Estimation of Planetary Masses within our Solar System can be calculated with the precision of the calculated speed of light approximately 186282fps.
I just looked up the mass of the Earth as being 5.98 x 10^24.
That couldn't be wrong by much more than ten million million million tons should it.
The mass of a planet can be calculated by a spacecraft by measuring a radio-signal spectral shift when the craft from a known and appropriate angle begins to accelerate from the gravitational field of the planet.