I think that should be 100th of an atmosphere <img src="/images/icons/smile.gif" /><br /><br />You would want an altitude margin in operate over much of Mars, so it would have to be designed for 1/200 atm (5 mB). Terrestrial high altitude balloons operate at these pressures, on Mars you have the advantage of the lower gravity, higher molecular atmosphere, and being able to use hydrogen.<br /><br />Other than the low pressure, the big drawbacks are the need to operate in high winds (lots of power) and to carry your on oxidiser. There are a few fuels (like silane - SiH4) that burn is CO2, but I don't know practical these are.<br /><br />Clapp compared a balloon, heaviler than air craft, and an airship which derived 10-14 of its lift from bouyancy and the rest from aerodynamics. A pure balloon on Mars would need 67000 m3 to lift 1000 kg, equivalent to a 100 kg payload. Obviously the hybrid would be more efficient. <br /><br />Such an airship with a 100 kg payload and a range of ~200 km (i.e. an aerobot or unmanned probe) would by 38 m long, 20 m wide, and 10 m high. It could cruise at up to 200 kph. The large "wing area" (740 m2) means that solar cells are feasible, and at high latitudes, could give the craft almost unlimited range.<br /><br />Manned airships would be colossal, needing payloads of 1 tonne ands thus 10 times the volume.<br /><br />Jon<br /><br /><br /> <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em> Arthur Clarke</p> </div>