For well known objects like these, the mass and approximate size is known. You can Google them to find values even. The positional uncertainty and lack of data

For well known objects like these, the mass and approximate size is known. You can Google them to find values even. The positional uncertainty and lack of data about attitude lead to modelling the objects as spheres as not so bad a choice. One algorithm that is sometimes used: https://link.springer.com/article/10.1007/BF03546397 https://link.springer.com/article/10.1007/BF03546397