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They do not call Hafþór Júlíus Björnsson "the mountain" for nothing.
In 2015, the strongman and actor of Game of Thrones broke a millennia old record by doing – or more accurately, staggering – five steps with a 650 kilogram log on his back.
For most of us, it was simply an extraordinary example of heroic strength. For scientists, this feat marked an overwhelming limit to the gravitational pull that any mortal could hope to endure, setting a limit on the mass of planets we could expect to colonize.
According to a small team of physicists from the University of Zagreb in Croatia, we really do not want to target planets whose mass is four times greater than that of our Earth. And this is not nothing after a little training.
All that is bigger, and we would need all the colony to be mountains.
Scientists came to this critical figure after taking into account the compressibility of human bones, the flexing of our muscles and the balanced nature of our legs as we walked on the surface of the planet.
Our bones are impressive structures in engineering. In fact, our shin could withstand something like 90 times the gravity of the Earth (g) before splitting.
It's a senseless force, but things change if we try to step into such a gigantic world. Dynamic stresses and twisting effects would simplify the task of our skeleton, lowering the actual limit to something closer to 10g.
That's until you take into account the fact that we had to move. Researchers have determined, through rigorous training, that we might be able to tackle gravity with a force not exceeding about 5 grams.
Standing up is a different story, though. We humans are pretty good at walking right. We have been doing this for about 3.6 million years, taking advantage of its energy efficiency and, without being too pretentious, we already know it well enough.
Nevertheless, our bipedal approach has been mastered under a gravitational pull equivalent to about 9.8 Newtons of force for every kilogram of body weight.
The physicists therefore wanted to know what would increase the effect of additional gravity on the swing of the pendulum of our meat sticks.
Walking is essentially a controlled fall cycle, where the sway of each leg prevents our face from meeting the ground. This fall-reset-reset scheme causes the switchover of our center of mass, which is the bulk of the work.
Physicists have developed their own model for what is described as an "inverted pendulum gait," taking into account the oscillations of a person's center of gravity and the timing of his or her legs.
Björnsson's record in five stages establishes a pretty good benchmark for the upper limits of what a human step could achieve.
Combining the mass of his diary, his own weight and the size of his legs, the team determined that a human of his athleticism could slowly make his way around a planet with a gravity – or g – about 4, 6 times that of the Earth.
Our heart can hardly support about 5 g of gravity, above which we begin to faint. So it looks like this figure sets an absolute limit for any type of human exploration.
This absolute cut seems rather promising. The current competitor of the largest terrestrial exoplanet ever recorded is BD + 20594b, a globe beast whose diameter is equal to half that of Neptune and which has about the same mass. On its surface, the gravitational attraction would be a little more than three times that of the Earth.
BD + 20594b is also a bit strange, most rocky worlds have a mass less than 1.6 times that of the rays of our planet and much smaller.
But keep in mind that most future astronauts would not be monsters capable of carrying half a ton of wood on their backs.
The researchers opted for a much more realistic upper limit for the types of worlds in which we had never set foot – with training – to be between 3 and 4 g.
Of the 594 exoplanets on which we have enough information to estimate its severity, 422 would be technically traversable by humans (ignoring the multitude of other infernal conditions).
This research is currently on the pre-peer review website arxiv.org, but must be published in The physics teacher– Just the thing to think about the students.
It is unlikely that humans will ever touch these distant worlds, at least in the near future.
But maybe we'll go next week to the gym, in case NASA calls.
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