are two different types of twins; identical, known as monozygotic, which occurs when one sperm fertilises one egg; and nonidentical, or dizygotic, which is when two separate sperm fertilise two separate eggs. Identical twins are the less common of the two, as once the egg has been fertilised the cell will split in two, which as result means both embryos share the same DNA code.

Your DNA is responsible for making you who you are, which is why identical twins are the same sex and look so much alike. Slight differences in appearance and behaviour are a result of environmental factors, and as changes to our genes can still occur in the womb, it’s possible (although very rare) for one twin to develop a genetic condition while the other does not.

Discover more amazing space facts in the latest issue of How It Works. It’s available from all good retailers, or you can order it online from the ImagineShop. If you have a tablet or smartphone, you can also download the digital version onto your iOS or Android device. To make sure you never miss an issue of How It Works magazine, make sure you subscribe today!

Plus, make sure you also check out our digital-only specials, such as Explore Mars, A Guide To The Galaxy and Earthquakes, available to download onto your digital device now!

Interview: Tim Peake on tough astronaut training & the Principia mission

Why do astronauts train underwater?

Danger on the ISS: When spacewalks go wrong