Complex sentence: Due to space being a vacuum, the suits are required to have oxygen, which is stored in tanks allowing astronauts to work or remain outside for 6 to 8.5 hours before having to return to the spacecraft.
Split into simpler sentences: Due to space being a vacuum the space suits are required to have oxygen. The suits store the oxygen within tanks that allows the astronauts to undergo work and spacewalks for 6 to 8.5 hours before having to return to the spacecraft.