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Let's say an alien species discovered Earth and wants to land here and establish contact, however their homeworld's gravity is only 46 percent of Earth's, Would it be possible for them to be able to walk on Earth? Is it possible for them to adapt to Earth's gravity? They are 1-2 ft tall and walk on two legs. Weighing about 50-60 pounds on their homeworld.

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    $\begingroup$ no, but if an alien is capable of interstellar travel, they can have nano-suits or exo suits which help them to live in earth like gravity, a 46% (50) reduction would be similar to doubling your body mass, would be really hard to walk in two legs for long times. walking on 4 would be a lil easier. $\endgroup$ – Chinu Jul 19 '16 at 7:02
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    $\begingroup$ @Chinu Or develop some kind of drugs, as for example in "The expanse". $\endgroup$ – BЈовић Jul 19 '16 at 12:42
  • $\begingroup$ "46 percent of Earth's [gravity] ... 50-60 pounds on their homeworld" So we are talking about a force of approximately 110-115 N? ($ 0.46 \times 9.82 \times \frac{55}{2.2} \approx 113 $) $\endgroup$ – a CVn Jul 19 '16 at 13:48
  • $\begingroup$ Isn't this the same as humans surviving being centrifuged at 2 g? $\endgroup$ – Superbest Jul 19 '16 at 17:26
  • $\begingroup$ Why are they so small? I would think they'd be larger than humans since their world has such low gravity. $\endgroup$ – Xandar The Zenon Aug 7 '16 at 18:55
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Short answer: it depends. The anatomy and physiology of your aliens will determine a lot.

For instance, their hearts are going to be working much harder to pump blood around their bodies, particularly when pumping upwards. Compare aliens which look like a 2 foot tall dog and aliens which look like a 2 foot tall ostrich or giraffe. The vertical distance between heart and brain in the ostrich-aliens is much greater than the dog-aliens. So the ostrich-aliens' hearts will be under much more strain. Their hearts will be pounding away like they've just run up a flight of stairs, even when they are just sitting watching TV.

Similarly, skeletons come with safety factors built in. When those safety factors are exceeded, bones break. For instance in humans, the force your leg bones has to bear when standing still is equal to your body weight. When you walk, that force doubles. When you sprint or jump over things, it is higher still. However, your bones can take all these forces, plus a bit extra for the safety factor.

Your aliens have come to a gravity more than double their own. So standing still will be the equivalent of walking, walking will be like jogging, jogging will be like running an Olympic hurdles race, and so on.

If their species has evolved to bounce around like gazelles (huge leaps, gaits called stotting and pronking), or is like a monkey species which habitually hurls itself out of trees onto the ground (as opposed to climbing down or jumping tree to tree) then their bones will be designed to take a pounding and have HUGE safety factors. So they may still be able to walk and jog, but should avoid running and jumping. If their ancestors evolved a more 'normal' lifestyle - zebra rather than a gazelle, or a normal monkey rather than a 'parkour' monkey - then walking may be the limit of what they can do without breaking bones, getting sprains and strains, etc.

Consider the elephant. Zoos have concrete moats round the elephant enclosure, because elephants are at the limits of the safety factors their bones can take. Elephants can't jump over that moat - they'd break their legs. In fact, elephants can't even run or trot, because their leg bones can't take it. A charging elephant is speed walking!

EDIT TO ADD EXAMPLES: at 1g a horse can walk, trot, gallop and jump over fences. The much heavier rhino can walk, trot and gallop but not jump over fences. The Elephant can only walk (partly weight and partly due to not having bones as thick as it 'should' for an animal that heavy - i.e. reduced safety factors). So your aliens may be 'horses' on their homeworld, but 'rhinos' or - if they are really unlucky - 'elephants' on earth.

Your aliens muscles will slowly get stronger as they adapt to the gravity. They may lay down some more bone to reinforce their skeleton. But the bone they'll be making is more of the same 'low gravity' bone with low gravity safety factors. Evolution won't have given them the tools to make 1g bone.

There will also be a lot of wear and tear on soft tissues. Knee cartilages will be feeling the strain, spine compressing on disks, sole of the foot taking a pounding, and so on. They may spend all their time with bad backs and aching feet.

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    $\begingroup$ From the musculo-skeletal point of view, doubling your weight is tough but not that unmanageable. Take a reasonably fit person and give them a backpack equal to their own weight. They won't be happy and they sure won't be able to hurdle, but their ankles won't instantly splinter to pieces if they run. I think the limiting factor is the first thing you said: doubling the pressure required to get blood to the brain all day is a big ask for a vertical earth-like species. Humans in high-G centrifuges suffer from this quite quickly, even sitting still. $\endgroup$ – Steve Jessop Jul 19 '16 at 10:09
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    $\begingroup$ Ooh, apparently NASA have done quite long trials (22 hours at 2G, albeit sitting down): science.nasa.gov/science-news/science-at-nasa/2003/… $\endgroup$ – Steve Jessop Jul 19 '16 at 14:19
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    $\begingroup$ @SteveJessop You might want to make an actual answer around that link, actually. $\endgroup$ – a CVn Jul 19 '16 at 14:37
  • $\begingroup$ @MichaelKjörling: Maybe, but summarizing the results and saying, "we can now wildly speculate about all the activities not tested here" feels anticlimactic. Please, if anyone fancies it, don't hold off waiting for me ;-) I intended the link as supplying some more info about something DrBob already mentions in this answer. $\endgroup$ – Steve Jessop Jul 19 '16 at 14:45
  • $\begingroup$ @SteveJessop - good point. Will edit to add some 1g examples. $\endgroup$ – DrBob Jul 20 '16 at 6:40
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Walking for the low-gravity aliens would be like running a marathon. This is something that is best done in small doses.

