Complex sentence: Mars to Stay missions propose astronauts sent to Mars for the first time should intend to stay, with their unused emergency return vehicles recycled into settlement construction as soon as the habitability of Mars becomes evident to the initial pioneers.
Split into simpler sentences: Mars to Stay missions propose astronauts sent to Mars for the first time should intend to stay, possibly with emergency return vehicles. Unused emergency return vehicles would be recycled into settlement construction as soon as the habitability of Mars becomes evident to the initial pioneers.