填空题

Questions 47 to 51 are based on the following passage.
On March 18,1965, Leonov, the Russian astronaut, walked in outer space for the first time. Several months later, a similar feat was performed by the first American astronaut. Both of these space walkers had spent months previous to their flights learning how to control their movements under the strange conditions which exist in space. Wearing their thick space suits, they learned to deal with an environment where there is neither weight nor gravity, neither up nor down.
We don’t realize how much we depend on the earth’s gravity until we are deprived of it. Then our feet no longer stay on the ground; we float around in the air, the slightest touch may send us drifting off in the opposite direction.
In the laboratories where astronauts are trained for their journeys, they are subjected to conditions that resemble those of flight. It takes time for them to prepare for the great changes that occur in space. When the spaceship leaves tile earth at tremendous speed, the astronauts feel as if they are being crushed against the spaceship floor. Later, when they leave the zone of the earth’s gravitation, they are unable to stay in one place. Simple actions, such as eating and drinking, become very difficult to perform. You may get an inkling of what the astronauts have to deal with if you try to drink a glass of water while standing on your head or while just lying down.
Tremendous speeds must be attained to reach other planets in a reasonable time. If spaceships were launched from space, the absence of weight Would permit the ships to be launched with great speed at reduced pressures. To illustrate this point, remember how strong the wind feels if we are traveling fast in a car, then imagine a car traveling through an area where there is no wind. The windless condition is comparable to the condition in outer space.