  We calculate the mean nucleation distance, $d_{nuc}$, in a first order
cosmological quark-hadron phase transition. For homogeneous nucleation we find
that self-consistent inclusion of curvature energy reduces $d_{nuc}$ to
$\lesssim 2$cm. However, impurities can lead to heterogeneous nucleation with
$d_{nuc}$ of several meters, a value high enough to change the outcome of Big
Bang nucleosynthesis.
