Targeting Uranus or Neptune requires gravitational assists or high-energy propulsion due to orbital distance.

The claim relies on fundamental orbital mechanics and the sheer distances involved in reaching the outer solar system, which is common knowledge in astronomy and spaceflight. While paper [1] discusses ice giant exploration and mission design concepts, the core physical reality of needing gravity assists or high-energy propulsion for outer planet missions is a trivial consequence of celestial mechanics rather than an empirical hypothesis requiring formal study citation.