Читать книгу Foundations of Space Dynamics - Ashish Tewari - Страница 15

Space flight is primarily governed by gravity. “Governing” implies dictating the path a given body describes in a three‐dimensional space. Aircraft and rocket flights are not primarily governed by gravity, because there are other forces acting on the body, such as the lift and the thrust, which are of comparable magnitudes to that of gravity and therefore determine the flight path. Discovered and properly analyzed for the first time by Newton in the late century, gravity can be expressed simply, but has profound consequences. For example, by applying Newton's law of gravitation, it could have been inferred that the universe cannot be static, because gravity would cause all the objects to collapse towards a single point. However, this simple fact escaped the notice of all physicists ranging from Newton himself to Einstein, until it was observed by Hubble in 1924 that the universe is expanding at a rate which increases with the distance between any two objects. A reader may be cautioned against the complacency which often arises by treating the motion governed by gravity as simple (even trivial) to understand. There are many surprising and interesting consequences of gravity being the governing force in flight, such as Kepler's third law of planetary motion, which implies that the time period of an orbiting body depends only upon the mean radius, and is independent of the shape of the orbit. A larger part of a course on space dynamics involves understanding gravity and its effects on the motion of a body in space.