I sincerely doubt that any physicist or astronomer knows the exact value of the gravitational constant out to 10 decimal places.
They also don't know the exact mass of the spacecraft - there may be residual fuel onboard that wasn't used but isn't accounted for in the mass calculation.
The mass of the spacecraft doesn't matter. All objects are accellerated the same amount, by gravity that is, regardless of their mass. This is due to the gravitational mass being the same as the inertial mass.... Or is it? Perhaps at very low gravitational strength values they are not quite the same..quantum effects perhaps?