Venus through the clouds
Venus is wrapped in sulphuric acid clouds opaque to every visible-light telescope. In the 1960s and 70s, Arecibo and Goldstone pinged it with S-band radar and mapped its surface for the first time. Magellan finished the job from orbit in 1990, but radar got there first.
Asteroid radar imaging
When a near-Earth asteroid passes within a few million km, Goldstone or (until 2020) Arecibo could resolve it down to metres. Shape, spin rate, surface roughness, moons — all recovered from delay-Doppler images. Hundreds of asteroids have been characterised this way.
Planetary defence
Radar imaging is how we know whether an incoming asteroid is solid rock, rubble pile, or contact binary. Defenders need that data to decide between a kinetic impactor (DART), a nuclear option, or a gravity tractor. Radar provides it years in advance.
After Arecibo
Arecibo's collapse in December 2020 cut radar astronomy roughly in half. The Goldstone Solar System Radar continues. Plans for a new dedicated radar at Green Bank (the ngRADAR concept) would partially replace Arecibo's capability.