Complex sentence: Two orbiters, Mars 4 and Mars 5 were launched earlier in the 1973 Mars launch window, and were expected to relay data for the two landers, however Mars 4 failed to enter orbit, and Mars 5 failed after a few days in orbit.
Split into simpler sentences: Two orbiters, Mars 4 and Mars 5 were launched earlier in the 1973 Mars launch window, and were expected to relay data for the two landers. However, Mars 4 failed to enter orbit, and Mars 5 failed after a few days in orbit.