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Sputnik's ball shape is instantly recognizable to most of us:

Sputnik 1

I was looking at the Wikipedia page for Luna 1, and saw it was also a ball-shaped spacecraft:

Luna 1

Why do these spacecraft have a spherical shape? Possible partial reasons (without full explanation) that I can come up with are:

  • The sphere minimizes the surface area to volume ratio. I don't know why this would be a specific advantage though.
  • The sphere is the strongest shape for a pressurized vessel. According to the Sputnik Wikipedia page, it was pressurized with nitrogen. However a spherical shape is not a necessity for a pressure vessel, as demonstrated by other spacecraft.
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    $\begingroup$ "However a spherical shape is not a necessity.." The first attempts to put objects into space were working within extremely tight constraints of the mass of the craft being launched. Any design decision that shaved off 1 or 2% of the weight needed for the required volume was highly valuable. $\endgroup$ – Andrew Thompson Jul 28 '15 at 21:14
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From this pdf (Reconsidering Sputnik: Forty Years Since the Soviet Satellite) at NASA.gov referring to Sergei Korolev, the lead rocket engineer for the Sputnik project:

There were many debates on the shape of the first satellite, with most senior OKB-1 designers preferring a conical form since it fit well with the nose cone of the rocket. At a meeting early in the year, Korolev had a change-of-heart and suggested a metal sphere at least one meter in diameter. There were six major guidelines followed in the construction of PS-l:

  • the satellite would have to be of maximum simplicity and reliability while keeping in mind that methods used for the spacecraft would be used in future projects;
  • the body of the satellite was to be spherical in order to determine atmospheric density in its path;
  • the satellite was to be equipped with radio equipment working on at least two wavelengths of sufficient power to be tracked by amateurs and to obtain data on the propagation of radio waves through the atmosphere;
  • the antennae were to be designed so as to not affect the intensity of the radio signals due to spinning;
  • the power sources were to be onboard batteries ensuring work for two to three weeks; and
  • the attachment of the satellite to the core stage would be such that there would be no failure to separate.

Also (interestingly, in my opinion at least), according to the contents of that same pdf, apparently it was polished in an attempt to help prevent it from overheating:

"Korolev came over to the shop and insisted that both halves of the sputnik's metallic sphere be polished until they shone, that they be spotlessly clean," recalled Konstantin Feoktistov, who would be the first engineer-cosmonaut to go into orbit in the three-man Voskhod 1 seven years later. "The people who developed the radio equipment were actually the ones demanding this. They were afraid of the system overheating, and they wanted the orbiting sphere to reflect as many rays of the Sun as possible."

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