单项选择题

TEXT A
An airliner travelling from London to New York may take from five to fifteen hours to cross the Atlantic, while a space capsule makes one complete circuit of the earth in about ninety minutes. The sequence of events is very similar in both types of flight: the vehicle must take off, climb to a suitable height, fly in the right direction at a relatively constant speed for an appropriate time, descend, and land at the destination. Yet although flights to New York are routine affairs which almost anyone may safely undertake, a flight into space is a hazardous adventure for which only a few selected men are at present considered suitable.
The most obvious difference between an aircraft and a space vehicle is that of speed, but this alone cannot account for the greater stamina required of astronauts. The human body is unaffected by speed alone and we are normally quite unconscious of the earth’s rotation on its axis, or of its rapid motion around the Sun. Of much greater importance is the rate at which the final speed is achieved, for the body is extremely sensitive to alterations of velocity, or accelerations, especially if they are sudden. An airliner can take a comparatively long time to reach its cruising speed of, say, 400mph, and its passengers will experience acceleration only to a mild degree. The space capsule, however, must be hurled through the atmosphere to reach its final speed of 18,000 mph as quickly as possible, and the acceleration applied by the launching rocket must be correspondingly high. The first problem of manned space flight, therefore, is to match the performance of the rocket to the body’s tolerance for
acceleration, and this naturally involves a study of the physiological effects of acceleration.
Like all other accelerations, gravity acts upon objects to produce a force, and this force is experienced as weight, or as pressure. It is usual and convenient to regard the earth’s gravity as a standard unit, referred to as lg, and also to use the expressions "force" and "acceleration" as interchangeable.
Most of our knowledge of the physiological effects of acceleration has come from studies on human centrifuges, in which acceleration is produced by rotation instead of by changing speed. It has been found that human tolerance is greatly affected by the direction in which the force acts. When the acceleration is applied in line with the long axis of the body, the early symptoms are merely of difficulty in lifting the arms and legs, and of being thrust down into the seat. If the acceleration is raised to 3g or so, vision becomes slightly misty or veiled. As the stress is increased further, the field of view contracts from the edges, until at about 4.5g only a small patch of central vision remains. With yet higher accelerations, even this small area is lost, and this is the state well known to fighter pilots as "black-out". Finally, at about 5.5g to 6g consciousness is lost.
The remedy follows logically: if tolerance depends upon the ability of the heart to push blood to the head, it should be possible to reduce the load by shortening the distance between heart and brain. Crouching, or bending the head forward, would be one solution, but an even more satisfactory result can be achieved by placing the body across the line of thrust. The effort needed to pump blood to the brain is then quite small, for the heavy fluid does not have to be lifted very far. In this position men have. withstood an acceleration of 17g for a period of three or four minutes without loss of consciousness.
Gravity and acceleration become important once more during the re-entry of the space capsule through the earth’s atmosphere. In this phase, all the speed acquired at the cost of so much fuel during the launch must be lost. Deceleration has exactly the same properties and physiological effects as acceleration, and the same precautions must be taken to avoid exceeding the limits of tolerance. This is why the American plan involves turning the whole capsule round shortly before re-entry, so that the man is again pressed back into his protective couch.
The highest, and shortest, deceleration of the entire flight comes at the moment of impact with the land or water. Here the last remnants of the speed must be lost very suddenly, and forces of up to 30g can easily accompany descent to an unyielding surface. The duration of this final insult is so short, however, that its physiological effects are negligible. No doubt the astronaut would regard the jolt as a welcome indication of his return to a normal 1g environment.
What common experience suggests that the human body is quite accustomed to travelling at high speed