  We study the projection constant of the space of operators on $n$-dimensional
Hilbert spaces, with the trace norm, $\mathcal S_1(n)$. We show an integral
formula for the projection constant of $\mathcal S_1(n)$; namely $
\boldsymbol{\lambda}\big(\mathcal S_1(n)\big) = n \int_{\mathcal U_n} \vert
\text{tr}(U) \vert \,dU \,, $ where the integration is with respect to the Haar
probability measure on the group $\mathcal U_n$ of unitary operators. Using a
probabilistic approach, we derive the limit formula $ \lim_{n\to \infty}
\boldsymbol{\lambda}\big(\mathcal S_1(n)\big)/n = \sqrt{\pi}/2\,. $
