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Another neat thing about this, to think about : if we're defining a kilogram as a number of atoms, that's how much they weigh in certain relation to earth's gravity, but it's still a certain number of atoms. What happens when you take that same number off-planet? Is a kilogram still based on the number, or what that number represents in certain circumstances?
Which is kind of the point of the Avogadro project (and other atom-counting definitions). If they can define a kilogram as X number of atoms of silicon, then a kilogram would be defined as the mass of those X number of silicon atoms under any gravitational force. If a kilogram weighs 2.2046 pounds on Earth, we can also know what that kilogram weighs where gravity is more or less than Earth. (We already know that now, I know, but atom-counting definitions take away the weight-based definition, and create one that is universal regardless of the force of gravity.)
The kilogram lags behind the other six SI base units (meter, mole, second, kelvin, ampere, candela) in being assigned a definition based on a fundamental physical property.
The problem is that of mass and weight. You can't really define weight in fundamental terms because it can change. Weight is the force gravity exerts on an object and derives from mass. If a meteorite hits Earth, everything weight will increase very, very slightly because Earth's mass (and gravitational pull) has increased due to the mass of said meteor. Likewise when we send satellites into orbit, Earth's mass decreases by the mass of those various satellites.