Why are comets sometimes called “visible nothing”? How was it proven that comets do not have any significant solid core?

Comets do not cause any disturbances in the motions of the planets near which they pass, but themselves, on the contrary, are subjected to strong disturbances from their side. It is assumed that their total mass is equal to a thousandth of the mass of the Earth. When comets pass in close proximity to the Sun (as if along the solar disk), the comets completely merge with the general solar background, and no dark spots have ever been noticed against this background. In the pre-space era, this observable fact was interpreted by the fact that comet nuclei are so small that they cannot be seen even with optical instruments. Direct measurements of the diameters of cometary nuclei, which were carried out from various spacecraft, showed that they are less than 20 km. The existence of comets with large sizes is not excluded.

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