The concept of large space habitats hosting hundreds or thousands of inhabitants has been independently explored by many spaceflight theorists in the 20th century, from Konstantin Tsiolkovsky onward. Spaceflight experts, however, have called into question the efficiency and safety of such designs and the need for such large populations in space. On the other hand, advanced life-support systems are being developed for very long manned space missions both on space stations and on interplanetary expeditions. Their aim is to increase the recycling of consumable supplies. Recovering water from expired air is relatively easy, for example, whereas distilling urine in space has presented serious engineering problems. Carbon dioxide can be recovered from cabin air and used in producing other needed supplies. Some food plants can be grown from human waste, but the weight of the required equipment is so high that this technique would become practical only on missions lasting several years. In general, research suggests that the best recycling efficiency that can be hoped for lies between 70 and 80%. Higher levels would require overly complex systems.
Returning to the Moon on a permanent basis is a serious option for the near future. Oxygen, the major component of rocket propellants as well as a key element in life-support supplies, can be produced readily from the lunar soil. It may also prove feasible to obtain building materials from the lunar soil. Advanced "space tugs" would be adequate to provide transportation, but at least one near-Moon substation would have to be erected either in lunar orbit or at another suitable point for the support of efficient and flexible flight schedules between Moon and Earth. Manned missions to Mars would require heavy boosters and efficient transport systems such as the space tugs just mentioned. Beyond these and similar goals in space exploration lie far more imaginative and difficult adventures.