Geek Answers Why do objects burn up when entering the atmosphere

first_imgMovies will get nearly everything wrong when it comes to space, from gravity to sound, but they do always seem to remember to engulf the spaceships in flames during a descent toward the Earth. That’s probably just because it’s an excuse for a dramatic moment; why just have an approach from orbit when you can have an approach from orbit on fire? Plus, it’s one of the most memorable facts about space missions, so people would notice if it was missing. But do these people know why it happens?Put simply, the answer is friction. When traveling through the vacuum of space, where there is no atmosphere to slow movement in any way, objects can get up to speeds that are simply impossible on Earth — partly because of drag, and partly because going that fast makes you go on fire. In space, meteors are free to maintain the velocity of whatever cataclysmic event set them moving in the first place. Most meteors enter the atmosphere at between 10 and 70 kilometers per second, and the friction that results when their surfaces whip past flammable atmospheric gasses causes those gasses to ignite. In addition, their extreme velocities cause the gasses in front of them to compress very quickly, and compressing a gas increases its temperature.This air density map shows how different designs produce different shockwaves at high velocities.Space craft can actually take advantage of this atmospheric compression through something called blunt body design. The principle is simple: by making a high-drag design that will push against as much air as possible, high-velocity objects create a cushion in front of them made of air that cannot get out of the way quickly enough. This shock-wave helps to keep the flaming atmospheric gasses at a distance from the vehicle; it works so well that the amount of heat experienced by a lander is directly and inversely proportional to the amount of drag it creates against the air. This is why the reentry pods of old seem to enter backwards, creating as much drag as possible rather than going in pointy-end first (see header image.)A vehicle in low Earth orbit will be traveling somewhere around 7 kilometers per second, easily fast enough to ignite the atmosphere. Single-use reentry vehicles were coated with ablative material that was designed to burn away in a controlled fashion to dissipate heat. A reusable shuttle has need of insulated tiles, however. The Space Shuttle uses insulated tiles that can withstand heats of thousands of Kelvin, but which break if exposed to even a little bit of rain.Eventually the drag of the surface against the atmosphere will slow the incoming body enough to reduce friction to below the atmosphere’s threshold of ignition. At around this point, the fire goes out, though some materials will keep it going a bit longer due to the heat they’ve stored up on their own surface. Reentry vehicles often glow red-hot even after the flames have subsided.Hypothetically, a fireball could be avoided by approaching too slowly to ignite the atmosphere in the first place, but this would require a constant downward burn, and at least as much fuel as the initial launch. SpaceShipOne used a feathered reentry technique that slowed its speed at a much higher altitude than NASA’s Space Shuttle, but it did not ever approach the orbital speeds generally required for application of the term “reentry.” As of now, there is no way to approach the Earth at anywhere near orbital speeds without creating a fireball.last_img read more

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