Complex sentence: Yuri Alekseyevich Gagarin (9 March 1934--27 March 1968), a Russian Soviet pilot and cosmonaut, became the first human to journey into outer space when his Vostok spacecraft completed an orbit of the Earth on 12 April 1961.
Split into simpler sentences: Yuri Alekseyevich Gagarin (9 March 1934--27 March 1968) was a Russian Soviet pilot and cosmonaut. He was the first human to journey into outer space, when his Vostok spacecraft completed an orbit of the Earth on 12 April 1961.