Active Sensing

Cassini's radio-science experiment, which used the spacecraft's radio link — tracking tiny frequency shifts and radio occultations — to weigh Saturn, map its gravity field, and probe the structure of its rings and atmosphere.

An active instrument that fires laser pulses and times their return to measure distance precisely — mapping the topography of a planet, moon, or asteroid with height accuracy of metres.

Magellan's imaging radar, which pierced Venus's thick clouds to map 98% of the planet's surface at high resolution.

MESSENGER's laser altimeter, which measured the topography of Mercury's northern hemisphere and helped confirm water ice in its permanently shadowed polar craters.

An active instrument that transmits radio waves and analyses the echoes to map surfaces through cloud and darkness, or to probe beneath the surface. Synthetic-aperture radar (SAR) achieves high resolution from orbit.

A technique that uses the spacecraft's own radio signal as an instrument — tracking tiny frequency shifts to weigh a planet, map its gravity field, and probe its atmosphere and rings by how they bend the signal.

InSight's ultra-sensitive seismometer, which detected hundreds of marsquakes and, for the first time, revealed the internal structure of Mars.

An instrument that senses the tiny ground motions of quakes and impacts, probing the interior structure of a world by how seismic waves travel through it — as InSight did for Mars.