It is incorrect to ask this question in the present tense. The present tense is used for routines, habits, and present facts. You are asking about something that occurred in the past, so you must use the past tense to ask the question. "Who was the first man in space?" This forum should be used for language questions, not history questions. Look it up.

Why don't you ask Google rather than posting your question here

I think that surely Sergey knows the answer to the queation, since it was his countryman, Yuri Gagardin, April 12, 1961 The question needs to be framed differently as others noted. Another way to say it in English, keeping the present tense for the person but correctly referring to a past event could be "Who is the man who was the first to fly in outer space."

I didn't see a question about English. And what's foolish about basic science?

Foolish answers don't help students.