Volatiles on Earth may partly originate in comets. Accretion in binary star systems. X-ray binaries, cataclysmic variables. Accretion onto compact objects are an extremely efficient energy source.
Accretion often heats gas to X-ray emitting temperatures.
In a pure gravitational 2-body system, the smaller body can never enter or leave the orbit. The probability of collision is rather low. A schematic diagram of a quasi-radial accretion onto a white dwarf. Accreting matter initially free-falls towards the stellar surface, whose velocity is highly supersonic . This result in the formation of a strong shock just above the surface.