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In the first few seconds of video taken at the Arecibo radio telescope on December 1, 2020, everything looks normal. Sure, support cables had broken the previous August and November, damaging the 300-meter-wide dish. And sure, the National Science Foundation was already planning to decommission Arecibo, an instrument that began scanning the sky in 1963. So things weren't great for the telescope. But it was at least still there.

That changed just before 8 A.M. when, as if on command, a bit of dust puffed out from a support pillar. That was, it turns out, a cable beginning to snap off. Left with extra load, other cables began to break, too. Soon the massive equipment platform, once suspended over the bowl-shaped observatory, began to tip. After an agonizing swing downward, the platform crashed. More cables snapped, and debris flew around like in a demolition. At the end of the footage, giant holes were visible in the iconic telescope, and dust rose all around. Arecibo, at least as scientists knew it, was gone.

When Edgard Rivera-Valentín, a staff scientist at the Lunar and Planetary Institute and formerly part of the planetary radar group at Arecibo, clicked on the video, they could stomach only a few seconds. It took them days to get through the full two minutes. “When everything went down, it was—I use the word ‘tragedy,'” says Rivera-Valentín, a native of Puerto Rico.

Arecibo had a long and storied legacy of scientific discovery, studying space weather, searching for extraterrestrials, timing pulsars, mapping neutral hydrogen gas. But it also had an unconventional claim to fame: It boasted the world's most powerful, sensitive and active planetary radar system. That radar could peer through Venus's thick atmosphere and map the dusty Martian surface, but it also helped protect Earth from asteroids. The data showed scientists those rocks in detail, revealed whether they might present a threat, and helped humans figure out what they could reasonably do if an asteroid was heading our way. “One of the neat things about doing radar is that you're actively defending the entire Earth,” Rivera-Valentín says. “So if anyone asks you, ‘Why should I care?,' it's like, ‘I'm going to make sure that asteroid doesn't come for you.'”

Arecibo's radar efforts fell under the umbrella of “planetary defense”: the attempt to identify and prevent potential collisions between asteroids (and comets) and this planet, which, ideally, we would like to keep intact.

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