Planets - Gravity, Mass, Size?

thespiritcoyote April 19, 2011 - 3:52am | How interrelated are the 'big triplet' factors? Consider that Gravity needs no more than a two decimal point granularity, and is used in the basic mechanic effects with only a one decimal point granularity.
But can it be simplified close enough, to match the single-decimal-point granularity of SF's planetary gravity statistic? Are there other factors that be accounted for, to make this basic triplet function properly, even given such a low resolution requirment?mustOh humans!! We discover a galactic community filled with multiple species of aliens, and the first thing we think about is " how can we have sex with them?".~ anymoose, somewhere on the net... so... if you square a square it becomes a cube... if you square a cube does it become an octoid? |

TerlObar April 19, 2011 - 5:40am | The surface gravity on a planet (at least one that has a surface) is given by g=GM/r ^{2} where G is the gravitational constant (6.67x10^{-11} m^{3}kg^{-1}s^{-2}), M is the mass of the planet, and r is the planet's radius. But the mass, M, is also given by M=4??r^{3}/3 where ? is the mean density of the planet. Plugging this in gives g=4G??r/3. So your triple is really gravity, radius and density.Terrestrial (rocky) planets like the Earth and Mars have densities on the order of 5500 kg/m ^{3}. The icy moons of the outer planets and the Kuiper Belt and Oort cloud objects have densities on the order of 2000 kg/m^{3}. And the Jovian planets have densities on the order of 1000 km/m^{3}.You could construct a table but it would have to be more than three columns, it would need to be a grid. the column header would be radius, the row header would be density and the values in the cells would be the gravity of the planet. Ad Astra Per Ardua! Webmaster - The Star Frontiers Network & this site Founding Editor - The Frontier Explorer Magazine Managing Editor - The Star Frontiersman Magazine |

thespiritcoyote April 19, 2011 - 7:14am | Would HAVE to construct a grid, was pretty much my thought too.I know the math works, but how do we get it into a simple format, for those who are under the impression that a slide-rule is a complicated tool. simplified to, the single decimal Gravity constant that is already used in the rules the size (in a single decimal of earth diameters preferably, so a second radian to diameter chart isn't neccessary) the density/mass (not the same thing I know but mass is a bit more usefull as a system mechanic value, though neither is necessary for much in a common use mechanic, and density would probably work fine.)So you suggest a grid-table... where if you know what gravity you are looking for, you look for the the two numbers that match, and then convert between radians and diameters on a second chart. If you know what diameter, you convert to radians, then check what gravities work at what densities. Can't really think of a reason you would want to start with densities or masses of earth, but it is a plausable unforseen necessity. cumbersome, but if it is unavoidable.... it might work... I was hoping for something a little more intuitive. though I realize that my rationale sets a counter intuitive requirment from the perspective of the typical random planetary generator. Oh humans!! We discover a galactic community filled with multiple species of aliens, and the first thing we think about is " how can we have sex with them?".~ anymoose, somewhere on the net... so... if you square a square it becomes a cube... if you square a cube does it become an octoid? |