  One of the great challenges of QCD is trying to understand the origin of the nucleon spin. Several decades of experimental measurements have shown that our current understanding is incomplete if only the quark and gluon spin contribution is considered. Over the last few years it has become increasingly clear that the contribution from the orbital angular momentum of the quarks and gluons has to be included as well. For instance, the sea quark orbital contribution remains largely unexplored. Measurements accessing the sea quark Sivers distribution will provide a probe of the sea quark orbital contribution. The upcoming E1039 experiment at Fermilab will access this distribution via the Drell-Yan process using a 120 GeV unpolarized proton beam directed on a polarized proton target. At E1039 kinematics the $u$-$\bar{u}$ annihilation process dominates the Drell-Yan cross section ($x_{Target}$ = 0.1 $\sim$ 0.35). If the $\bar{u}$ quark carries zero net angular momentum, then the measured Drell-Yan single-spin asymmetry should be zero, and vice versa. This experiment is a continuation of the currently running SeaQuest experiment. 
A Future Polarized Drell-Yan Experiment at Fermilab