# Why artificial gravity is not possible?

1. Jun 25, 2010

### wllsrvive

What are some of the obstacles scientist face with the idea of creating artificial gravity for a space station?

2. Jun 25, 2010

### Yaridovich

In order to create artificial gravity you would need to have the space station constantly accelerating in one direction so that the people inside would experience a force equal to the rate of acceleration times their body mass. This would create the illusion of weight. This is impractical because the station can't just keep accelerating; it would run out of fuel at some point. Utilizing centripetal acceleration could work, but the space station would need to be orbiting very quickly for any significant effects.

3. Jun 25, 2010

### Filip Larsen

Regarding using a rotating space station to simulate gravity there is a so-called comfort zone where only some combinations of rotational radius and speed is considered applicable for human living conditions.

If you don't mind reading a bit you may find http://www.artificial-gravity.com/ a useful site.

4. Jun 25, 2010

### Mikeral

If you want to create gravity in it natural state. By that, i mean attraction of two masses. you would need to place an object with the same mass as the earth, maybe a black hole, in the belly of the craft. Only problem with that is going to be moving. when you accelerate, you are going to have to move the mass of an entire planet. But by the time we are able to safely handle black holes like they were toys, i think we would be able to play around with accelerating enormous masses. But i'll leave that to the science fiction authors.
Like the two posts above me have said, the only obstacles that are faced are constant acceleration, fuel is expensive, or building a rotating station is just a question of size and maintaining the speed of rotation.

5. Jun 25, 2010

### Filip Larsen

Since the spacecraft would be much closer to such a black hole than the center of earth you would only need a fraction of the mass. For instance, to get 1 g at 100 m from the black hole you would only need around 4-billionth of the mass or around 1.5*1015 kg. Still a great deal to accelerate though and there would also be some unpleasant gravity gradients.

Rotation is usually easy to maintain in vacuum since rotational momentum is conserved. If you need to "start and stop" rotation a lot you can use two or more counter-rotating rings instead of ejecting a lot of rocket reaction mass to change rotation. This way you only need energy to change rotation speed.

6. Jun 25, 2010

### Staff: Mentor

What is "artificial gravity"? Is that like artificial light?

7. Jun 25, 2010

### Dickfore

In order that you provide gravitational field of the order g at distances of the order L, using Newton's Law of Universal gravitation we can deduce the necessary mass:

$$M = \frac{g \, L^{2}}{G}$$

where:

$$G\ =\ 6.673 \times\ 10^{-11}\ \mathrm{m}^{3} \mathrm{kg}^{-1} \mathrm{s}^{-2}$$

is the Universal gravitational constant.

Since the object surely has to be within a sphere with radius L, the density of the object has to be no less than:

$$\rho = \frac{3 m}{4 \pi \, L^{3}} = \frac{3 \, g}{4 \pi \, G \, L}$$

i.e. it scales inversly proportional with the distances over which we want to create the field. Let us look at what distance a sphere with the density of water (typical density of ordinary substances plus minus one order of magnitude) will create the same field as the acceleration of free fall:

$$L = \frac{3 \times 9.81 \, \mathrm{m} \, \mathrm{s}^{-2}}{4 \times 3.142 \times 6.673(10)\ \times\ 10^{-11}\ \mathrm{m}^{3} \, \mathrm{kg}^{-1} \, \mathrm{s}^{-2} \, \times 10^{3} \, \mathrm{kg} \, \mathrm{m}^{-3}} = 3.51 \times 10^{7} \, \mathrm{m}$$

Not surprisingly, this is of the same order of magnitude as the radius of the Earth. For laboratory sized distances, which are a million times smaller, one would need a material that is a million times denser than water. Fortunately, the mass scales with the square of linear dimensions. This would mean that the mass of the object would be one trillion (1012) times less than the mass of the Earth ($M_{E} = 5.96 \times 10^{24} \, \mathrm{kg}$), so only about $6 \times 10^{12} \, \mathrm{kg}$. Assuming the weight of an average human to be 70 kg (between both sexes), this mass would be equal to the total combined mass of 85 billion average humans!

8. Jun 25, 2010

### mheslep

At first glance I assume the OP meant the creation of artificial gravity by means of acceleration, i.e. a=g. The 'problems' then a spacecraft construction would include structural mechanics, docking dynamics, and the like.

9. Jun 25, 2010

### Staff: Mentor

That's not what I concluded, because clearly, that method of creating an effect similar to gravity is possible.

10. Jun 25, 2010

### mheslep

<shrug> Maybe. OP's phrase was "what are the obstacles", which is ambiguous in its implied assumptions.

