In World War II, both sides detected enemy aircraft and ships by bouncing microwaves off their surfaces. This was the first application of radar, a technique that has become one of the most powerful tools for studying other planets. Planetary scientists didn't use the method until the 1960s, when they realized they could use it to study very distant bodies like planets and asteroids. Using large radio telescopes like Arecibo in Puerto Rico, scientists transmit a radar signal, and then detect the echo that bounces off the object. Radar, which stands for Ra(dio) D(etection) a(nd) R(anging), uses electromagnetic waves with wavelengths ranging from one centimeter to one meter (or, equivalently, frequencies ranging from 50 megahertz to several hundred gigahertz). Because these wavelengths are so much longer than those of visible light, radar has the ability to "see" through clouds or dust layers that make visual observations impossible.
The most straightforward application of radar is ranging: measuring the time it takes a signal to travel to another object and return. Using the speed of light, you can then measure how far away the object is (its "range"). Radar was used in this way to refine the exact distance from the Earth to the Sun, which defines the Astronomical Unit. Measuring the elevation of a planet's surface is a similar application. This is how the Magellan spacecraft produced topographical maps of Venus.
By bouncing a radar signal off a rotating object, its rotation period can be measured. Imagine a spinning asteroid, for example. One side would be moving towards you, and one side would be moving away. The radar echoes from the two sides will be Doppler-shifted in opposite directions, so the wavelength of the echo from the approaching side will be shorter, and the wavelength of the echo from the receding side will be longer. By looking at how the wavelengths of the echo are spread out, you can measure how fast the asteroid is rotating. Scientists have used this method to measure rotation rates of many asteroids near the Earth and in the main belt.
The surface properties of an object determine what fraction of a radar signal is reflected back. For example, if an area is very rough, the radar signal will bounce off multiple surfaces and be scattered in different directions before it's finally reflected back to the observer. This can be detected in the echo. In a radar image, a rough surface will show up as a bright area. The composition of the surface material can also affect the strength of the radar echo. This makes interpreting radar images tricky. They're not like photographic images you're accustomed to looking at — a bright patch on a radar image doesn't mean the area is visually bright, or light-colored. Instead, it usually represents a rough area, although radar brightness can also be due to chemical or physical differences in the surface materials.
Radar is the only way we can "see" through the thick atmosphere of our neighbor Venus. The NASA space probe Magellan used radar to produce a detailed topographical map of the planet. In addition to measuring surface elevation, radar images also showed craters, mountains, lava flows, and tectonic features. Radar-bright material was discovered surrounding impact craters and on the tops of high mountains. The crater ejecta is probably bright because it's rough, while the mountains are thought to be covered in material that reflects radar differently due to its composition.
When radar signals were trained on Mercury, scientists got a surprise from the planet closest to the Sun: radar echoes from the polar regions were characteristic of ice! Subsequent radar mapping found ice deposits in permanently shadowed craters at the poles. Radar imaging has been used on many other planetary targets as well: comets, Saturn's rings, and the Galilean satellites of Jupiter, to name a few. Shape models of asteroids from radar data have revealed unusual shapes, and even binary objects. Satellites also use radar on the surface of the Earth itself, to study its weather, oceans, and agriculture.