Space Travel

Motion in Space

Although at extremely high speeds the theory of relativity is invoked, the basic motions of space travel are governed by Newton's laws of the motion of bodies and Kepler's laws of planetary motion. Newton and Kepler showed that orbital motions are predictable both backward and forward through time. Through careful recording of where objects in space have been, it is possible to plan the optimum moment for a future launch of a space vehicle.

Space travel is possible because of the “clockwork” nature of the universe in classical physics. Everything in the universe interacts primarily via gravity. All destinations of any significant size are traveling in specific regular orbits. These orbits behave for the traveler as asynchronous clocks. Each object revolves in a regular orbit with a regular period, but every object has its own period. Navigation in space is therefore concerned with finding the optimal time to travel from one orbit to another.

Orbital mechanics dictates that there are an infinite number of possible free-flight paths between [Page 1185]two objects. Space navigators are concerned with the trade-off between the path that requires the least fuel and time spent in transit. Operating within these limitations has meant that, from the beginning, space travel has been associated with certain time-specific terms such as countdown, launch window, and mission elapsed time

A practical example of the timing needed would be predicting the optimum time to leave Earth for the Moon. This is referred to as a launch window or orbital window. The Moon revolves around the Earth in an elliptical orbit, so there is an optimal time of the month to launch when the Moon's orbit brings it closest to Earth. The calculation also should consider that the Earth rotates on its axis. Every 24 hours there is an optimum time for the rocket to leave Earth pointing at the moon. The interaction of two cyclic clocks gives a best time and day every month to launch. The planner also needs to account for the motion of the Moon during the rocket's time of transit, aiming for where the Moon will be when the rocket reaches lunar orbit path.

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