11. Jun 26, 2010

### cjameshuff

Centrifugal simulated gravity is entirely possible. The radius of rotation required for comfortably low coriolis effects is large, but that can fairly easily be handled by using two sections attached by a long tether. It would be harder to dock with, but even this isn't really the biggest problem.

One of the biggest reasons to put a station into orbit in the first place is for the freefall environment...a rotating station would be rather unsuited for all the microgravity experiments we want to do. Separate rotating and non-rotating sections would be one way around this, but would be far more complex and expensive than either a rotating or non-rotating station. An ISS module containing a much smaller centrifuge was planned for doing small experiments at a variety of "gravity" levels, but was cancelled due to cost overruns and lack of available Shuttle flights.

12. Jun 30, 2010

### nismaratwork

I believe that we'll be genetically modifying ourselves for space LOOOONG before we bother to start moving worlds, or orbiting superdense objects. I am not including centrifugal simulation in this statement.

13. Jul 2, 2010

### Chronos

Taming even a tiny black hole would be a formidable challenge. How would you make it 'sit' while building a space craft around it? It would be necessary to have an 'on/off' switch. The science necessary for black hole obedience school remains beyond our reach.

14. Jul 2, 2010

### Filip Larsen

Using (a lot of) electrical charge seems like a possibility.

15. Jul 2, 2010

### nismaratwork

You see, this is why I'm thinking cybernetics and genetic modification before tame black holes.

16. Jul 22, 2010

### DLuckyE

You could probably also create a very strong magnetic field to pull everything to one direction. Problem being ofcourse you won't be able to carry any ferromagnetic material since it would be pulled too hard towards the field.

But it's probably easier than creating a mini black hole ;)

17. Jul 22, 2010

### nismaratwork

Anything that strong would probably kill you, although I'm not sure. Certainly it wouldn't pay to have fillings... *wince*

18. Jul 23, 2010

### FawkesCa

that would be great except the whole point to creating gravity would be so HUMANS (andthe such) could walk around. a magnetic field would only hold down metal things, not people... unless, like the nismaratwork insinuated, you had lot of fillings... or a Prince Albert... oooowwwwww:surprised

19. Jul 24, 2010

### utkarsh009

i totally agree with what cjameshuff said. scientist are looking forward to build a rotating spaceship to send people to mars thus reducing the risk of muscle weakening or decay.

20. Jul 25, 2010

### DLuckyE

Magnetic fields do work on everything, given they are strong enough. But i'm not sure if they would just hold you in place (like they do at the HFML) or if you could make them push you away.

You could of course also keep accelerating at 1g continuously, no idea where you'd get the fuel for that though =P

21. Jul 25, 2010

### nismaratwork

I'm pretty sure a field that strong would interfere with your nervous system long before it was capable of producing anything like 1g equivalent for a human. You'd basically be in a non-stop EMP from hell, and by "interfere with" I mean kill horribly.

Rotational microgravity makes more sense anyway, substituting one pseudo force for another.

22. Jul 26, 2010

### Brett13

couldnt you just place magnets in the floor of the station and on the bottom of your boots and on certain places of your body?

23. Jul 26, 2010

### nismaratwork

Magnetic boots would hold you to the deck, and assuming you had pressure switches to turn the magnets on your boots on and off to allow you to walk, it works. That has nothing to do with the lack of gravity on the rest of the body, and magnets on your body would simply cause local strain. Gravity is more than just "glue" keeping you on the floor.

24. Jul 26, 2010

### DaveC426913

You wouldn't need to turn the magnets on/off.
1] You'd make the floor/boots so that there was only enough attraction to hold you to the deck, easy enough to pull away from with moderate effort.
2] You wouldn't have to pull away directly, you'd use a labour saving mechanism. Consider the lowly fulcrum. Your boots (or sandals) would have a convex sole, with magnets only at the toe and heel. Rolling forward on them would pull the magnets away from the deck using a very fluid, natural motion. Once the magnets are more than a small distance from the deck, the pull is vastly reduced* (magnetic field falls off as the cube. This is why fridge magnets fall off the fridge so easily.)

*This is also why attaching magnets at various points on your body wouldn't work. A magnet attached to your elbow could be calibrated to provide a natural amount of "weight". But if your elbow for some reason were to go from a height of 3.5feet to 1.75feet, the pull on it would increase drastically - by eight times.

25. Jul 26, 2010

### nismaratwork

Good point, and even if you did simulate the weight with electromagnets on the body that altered intensity with distance from the deck, I'm not sure that it would change the effect of microgravity on the bones and organs. Needless to say, you would also be walking death for any unshielded electronics.