What gas was produced in this orbiting space-balloon from 33 pounds of benzoic acid and anthraquinone? Why these two chemicals?
There are three materials to investigate: the two solids in question have been described as having fast and slow rates of sublimation. The balloon was sent up with 4.5 kg of benzoic acid (bp 250C) and 10.6 kg of anthraquinone (bp 380C). https://en.wikipedia.org/wiki/Project_Echo
The third material is the film: 12.7 micron biaxially oriented polyethylene terephthalate metallized with 0.2 micron aluminum. The two sublimatible solids were used to rapidly inflate the balloon and then to keep it inflated in case of micrometeorite strikes.
Echo-2 was made with a thinner PET film (9 microns) sandwiched between two thicker aluminum films (4.5 microns each), and the balloon was inflated to a pressure that slightly deformed the aluminum sandwich and kept the whole structure more rigid, so continual sublimation was not required.
The PET films are so thin! By comparison, a human hair is typically 100 microns in diameter. Perhaps the choice of sublimatible materials was made for non-reactivity (with the film) rather than with any reaction to form a gas. But the rationale is still not clear - it certainly couldn't have anything to do with cost.