# Spacecraft and Earth's gravity

Spacecraft must be accelerated by a rocket engine to overcome the Earth's gravity. Is it possible to use for this not an engine, but... the very attraction of the Earth? It turns out that in one case such an option is possible.

The thing is that in calculating the motion of a spacecraft, it is usually taken for a material point: the dimensions of a ship are negligible in comparison with the dimensions of celestial bodies. But the spaceship is not a point, but an extended body that has certain dimensions and shape. Therefore, the attraction of the Earth acting on it is somewhat different from the one that would act if the entire mass of the ship was concentrated at one point. This difference is quite noticeable if the spacecraft has a considerable length. But for ordinary ships and satellites it is small, and it can be ignored.

For example, if a spacecraft consists of two balls that can be pulled to each other and spread over long distances connected by a rod perpendicular to the continuation of the Earth's radius, then an attractive force acting at an angle to the connecting rod acts on each of the balls. The power of these forces is somewhat less than the gravitational force that would act on the center of the rod if the entire mass of the spacecraft were concentrated in it.

It turns out, "stretching" such a spaceship, there is some repulsive radial force and its movement around the Earth occurs in an orbit, somewhat different from the "Keplerian".

By connecting the balls at the farthest point of the orbit - the apogee, the spacecraft will turn almost into a material point, and its motion will occur already along the Keplerian orbit. If the perigee to divide the balls into the same distance, then the mentioned "repulsive force" will appear. In this case, the orbit of further movement will be more elongated and on the second turn the apogee distance will be somewhat larger than at the first. Thus, it is possible to force the spacecraft to move along an untwisted spiral until it overcomes the attraction of the Earth.

How long does it take to overclock this method? If a spacecraft with a length of 140 km begins to move at a distance of 2 thousand km from the center of the Earth, then the dispersal will take about two years. 80 years will be needed to overcome the attraction of the Sun at an initial distance of about 700 thousand kilometers from it.

Scientists have calculated that the more the mass of the celestial body and the closer to it the spacecraft, the faster it is possible to overcome the attraction with this method. For example, being twenty thousand kilometers away from the center of the famous superdense star - the white dwarf Sirius B, the spacecraft could have gone on an untwisted spiral into space in just an hour and a half.

Is it possible to create such a pulsating spacecraft? Probably, this is the business of the technology of the future. In any case, there is a theoretical possibility.