"This post has given a surprise for those players who love fast flights on short distances using explorers II"

Exactly.

"Can you show this nonlinear equation in algebraic view (like y = exp(x) or something else), for our calculation itself?"

=5.5*10^-2 * EXP(-60 / distance)

"Whether there are some real limits for distance in space of NextColony?"

The entire Deep Space is 11 times 11 galaxies big but potentially unlimited.