Classical three body problem has not been solved exactly. Still we don’t know how long it will take to sense any distant gravitational perturbation. It is simpler to think with classical three body compared to that of Helium atom. Existing (classical) algorithms could be reference for any new (quantum) to compare with …..
….. however, due to your counter question I have realized that any classical problem which are not tractable with classical algorithm also doesn’t have any quantum solution. Also, quantum algorithms are to solve only quantum problems. Thank you.