I wonder if these numbers could be improved by considering an oxygen-helium atmosphere rather than an oxygen-nitrogen atmosphere. I'm not sure how feasible thi

I wonder if these numbers could be improved by considering an oxygen-helium atmosphere rather than an oxygen-nitrogen atmosphere. I'm not sure how feasible this would be in a large enclosed space, my initial concern would be the development of pockets of high helium and low oxygen concentration but perhaps a simple ventilation system could keep the air mixed at an acceptable ratio. I believe oxygen-helium was considered for the Apollo missions but not used. According to some quick calculations a mixture of 22% oxygen and 78% helium would have a gas density of 0.423 kg/m3 at 20 deg C. I used (density1vol1 + density2vol2) / (vol1 + vol2) with density1 = 1.331 and density2 = 0.1664 and a total volume of 1000m3. The fact that Venus' gravitation is only 90% of Earth's and further (though slightly) modified due to the height of 50 km above the surface would mean that the gravitational attraction would be somewhere around 8.722 m/s2 which is about 88.9% Earth standard. A Venusian (Venerian?) habitat also has the benefit of less disastrous consequences of developing a small leak in the wall of the habitat since the pressure differential would not be so great though an all out failure of atmospheric containment would unfortunately not be at all survivable even if protective equipment were being worn at the time. The only chance would be immediate activation of the ascent vehicle. Although I would be curious to know if the entire habitat could be constructed in such a way as to remain a rigid shape once it had been deployed, possibly in the form of a Hybrid Airship or Dynastat[1] This could allow a longer grace period on evacuation should it be necessary and might have added benefits to maneuverability during the mission. Take off and landing concerns could be ignored since there would be none other than the initial mid-descent inflation and fuel capacity would only be limited by the available sunlight. [1] http://en.wikipedia.org/wiki/Hybrid_airship#Dynastats http://en.wikipedia.org/wiki/Hybrid_airship#Dynastats