The Earth is the cradle of Humanity. But one doesn't always live in the cradle.

Consider a cask filled with a highly compressed gas. If we open one of its taps the gas will escape through it in a continuous flow, the elasticity of the gas pushing its particles into space will continuously push the cask itself. The result will a continuous change in the motion of the cask. Given a sufficient number of taps (say, six), we would be able to regulate the outflow of the gas as we liked and the cask (or sphere) would describe any curved line in accordance with any law of velocities.

A planet is the cradle of mind, but one cannot live in a cradle forever.

The Earth is the cradle of Humanity. But one doesn't always live in the cradle.

Man must at all costs overcome the Earth's gravity and have, in reserve, the space at least of the Solar System. All kinds of danger wait for him on the Earth. . . . We have said a great deal about the advantages of migration into space, but not all can be said or even imagined.

Man will not always stay on Earth; the pursuit of light and space will lead him to penetrate the bounds of the atmosphere, timidly at first, but in the end to conquer the whole of solar space.

Man must at all costs overcome the Earth's gravity and have, in reserve, the space at least of the Solar System.