|MadSci Network: Astronomy|
Hi Chris! An antenna has to be carefully tuned in order for it to resonate with a transmitted signal. This resonance manifests itself as a very small varying voltage across the antenna, and has to be carefully amplified to enormous levels - many millions of times and more - to keep random noise as low as possible, and to enable the signal to be of any practical use. Whilst it is certainly possible to alter another planet's electromagnetic field (the Sun has an effect on the Earth's field, and Jupiter has an effect on Io's field, for example) it would be impossible at current radio power levels to alter a distant body's electromagnetic field to any useful degree. Additionally, there would be no distant "on-site" control and amplification of the received signal at the body, thus ruling it out as a method of repeating and thereby increasing the range of sensors. That said, astronomy almost solely relies upon the passive detection of electromagnetic fields of distant bodies in order to study them: whether visible light, radio waves, x-rays, etc. Only in a few cases are active sensors employed: these include radar and lidar (radio waves and light waves, respectively) to directly probe the distances to, and surfaces features of, distant bodies. Andy Goddard
Try the links in the MadSci Library for more information on Astronomy.