- Radar is the use of radio waves reflected from distant objects to yield information about them. For astronomers, it has the disadvantage that radar methods can only be used on object near enough for a usable return to be recorded. In the days before space probes were available, radar was useful as a means of discovering details of the surface topography of the Moon and the terrestrial planets. Radar signals are also reflected by meteors in the upper atmosphere, since the flash of light seen by optical astronomers is accompanied by a streak of ionised air which returns radar waves. The use of this method allowed daylight meteor showers invisible to optical astronomy to be discovered, revealing meteoroids coming towards the Earth from the direction of the Sun, in contrast to the nighttime showers whose members approach us moving towards the Sun. The Magellan spacecraft, used to map Venus in radar wavelengths, was spectacularly successful. Radars are also used on earth-orbiting satellites to observe the surface of the Earth, since they can ‘see’ at night and through cloud.
- The use of radar to study astronomical phenomena, such as the location and movements of celestial bodies.