I've been reading the book Planetary Geology. In chapter 7, section 7.6, the author states that

The angle of incidence at which asteroids or comets encounter the surface of a planetary body follows a Gaussian probability distribution with vertical and horizontal collisions being extremely unlikely and the highest probability existing for 45°

Why is this the case? Why doesn't the angle of incidence for meteoroids follow a uniform law? Isn't there just as much chance for an asteroid or a comet to arrive from any direction?


1 Answer 1


Horizontal atmospheric entries are unlikely because the object would have to almost miss the Earth and brush against it rather than impact it (and probably would disintegrate in the atmosphere if it's a small one, due to the long flight through it in such case).

Vertical entries are unlikely for the opposite reason: the object would have to fly at precisely 90 degrees relative to the Earth, meaning the Earth is in the very center of the object's orbital path.

But if an asteroid or comet comes at another angle its path may be changed by the Earth's gravity towards about 45 degrees. This is an approximation, it is about 40-50 degrees, but 45 degrees is the most likely one out of specific angles.

  • $\begingroup$ So if an meteor fell on a hypothetical body with no mass, its incidence angle would follow a uniform law. But since the gravity of the impacted body changes the trajectory of the meteoroid, the incidence angle distribution is changed too. $\endgroup$
    – usernumber
    Jul 20, 2020 at 8:42
  • 1
    $\begingroup$ @usernumber Right, gravity changes a meteoroid's path so that it goes towards the planet's center. Therefore, a horizontal entry is the least likely. For that, it would have to be a very fast comet. $\endgroup$
    – Ioannes
    Jul 20, 2020 at 9:14

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