The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence.

It takes time for them to prepare for the great changes that occur in space. When the spaceship leaves the earth at tremendous speed, the astronauts feel as if they are being crushed against the space-ship floor. Later, when they leave the zone of the earth's gravitation, they are unable to stay in one place.
Problems for astronauts to （） when traveling to space.