If an asteroid were headed for Earth, there is nothing we could do
The old claim turned out to be wrong and was rejected by evidence.
TaughtThe dinosaurs had no defense against the asteroid that killed them, and neither would we. If a large one were found on a collision course, our only real options would be to evacuate and hope. Actually deflecting one is science fiction.
NowOn 26 September 2022 NASA's DART spacecraft deliberately crashed into the asteroid moonlet Dimorphos and measurably changed its orbit, shortening it by about 32 minutes. It was the first time humanity has altered the motion of a celestial body on purpose.
What actually happened
The technique is a billiard shot across eleven million kilometers. DART was a half-tonne spacecraft carrying no explosive; it simply hit Dimorphos head-on at about 6.1 kilometers per second. Telescopes on Earth then timed how long the little moon took to pass in front of and behind its larger partner Didymos, and clocked the change in its orbit.
The result beat expectations because of ejecta. The impact blasted thousands of tonnes of rock off the surface, and the recoil from that debris plume shoved far harder than the spacecraft alone could. The orbital period shortened by roughly 32 minutes, against a threshold for success of just 73 seconds.
This does not mean we can now stop anything. Dimorphos is small, it was never actually a threat, and the method needs years of warning to nudge a rock off course in time. What changed is the category the whole idea lived in: planetary defense moved from a thought experiment to something with one confirmed data point, the first time a species rearranged its own solar system on purpose.
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Who was taught this
Still standard through 2022, so anyone who finished school between 1950 and 2022 learned the earlier version.