Just plain ol’ gravity.
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The iron chicken will get its revenge.
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How much mass did we hit it with? What’s the mass of asteroid? is this like a bug hitting my windshield on the freeway?
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Yeah but it still seems remarkable such a small object can keep it all together like that. Gravity is both weak in one respect, and strong in others. It is just hard to wrap your head around.
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It looks like a giant flying turd. Do we really want to punch a flying turd and have that shit splatter all over up?
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Per this paper there are other forces at work as well:
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Didymos’ mass is about 5,000,000,000 kg. DART’s mass was 500 kg at the time of impact.
By contrast a smaller car’s mass may be 1,500,000g, and the largest dragonfly’s mass is about 3 g. But if the dragonfly was made of steel, and was traveling at a hypersonic speed of 14,000 MPH, it’s going to transfer a lot more energy to your car than a typical bug on the freeway.
The prediction is that the impact slowed Didymos by about 2cm/second. That’s not much, of course, but it should be enough to measure. But measuring something that small at that distance is difficult.
The reason Didymos was chosen for the experiment is that it’s in a binary pair with another asteroid, giving them a frame of reference. By changing one body’s motion, they hope to see that perturb the path of the pair, which they can more easily measure with precision.
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Go do a google search for “NASA DART” on google’s main search page right now for a fun gag.
An animated impact knocks the whole search page off-kilter)
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American Football Stadium = The Big House in Ann Arbor, MI. Go Blue!
Grains of sand will hold each other together by gravity alone in space. When there’s no air, there’s nothing besides gravity to act on them. Force is force, no matter how weak, and over time all things in space draw toward each other. Asteroids that are a solid chunk of rock like in movies and games are the exception. Most are so-called “rubble piles”– clumps of small rocks floating along together.
To the larger story, I want to say this is amazing. We could be the first generation in human history to be able to prevent the most likely type of extinction event*. We’ve been rolling the dice for millions of years, but now maybe we have some say in it!
*climate change notwithstanding 
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Per the NASA article I posted upthread, gravity alone is not enough to hold together small rubble pile asteroids, at least the ones that are rotating. The centripetal forces from the rotation would be more than enough to counteract the weak gravity from that much mass. So scientists believe that
Van der Waals forces and/or Electrostatic forces are a big part of what holds the smaller rubble pile asteroids together.
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Slow asphyxiation vs blunt force trauma?
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wheatpaste also works surprisingly well…
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