Cassini–Huygens is an unmanned spacecraft sent to the planet Saturn. Therefore, option 1 is wrong, this eliminates option (a), (c) and (d). Now we are left with final answer B only 2 and 3. MESSENGER is a robotic NASA spacecraft orbiting the planet Mercury. Voyager 1 (September 1977) and voyager 2 (Aug 1977) were launched to study the outer Solar System.