Rule number one of space exploration in the Walters-verse: always have a spare ship on hand for a rescue. Rule number two: have Chris Godfrey available to pilot it.
Pluto is acting weird, so the other three boys go out to investigate. We can now travel to 12 million MPH, folks! Which is good, because when things go pear-shaped, there's just enough time for Chris to work his heroics.
Although the real hero is Tony Hale, as it turns out (I won't spoil that part).