When the missile is about to strike Santa’s sleigh he opens his bag of tricks and inside it goes into an extra dimensional space. Then later he gives a nice missile, refueled and deactivated with a bow on it, to a Ukrainian artillerist who’s been good this year.
Why is it hard? At least to get an approximation since you can’t measure everywhere.
We know temperatures of the mantle and both cores. We know their size. We can ignore the crust as a rounding error. This approximation will improve as our measurements get better.