  The so called Casimir energy embodies the O($(N_c)^0$) contribution to the
skyrmion mass according to the semi-classical expansion theory of solitons. We
claim that this contribution can be accurately estimated despite the fact that
some of the counterterms, provided in principle by chiral perturbation theory,
are not known in practice. Using $\zeta$ function techniques we show that
$E_{cas}=E_{cas}(\mu)+E_{ct}(\mu)$ where $E_{cas}(\mu)>>E_{ct}(\mu)$ because it
incorporates all the zero-mode contributions and it can be exactly calculated.
Our results confirm that the fourth order Skyrme lagrangian does not seem to
provide a correct description of the lightest baryons as solitons. We show that
a simple extension to order six gives, on the contrary, good results without
tuning the parameters of the chiral lagrangian.
