The astronaut, who is about to take off, is saying goodbye to his family and friends.

  We can say the astronaut is saying goodbye to his family and friends and there is no doubt who we are talking about because there is only one astronaut in the picture . But if we leave out a defining relative clause we do not know who or what we are talking about: The man who lives next door is thinking of moving to Spain. The man is thinking of moving to Spain.  (which one?)

Non-defining relatives clauses:

  are separated from their antecedent by commas: John does not like Peter, who has been his boss for 5 years.

  always need a relative pronoun. The relative pronoun is never omitted: Robert loves Mary, who he has gone out with for 3 months.
 
always need who to refer to people and which to refer to things. "That" cannot be used instead of "who" or "which": Every year millions of people visit the Eiffel Tower, which was built at the end of the 19th century.

  The relative pronoun, like other pronouns, can be the subject, object of a verb or the complement of a preposition:
 
        John does not like Peter, who has been his boss for 5 years. (who is the subject of has been)

        I have not met Sharon, who Paul loves so much. (who is the object of loves)

        Robert loves Mary, with whom he has gone out for 3 months.