The DART hit could be the beginning of a programme to test ways for deflecting asteroids.
When NASA's Double Asteroid Redirection Test (DART) collides with the tiny asteroid Dimorphos, it causes a chain reaction.
It will be our first attempt to show that we can deflect dangerous oncoming asteroids.
For decades, experts all across the world have been studying the skies for potentially catastrophic asteroids near Earth.
And, as astronomers discover more and more near-Earth asteroids,
The focus is now shifting to how we may defend Earth if an asteroid on a collision course is identified.
Dimorphos is part of a binary system with another asteroid, Didymos, which is 2,600 feet (780 metres) broad.
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