  We investigate the accuracy attainable by forthcoming space-based
observations of the cosmic microwave background (CMB) temperature and
polarization anisotropy in constraining the dark energy density parameter $\Oq$
and equation of state $\wq=p_{\rm Q}/\rho_{\rm Q}$. Despite degeneracies among
parameters, it is possible for high precision observations such as those from
WMAP and Planck to provide interesting information on the nature of the dark
energy. Furthermore, we show that imposing a flat universe constraint makes it
possible to obtain tight limits in the space of dark energy parameters even
from the CMB alone.
