Radar astronomy

Radar astronomy transmits radio waves toward celestial objects and analyzes their echoes to measure distance, motion, surface properties, and shape.

Radar equation: An equation relating transmitted power, target reflectivity, distance, and received radar power. It connects echo strength to target properties and distance.
Lunar ranging: Measurement of the Earth–Moon distance using laser pulses reflected from lunar retroreflectors. It provides a notable ranging counterpart, though it uses laser rather than radar echoes.
Radio astronomy: The study of celestial objects through their naturally emitted or reflected radio waves. Radar astronomy is a specialized branch that actively transmits signals rather than only receiving emissions.
Optical astronomy: The study of celestial objects using visible and nearby wavelengths of light. Optical observations rely on light arriving from a target, not a transmitted radar echo.
Radar cross-section: A measure of how strongly an object scatters radar energy back toward a transmitter. Echo strength depends on the target’s effective radar reflectivity.
Planetary radar: Radar observations of planets and small bodies within the solar system. This is the main observational domain of radar astronomy.
Project Diana: A 1946 United States experiment that bounced radar signals off the Moon. It demonstrated that radar echoes could be detected from a celestial body.
Passive radar: Radar sensing that uses signals transmitted by other sources rather than operating its own transmitter. Radar astronomy generally transmits a signal toward the celestial target.
Radar Doppler effect: The frequency shift in a radar echo caused by relative motion between transmitter and target. Echo frequency shifts reveal rotation and motion along the line of sight.
Asteroid radar astronomy: Radar observation of asteroids to determine their distances, rotation, shapes, and surface features. Echo imaging can reveal asteroid shapes and refine their orbits.

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