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/
To get to Pluto in a reasonable amount of time you have to be going faster than the escape velocity of the Sun. The Sun's gravity is decreasing faster than your velocity