Complex sentence: Its main objective was to investigate the atmosphere on the station to see if it had become toxic and had an effect on the crew of the Soyuz 21 contributing to the problems they had at the end of their flight.
Split into simpler sentences: Its main objective was to investigate the air supply inside the station to see if it had become toxic. This may have had an effect on the health of the Soyuz 21 crew, contributing to the early termination of their flight.