You are right, it is an atom of Helium is twice as heavy as a molecule of H2. What, however, is the volumetric density? Hydrogen gas is .000089 g/ml, while Helium is .0001786 g/ml. This is at 0 deg C. Hydrogen slightly less than half as dense. However, various sources I've found claim that Helium provides over 95% of the lifting capability of Hydrogen, but do not explain why, given the significant density difference. <br /><br />Ah, I see, it is because the ratio of the displaced air to Hydrogen masses vs the same for Helium vs the displaced air is so close that there is less than 5% difference between the two. If this is indeed the case, then there really isn't much advantage to going with vacuum vs hydrogen, one would think.<br /><br />However, the original poster asked, I believe about improving performance at high altitudes, where the atmospheric pressure is much lower. The obvious answer is that since we are dealing with displacement ratios, it really doesn't matter, since ratios stay the same irrespective of the values they derive from. So long as the pressure inside the balloon remains the same as the outside, the density ratio is what does the lifting.<br /><br />However, this ignores one important factor: the makeup of the atmosphere changes with altitude. Firstly, moisture content of the atmosphere drops with altitude, which makes the air less dense. Ozone content goes up, which should make the air more dense, but it makes up such a smaller proportion than water that it doesn't make up the difference much. It isn't until we reach near the official boundaries of space that the atmosphere starts to gain significant proportions of hydrogen. It is at this point that balloon ships to orbit run into problems, as the gas inside has the same density as that outside, and no lift effect results (or, if helium is being used, you get negative lift and are stuck to strictly high density layers.) While the lower layers of the atmosphere are pretty well mixed (these are call