As pointed out in this answer, on page 4 of NASA Technical Note TND-1366 The Orbital Behavior of the Echo I Satellite and its Rocket Casing During the First 500 Days (June, 1962) says:


Echo I has an effective cross-sectional area of 7,854 square feet. The launch weight of 157 pounds decreased to 124 pounds with the loss of 33 pounds of benzoic acid and anthraquinone which were used to maintain inflation for the first few weeks in orbit. Thus, the initial ratio of the weight to mean drag area for Echo I was 0.020 pound per square foot and reduced to 0.016 pound per square foot after several weeks in orbit.

Echo-1 was about 100 feet in diameter, so that change in weight of 0.004 pound per square foot of drag area corresponds to the 35 pounds mentioned.


  1. What kind gas would have been produced by this mixture, and what fraction of the initial mass would have been converted to gas? Could it have been all of it as suggested? (the idea is that after the gas leaked out the balloon would have become rigid enough to retain its shape while in orbit.
  2. Why both benzoic acid and anthraquinone? Did they react? Did they sublimate only? Is there any chemistry-related reason why both substances were used?

below x2: cropped and full size from Echo, NASA's First Communications Satellite

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  • $\begingroup$ From the answer to the queistion you have linked, it would seem that there was no reaction; that the two substances merely sublimated. I cannot think of a reaction that would provide a gaseous product per se. The choice for these two substances may simply be down to: they evaporate at the right conditions and do not cost too much. $\endgroup$ – TAR86 Mar 18 '19 at 19:00
  • $\begingroup$ @TAR86 oh, it's been so long since I've read through that answer. Yes it says so quite clearly. Okay I will probably delete this question soon since it no longer makes sense, then perhaps rewrite it. Thank you for bringing that to my attention! $\endgroup$ – uhoh Mar 18 '19 at 19:06
  • $\begingroup$ deleting so I can rewrite... $\endgroup$ – uhoh Mar 18 '19 at 19:11
  • $\begingroup$ @TAR86 I've just stumbled upon this question, edited and undeleted it. I think it can be answered but there may be some interesting reasons why two compounds were used. $\endgroup$ – uhoh Jan 1 at 3:54

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.

  • $\begingroup$ Interesting! If the fast and slow rates of sublimation were very different, one might be for initial inflation so it could start performing as a signal reflector quickly, and the other might be to maintain inflation over time. But If I understand correctly the metalization was thought to maintain the shape even if the pressure went to zero. Did Echo 2 remain spherical without requiring gas pressure? If so, how is this known to be true? The answer there is extensive! $\endgroup$ – uhoh Jan 1 at 15:18

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