Until recently the entire astronomical endeavor was restricted to passive observations from the surface of the Earth. Natural experiments — such as stellar occultations, eclipsing binaries, or supernova explosions — might fortuitously arrange themselves, and systematic inventories were performed of information carried by reflected or emitted electromagnetic radiation. But it is only in the last twenty years that our species has been able to carry out in situ experiments on astronomical objects.
Radar astronomy permits a kind of experimentation on solar system objects not too distant from Earth, and has made a number of major discoveries, including the determination of the 3:2 spin/orbit coupling of Mercury, the Earth-locked retrograde rotation of Venus, and major elevation differences on Mars. Radar is the tool of choice for geomorphological mapping of cloud-enveloped Venus, and efforts now underway at the Arecibo Observatory should lead to complete mapping of one hemisphere of Venus down to 10 km resolution. The radar reflectivity of the rings of Saturn has forced a rethinking of the nature of the constituent particles in the rings; and a determination of the radar cross-section of Titan is an experiment which can resolve much of the uncertainty about the atmospheric structure and surface pressure for that enigmatic and fascinating object.