Complex sentence: Soyuz 11 was the first successful visit to the world's first space station, Salyut 1, but ended in disaster when an air leak killed the three-man crew during preparations for re-entry.
Split into simpler sentences: Soyuz 11 was the first successful visit to the world's first space station, Salyut 1. However the mission ended in disaster when the crew capsule depressurised, killing the three-man crew during preparations for re-entry.