Is prolonged micro-gravity really a problem for inter-planetary spaceflight?
May 10, 2011 5:48 PM Subscribe
Why are the lack-of-long-term-gravity health problems still an issue for space travel? Isn't it easily dealt with by designing craft that generate gravity by spinning?
I am not a scientist, but do like to follow discussion about space travel. When trips to Mars are discussed, the health risks due to prolonged exposure to micro-gravity are routinely brought up. But isn't this one of the easiest things to resolve, by having a spinning craft design? Or am I missing something?
I am not a scientist, but do like to follow discussion about space travel. When trips to Mars are discussed, the health risks due to prolonged exposure to micro-gravity are routinely brought up. But isn't this one of the easiest things to resolve, by having a spinning craft design? Or am I missing something?
I'm pretty sure in order to generate enough gravity you'd need a much, much bigger ship then we've ever built before.
Also I keep hearing radiation flares as a huge risk, and the sheer time it would take - along with providing a return trip, it would be a resource dump of epic, global proportions.
posted by The Whelk at 5:53 PM on May 10, 2011 [1 favorite]
Also I keep hearing radiation flares as a huge risk, and the sheer time it would take - along with providing a return trip, it would be a resource dump of epic, global proportions.
posted by The Whelk at 5:53 PM on May 10, 2011 [1 favorite]
It doesn't take a lot of energy to spin a space craft, but you need to build a big enough space craft in earth orbit in the first place and that's the tricky thing. The reason we haven't been to Mars is that no-one wants to spend the money, it's not the micro-gravity thing.
posted by joannemullen at 5:54 PM on May 10, 2011 [1 favorite]
posted by joannemullen at 5:54 PM on May 10, 2011 [1 favorite]
There's an issue of scale when it comes to spinning a craft in order to generate gravity. The craft needs to of large enough scale so that the occupants would not be aware of the spinning, for both psychological and physical reasons. Such a craft would be far larger than anything we've attempted to put into space.
posted by Thorzdad at 6:03 PM on May 10, 2011
posted by Thorzdad at 6:03 PM on May 10, 2011
Best answer: There is lots of good info here: http://en.m.wikipedia.org/wiki/Artificial_gravity
posted by dfriedman at 6:04 PM on May 10, 2011
posted by dfriedman at 6:04 PM on May 10, 2011
A spinning segment of a space ship would be subject to considerably greater force than a comparable non-spinning segment, which means it would have to be built sturdier, which means it would be heavier. (which in turn increases the force, requiring still more structure, and so on. Space ship design constantly battles run-away design traps.)
All of that means it would cost a hell of a lot more, and require a lot more fuel.
posted by Chocolate Pickle at 6:12 PM on May 10, 2011
All of that means it would cost a hell of a lot more, and require a lot more fuel.
posted by Chocolate Pickle at 6:12 PM on May 10, 2011
Best answer: Science fiction author Karl Schroeder has blogged about this issue more than once. See here, here, and here, for instance. From the last link:
The objections all sound reasonable: too much radiation! Too far away! Zero gravity is too debilitating! Too expensive!posted by Conrad Cornelius o'Donald o'Dell at 6:39 PM on May 10, 2011 [3 favorites]
All of these objections are true, while at the same time they're all wildly wrong, and largely for the same reasons. In fact they're all true only if getting from Earth to orbit remains as expensive as it is now.
Consider the seemingly insurmountable problem of radiation that Krauss complains of in his piece. What's the solution to radiation? Shielding. Is shielding a spacecraft impossible, or even difficult? No, actually it's easy. Two meters of water around the crew cabin are enough to solve the problem of radiation in the inner solar system. The problem is not the shielding; it's the cost of shipping the water up to orbit that is the problem.
Ditto for, oh, let's say zero gravity. No astronaut should ever have to put up with zero gravity for more than a day or two at a time; the simple solution to the debilitating effects of freefall is to spin the spacecraft. To do it in a manner comfortable to to the astronauts, you need a long boom arm, which might be heavy and awkward to lift from Earth. The point is, the solution is easy.
