Travelling in space
Travelling in space
German astronomer Johannes Kepler first described mathematically the way celestial objects move through space in the early 1600s. His three laws of planetary motion have become the cornerstones of our modern understanding of spacecraft orbits because they apply equally to any object, whether it is a planet or a spaceprobe, moving through space.
All celestial objects follow orbits. Chemical rockets burn their fuel rapidly, then, the forward momentum of the spacecraft and the gravity of the Sun (or other nearby planet or moon) bend the spacecraft's path, creating the orbit. If the spacecraft has to change orbit, it has to burn more fuel.
To travel to another planet usually requires three orbital changes. The first rocket lifts the spacecraft out of the Earth's gravitational field, putting it into orbit around our planet. Another rocket, called the upper stage, then places the spacecraft on an orbit around the Sun. When the spacecraft approaches its target planet, it must then fire a third rocket so that it enters orbit around its destination.
Ion engines do not work in this way. They supply a lower instantaneous drive than chemical rockets but can fire for much longer. Instead of sudden bursts to change orbit, an ion engine runs most of the time, constantly changing orbit, gradually propelling the rocket to its destination.
Ion engines are not the only exotic technology under investigation at ESA. Together with the German Space Agency (DLR), ESA plans to test-launch a solar sail. This device fits into a container the size of a suitcase but, once in space, unfurls to become thin, reflective sails as large as a soccer field. The pressure of sunlight on the sails propels the craft through space.