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And it makes it amazing that we actually did use Earth-based radar to study the planet Mercury. Before the Messenger spacecraft, some of our best science about Mercury came from Arecibo radar observations.

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For an interesting outline of this, see:

http://www.naic.edu/~pradar/radarpage.html

Title: "Solar System Studies at Arecibo Observatory"

Subtitle: "Introduction to Planetary Radar"

Arecibo is a radio telescope, not radar. It receives radio waves transmitted by objects in space, but doesn't send and bounce them off objects.

Arecibo is and has been used for radar astronomy as well https://en.m.wikipedia.org/wiki/Radar_astronomy

Whoa, I had no idea - thanks! Should have mentioned that IANAAstronomer.

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