While exosuits are feasible. An alternative is mobile flotation tanks. The aliens will wear a protective suit, and rest inside the mobile tanks in a floating position. With medical equipment in case of "gravity stress" problems and possibly cardiovascular apparatus to take the load off their bodies.

One aspect about low-gravity aliens visiting Earth is the fact that their spacecraft, when they land and take-off from planet Earth, will have to be at relatively low acceleration, compared to the kind of rocket take-offs we know, and be for a much longer duration. Unless their ships are propelled by a super-advanced field-drives which either neutralises or considerably reduces the g-forces accompanying the acceleration.

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  • $\begingroup$ Yes, let's indeed hope that these aliens' rocket scientists did their homework and adjusted their delta-v budgets accordingly. $\endgroup$ – a CVn Jul 19 '16 at 13:46
  • $\begingroup$ I know what you mean, but the Mixed Metaphor Police say that if you run a marathon in small doses, that's just running ;-) $\endgroup$ – Steve Jessop Jul 19 '16 at 14:14
  • $\begingroup$ @SteveJessop As long as they don't run me out of town. $\endgroup$ – a4android Jul 20 '16 at 7:54
  • $\begingroup$ @MichaelKjörling, alien rocket scientists always do their homework like good little alien rocket scientists. Otherwise their careers tend to crash and burn. $\endgroup$ – a4android Jul 20 '16 at 9:01
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If their bone structure can withstand the stress, then they can, but it will be exhausting and highly uncomfortable. For comparison, imagine carrying around your own body weight in extra weight all day and night, evenly distributed over your body. Every step will be a chore, lifting your arms and even your head will feel like workout, every breath you take is a struggle. Not even lying down is much comfort because you will be pulled down so hard.

You will want only your most able-bodied aliens to ever visit Earth, and even they will require some sort of mechanical exosuit or strength-boosting medication to get through the day without too many problems.

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If the aliens have arrived via a long spaceship journey, and if they knew what was waiting for them when the got here, then they would have been well advised to use the journey as preparation.

Their spaceship should have some kind of artificial gravity. Whether by some high tech plating (per Star Trek) or simply the craft revolving (per Space Oddesey), the strength of this gravity could be increased steadily through the journey time to help the travellers adjust to the conditions at the destination planet. A regime of weight training and exercise en route would be necessary as well.

Picking suitably strong individuals for the journey in the first place would also be a consideration of course. Weaker individuals would struggle to adapt, but for stronger individuals coming off the ship after a journey helping them to adapt, they should be in good shape to cope with conditions on Earth.

There is one other caveat to mention though. Breathing.

If the aliens' planet has a lower gravity, then it follows that it also has a significantly lower atmospheric pressure. Even if their air composition is similar to ours, breathing air on Earth may cause them to hyperventilate. They will therefore likely need assisted breathing aparatus of some kind. This will appreciably add to the weight that they need to carry with them, which in turn will make adapting to the increased gravity a bit harder still.

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  • $\begingroup$ Venus has a lower gravity than Earth (0.89g), yet has a much higher atmospheric pressure (about 90 atmospheres). Of course, most of that is CO2, and life on Earth has been merrily turning CO2 into limestone for eons, so maybe there would be a thinner atmosphere on Venus if it had life...? I suppose that 46% gravity might not be able to hang onto lots of lighter gases? Hopefully it can keep oxygen! $\endgroup$ – DrBob Jul 20 '16 at 6:52
  • $\begingroup$ It's still possible to have Earth's standard air pressure at 0.46 G, Titan with only 0.12 G has a denser atmosphere than Earth. $\endgroup$ – Stephanie Jul 28 '16 at 7:42
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Assuming they are biologically similar to humans, question boils down to this. Would you be able to carry 117% of your own weight? For me the answer to that question would be no, but for some, this might be possible.

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    $\begingroup$ But you weren't selected for a landing on a planet with 2.17x your own planet's gravity. Like Hackworth said, you'd probably pick the individuals to make such a landing based in large part on their individual physique. Even among humans there is a huge variety in physique; the question gives no reason to believe that these aliens would be any different in that regard. $\endgroup$ – a CVn Jul 19 '16 at 13:44
  • $\begingroup$ I guess there would be people who could pull that off, but even for them it won't be easy and practical, not that it cannot be done. It would be much easier for them to communicate from orbit or a spacecraft or ask humans to come to their spaceship, as lower gravity does not have any adverse effects in the short term. $\endgroup$ – Cem Kalyoncu Jul 19 '16 at 14:28
  • $\begingroup$ I'm not saying it would be easy for the aliens if they were to land on Earth, I'm just pointing out that a single example (yourself, "for me") is hardly proof that it couldn't be done. $\endgroup$ – a CVn Jul 19 '16 at 14:36
  • $\begingroup$ I didn't stated as proof that is why I said for me no. That depends on the person visiting. Editing to make it clearer. $\endgroup$ – Cem Kalyoncu Jul 19 '16 at 14:39
  • $\begingroup$ It's definitely much clearer now. Which, incidentally, is what comments are for. :-) $\endgroup$ – a CVn Jul 19 '16 at 14:47

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