Atmospheric Reentry
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Basic Information

Atmospheric Reentry is what happens when a spacecraft or satellite enters the atmosphere of a planet. It results in significant aerodynamic heating of the object and the air it passes through.

Reentry capable vehicles tend to have blunt or rounded body shapes, as this helps reduce the heat they experience. The air can't pass by them as quickly - instead of heating up via friction, the blunt shape creates an air cushion that insulates the craft. The temperature of the "shock layer" of heated air pushed ahead by the craft is subjected to (in Kelvins) is roughly equal to the speed it is traveling in meters per second. A spacecraft entering the atmosphere at 7.8 km/s would experience a peak shock layer temperature of 7800 K. That's pretty hot! By comparison, lava is liquid at temperatures of around 950 K to 1,500 K, and the surface of The Sun is only 5800 K. Luckily, this super-heated air is quickly dissipated into the upper atmosphere.

The other fun effect of this envelope of super-heated air is related to the fact that being heated and compressed like that tends to force it to ionise and, as a result, interferes with radio signals coming to and from the craft in question, leading to a phenomenon known as ionisation blackout or re-entry blackout. These blackouts can last several minutes (depending on the thickness of the atmosphere, speed and angle of descent) and have traditionally been a nervous time for everyone involved.

The concept of the ablative heat shield was described as early as 1920 by Robert Goddard. Such heat shields often feature layers of materials that conduct heat poorly, sandwiched between layers of very hard materials.

In 2004, aircraft designer Burt Rutan demonstrated the feasibility of a shape changing airfoil for reentry with the suborbital SpaceShipOne. The wings on this craft rotate to provide a shuttlecock effect. Notably, SpaceShipOne does not experience significant thermal loads on reentry.

Everything written above assumes an earth-like planet with gravity and atmosphere not unlike our own. Planets with greater gravity and thicker atmosphere (with correspondingly more force or more friction) may be even more perilous. Chemical composition of the atmosphere matters quite a bit as well: you might have to worry about literally igniting the atmosphere.

Unmanned Entry

Asteroids, space junk, and inorganic debris faces the same heat and shock dangers as manned vessels, but without the benefit of engineering and piloting to soften the blow. Often this results in an aerial fireball known as a bolide. See also: asteroid impact.

Likely the same fate would a derelict spaceship that arrived with no living crew or functioning robots, unless it had a really good autopilot or ship's computer installed. Which it probably does unless whatever ate the crew also drank all the computronium. I mean, if your civilization is advanced enough to cross light years, you're probably also advanced enough to have an automatic parking mode.


Game and Story Use

  • Any Orbital Shuttle, Atmospheric Probe, or Landing Module should feature a blunt nose with a heat shield, rotating wings, or both.
    • Damage to the heat shield of a spacecraft could make landing disastrous. That's how the Space Shuttle Columbia Disaster happened.
      • If the players are foolish enough to try penetrating an atmosphere with a ship that lacks such features, it'll burn up with them in it.
      • And don't try stowing away on the hull of a landing craft!
    • One way to distinguish between the technologies of different space empires would be that one uses high-temp heat shields and flaming reentry, while the other uses a more graceful low-temp shuttlecock effect.
  • So the heat dissipates, eh? If that turns out to just be wishful thinking, it could be a contributing factor in global warming.
    • This is not as silly an idea as it sounds - arguments about current climate change aside, there's every possibility that a spacefaring civilisation would have to take the energy output of its boosts and put-downs serious once they reached higher frequencies. These sort of considerations would probably hasten the development of space elevators and similar things.
    • Or for a crazy disaster movie feel, every reentry could have a chance of igniting the atmosphere and destroying the world!
      • Actually, if you happened to be putting down on a high oxygen "early earth" there's a strong possibility that you might cause some kind of fire in the process.
  • "Amphibious Assaults" in the space age are going to have to take account of the perils of re-entry, not least being a predictably moving target with a massive energy signature and having all of your communications and sensors blacked out for minutes at a time.
    • That probably should apply to alien invasion scenarios as well. Logically, the invaders are at their most vulnerable while en route down to the surface. Starship Troopers (the novel, not the movie) put some thought into it, but there's a lot of sci-fi that just ignores that particular difficulty.
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