Asteroid shape and rotation
Asteroids are so small that gravity is negligible. She is not able to give them the shape of a ball, which gives the planets and their large companions, crushing and ramming their substance. An important role is played by the phenomenon of fluidity. The high mountains on Earth at the bottom “sprawl”, because the strength of the rocks is insufficient to withstand loads of many tons per 1 cm3, and the stone, without crushing, without breaking, flows, although very slowly.
On asteroids with a diameter of up to 300-400 km, due to their low weight, such a phenomenon of fluidity is completely absent, while on the largest asteroids it occurs extremely slowly, and even then only in their bowels. Therefore, only the deep bowels of a few large asteroids can be “rammed” by gravity. If the material of the asteroids did not go through the melting stage, then it should have remained “poorly packed”, approximately the same as it had arisen at the stage of accumulation in the protoplanetary cloud. Only collisions of bodies with each other could lead to the fact that the substance gradually crumpled, becoming less loose. However, new collisions were supposed to crush the compressed substance.
Low gravity allows broken asteroids to exist in the form of aggregates consisting of separate blocks held together by gravity, but not merging with each other. For the same reason, their companions, which have descended to the surface of asteroids, do not merge with them. The Moon and the Earth, having touched each other, would merge as the droplets in contact merged (albeit for a different reason), and after a while one would produce the same spherical body, in 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 rather large bodies that failed to turn into independent planets or satellites. Now their tracks are gone.
Only the largest asteroids can maintain their spherical shape, acquired during the formation period, if they manage to avoid a collision with a few bodies of comparable size. Collisions with smaller bodies will not be able to significantly change it. Small asteroids, however, must have and indeed have an irregular shape, formed as a result of many collisions and not subjected to further alignment under the influence of gravity. Craters that emerged on the surface of even the largest asteroids in a collision with small bodies do not “swim” over time. They remain until they are erased during the next impact on the asteroid of small bodies or immediately destroyed by the blow of a large body. Therefore, mountains on asteroids can be much higher, and troughs much deeper than on Earth and other planets: the average deviation from the level of the smoothed surface on large astroids is 10 km or more, as evidenced by radar observations of asteroids.
The irregular shape of asteroids is also confirmed by the fact that their brightness unusually rapidly decreases with increasing phase angle. For the Moon and Mercury, a similar decrease in brightness is entirely explained only by a decrease in the fraction of the surface illuminated by the Sun visible from the Earth: the shadows of mountains and troughs have a weak effect on the overall brightness. The situation is different with asteroids. The mere change in the sunlit fraction of the surface of the asteroid does not explain such a rapid change in their brightness that is observed. The main reason (especially for small asteroids) of this nature of the brightness change is their irregular shape and extreme degree of “outburst”, which is why on the side illuminated by the Sun some parts of the surface shield others from sunlight.
Planetary disturbances lead to continuous mixing of the orbits of the asteroids, and, consequently, to the mixing of objects moving along them. This makes asteroids colliding with each other. Over the past 4.5 billion years, since the existence of asteroids, they have experienced many collisions with each other. The tilts and eccentricities of the orbits lead to the non-parallelism of their mutual motions, and the speed with which asteroids sweep past one another is on average about 5 km / s. Collisions with such speeds lead to the destruction of bodies.