Sperm have trouble finding their way in zero gravity, according to new research.
A study has found that sperm are affected by weightlessness, losing their sense of direction and struggling to navigate their way through the reproductive tract.
The results could have implications for humanity's ambitions to become a true space-faring species and our ability to travel to distant stars, which would – in theory – require journeys lasting multiple human lifetimes.
The need for off-Earth reproduction
What will it take to for humans to become truly interplanetary; for the first humans to be born, live and die beyond Earth?
There aren't many places we could travel to in the Solar System and expect a warm welcome, environmentally speaking.
More on spaceflight
So where's our nearest Earth-like exoplanet? That's Proxima Centauri b, a rocky world with a mass similar to Earth and which orbits in its star's habitable zone, or Goldilocks zone.
There's no guarantee it's suitable for humans to live on, but if we were to travel there, it would take us 4.2 lightyears.
That means it would take us just over four years, if we could travel at the speed of light (we can't).
The fastest thing humans have ever built is the Parker Solar Probe, which is a NASA spacecraft dedicated to studying the Sun.
At its fastest, Parker reached speeds of up to 692,000 km/h (430,000 mph).
Even if we could load humans on to such a spacecraft, keep it at that maximum speed and not do the human passengers any damage, it would still take us over 6,600 years to reach Proxima Centauri b.
That would require generations upon generations of humans to be born within that same spaceship, in order for humanity to make it to the nearest planet beyond our Solar System that's even remotely like Earth.
Clearly, becoming an interplanetary species is going to require lots of sex in space.
The problem with sperm in space
Researchers at Adelaide University in Australia have found that one of the biggest problems with pro-creation in space is a lack of gravity, which seems to negatively impact the navigational abilities of sperm.
Researchers at the University’s Robinson Research Institute, School of Biomedicine and Freemasons Centre for Male Health and Wellbeing, looked at how off-Earth conditions might affect sperm navigation, fertilisation and embryo development.
They took sperm samples from three different mammals, including humans, and put them through a machine that simulates zero gravity conditions by flipping cells, to make them become disorientated.
The sperm were then tasked with travelling through a maze that mimics the reproductive tract; the journey they would need to make in order to fertilise an egg.
"This is the first time we have been able to show that gravity is an important factor in sperm’s ability to navigate through a channel like the reproductive tract," says senior author Dr Nicole McPherson from Adelaide University’s Robinson Research Institute.
"We observed a significant reduction in the number of sperm that were able to successfully find their way through the chamber maze in microgravity conditions compared to normal gravity.
"This was experienced right across all models, despite no changes to the way sperm physically move. This indicates that their loss of direction was not due to a change in motility, but other elements."
A solution for sex in space?
The team added the sex hormone progesterone – which is important for pregnancy establishment – into the mix, and found this helped more human sperm overcome the effects of simulated microgravity.
"We believe this is because progesterone is also released from the egg and can help guide sperm to the site of fertilisation, but this warrants further exploration as a potential solution," says Dr McPherson.
The team also looked at the impact of exposure to microgravity during fertilisation on embryo development in animal models.
The results were pretty significant, amounting to a 30% reduction in the number of mouse eggs that were fertilised after four hours of exposure to zero gravity, compared to conditions on Earth.
"We observed reduced fertilisation rates during four-to-six hours of exposure to microgravity," says Dr McPherson.
"Prolonged exposure appeared to be even more detrimental, resulting in development delays and, in some cases, reduced cells that go on to form the fetus in the earliest stages of embryo formation."
"These insights show how complex reproductive success in space is and the critical need for more research across all early stages of development."
The team say previous studies have looked at sperm motility in space, but this is the first to study a sperm's ability to navigate through a reproductive channel under microgravity.
Sex on the Moon and Mars
The researchers say they'll now look at how environments like those on the Moon, Mars and in artificial gravity systems might impact humans' ability to get pregnant.
Do these changes in development happen gradually as gravity gets weaker, or is it an 'all-or-nothing' situation?
They say this research could be essential for future human reproduction in extraterrestrial environments, including Moon and Mars settlements.
"In our most recent study, many healthy embryos were still able to form even when fertilised under these conditions. This gives us hope that reproducing in space may one day be possible," said Dr McPherson.
"As we progress toward becoming a spacefaring or multi-planetary species, understanding how microgravity affects the earliest stages of reproduction is critical," says said Associate Professor John Culton, Director of the Andy Thomas Centre for Space Resources.
You can read the full paper via Communications Biology (Nature Portfolio).
