The *really* interesting thing about
Flight of the Navigator was the concept that the boy did not
age (well, he did, but an undetectable four hours) because the majority of his 8 missing years were spent
travelling at (or near) light speed! Of course this was due to the
time dilation experienced by matter travelling near
C, as explained in
the Special Theory of Relativity. Pretty heavy concept for a
kids' movie to be based on. To be honest though, I can't remember how explicitly this was addressed in the movie. And of course they kind of copped out, letting him
go back in the end... I suppose
tachyons were somehow involved...
Addendum: According to the "Relativity Calculator" (http://www.1728.com/reltivty.htm) the Navigator's travel speed was 0.9999999983710723 * C, or 299792.45751165977 km/sec. That's assuming that the ship accelerated instantaneously (a dubious prospect) and maintained the same speed afterwards.