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What other uses are made of latent heat in space systems?

    Returning to where we started from, latent heat is that energy which is being expended in changing the state of matter without there being a change in the temperature of the substance. In this respect we can use this as a kind of thermostat to limit the temperature. And thermostats are used, not only to regulate things, but to protect things as well.

    The most valuable thing that needs to be protected is human life and the re-entry of manned spacecraft is a very dangerous phase of any manned mission because of the enormous amounts of energy which have to expended (equivalent to the energy required to launch the vehicles into space in the first place).

    In the early days of space exploration this was achieved by having a heat shield comprising a material which could be 'burned off'. The frictional heat generated at the leading surface of the craft during re-enrty was used to provide the latent heat of vaporisation rather than to continually raise the temperature of the craft. Clearly there had to be sufficient mass of material so that it was not all burned off. An examination of some of the early craft which can be seen in space museums show what a dramatic effect this has.

    But there is more to the story. As the re-entry vehicle hits the denser atmosphere a shock wave is set up ahead of the vehicle in which very high temperatures are established. When temperatures of several thousands of degrees are reached some of the energy in the shock is taken up ionising the gas in the shock; a kind of latent heat of ionisation. It is this ionisation, in particular the free electons which are produced in this plasma, which cause the drop out of telecommunications at this critical time.

    Although the space shuttle uses highly insulating tiles which do not sublimate (because of the requirement for the shuttle to be re-usable) the drop out in communications still occurs when the shuttle re-enters the atmosphere.

    Turning the situation around just shows how the atmosphere protects us from the hostile space environment. We are shielded from the Sun's ultraviolet and X-rays radiation since their energy is absorbed in ionising the upper atmosphere thus creating the ionosphere, and we are shielded from the bombarding matter which manifest itself as shooting stars. Fortunately for us most of the meteorites (as well as general space debris) never reach the ground in any sizeable chunks.

The End of the LATENT HEAT topic.

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The University of Birmingham 

Physics and Astronomy Department, The University of Birmingham