We know that just before reentry started, there was concern over a signal which indicated that the heat shield which would protect the capsule from the heat of reentry had become detached. This would have meant catastrophe, and orders were given to keep the retrorocket packs intact to hold the heat shield in place. However, it turned out that it was merely a faulty signal and there was no problem with the heat shield.

My question, or questions are: Why would the heat shield possibly come loose? Wasn’t it permanently attached to the rest of the capsule? Since there was a sensor that indicated that it had possibly come loose, this implies that in some circumstance, the heat shield was, for lack of a better term, detachable, or at least at risk of coming loose. Why would this be?

  • $\begingroup$ I recall they had landing bags/floation bags under the heat shield so during landing, after thru the atmosphere it would drop off? Kind of like the CST-100 does. $\endgroup$
    – geoffc
    Jul 17, 2017 at 18:20

1 Answer 1


To reduce the force of the impact on landing the heat shield was designed to separate and be held on by a skirt that acted as an airbag. From the NASA list of Mercury illustrations:

Impact attenuation

Figure 46: Impact attenuation

When the heat shield was released the impact skirt would fill with air, but when the heat shield hit the water the air being forced out the holes at the bottom (which would be under water) would provide a bit of softer deceleration before the capsule was fully landed.

More detail on how the system worked here (from the same source):

Landing shock attenuation system

Figure 33: Landing shock attenuation system

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    $\begingroup$ The Apollo capsules implemented the same functionality by mounting the couches in shock absorbing struts. The Soyuz capsules fire rockets just before touchdown. It seems that people try to not break their astronauts. $\endgroup$ Jul 18, 2017 at 0:39
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    $\begingroup$ So with Mercury, the thinking was, the entire capsule needed impact protection, but by Apollo, it was decided that only the crew needed protection? Presumably, the capsule and everything else could either withstand the splashdown shock or didn't need to? $\endgroup$
    – Anthony X
    Jul 18, 2017 at 3:46
  • $\begingroup$ @AnthonyX Maybe it was considered a more cost-effective solution - using a skirt like this is rather clever, using ambient air pressure and making a relatively large shock absorber with relatively little material. Perhaps later developments asserted that integrating that into the couches is even cheaper. $\endgroup$
    – Luaan
    Jul 18, 2017 at 8:33
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    $\begingroup$ @AnthonyX The heat load on reentry was much more for Apollo. A gap in the heat shield for the use of an impact skirt was a risk for the integrity and functionality of the heat shield. $\endgroup$
    – Uwe
    Jul 18, 2017 at 9:40
  • $\begingroup$ @AnthonyX, I think it was more a matter of space. The Mercury capsules were incredibly cramped -- astronauts didn't so much "enter" them as "wear" them -- so a shock absorber had to be built on the outside. $\endgroup$
    – Mark
    Mar 4, 2020 at 4:05

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