Complex sentence: For example, in nuclear fusion or fission, only a small proportion of the mass is converted into energy so enormous quantities of material would be needed thus a nuclear starship would greatly deplete Earth of fissile and fusile material.
Split into simpler sentences: For example, in nuclear fusion or fission, only a small proportion of the mass is converted into energy, so enormous quantities of material would be needed. Thus, a nuclear starship would greatly deplete Earth of fissile and fusile material.