Editing 2908: Moon Armor Index
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This visual index illustrates that the moons of both Earth and Pluto are unusually massive in comparison to their planet. The large relative size of Earth’s moon — and its protective role in deflecting asteroids — is one reason that’s been suggested by astronomers for why intelligent life successfully evolved on Earth. | This visual index illustrates that the moons of both Earth and Pluto are unusually massive in comparison to their planet. The large relative size of Earth’s moon — and its protective role in deflecting asteroids — is one reason that’s been suggested by astronomers for why intelligent life successfully evolved on Earth. | ||
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| + | The interesting difference here is that the usual means of comparison is of size of moon to size of 'planet', both described/shown by: | ||
| + | * linear qualities, such as the given radius, diameter or circumference, | ||
| + | * 2D qualities, e.g. a side view (representing a cross-sectional area), but could also be surface area, or | ||
| + | * volumetric values, including (where applicable to bodies of similar types, and thus density) the relative masses. | ||
| + | These produce different ratios, according to their chosen dimensionality: A linear doubling factor would relate to a quadrupling of an area as well as an octupled volume. But the version used here derives a ''linear'' indicator (the thickness of the new material) by dividing the ''area'' of the main body (proportional to the square of its uncounted radius) into the ''combined volume'' of all other bodies (proportioned cubes of their own radii), which gives an unusual dimensional analysis. | ||
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| + | Whether intended or otherwise, this particular methodology makes the Pluto-Charon system (Charon being roughly half the diameter and one-eighth the volume of Pluto, before even adding that of the other moons) surprisingly similar to the Earth-Moon one (our sole Moon is around one-quarter Earth's diameter, and therefore less than 2% its volume), but leaves them ''both'' as still standing out significantly against all other planetary comparisons. | ||
Mars's moons {{w|Phobos (moon)|Phobos}} and {{w|Deimos (moon)|Deimos}} are small compared to Mars, so they would contribute a thin 2-inch layer of 'armor' around Mars, in contrast to the 20-inch (0.5 m) diameter of a {{w|Mars rover}} wheel. Huge Jupiter would be covered with almost 3 km of "moon" matter, which indicates just how much moon mass orbits Jupiter, a situation mostly similar for Saturn, Uranus, and Neptune. | Mars's moons {{w|Phobos (moon)|Phobos}} and {{w|Deimos (moon)|Deimos}} are small compared to Mars, so they would contribute a thin 2-inch layer of 'armor' around Mars, in contrast to the 20-inch (0.5 m) diameter of a {{w|Mars rover}} wheel. Huge Jupiter would be covered with almost 3 km of "moon" matter, which indicates just how much moon mass orbits Jupiter, a situation mostly similar for Saturn, Uranus, and Neptune. | ||
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* moons already serve a protective purpose by deflecting and even intercepting some incoming asteroids (with a ''slight'' chance of turning a future miss into a hit). | * moons already serve a protective purpose by deflecting and even intercepting some incoming asteroids (with a ''slight'' chance of turning a future miss into a hit). | ||
* the four gas giants — Jupiter, Saturn, Uranus, and Neptune — lack a solid surface to practically sustain a layer of armor without even ''more'' ambitious engineering than the already complicated process of somehow distributing soft-landed fragments of disassembled satellite evenly all across a planet. | * the four gas giants — Jupiter, Saturn, Uranus, and Neptune — lack a solid surface to practically sustain a layer of armor without even ''more'' ambitious engineering than the already complicated process of somehow distributing soft-landed fragments of disassembled satellite evenly all across a planet. | ||
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The title text continues that NASA's [https://what-if.xkcd.com/117/ Planetary Protection Officer] is purportedly in favor of the idea. In reality, this officer is actually responsible for keeping other celestial bodies safe from Earth's contamination, not for shielding planets in armor. Theoretically, though, armoring other planets could indeed protect them from further Earth-sourced contamination, and armoring Earth would also theoretically protect other planets by burying the biosphere and all of Earth life not already sent into space — a potentially civilization-smothering action, though a surprisingly unapocalyptic result compared to many of Randall’s “What If?” scenarios. | The title text continues that NASA's [https://what-if.xkcd.com/117/ Planetary Protection Officer] is purportedly in favor of the idea. In reality, this officer is actually responsible for keeping other celestial bodies safe from Earth's contamination, not for shielding planets in armor. Theoretically, though, armoring other planets could indeed protect them from further Earth-sourced contamination, and armoring Earth would also theoretically protect other planets by burying the biosphere and all of Earth life not already sent into space — a potentially civilization-smothering action, though a surprisingly unapocalyptic result compared to many of Randall’s “What If?” scenarios. | ||
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| {{w|Eris (dwarf planet)|Eris}} || (1.70±0.02)×10^7 || {{w|Dysnomia (moon)|1}} || 1.61×10^8 || 9.47 km (5.88 mi) | | {{w|Eris (dwarf planet)|Eris}} || (1.70±0.02)×10^7 || {{w|Dysnomia (moon)|1}} || 1.61×10^8 || 9.47 km (5.88 mi) | ||
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===The complexities of armor thickness calculations=== | ===The complexities of armor thickness calculations=== | ||
| − | The comic uses the ≈ sign to show that the formula is only an approximation: it does not take account the increase in armor surface area as it gets thicker. This approximation would be perfect for a shield of thickness zero, but for the thickest shield (Pluto) around a small celestial body the error is around 4% (52.5 km by this approximation, but 50.4 km by more thorough calculation). To find the correct value, we can use the formula for the volume of a sphere, V = 4/3 * pi * | + | The comic uses the ≈ sign to show that the formula is only an approximation: it does not take account the increase in armor surface area as it gets thicker. This approximation would be perfect for a shield of thickness zero, but for the thickest shield (Pluto) around a small celestial body the error is around 4% (52.5 km by this approximation, but 50.4 km by more thorough calculation). To find the correct value, we can use the formula for the volume of a sphere, V = 4/3 * pi * r^3 (where V is the volume and r is the radius). Using this formula, we can find and add together the volumes of each moon, as well as the volume of the planet, to get a total volume of the new shielded planet. Then we can find its radius using the formula r = (V / (4/3 * pi))^1/3, derived from the previous formula. Subtracting the radius of the previous planet from the radius of the new planet gives us the thickness of the armor. |
This process described above assumes that all objects involved are completely spherical, which may not be the case. The act of tearing apart a solid moon, perhaps into rough gravel, might add microvoids to the new layering that bulk up the volume slightly. But neither are gravitational compression effects taken into account on an originally loose material; the planet's gravitational pull could settle some of the moon material into a slightly smaller volume than the one it occupied as lower-gravity moon. | This process described above assumes that all objects involved are completely spherical, which may not be the case. The act of tearing apart a solid moon, perhaps into rough gravel, might add microvoids to the new layering that bulk up the volume slightly. But neither are gravitational compression effects taken into account on an originally loose material; the planet's gravitational pull could settle some of the moon material into a slightly smaller volume than the one it occupied as lower-gravity moon. | ||