As far as humans traveling to Mars, it might not be an issue with a Mars Cycler, which would take about five or six to travel between Earth and Mars. Astronauts and sosmonauts have done that with no serious ill effects.
Otherwise, a spinning space station or craft has never been built before. It could be done, but it would have to be designed and tested, which takes money and direction, something NASA doesn't seem to have much of these days.
posted by Brandon Blatcher at 6:44 PM on May 10, 2011
Otherwise, a spinning space station or craft has never been built before. It could be done, but it would have to be designed and tested, which takes money and direction, something NASA doesn't seem to have much of these days.
posted by Brandon Blatcher at 6:44 PM on May 10, 2011
The real problem here comes from the radius of rotation and how that would affect humans. Centripetal acceleration equals (v^2)/r. To get Earth-normal gravity at the perimeter, plug in 9.8m/s for v, and 9.8m for r.
Except...
An average human stands 1.7m tall. At a radius of 8.1m at the same rate of rotation (1 radian per second), on the same craft, you have a centripetal acceleration of 8.1m/s^2.
Now, important detail - Humans can stand between 0 and 5ish (more with training and specialized gear) times Earth's gravity, overall; We can only stand tiny variations in acceleration between our heads and our feet (on the order of 0.01g), however.
If you solve for that difference as an upper acceptable limit, you get a minimum radius of rotation of a whopping 196 meters.
So, until we have the technology to get an object the size of the Superdome (r=207m) into space, we can't realistically use rotation to keep the crew healthy. And as others have already mentioned, it takes a LOT of energy just to launch the tiny (think cargo-container-sized) craft we can get to orbit today.
posted by pla at 7:32 PM on May 10, 2011 [2 favorites]
Except...
An average human stands 1.7m tall. At a radius of 8.1m at the same rate of rotation (1 radian per second), on the same craft, you have a centripetal acceleration of 8.1m/s^2.
Now, important detail - Humans can stand between 0 and 5ish (more with training and specialized gear) times Earth's gravity, overall; We can only stand tiny variations in acceleration between our heads and our feet (on the order of 0.01g), however.
If you solve for that difference as an upper acceptable limit, you get a minimum radius of rotation of a whopping 196 meters.
So, until we have the technology to get an object the size of the Superdome (r=207m) into space, we can't realistically use rotation to keep the crew healthy. And as others have already mentioned, it takes a LOT of energy just to launch the tiny (think cargo-container-sized) craft we can get to orbit today.
posted by pla at 7:32 PM on May 10, 2011 [2 favorites]
Launching a Superdome isn't needed though, right? It could be assembled in orbit.
posted by Brandon Blatcher at 7:37 PM on May 10, 2011
posted by Brandon Blatcher at 7:37 PM on May 10, 2011
Correction, make that 163m. Math/editing error (I started assuming humans stand 2m, which I decided counts as simply too much to idealize, but didn't fix my intermediate calculations). Still "really freakin' big", though.
Brandon Blatcher : Launching a Superdome isn't needed though, right? It could be assembled in orbit.
If we had any serious capacity to assemble things in orbit, yes. At present, we can barely get one prefabbed lego-like ISS module in place per year, successfully.
posted by pla at 7:41 PM on May 10, 2011
Brandon Blatcher : Launching a Superdome isn't needed though, right? It could be assembled in orbit.
If we had any serious capacity to assemble things in orbit, yes. At present, we can barely get one prefabbed lego-like ISS module in place per year, successfully.
posted by pla at 7:41 PM on May 10, 2011
We can only stand tiny variations in acceleration between our heads and our feet (on the order of 0.01g), however.
