In the laboratories whereastronauts are trained for their journeys, they are subjected to conditionsthat resemble those of flight. It takes time for them to prepare for the great changesthat occur in space. When the spaceship leaves the 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 tostay in one place. Simple actions, such as eating and drinking, become verydifficult to perform. You may get an inkling of what the astronauts have todeal with if you try to drink a glass of water while standing on your head orwhile just lying down.
The beginnings of man's conquest of spacetook place in 1958, seven years before Leonov's trip. The first successfullaunching of"Sputnik"demonstrated that it was indeed possible to sendobjects far enough out of range of earth's gravity so that they would not fallback to earth. Rather, such objects could be forced to revolve about the earth,just as the moon does. However, while the moon is so far from earth that ittakes it a month to revolve around the earth, man-made satellites, which arecloser to earth, can make a complete revolution in a few hours.
It was three years after the firstsatellite launching that a spaceship containing a man made a successful flight.The flight lasted less than two hours, but it pointed the way to futuredevelopments.
Other planets are so far away thatspaceships must attain tremendous speeds to reach them in a reasonable time. Ifspaceships were launched from space or from the moon, the absence of weightwould permit the ships to be launched with great speed at reduced pressures. Arelatively small explosion would be enough to send ship off at very fastrate.And, since there is no atmosphere in space as there is on earth, thespaceship would meet with no resistance.
the lack of something (Para. 4)
【正确答案】：absence