  We study the fragmentation of He$_2$ dimers into He$^+$ ions by relativistic
highly-charged projectiles. It is demonstrated that the interaction between an
ultrafast projectile with an extremely extended object -- the helium dimer --
possesses interesting features which are absent in collisions with "normal"
molecules. It is also shown that such projectiles, due to their enormous
interaction range, can accurately probe the ground state of the dimer and even
be used for a precise determination of its binding energy.
