Why space exploration limits your time here on Earth
An astrophysicist once commented that "dinosaurs are extinct today because they lacked the opposable thumbs and brainpower to build a space programme".
Yet although we now have the technological ability to leave the Earth, scientists have found another stumbling block to colonising new worlds - our own immune system.
Scientists from Russia and Canada analysed the effect of microgravity on protein in blood samples of 18 cosmonauts who lived on the International Space Station for six months.
They found alarming changes to the immune system, suggesting that they would struggle to shake off even a minor virus, such as the common cold.
"The results showed that in weightlessness, the immune system acts like it does when the body is infected because the human body doesn't know what to do and tries to turn on all possible defence systems," said Prof Evgeny Nikolaev, of the Moscow Institute of Physics and Technology.
It is known that microgravity influences metabolism, heat regulation, heart rhythm, muscle tone, bone density and the respiratory system. Last year, US research also found that astronauts - fitter than the general population - on lunar missions were five times more likely to have died from cardiovascular disease than those who went into low orbit or never left Earth. Those of comparable age but who never flew, or only achieved low Earth orbit, had less than a one in 10 chance of death from cardiovascular disease.
But that rose to 43pc for those who reached the Moon or deep space, probably because of the impact of deadly space radiation. The time astronauts can spend in space is limited because of fears that the radiation causes cancer.
A 2008 study by scientists from Georgetown University Medical Centre in Washington found that mice exposed to the high-energy particles found in space developed a harmful stress response, which could damage DNA and cause cancer.
However, until now the molecular mechanisms that drive the physiological changes caused by space flight has remained a mystery. To understand better the changes in physiology during space travel, the researchers quantified concentrations of 125 proteins in the blood plasma of cosmonauts 30 days before they travelled to the ISS and then on their return to Earth.
"When we examined the cosmonauts, their immune system was weakened," said Dr Irina Larina, also from the Moscow Institute. "They were not protected from the simplest viruses. We need new measures of disorder prevention during a long flight. We must understand the mechanism that causes disorders."
The research was published in the journal Nature Scientific Reports.