Shape and rotation of asteroids

(ORDO NEWS) — Asteroids are so small that the force of gravity on them is negligible. She is not able to give them the shape of a ball, which she gives to the planets and their large satellites, crushing and tamping their substance.

Fluidity plays an important role in this. High mountains on the Earth at the sole “spread”, as the strength of the rocks is insufficient to withstand loads of many tons per 1 cm 3 , and the stone, without crushing, without splitting, flows, albeit very slowly.

On asteroids with a diameter of up to 300-400 km, due to the low weight of rocks there, such a phenomenon of fluidity is completely absent, and on the largest asteroids it occurs extremely slowly, and even then only in their depths.

Therefore, only the deep interiors of a few large asteroids can be “rammed” by gravity. If the substance of asteroids did not go through the melting stage, then it should have remained “poorly packed”, approximately as it appeared at the stage of accumulation in a protoplanetary cloud.

Only collisions of bodies with each other could lead to the fact that the substance was gradually crushed, becoming less friable. However, new collisions should have crushed the compressed substance.

Low gravity allows broken asteroids to exist in the form of aggregates, consisting of separate blocks, held near each other by gravity, but not merging with each other. For the same reason, their satellites that have landed on the surface of asteroids do not merge with them.

The Moon and the Earth, having come into contact with each other, would have merged, as the adjoining drops merge (although for a different reason), and after a while one, also a spherical body, would have turned out, from the shape of which it would be impossible to guess what it came from.

However, all the planets of the solar system at the final stage of formation absorbed fairly large bodies that failed to turn into independent planets or satellites. Now there are no traces of them.

Only the largest asteroids can retain their spherical shape, acquired during the formation period, if they manage to avoid collision with a few bodies of comparable size. Collisions with smaller bodies will not be able to significantly change it.

Small asteroids, on the other hand, must and indeed do have an irregular shape, formed as a result of many collisions and not subjected to further alignment under the action of gravity. Craters formed on the surface of even the largest asteroids when they collide with small bodies do not “float” over time.

They are preserved until they are erased during the next impacts of small bodies on the asteroid, or are immediately destroyed by the impact of a large body. Therefore, mountains on asteroids can be much higher, and depressions much deeper than on Earth and other planets:

The irregular shape of asteroids is also confirmed by the fact that their brightness decreases unusually rapidly with increasing phase angle.

For the Moon and Mercury, a similar decrease in brightness is fully explained only by a decrease in the fraction of the surface illuminated by the Sun visible from the Earth: the shadows of mountains and depressions have little effect on the overall brightness.

The situation is different with asteroids. Such a rapid change in their brightness, which is observed, cannot be explained by a mere change in the part of the surface of the asteroid illuminated by the Sun.

The main reason (especially for small asteroids) of this nature of the change in brightness lies in their irregular shape and extreme degree of pitting, due to which, on the side illuminated by the Sun, some parts of the surface shield others from the sun’s rays.


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