QUESTION:
How much speed can a body's organs take in space?

ANSWER from Bruce Thompson on September 20, 2000:
The question is not how much speed the body can take, but rather, how much 
*acceleration*. It has long been known that speed alone has no effect on our 
organism. Accelerations, on the other hand, can cause severe injuries, as any 
paramedic attending a road smash knows.

The study of the effects of acceleration on the body began towards the the 
end of World War I, when aircraft were becoming robust enough to perform 
violent aerial manoeuvres without falling apart in mid-air. Pilots reported 
that those same manoeuvres were giving them temporary vision problems, as 
well as making them feel heavier or lighter, depending on what their aircraft 
were doing at the time. Over the decades that followed, an understanding was 
developed of so-called g-forces and their effects on the body. When space 
flight began, a new element - that of weightlessness - was added to the 
research.

The human body can endure any speed in space, or on Earth, but the time taken 
to reach that speed is governed, among other things, by the body's ability to 
withstand the forces of acceleration.

Every moment of our lives is subject to an acceleration of one sort or 
another. In a car, we feel the bumps and irregularities in the road by the 
small vertical accelerations they impose on the car, and when the car turns a 
corner, we tend to lean over, because our bodies are trying to continue in a 
straight line, while the car's turning imposes a sideways acceleration on us. 
When the car brakes, we lean forward, because the car is decelerating. 
Talking about "acceleration" and "deceleration" at the same time might sound 
confusing, but they are describing the same thing - a change in velocity; 
either faster or slower.

Even when we are sitting in a chair, we are still subject to the constant 
acceleration of the Earth's gravity. It is trying to pull us down at 32 feet 
per second squared, but whatever we are sitting on, or standing on, stops us, 
so we stick to that surface. Take away that surface and gravitational 
acceleration takes over, until we encounter another surface, at which point, 
deceleration takes place.

In space, a space ship in orbit around the Earth is travelling at 17,000 
miles per hour. The crew inside the space ship are also travelling at the 
same speed, so they feel no forces acting on them. However, in reaching that 
speed, the space ship and crew had to accelerate from stationary, on the 
ground, to orbital speed in only a few minutes.

In the case of the Space Shuttle, this acceleration is around 3 times the 
force of gravity - 3g - making the crew feel three times their normal weight, 
but in the early days of manned space flight, the astronauts had a rougher 
ride. The Mercury capsules launched by the Atlas booster reached a peak 
acceleration of 8g during ascent to orbit, then decelerated during re-entry 
at loads as high as 7.8g. The Titan rockets launched the Geminis at 7.25g, 
and the Saturn 5 peaked at 4g. However, the Apollo capsules returning from 
the Moon re-entered the atmosphere at over 6g.

The highest acceleration/deceleration that a human volunteer has been 
subjected to was in an experimental sled called the "Daisy Decelerator", used 
in the 1970s to study the effects of sudden deceleration - violent braking, 
that is - on the human body. A Major Beeding experienced a sudden 
deceleration with a peak force of 83g, lasting 0.04 of a second. Major 
Beeding was subjected to the same force when his rocket sled accelerated as 
he was when it decelerated, but because it was more gradual, the 
rocket-powered acceleration was more gentle than the violent brick-wall 
deceleration at the other end. The Daisy Decelerator experiments were 
carefully controlled, with the health of the human subjects always taking 
priority.

The following terms have been adopted for various kinds of acceleration: 
impact acceleration. less than 0.2 of a second; abrupt acceleration, 0.2-2 
seconds, brief acceleration, 2-10 seconds; long term acceleration, 10-60 
seconds; and prolonged acceleration, more than 60 seconds.

The human body can tolerate violent accelerations for short periods, 
including the the prolonged high-g acceleration necessary to reach Earth 
orbit. However very prolonged periods of high-g acceleration during travel 
between planets would be very harmful to the body, and therefore out of the 
question. Imagine travelling to Mars, accelerating all the way at 3 
gravities. You would weight three times your normal weight for the duration 
of the trip and would barely be able to move, but what would the unrelenting 
acceleration be doing to your body? Heavy acceleration is a speeded-up aging 
process. Tissues break down, capillaries break down and the heart has to do 
many times its proper work. You could not count on being in good shape when 
you arrived.

In his 1953 short story, "Sky Lift", Robert Heinlein explored the effects on 
the body of several days of space travel at accelerations as high as four 
gravities. The effects were not pleasant.