This fascinating answer says that "The Gemini capsule floated because it had thousands of hollow balls made of extremely thin aluminum, each composed of two half-spheres welded together, the size of ping pong balls, which were embedded between the inner and outer wall, for the exact purpose of flotation". I'd never, ever, heard of such a thing, so I was fascinated.

enter image description here

And sure enough, one of the several (please go take a look) links in that answer show a design of something clearly labeled "ball - flotation". I'll double back to this.

But I'm really skeptical. I guess I'm actually just confused. How would this aid flotation, other than just lightening the mass of the capsule walls? I'm certain they weren't external to the capsule, so they didn't increase displacement.

One of the comments in yet another link in the original answer seems to suggest that they were a reserve of buoyancy in case the capsule [walls] were flooded, which, okay, that makes sense if water gets into the walls, I guess. But the walls of the capsule were obviously airtight, so surely they were watertight too--and I can't imagine that a handful of these things would keep the entire capsule from sinking if, like, the whole thing filled up with water.

Also, look at the requirements in that document: has to resist 1000°F for 30 minutes! What?? Is this a reentry thing? But if the walls get that hot, you've got a huge problem--far, far more so if they get that hot for 30 minutes.

Reading through that discussion thread (the second link) just seems to make things more confusing. Did the things really fly? On Gemini or Mercury? Did they really ever exist? Is it all a scam?

What the heck are these balls??

  • $\begingroup$ It seems the outer wall is not water tight. $\endgroup$ Feb 19, 2022 at 5:27
  • $\begingroup$ I am skeptical too. It seems mass inefficient. $\endgroup$ Feb 19, 2022 at 5:29
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    $\begingroup$ A lot of conflicting info in this collectspace thread but one claim is that they were used just in the nose of the Gemini to control its resting attitude in the water (Gemini lay horizontal in the water, rather than upright like Mercury and Apollo), so the overall weight impact would have been small, but that still leaves a lot of questions regarding what was and wasn’t watertight in Gemini. $\endgroup$ Feb 19, 2022 at 16:09
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    $\begingroup$ Considering the strength of impact against water during splashdown, I'd bet the walls were watertight right up until the moment their being watertight would start being useful... $\endgroup$
    – SF.
    Feb 21, 2022 at 16:57
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    $\begingroup$ Chapter 21 in the Gemini Mid-Program Conference says "Spacecraft changes included the addition of extra flotation material in the reentry control system section, thus trimming the floating spacecraft to an approximately horizontal attitude in pitch." But sadly does not say what it was. $\endgroup$ Feb 22, 2022 at 1:02

2 Answers 2


A damaged hollow structure would sink; a damaged hollow structure filled with hundreds of hollow structures can't sink:

enter image description here

I would call it "hyper-redundacy" of buoyancy.

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    $\begingroup$ Partial answer to " How would this aid flotation, other than just lightening the mass of the capsule walls?" So +1 $\endgroup$ Feb 20, 2022 at 13:50
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    $\begingroup$ > a "hyper-redundacy" of buoyancy". or, perhaps, "a reserve of buoyancy in case the capsule was flooded"? :) Thanks! I guess it begs the question: were the walls watertight? If so, were these present just in case the ways were damaged? What scenario would damage the walls enough to let them become waterlogged without flooding the entire capsule? Was this really the motivation? $\endgroup$ Feb 21, 2022 at 16:33
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    $\begingroup$ not the capsulr,e, just the walls. $\endgroup$
    – jumpjack
    Feb 21, 2022 at 17:49
  • $\begingroup$ hierarchical buoyancy or buoyant metamaterial or granular buoyancy or... $\endgroup$
    – uhoh
    Feb 25, 2022 at 0:47

The capsule was only unsinkable if one didn't exit the hatch of the vehicle like on Liberty Bell 7: https://space.nss.org/space-myths-busted-gus-grissom-didnt-blow-the-hatch-on-liberty-bell-7/

“A percussion-activated, explosive primer cord surrounded the new hatch, and the astronaut had to activate a switch in order to arm the mechanism. When he was ready for recovery the astronaut would place the switch in the armed position, and a recovery loop on top on the capsule became the trigger. When the recovery helicopter’s hoisting cable was hooked onto the loop, the pressure created by lifting the capsule fired the mechanism and blew the hatch off.”

One is for sale if you want it: https://historical.ha.com/itm/explorers/space-exploration/gemini-3-flown-floatation-ball-with-original-mcdonnell-spacecraft-parts-tag/a/6007-41149.s

Gemini 3 Flown Floatation Ball with original McDonnell Spacecraft Parts Tag. A 1.25" diameter aluminum sphere of the type that was used in the space program to help maintain the spacecraft's buoyancy in the water. After Grissom's bad experience with his Liberty Bell 7 sinking after splashdown, he and Young nicknamed their Gemini 3 capsule "The Unsinkable Molly Brown." The ball is attached to a 3" x 6" parts tag noting that it was removed from the GT-3 Reaction Control System on March 26, 1965 at Cape Kennedy, Florida. A great flown souvenir from America's first two-man spaceflight. Fine condition.

In addition, there is a discussion about these spheres, apparently some were made of titanium (which in the below discussion they do appear to be titanium grey): http://www.collectspace.com/ubb/Forum14/HTML/000627-2.html

David Carey

"Perhaps the small displacement, while not enough to save a swamped capsule, helped provide a 'buoyancy torque' for righting. Your 0.5lb H2O/sphere is right for a 3" sphere but - if I'm understanding correctly - the 1.4" spheres of Gemini usage would be more like 0.05lb H2O displacement each. Still, assuming 1700 spheres as uncovered by John you get ~90lb of buoyancy biasing against a ~3500lb (sources varied) Gemini capsule splashdown weight."


"The purpose of the spheres in Gemini was to raise the nose section so that water would not so readily enter the cabin through the forward corners of the open spacecraft hatchways. The buoyancy was biased in such a way that the command pilot's hatch was slightly more out of the water. After Gemini 3 foam inserts were used instead."

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    $\begingroup$ Thanks, but this doesn't provide any new information. "used in the space program to help maintain the spacecraft's buoyancy in the water" is something we already know is claimed about these objects. Unfortunately, your answer doesn't provide any sources for that claim, so we've learned nothing new. $\endgroup$ Feb 19, 2022 at 6:08
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    $\begingroup$ @AntonHengst - uninformed speculation, but that 'removed from RCS' wording on the tag might indicate a small (1?) number of these mystery parts were intended to sit amongst the RCS pipework in a small section that vents to space, to reduce the volume that flooded on landing. $\endgroup$ Feb 19, 2022 at 7:22

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