Twin Nasa astronauts Mark and Scott Kelly are to put Einstein to the test in a pioneering study that could pave the way for humankind reaching Mars

Scott Kelly has found an unusual way to maintain a youthful advantage over his twin brother Mark. Next month, he will blast off on a year-long mission on board the International Space Station, a journey that will have one unintended consequence. During his 12 months in space, Scott will age less than his earthbound brother – thanks to the tenets of Einstein’s theories of relativity.

“The effect is known as the twin paradox, though it is not actually a paradox, it is a straightforward consequence of the laws of relativity,” says Mark, who has also flown on several space missions including two as a space shuttle commander. “Essentially, time will pass slightly more slowly for Scott than for me because he will be travelling at a greater speed relative to me.”

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Not that Mark is too worried about the age differential that will develop. The effects of time dilation only become noticeable when spaceships travelling from Earth approach speeds near that of light. By contrast, the space station moves at a sedate 17,000mph relative to Earth’s surface – which means that at the end of Scott’s year of space travel, when he returns to Earth in March 2016, he will have aged only about three milliseconds less than Mark who was left behind on Earth. At least, that is what relativity dictates. “Scott and I are identical twins though I am the elder having been born six minutes before him,” adds Mark.

“Certainly his spaceflight is not going to make that much difference.” Nevertheless, it is intriguing that the Kelly brothers will realise the twins-in-space thought experiment, dreamed up by Einstein and others to illustrate the effects of time dilation, a point that is stressed by Mark. “I cannot imagine that when Einstein came up with that idea that he ever thought there would be twins flying in space at some point in the near future.”

In fact, Mark and Scott – who will be 51 this month – are the first siblings to fly in space and that certainly makes them special. For good measure, they are identical twins, which makes them even more important to science, for it means that Scott’s biological reactions to unearthly phenomena – zero gravity or elevated levels of radiation – can, for the first time, be compared to the responses of a genetically identical individual on the ground.

Facebook Twitter Pinterest NASA astronaut Scott Kelly on board the International Space Station in 2010. Photograph: Reuters

As a result, Scott and Mark have agreed to take part in a remarkable experiment, a year-long investigation of the impact of space travel using identical twins as subjects. “It is a once-in-a-space-programme opportunity,” says John Charles, a senior executive with Nasa’s human research programme. The Kelly Twin project will examine how individuals with the same genetic profiles respond to radically different environments. Researchers already know that life in orbit can have profound changes on astronauts. They experience the loss of bone and muscle, as well as vision problems and changes affecting motion and balance – and all these will be examined in detail.

In addition, researchers will compare the men’s immune systems, reaction times and the performance of their hearts. Blood and urine samples will be taken regularly and, in Scott’s case, flown to Earth. Analyses will then tell scientists if Scott’s DNA is changing in space. MRI images will also be taken of Scott, while the trillions of bacteria – the microbiota – that live inside the astronauts’ digestive tracts will be sampled and analysed. And all these studies will be mirrored in tests carried out on Mark on Earth.

Nor will the experiments on the two men cease at the end of Scott’s mission to the space station – the longest in which a Nasa astronaut will have travelled in space. “The tests are scheduled to last at least another year after Scott lands and hey, who knows, in 10 years, they may still be coming back to Scott and I to see how we are doing,” adds Mark. “We are the first of a kind, after all.”

Mark, left, and Scott Kelly in 1968. PR Photograph: Mark and Scott Kelly

Mark and Scott were born on 21 February 1964 in West Orange, New Jersey, to parents who were both police officers.“Our Mum dressed us alike and stressed our similarities but I don’t think she overdid it,” says Mark. “We do have personalities that differ but you will have to ask someone else what those differences are.”

The twins did well at school, co-captained its swimming team, and subsequently became US Navy test pilots before joining Nasa’s astronaut corps. Scott was the first into space, in 1999, on a shuttle mission to service the Hubble telescope. Mark followed a couple of years later on a flight to the space station. Both became commanders for later space shuttle flights. Scott has notched up more than 180 days in space, while Mark has logged just under 60. During Scott’s last flight, Mark’s wife – Gabrielle Giffords, the Democrat congresswoman – was shot in the head in January 2011 during a gun attack in Tucson Arizona in which six others were killed. Scott was told of the attempted assassination of his sister-in-law while he was in the space station. He ended up calling his brother once or twice a day from space to offer him help and advice.

In the end, Mark flew on one more mission, commanding the shuttle Endeavour on its last flight later that year. Then he retired, citing the need to help his wife during her recovery. (Giffords is still paralysed on her right side though she has returned to active life, marking the third anniversary of her shooting by skydiving.) But if Mark has given up flying Nasa missions, he is still an enthusiastic supporter of the agency’s goals, in particular its long-term plans to fly astronauts to Mars. Such a mission would take men and women on a two-year jaunt into interplanetary space. Understanding what will happen to these individuals is crucial for Nasa if it wants the first humans it sends to another planet to arrive in a healthy condition.

Facebook Twitter Pinterest Mark Kelly and his wife Gabrielle Giffords in 2013. Giffords had survived a gunshot to the head two years earlier. Photograph: J. Scott Applewhite/AP

“If we want to put people on Mars we have to have every possible bit of information we can get about the effects of living in space for years at a time,” says Mark. “That is why I am taking part in this study. Its individual component tests are incredibly specific and they will keep us both busy, but the benefits could be enormous.” The effects for both men are likely to be gruelling, especially Scott who will have plenty of other activities to keep him busy on the space station. “They are both being very generous about this,” says one of the scientists involved in the project, geneticist Andrew Feinberg, who is based at Johns Hopkins University, in Baltimore. “Just about every hour of the day, they will be tested in some way, it would seem.”

