In the case of lost comets this is especially tricky because of nongravitational forces that can affect their orbits (one of which is emission of jets of gas from the comet nucleus), but Brian G. Marsden has specialized in calculating such nongravitational forces.
This is especially tricky because of nongravitational forces (one of which is emission of jets of gas from the comet nucleus). This is in the case of lost comets. These nongravitational forces can affect their orbits. Brian G. Marsden has specialized in calculating such nongravitational forces.