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Have you ever wondered how space craft can reenter the Earth’s atmosphere without being vaporized? If you know something about our atmosphere you know that it acts as a natural shield against not only radiation but also high speed objects from space such as meteors and asteroids. The mechanism is simple but it effective. The principle of friction causes any high speed object entering the Earth’s atmosphere to burn up as it rubs against the air molecules in the Earth’s atmosphere. Because of this most objects that enter our atmosphere never even make to the surface. The challenge for space craft is similar and the solution devised is what we call a heat shield.
Space craft often have to deal with the challenge of reentry. Leaving the Earth is relatively easy when compared to this task. When a space craft leaves the Earth it just needs enough fuel to reach escape velocity. Reentry presents a whole different set of challenges. The first is that a space craft needs to reenter the Earth’s atmosphere at the right velocity. If if it moves too slow if will simply bounce of the outer edges of the atmosphere. If it moves to fast without some sort of protection it will simply break apart and burn up in the atmosphere. On top of this a craft has to go undergo reentry without the assistance of rockets. To deal with this problem scientists came up with two types of heat shields.
The first type of heat shield that was devised is one called an ablative shield. Ablative shields work according to the concept of convection. The goal is to make a space craft on reentry loose the heat as quickly as it gains it. The ablative heat shield uses a plastic coating that vaporizes. As it does so the shielding carries away heat. This way the craft is protected from burning up and is able to use the atmosphere to slow down to suitable speeds.
The second type a heat shield is a heat soak type. This type of shield works by soaking up heat from reentry and radiating it out again. This type of shield is the kind used on the space shuttle. Heat soak shields normally are made up of a series of ceramic tiles. It is considered the better model because it makes a space craft easier to reuse. All that has to be done is for the craft to have its tiles replaced.
We’ve recorded an episode of Astronomy Cast all about the space capsules. Listen here, Episode 124: Space Capsules, Part 1: Vostok, Mercury and Gemini.