Previous Russian studies have shown that when male and female rats were sent into space in 1979 they did not mate at all. Another study found that when male rodents were placed in simulated zero gravity conditions they could no longer produce sperm.

Dr Joseph Tash, of the Department of Molecular and Integrative Physiology at the University of Kansas told a conference in Hawaii that there was concern that astronauts could experience the same effects.

We dont really have the human data to really determine whether what we are seeing in the animals is translatable to humans. But we are seeing big impacts in the animals, he said.

And the animals still had a strong mating urge. So a male astronaut could come back without any change in behaviour but they may be shooting blanks and maybe they are unaware there is a problem if they are trying for a baby.

It is a potential issue that needs to be looked at. Certainly the flight surgeons down at Johnson Space Centre suggest that the male astronauts cryopreserve semen and some female astronauts have elected to preserve eggs just in case it is a factor.

Dr Tash believes that space flight can disrupt important chemical reactions which must happen into the body to allow reproduction.

Astronauts report a range of problems when they are at zero gravity in a condition known as space flight syndrome. It causes bone mass loss and accelerated ageing of cells.

Dr Tash discovered that many of the systems that are shutting down in space flight syndrome are linked to the hormone oestrogen.

Cosmic radiation may also be harming fertility and is expected to increase over the next 20 years because the Sun is entering a quiet phase where there are fewer solar winds to keep radiation out of our galaxy.

Around 80 per cent of male astronauts have experienced some kind of reported visual impairment after around six to eight weeks in space. The majority need glasses when they comes back to Earth.

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Space may make astronauts infertile, scientists fear