I couldn't find mention of this in the Wiki article, is there an entry for it anywhere?
posted by bonobothegreat at 7:42 PM on May 10, 2011 [1 favorite]
I couldn't find mention of this in the Wiki article, is there an entry for it anywhere?
posted by bonobothegreat at 7:42 PM on May 10, 2011 [1 favorite]
The answer to the different acceleration problem from head to foot is use a long tether to connect to portions of the space ship while you are in transit. Say put the propulsion stuff on one end (it will not be used for propulsion while you are in tether mode) and the life support segement on the other end. Say make the cable...oh....200 meters long? and spin them. You will actually start the rotation while the ship is still rigidly attached and then slowly let the tether play out. It also appears that 9.81 m/s^2 is not really required to maintain human health so maybe half that will do (we don't really know what % is required it is just unlikely to need to be 100% earth surface normal). This eliminates the Coriolis effect problem and also the water hammer problem you can get in a variable acceleration field like you get from centrifugal acceleration.
posted by bartonlong at 7:43 PM on May 10, 2011
posted by bartonlong at 7:43 PM on May 10, 2011
bartonlong, most of the methods of propulsion we have going would involve constant acceleration towards the target until about half way there, turning the ship around, and then decelerating the rest of the way.
posted by Blasdelb at 8:59 PM on May 10, 2011
posted by Blasdelb at 8:59 PM on May 10, 2011
Really? I'd assume any practical Mars mission would involve something very close to a Hohmann transfer orbit (one burn at departure, long coasting phase, one burn at insertion).
posted by hattifattener at 9:09 PM on May 10, 2011
posted by hattifattener at 9:09 PM on May 10, 2011
pla writes "So, until we have the technology to get an object the size of the Superdome (r=207m) into space, we can't realistically use rotation to keep the crew healthy. And as others have already mentioned, it takes a LOT of energy just to launch the tiny (think cargo-container-sized) craft we can get to orbit today."
Your actual craft doesn't have to be that big. All you need is a couple masses joined by a cable. Even if you wanted a rigid connection between your two modules something like a conventional self lifting building crane (obviously the cross sections would be different because of the different loading) could be shipped up in sections and you'd just have to bolt them together. Soyuz can launch pieces 7m long (I think) so 200 metres is only 30 launches. We could do this today, all you'd need is someone to step up with the money. Designed right it sure as heck wouldn't take a year to bolt each piece together.
posted by Mitheral at 9:13 PM on May 10, 2011
Your actual craft doesn't have to be that big. All you need is a couple masses joined by a cable. Even if you wanted a rigid connection between your two modules something like a conventional self lifting building crane (obviously the cross sections would be different because of the different loading) could be shipped up in sections and you'd just have to bolt them together. Soyuz can launch pieces 7m long (I think) so 200 metres is only 30 launches. We could do this today, all you'd need is someone to step up with the money. Designed right it sure as heck wouldn't take a year to bolt each piece together.
posted by Mitheral at 9:13 PM on May 10, 2011
The real problem here comes from the radius of rotation and how that would affect humans. Centripetal acceleration equals (v^2)/r. To get Earth-normal gravity at the perimeter, plug in 9.8m/s for v, and 9.8m for r.
But if we're sending them on a trip to Mars, is there any point building a ship that simulates a gravity greater than Mars's?
posted by justkevin at 9:19 PM on May 10, 2011
But if we're sending them on a trip to Mars, is there any point building a ship that simulates a gravity greater than Mars's?
posted by justkevin at 9:19 PM on May 10, 2011
You want to have enough "gravity" to prevent osteoporosis. We know 1G will do the job; I don't think we know for sure what the cut off is for long term fractional Gs besides knowing zero G is insufficient.
posted by Mitheral at 10:35 PM on May 10, 2011
posted by Mitheral at 10:35 PM on May 10, 2011
Mitheral: "You want to have enough "gravity" to prevent osteoporosis. We know 1G will do the job; I don't think we know for sure what the cut off is for long term fractional Gs besides knowing zero G is insufficient."
Indeed - this lack of knowledge is one of the things Schroeder decries in one of the posts I linked above:
Indeed - this lack of knowledge is one of the things Schroeder decries in one of the posts I linked above:
For instance, if NASA were actually interested in putting people on, say, Mars, for extended periods--or on the moon or indeed anywhere but low Earth orbit--they would logically have long ago embarked on a research program to learn what the biological effects of Martian or lunar gravity are. Instead, they've invested decades and billions into learning how humans react to zero gravity--an almost useless scientific endeavor, because the clear lesson from the start of that program was that living in freefall is a bad idea. Conclusion: whenever people are going to spend more than a few weeks in orbit, provide them with artificial gravity in the form of a rotating spacecraft. There's no reason not to; the technology involved in spinning things around is not actually rocket science.posted by Conrad Cornelius o'Donald o'Dell at 11:22 PM on May 10, 2011 [3 favorites]
No amount of data about how the human body reacts to zero-G is going to answer the important question, which is: how does the human body react to extended periods under fractional gravity--like the moon's 1/6 G or Mars's .38 G? If there's a potential show-stopper to colonizing other worlds, it's going to be how our physiology responds to fractional gravity, not zero gravity.
