xkcd #3310: Rejected Space Mission Profiles
Title text:
Although NASA agreed that Voyager 2 'seemed lonely' they decided against launching a companion orbiter mission after further measurements indicated that the spacecraft actually preferred the peace and quiet.
Transcript:
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Source: https://xkcd.com/3310/
It would be really difficult to design a spacecraft to go to Pluto and then return. You'd have to carry enough fuel to come to a stop at Pluto, then reverse back to Earth, then come to a stop at Earth. The extra fuel adds up quick because the more you carry, the more you need. A ship that could carry enough fuel for a round trip would be incredibly large and would have to be constructed in orbit rather than on the surface of the Earth.
You could reduce fuel requirements somewhat by going slower, but it already takes so long to get to Pluto that you'd soon be limited by human life expectancy.