However, there is a limit to how much information can be gleaned from just one pair of data sets, a point acknowledged by Feinberg.

“In any proper scientific study, you really need more than one paired sample to make conclusions. It is not statistically valid to have only one set. Nevertheless, if you find a sequential change in Scott’s physiology once he is in space and then it reverses when he comes back, that is a pretty good indicator that something is happening. Then we can follow that up.”

Scott and Mark do not have DNA that is absolutely identical, it should be noted. More than five decades of living different lives under different circumstances will have produced epigenetic changes in their makeup. Nevertheless their basic biologies will still be very similar and that will be vital for the kind of experiments to which they will both be subjected.

One such analysis will involve both being given flu vaccinations in a bid to understand how space changes the immune system. On the ground, Mark is expected to have a normal response to the flu vaccine. But in orbit, it is thought Scott may react slightly differently. “We may get a clue, but no more than a clue, to the fact that something fundamental might be affecting his biology in orbit,” adds Feinberg. This sort of research raises considerable logistical problems, however. Researchers need ambient – fresh – samples rather than frozen ones because the former provide much more information.

Facebook Twitter Pinterest Scott and Mark on the porch in 1965. Photograph: The Kelly family

This will place a strain on how astronauts use the space station’s main link with Earth, Russia’s fleet of Soyuz capsules. These will be making regular flights to the space station while Scott is on board. (One of the astronauts who will be ferried up during Scott’s year in space will be Tim Peake, the UK’s first official astronaut. He is set to fly to the station on a Soyuz capsule in November.)

This space ferry schedule will have to be monitored with considerable care by Scott. Just before a Soyuz is set to return to Earth, he will take a sample of his own blood and then run it through a centrifuge to separate its white cells, which contain DNA, from its red cells, which do not. Then the white cells and their constituent DNA will be placed in a container that will be placed inside Soyuz and ferried down to Kazakhstan.

From there, the package will be picked up by a Lear jet and flown to Houston. “Twenty four hours after giving a blood sample in orbit, we will be able to study it on Earth – along with a sample of earthbound Mark’s blood, of course,” says Feinberg. “I think that is really cool!”

Such procedures will require incredible precision and care in planning, however. For example, the space station centrifuge, which will separate out Scott’s blood cells, will use electricity and that will have to be scheduled carefully into the space station timetable so that other instruments on board are not affected.

As to the Kelly’s DNA samples, Feinberg will be looking for chemical changes in their DNA and in the proteins, called histones, to which DNA is attached. “We will be looking to see if there are any important changes over the course of the time that Scott is in orbit,” says Feinberg.

“It is intriguing work, for we are starting to study the genomics of humans as they prepare to move away from their home planet. We are trying to find out what will happen to our DNA when we leave Earth. It is completely different to the kind of science that we all do normally.

“We scientists are used to doing experiments in splendid isolation. With this study, we are all just small cogs involved in a very big project that has one aim: to help humankind explore space. This is the start of a study of humanity that will find out if we can cope with the rigours of travel between the planets.”

PARALLEL LIVES

Einstein’s special theory of relativity is based on the simple but counter-intuitive idea that all observers should agree on the speed of light – that is, it is independent of the speed at which an observer travels.

This theory makes counterintuitive predictions on many levels. For example, special relativity predicts that if you watch a clock moving faster than about a third of the speed of light, it runs noticeably slow (compared with one at rest) – an effect known as time dilation. This can be any clock, including the human metabolism!

You could in theory send astronaut Amy off on a long trip at near to the speed of light. When she returns to Earth, she is much younger than her twin sister Sarah, who stayed behind. It might seem strange for people to age at different rates but even if you accept this, there is another, more subtle problem. If you are Amy, it is Sarah who appears to be moving fast, so many people get worried that as far as Amy is concerned, it is Sarah who should age more slowly.

Aren’t these predictions contradictory? Well, it turns out that the answer is no, because while Sarah need feel no acceleration at any time, Amy must experience acceleration during her journey when she turns around to come back.

Therefore the physical experiences of the two twins are not the same. One accelerates, the other doesn’t. If you make their experiences totally symmetric, by having them both set off in opposite directions, then each turn around, accelerate, and return, they reunite at the same age. So there is no real paradox, only the illusion of one.

Dr Ed Daw, reader in physics, University of Sheffield

WHAT THE RESEARCHERS PLAN TO STUDY

Visual impairment and intracranial pressure

Microgravity pushes body fluids towards the head, affecting pressure in the skull and hence vision. Scientists will be looking at proteins linked to the flow and regulation of fluids.

Cognition

Researchers will compare the twins cognitive skills to see whether spaceflight affects decision-making ability, proficiency in emotional recognition, spatial orientation, memory and attention levels.

Microbiome

Your gut is home to trillions of bacteria – some beneficial to you, some not. Scientists will use stool samples to see how space affects these organisms. Results could help with future dietary strategies.

Atherosclerosis

Scientists will take blood and urine samples and look at the structure and function of blood vessels to see how spaceflight affects the cardiovascular system, in particular artery hardening.

Ageing

Scientists are hoping the study will help find out how spaceflight might affect biological processes behind ageing, in particular looking at the rate at which regions at the end of chromosomes, called telomeres, shorten.

Genetics

While monozygotic twins have near identical genomes, environmental factors can lead to chemical changes to DNA and RNA (epigenetic effects). Scientists will see how spaceflight affects such changes.

Immune system

Scientists are keen to examine whether spaceflight has an effect on the immune system. They will give Scott and Mark seasonal flu jabs and looking at their responses.

Biochemical profiling

Regular blood and urine samples will be used to check numerous biochemicals, such as hormones and proteins, to analyse how they are affected by diet, stress and weightlessness.