At what gravitational level does osteoporosis start in human bones? What's the minimum level for maintenance of cardiovascular health? At what level do embryonic and infant development begin to suffer? Maybe these questions can be tentatively answered from studies in zero-G, but any conclusions reached that way need to be empirically confirmed. In other words, what manned spaceflight needs as its next step is a variable-gravity research station. The ISS is useless for learning what we really need to know; what's needed is a very simple, rotating station whose gravity can be tuned up or down to simulate life on worlds ranging from Mercury to the moon to Mars, or Ganymede or Titan.
I highly recommend the book Packing for Mars by Mary Roach. It is all about this question.
posted by smackfu at 6:53 AM on May 11, 2011
posted by smackfu at 6:53 AM on May 11, 2011
Instead, they've invested decades and billions into learning how humans react to zero gravity--an almost useless scientific endeavor, because the clear lesson from the start of that program was that living in freefall is a bad idea.
I've been saying this for years.
Testing the effects of zero-G on human bodies in preparation for space exploration would be like Columbus spending four months immersed in salt-water in preparation for crossing the Atlantic.
posted by General Tonic at 6:56 AM on May 11, 2011
I've been saying this for years.
Testing the effects of zero-G on human bodies in preparation for space exploration would be like Columbus spending four months immersed in salt-water in preparation for crossing the Atlantic.
posted by General Tonic at 6:56 AM on May 11, 2011
bartonlong, most of the methods of propulsion we have going would involve constant acceleration towards the target until about half way there, turning the ship around, and then decelerating the rest of the way.
If we can do that, hell we can go visit Jupiter or even Pluto with relative ease. Even using Ion Drives I don't think we can make the craft accelerate out for 1/2 the distance and then slow down the other half. I believe Jerry Pournelle wrote a really good article on this called Those pesky belters and their torchships. It is included in his A step Further out that is available on Kindle and out of print on real books.
Say we do use some kind of constant acceleration drive or solar sails that make the tether rotation unfeasible than you have a large central mass, like the propulsion/fuel unit and then you rotate the life support and say the descent stage around this mass using tethers to get the same effect. The acceleration from such drives are so small I don't think they would significantly impact the rotational vector of the modules. The point is it is an engineering exercise not a laws of physics problem. We can solve the engineering exercise problems. Discovering new scientific principles or materials are much more difficult problem. See Space Elevator for a really good idea that is up agaisnt a materials problem as well as REALLY big engineering problems.
posted by bartonlong at 9:11 AM on May 11, 2011 [1 favorite]
If we can do that, hell we can go visit Jupiter or even Pluto with relative ease. Even using Ion Drives I don't think we can make the craft accelerate out for 1/2 the distance and then slow down the other half. I believe Jerry Pournelle wrote a really good article on this called Those pesky belters and their torchships. It is included in his A step Further out that is available on Kindle and out of print on real books.
Say we do use some kind of constant acceleration drive or solar sails that make the tether rotation unfeasible than you have a large central mass, like the propulsion/fuel unit and then you rotate the life support and say the descent stage around this mass using tethers to get the same effect. The acceleration from such drives are so small I don't think they would significantly impact the rotational vector of the modules. The point is it is an engineering exercise not a laws of physics problem. We can solve the engineering exercise problems. Discovering new scientific principles or materials are much more difficult problem. See Space Elevator for a really good idea that is up agaisnt a materials problem as well as REALLY big engineering problems.
posted by bartonlong at 9:11 AM on May 11, 2011 [1 favorite]
This thread is closed to new comments.
posted by dfriedman at 5:51 PM on May 10, 2011 [1 favorite]