From Newsgroup: alt.astronomy

Never !

Interstellar distances are so great, as to preclude any chance of
travel or practical comunication.

Do the simple math : Time = Distance / Speed

Even at speeds approaching c , which is highly unlikely in any case,
times for even the shortest trips would be prohibitive.

This is why, although intelligent space faring species likely abound
throughout the universe, we are never visited by any.
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