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Last Word is Â鶹´«Ã½â€™s long-running series in which readers give scientific answers to each other’s questions, ranging from the minutiae of everyday life to absurd astronomical hypotheticals. To answer a question or ask a new one, email lastword@newscientist.com
Sci-fi shows often feature “space ports” spinning slowly to create artificial gravity. Is this feasible, and how soon?
Alex McDowell
London, UK
They are entirely feasible and, with sufficient funding, we could build one within months or years!
Due to the effect of the Coriolis force on the semicircular canals (three tubes in the inner ear), humans would feel dizzy if the rotational speed of the space port exceeded about 2 revolutions per minute (rpm), in which case the centrifuge would need a diameter of 448 metres for the artificial gravity to be equivalent to that on Earth.
However, humans can adapt to rotational speeds of up to 23 rpm, in which case the diameter of such a centrifuge would need to be only 3.4m. Such centrifuges would overcome the detrimental health effects of prolonged weightlessness, like the loss of bone and muscle mass, for example on missions to Mars and beyond or extended trips to space stations.
In 1975, the Kosmos 782 satellite carried a centrifuge that gave animals on board artificial gravity. And on Skylab in the 1970s, astronauts ran round the circular wall of the capsule, giving the effect of running with some gravity.
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Astronauts could spend time inside centrifuges to avoid the health problems caused by living in weaker gravity
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These centrifuges could also be used where gravity is weak, for example, on the moon. Astronauts in such places could spend time inside centrifuges to avoid the health problems that prolonged living in weaker gravity would cause. In these centrifuges, the “floor” wouldn’t be perpendicular to the ground outside: it would be at an angle where the components of the real gravity and the artificial gravity vectors add up to be perpendicular to the floor.
Ron Dippold
San Diego, California, US
The beautiful thing is that objects in space continue moving as long as nothing slows them down (in accordance with Isaac Newton’s first law of motion). If you had a 30-metre-diameter space station (about 95m in circumference), you would only need it to be rotating every 8 seconds to get full Earth gravity at the edge. And once you got it up to speed, it would spin forever, except for space dust slowing it down. You could, of course, go half as fast, but any less than full Earth gravity seems to lead to nasty health problems.
There are two big problems. First, the difference in speed between feet and the head: in a 30m-diameter space station, a 2m-tall person’s feet are 15m from the centre, but their head is 13m from the centre. So their feet are rotating at 12 metres per second and their head is at 10.5 m/sec. This is enough to cause disorientation. You can help this by making the station larger or (in a 30m-diameter station) just letting people get used to it. The smaller you make it, the worse it gets.
The second problem is docking rockets or shuttles. Good luck docking anything to the rim that is flying by at 12 m/sec! So rotating-wheel stations need a docking station at their axle. A fixed 3m docking hatch on a station rotating every 8 seconds would still be rotating at 1.2 m/sec at its edge, so you need a “despun hub” that is rotationally detached from the spinning station.
It would still need a little propellant to stay relatively still because it can’t be 100 per cent detached and the rotation of the rest of the station would tend to make it rotate a bit, but it wouldn’t take much propellant to counter that. Once a ship docked to the non-spinning hub, astronauts would ease into the centre of the spinning station, where it isn’t spinning so fast, then get up to speed by hanging on to handles and slowly easing outwards to Earth gravity.
The International Space Station doesn’t do this because, while it has been a huge success, it was accreted instead of orbited in its final form. It was designed so you could just bolt new sections on as needed, and they have done that. But if you want a spinning wheel design, you have to build the whole thing to start with, and that was too expensive and too much work at the time. But it could be done right now if any country or coalition wanted to commit to it.
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