  In our paper we investigate the lower limit of collisional energy of test
particles near the Kerr black hole. In particular we examine the minimal
Lorentz factor between the freely falling particles and the particles orbiting
around a black hole. We consider collisions on the innermost stable circular
orbit (ISCO) and examine near--extreme case, where collisions take place near
an event horizon. By fine--tuning the particles' angular momentum, the Lorentz
factor of the collision can always be minimized to a value dependent on the
black hole's spin. We identified that this minimal value is always less than
$\frac{2\sqrt{2}-1}{\sqrt{3}}$ and more than $\frac{\sqrt{12}-1}{\sqrt{6}}$
(the limits are the values for an extreme Kerr and Schwarzschild,
respectively). It implies that this kind of collisions of compact objects are
expected to be highly energetic near supermassive black holes. In addition, we
show that an interaction between black hole's and particle's spins has an
influence on minimal Lorentz factor. This contribution is nonnegligible for
near--extreme black holes. We also discuss the relation between our results and
sci--fi movie Interstellar.
