In The Expanse the group responsible for the attack stole the most advanced stealth coating in the Expanse universe to paint the asteroids, leaving Earth unable to detect them with anything but their most powerful sensors.

The kilogram is defined as the mass equivalent of a photon of a specific energy via the Planck constant h thus linking the speed of light and the frequency of the hyperfine level of caesium-133. The relative uncertainty of the measured value of the gravitational constant G is 10^-5 which would lead to a definition of the kilogram that has a worse relative uncertainty than using the former definition defined via an international prototype. The Wikipedka article is more detailed than this short summary.