no way of guessing; the very nearest of them is a million times further away than Mars or Venus, those still remote goals of the next generation. But the barriers of distance are crumbling - one day we shall meet our equals, or our masters, among the stars.
  Men have been slow to face this prospect. Increasing numbers, however, are asking: "Why have such meetings not occurred already, since we ourselves are about to venture into space?"
why not indeed? 2001: A SPACE ODYSSEY offers one possible answer to this very reasonable question. But please remember: this is only a work of fiction.
the truth, as always, will be far stranger.