Complex sentence: Rendezvous was first successfully accomplished by US astronaut Wally Schirra on December 15,1965, who maneuvered the Gemini 6A spacecraft within of its sister craft Gemini 7.
Split into simpler sentences: Rendezvous was first successfully accomplished by US astronaut Wally Schirra on December 15,1965. Schirra maneuvered the Gemini 6A spacecraft within of its sister craft Gemini 7.