NASA’s unprecedented asteroid-deflection mission is more than ‘billiards in space,’ scientists say

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https://www.science.org/content/article/nasa-s-unprecedented-asteroid-deflection-mission-more-billiards-space-scientists-say

By Zack Savitsky, Science Magazine. 

Excerpt: On 26 September, an act of targeted violence will unfold 11 million kilometers from Earth, as a spacecraft about the size of a vending machine smashes into a small asteroid at 6 kilometers per second. Unlike some asteroids that stray worrisomely close to Earth’s orbit, Dimorphos—the 160-meter moon of a larger body—is an innocent bystander, posing no threat to our world. But the looming assault represents humanity’s first-ever field test of a planetary defense mission: NASA’s Double Asteroid Redirection Test, or DART. The hope is that the collision will nudge Dimorphos into a closer orbit around its 780-meter partner, Didymos, shortening its nearly 12-hour orbital period by a few minutes. A successful strike would support the idea that, in the future, similar efforts could deflect threatening asteroids onto safer courses. But new simulations and lab experiments show the fate of the mission depends heavily on a crucial question: Are such small asteroids solid boulders or—as astronomers increasingly believe—loose heaps of rubble? The answer, which should be revealed from the crater and ejecta produced by DART’s collision, could determine just how hard to hit an asteroid when the exercise is not a test. ...Dimorphos-size asteroids are thousands of times more likely to strike Earth than the larger ones that have triggered mass extinction events in the geologic past, and they are still capable of devastating a state or small country, making these smaller bodies the top priority for planetary defense efforts.…

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