I was just reading an article on the Space Shuttle Columbia tragedy and I began to wonder, why do they need to re-enter the atmosphere at roughly 17,000mph? Why not slow down to a less “burn up on re-entry” speed?
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Spacecraft need to be at that speed so they don’t fall back to Earth as they go around it. To reach this speed, spacecraft launch vehicles expend many, many times the payload’s mass in fuel.
To slow down again, all the kinetic energy in the vehicle’s velocity needs to be converted to something else. If the spacecraft was attached to another launch vehicle with the same amount of fuel as the one that boosted it to orbit, then it could just de-orbit itself and land. But as I said, the fuel needed for that is many times the spacecraft’s mass.
Instead, the reentry vehicle manoeuvres so that its orbit grazes the upper atmosphere. Friction between the atmosphere and the spacecraft converts energy from its velocity into heat — lots of heat! In fact, the amount of heat generated during reentry is the same amount as was generated by the launch vehicle’s rockets during launch, only spread out over the much longer reentry phase.
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