As the solar wind expands outwards, it covers more space, which means it loses density. Although the interstellar medium is usually called a void, it is not completely devoid of matter. IIRC it's something like one atom per square meter.
And since it has mass, it has pressure. As the solar wind expands, it's density decreases. At about 80 AU, it's pressure drops below that of the void's, and thus, it gets "pushed" back towards the sun. We call this point the termination shock.
Logic, logic, logic. Logic is the beginning of wisdom, Valeris, not the end.
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