Complex sentence: 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.
Split into simpler sentences: In the case of lost comets this is especially tricky because of nongravitational forces that can affect their orbits, such as emission of jets of gas from the comet nucleus. However, Brian G. Marsden has specialized in calculating such nongravitational forces.