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Most asteroids that go on a collision course to Earth disintegrate before reaching surface level, given that they are small enough that the surface 'shedding' is able to eliminate them in the atmosphere. Nevertheless meteors exist, which begs the question:

How big must an asteroid be to be able to reach the ground and not disintegrate in the atmosphere?

Note: Naturally this will vary with material composition, but just an estimate would be interesting.

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Surface shedding is called "ablation", and all asteroids are large enough to penetrate the Earth's atmosphere and reach the ground without ablating away.

The smallest asteroid is 2015 TC25, which is 2 meters. Anything smaller is an uncharted meteoroid. Any meteoroid bigger than a marble (1-2 cm) is large enough to become a meteorite (by striking the surface); however, I don't have a calculation for that estimate.

I recall reading anything larger than a bus could retain hypersonic velocity on impact (if it doesn't break up). Smaller meteors get on ballistic trajectories and fall at terminal velocity.

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    $\begingroup$ I upvoted your answer bc it's correct but just a point of clarification, don't larger objects break apart due to thermal gradients causing differential thermal expansion, which breaks them apart? Isn't that a bigger issue for incident objects than ablation, regarding whether they make it to the surface? When smaller, their surface area-to-volume ratio increases so ablation is more efficient. $\endgroup$ Aug 25, 2021 at 13:23
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    $\begingroup$ @honeste_vivere certain ablation doesn't take them apart. I had seen a RAND study on weaponizing asteroids, and they had detailed calculations. Heat sputters the surface, and doesn't heat the meteor. When they're coming in a 40 miles per second, they experience huge g-loads (100g-1000+g), and that leads to the meteor shattering. $\endgroup$
    – JEB
    Aug 26, 2021 at 19:54
  • $\begingroup$ Oh that's very interesting! I had always thought it was the heating of the object, since iron-based minerals have a relatively low specific heat. So the acceleration they experience due to air drag is more important than thermal expansion for fracture... that's really fascinating... $\endgroup$ Aug 26, 2021 at 20:19
  • $\begingroup$ @JEB Very interesting clarification, but it still leaves me with a question. Commenting on the original question, Keith McClary links to an article which affirms that atmospheric gasses which "inject" (I think it would be a proper term) into the asteroid through pores would be able to explode some of them. Would that effect be more of a determinant in the size of meteors than g-loads? $\endgroup$ Sep 3, 2021 at 18:00
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    $\begingroup$ @ElkinMontoya Could be. The RAND study was decades ago. The research you mention is from 2017: purdue.edu/newsroom/releases/2017/Q4/… $\endgroup$
    – JEB
    Sep 3, 2021 at 18:41
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The 2013 Chelyabinsk meteor is estimated to have been 20m in size. The object that caused the 2009 Sulawesi superbolide is estimated to have been 10m in size. Both of these objects disintegrated in the upper atmosphere.

The object that caused the Tunguska event may have been 50m in size, but although it also disintegrated in the atmosphere, this event is classified as an impact because of the relatively low height of the disintegration and the amount of damage at ground level.

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