A space probe is an unmanned space mission, usually a small spacecraft sent out to find out information about a planet or other far away thing. As a tool of space exploration a planetary probe may land on the planet, or orbit around it, or merely fly by closely. Probes use radio waves to send information back to Earth or sometimes to a manned spacecraft. A space observatory leaves the Earth to look at distant objects, but does not go near the objects, so it is not called a "probe".