Saturday, August 2, 2008

9.3 Centrifuges and reading in space

This is a standard desktop centrifuge. Each "hole" accommodates an Eppendorf centrifuge tube. The centrifugal force is about 2,000 x g, strong enough for separating most particulate matters from the solvents. For comparison, 1 x g is what most of us experience on the surface of Earth everyday.

Humans occasionally subject themselves to higher Gs. For example, most rides in amusement/theme parks are actually centrifuges or variations of centrifuges. While most are in the less than 2 x g range (an example is shown below), some in the form of roller-coasters, e.g., the Space Mountain in Disney World, can reach 4 x g.

(An amusement park centrifuge)

Momentary exposure to 4 x g (or even higher) is relatively harmless, unless there is an undiagnosed pre-existing medical condition - then it can end in tragedy. Indeed, heart attacks and brain hemorrhages are known to occur on some high-G rides.

NASA has the ultimate centrifuge for the humans:
This is NASA's 20 x g centrifuge used for testing the physiologic responses of astronauts. And this is where you see the astronauts' faces start to melt, among other things.

Technically, the g-force is calculated thus:

where g = Relative centrifugal force (RCF)
r = rotational radius (centimeters, cm)
N = rotating speed (revolutions per minute, rpm)

Of course, g is also the basic unit of the gravitational force. Without gravity (i.e., zero-g), as that in the outer space, some human reflexes that have worked previously now begin to unravel. And one of them is the simple vestibulo-ocular reflex (VOR).

Now pick one object in your room, stare at it, and then move/rotate your head to left and right, you'll notice the object remains in the same stable position. In other words, the image of the object is stabilized on your retina. This is the VOR at work.

The sensory aspect of VOR is handled in the inner ear by the semicircular canals (which sense angular acceleration to detect head rotation) and the otolith organs (which sense linear acceleration to detect both head translation and the position of the head relative to gravity). The information is processed centrally and the output sent to extraocular muscles.

In zero-G, the semicircular canals do not work well because the fluid contained within no longer convects effectively. So the coordination between head motion and eye movement is now disturbed. If you read too fast, the text lines will appear jerky. Effective reading can be achieved only at voice speed. Whether VOR eventually adapts to long periods of zero-G and normalizes is still yet to be tested. In a long journey to other galaxies, catching up on your reading will become an integral part of mental exercise. Certainly beats looking out of the window into darkness 24/7.

Of course, large centrifuges can be incorporated into space stations and star ships. The centrifugal force will provide simulated gravitation that can make the astronauts' lives easier.

The (presumably) rotating space station shown below is in 3D. You'll need to borrow a pair of red-green glasses from your eye doctor to see the effect.