Why is tethered artificial gravity hardly ever considered?

Question

When you ask about rotating artificial gravity (yes, yes, technically radial acceleration), most answers boil down to "you'd need a 200m diameter spacecraft, and we can't build that yet"

But, of course, why would you need the entire circle to be a continuous space station? Just tie two modules together with a cable and extend, right?

Are the engineering problems just too difficult and numerous to even consider that? I can see a few already:

1. Moving people between the modules, or to a stationary part of the station, would require an EVA. Not only that, but an EVA from a rapidly moving module.
2. Making orbital or attitude adjustments would be much more complicated.
3. Docking to visiting spacecraft would require a stationary module or stopping the rotation entirely.
4. Moving resources between the modules and/or a stationary section would require long cables and pipes, and possibly seals between moving pieces.

Of course, one can think of solutions to all of these. Most can be addressed by temporarily halting the rotation and maybe winching the modules back together. But none of the solutions seem simple. Is it all just too much to even contemplate?

Edit:
I didn't think I needed to, but I guess I'll clarify the motivation. I've read plenty of people questioning whether Mars gravity (0.38g) is sufficient to ameliorate the many negative effects of long-term weightlessness. It seems reckless to wait until we're there, in the middle of an 18-month mission, to find out.

Answer 1

Tethering two habitable modules together splits your already cramped living space into two smaller spaces that can't be easily traversed; this means a lot of equipment like life support and bathroom facilities need to be duplicated (presumably power and comms could be on an umbilical alongside the tether.) That's an unacceptable efficiency/mass hit, so let's reject it.

You could put an inert mass on a long tether; this would involve launching a lot of dead weight, which is also unattractive.

Gemini XI did some tether experiments with an Agena spacecraft, with some odd "jump-rope" oscillations and jerkiness; I don't know if that's an insurmountable problem. The Gemini experiment achieved only milli-gee levels of acceleration. http://www.spacesafetymagazine.com/space-exploration/gemini/m-equals-1-all-up-mission-gemini-xi-part-2/

You'd have to reel in the weight and de-spin every time you wanted to dock a visiting spacecraft.

Finally, one of the most significant purposes of having a space station is to do experiments in zero g. Spinning the station defeats that purpose.

Answer 2

There's no need for simulated gravity in human spaceflight the next several decades. It might be of interest to basic biological research, but it's up to them to buy a biological research station in LEO, it is of no concern for agencies and companies planning human space flight.

It will be some time before humans go further than Mars, and it only takes a 6-9 months economical Hohmann transfer to get to Mars. Hundreds of astronauts have spent that much time in microgravity and enjoyed it. No one has ever been hurt by microgravity, and reduced gravity isn't exactly epidemic on Earth, so simulated gravity has very low priority in both the space and the medical communities.

Microgravity has the benefit of increasing human utilization of spaceship space. One can squeeze more people into a weightless room. It helps reducing the size and mass of a spaceship. Structures such as solar arrays and antennas can be made lighter and more delicate in microgravity. A spinning spacecraft suffers a mass penalty throughout the design. Also, gravity hurts. It breaks backs, it kills people falling, you drop things on your toes. It is a blessing for human health to get rid of the great cause of accidents, wear and toil which gravity is.

Reduced gravity, such as 16% or 38% as on the Moon and Mars, should take care of several of the problems experienced in microgravity. It should be enough to somewhat normalize the fluid pressure in the upper body, to give load on muscles and skeleton, to greatly increase the effect of exercise, to make dust fall down instead of floating in everybody's faces (reducing the need and noise of ventilation), to deactivate microbes which in microgravity seem to react as if they were buoyant in water which activates them, to sit or sleep without strapping down.

The resources for a simulated gravity station are better needed on a real mission to Mars to lower real risks of launch, landing and any kind of hardware or software failure. The best place to find out about the effects of reduced gravity is on the Moon and on Mars! We don't need to recreate that which we already have got handy by nature. Gravitational effects interact with radiational, chemical and psychological effects. No examination could be more complete than actually spending a long time on the Moon or Mars. If people can stay on the Moon for a year, they certainly can stay on Mars with more than double the gravity for a year too. And actually, people have spent a year in microgravity, so there's the question about what purpose simulated gravity would have.

Answer 3

Both the question and the accepted answer list good objections to the idea of a tethered-pair approach to artificial gravity. But I think it's too strong to say it's "hardly ever" considered. The Wikipedia article on artificial gravity has a list of proposals, including both full rings and tethered pairs. One of the tether ideas is Robert Zubrin's very influential Mars Direct proposal.

So, it's not too much to even contemplate. It's just that, when you do contemplate it, you realize that it is basically a luxury for the work in space that we are doing now.

It is significant that the most well-known proposal is for a Mars mission. Then there would be a sustained period on the way to Mars and on the way back when artificial gravity would be helpful, and when there are no visiting spacecraft. The propulsion module could serve as a counterweight so you wouldn't have to split the habitable area or have dead weight. But lots of other objections remain. The cable is still dead weight, and also the whole thing would have to be sturdier and hence heavier.

Answer 4

You could use a technology that us space scientists call 'a metal ladder' to connect the two spaceships together. This would provide artificial gravity and massively reduce the risk to human beings and the mission. It should significantly outweigh the risk of dropping something on your toe.

Not only does this technology prevent the spacecrafts from flying apart it also provides a way for astronauts to travel between the two spacecrafts. The use of 'a ladder' to overcome gravity has been tested on Earth.

If the solution proves too simple and cheap the ladder can be computerized and gold plated.

Answer 5

In a space ship, if an astronaut doesn't touch any part of the inside of the ship then they will not feel centrifugal force after rotating the ship and hence no gravity will be established for them.