If we found another planet that was “exactly” like earth in many regards (ie, climate, atmosphere, sustains large complex life forms, etc. but minus the smelly humans) but had less gravity, what would be different about it?
I guess what I really want to know could be phrased this way: if God slowly turns a the Earth gravity dial from 100 percent down to 99, 98, 95, 80, etc. percent, what effects would we see? At what level could we jump around (whoopeee!) but still have a livable planet (i.e., no space suit required, still habitable and sustainable)?
In order for a planet to have retained a sufficient atmosphere to support us, it will have to be at least somewhat larger than Mars. The Red Planet lost its atmosphere, it is believed, because its smaller size allowed its interior to cool off much faster, causing the once-molten core to solidify. Once that happened, any magnetic field it might have had would have disappeared, and the solar wind eventually would have stripped away the atmosphere. The major difference on a slightly smaller planet with a similar atmosphere to Earth’s wold be air pressure. We can survive relatively comfortably(with some adaptation) at a pressure level equivalent to roughly an altitude of 10-15,000 feet here on Earth, so as long as our putative planet had sufficient gravity and/or atmosphere volume to maintain about that level of pressure, given a similar O[sub]2[/sub] concentration, we’d be OK. We could probably survive even lower pressure levels given a sufficiently high concentration of atmospheric O[sub]2[/sub], although with less total air there is the problem of having enough radiation shielding–less air means less ozone.
The lower the gravity is, the less muscle mass we’ll need to move about, and our descendants will most likely be too frail to withstand the gravity of Earth without some serious physical therapy. There is evidence from extended orbital missions that there is loss of bone mass in microgravity, and there will probably be some bone mass loss even under conditions of, say, 70-80% of Earth’s gravity. There may be other long-term physiological effects, but I think as long as we had at least 50% G or so, we’d be able to survive if all other conditions were within a suitable range.
the first problem with this is that as you crank down the gravity dial, the atmosphere starts to go away. You can’t really change that and leave the other things equal.
What ever happened to sci-fi rotating space ships that produce their own gravity. like in the movie 2001?
When I was a kid in the space-race days, all the ‘futurama’ type books talked about the soon-to-be space colonies built like rotating wheels where humans would live happily in artificial gravity.
But in the real space station, there is serious medical concern about long-term effects of zero gravity on astronauts. Why didn’t they build it like the story books?
I wonder if they took into account the effects of a reduction in atmospheric pressure. I would expect that as the atmospheric pressure of a planet is reduced, so too will be the rate of stripping due to the solar wind. Perhaps the current low rate of loss is merely due to the much lower pressure Mars’ atmosphere now has. Unless they’ve run models showing otherwise, we can’t automatically assume that the rate of atmospheric stripping now is the same as it was millions of years ago.
My WAG: to get a decent artificial gravity, your rotating space station either needs to be very big or very fast, or preferably both (though the bigger it is, the less fast it would need to spin, and vice versa). If you look at the station in 2001, it’s pretty big and going round quite fast.
The ISS simply isn’t that big… and isn’t built for cylindrical symmetry. And I imagine it would fly apart if they tried to spin it much.
Sure, if one day they can build a big enough space station, and get it spinning, I can see low-g environments no longer being an issue, at least in planetary orbit.
In a word: cost. You need large structures for this to work comfortably, other wise the Coriolis forces that would be experienced on a small rotating station would be quite severe and disorienting. The smaller the diameter of your rotating ring, the faster it mus spin, but you have the tradeoff of a very large difference in the apparent gravitational force between your feet and your head. But a large station, naturally, costs much, much more to build because you have to get more mass into orbit, and orbital payload is still very expensive.
Cost is certainly an issue to why a large 2001 wagon wheel-type station couldn’t be built, but it would be possible to build a station that is two tethered habitats (presumably connected by a tube that personnel can transfer through). You could make the tether arbitrarily long, subject to material limts, to bring Coriolis forces to managable levels. So why didn’t they?
First of all, one of the (ostensible) reasons for the ISS and its predecessors is to experiment in long-term freefall exposure. Another is the added complexity; you’re going to have to have some kind of non-rotating hub in the middle for vessels to dock. (Never mind spiraling in a Pan Am Clipper Shuttle as in 2001; the pilots would be disoriented and motion sick long before they matched spin rates.) This is non-trivial; you’re going to have to have some kind of free, full rotation joint that is totally sealed against vacuum. You’re also going to have to have a reaction and orientation control system to compensate for changes in loading and natural precession, as well as damping out harmful nutation. Of course, once you bring the station up to spin, it’s essentially impossible to add more modules; you’ll be hanging on at 1G (or whatever gravity your station is simulating) and anything you let go flies off tangentially at great speed. And significant load imbalances are going to impart an unstable rotational moment, so this is really only practical with a station that is substantially more massive (say 1:10000) than any amount of mobile equipment or supplies that may be moved from place to place.
Some of the early concepts for Space Station Alpha (the conceptual predecessor to the ISS) involved a counterbalanced “hampster wheel” habitat or exercise module where Earth-like gravity could be simulated, but it was deleted prior to any serious proposals for the above-mentioned reasons.
Is this for a station with the size and rotation rate of the one in 2001, or for a more current-feasible station? The one in 2001 was only rotating at about 1.3rpm, which I wouldn’t think is fast enough to cause disorientation in most people, let alone pilots.
Hmmm…it’s been a while since I’ve seen the movie, but it seemed like it was rotating faster. In any case, it makes docking operations that much more complicated, compounding any mismatch of velocity and/or rotation.
Regarding a more current feasibly station, it’s not that it couldn’t be done, but it just adds a lot of complexity for little gain (in terms of the ISS, which keeps being progressively scaled back). I think any long-term (multiyear) manned interplanetary craft is going to have to integrate spin-simulated gravity into its design, though, so it’s definitely a capability that needs to be developed.
I base that number off of timing a short YouTube clip. It’s in the right ballpark for a large station providing g/6 (according to Wikipedia, but that’s plausible for a lunar-orbiting station); there seems to be some contradictory information about the exact size of the station (diameter 300m~900ft or 1800ft?), and without watching the movie again I’m not sure which is correct.
If and when they do start building rotating environments in space, the “enough gravity” debate will stop being academic and become a serious matter of cost. The ideal would be full earth gravity ( = 9.7 meter*sec^2), but that means making the structure strong enough to take the strain. I can just see the bean counters moaning “but wouldn’t half a G be plenty?”
You mean 9.8 meter**/sec^2 or 9.8 meter*sec^-**2. Meter*sec^2 isn’t a unit of acceleration; I don’t know what that is, or even if it’s a sensible unit at all.
And the fact of the matter is, nobody really has a clue how much gravity is “enough”, and it’s even conceivable that some lesser amount is actually better than the full 1g. The longest any Earthly organism has ever spent in a field greater than 0 but less than 1 was the few days that the Apollo astronauts spent on the Moon, which isn’t nearly long enough to see any effects. There’s also the possibility of using a lower field, but compensating with the proverbial lead pajamas, to bring the total weight back up to normal, which should give the musculoskeletal system a decent workout (but who knows how the internal organs would react).
