  We present a method for measuring the cosmic expansion history H(z) in
uncorrelated redshift bins, and apply it to current and simulated type Ia
supernova data assuming spatial flatness. If the matter density parameter
Omega_m can be accurately measured from other data, then the dark energy
density history X(z)=rho_X(z)/rho_X(0) can trivially be derived from this
expansion history H(z). In contrast to customary ``black box'' parameter
fitting, our method is transparent and easy to interpret: the measurement of
H(z)^{-1} in a redshift bin is simply a linear combination of the measured
comoving distances for supernovae in that bin, making it obvious how systematic
errors propagate from input to output.
  We find the Riess et al. (2004) ``gold'' sample to be consistent with the
``vanilla'' concordance model where the dark energy is a cosmological constant.
We compare two mission concepts for the NASA/DOE Joint Dark Energy Mission
(JDEM), the Joint Efficient Dark-energy Investigation (JEDI), and the Supernova
Accelaration Probe (SNAP), using simulated data including the effect of weak
lensing (based on numerical simulations) and a systematic bias from
K-corrections. Estimating H(z) in seven uncorrelated redshift bins, we find
that both provide dramatic improvements over current data: JEDI can measure
H(z) to about 10% accuracy and SNAP to 30-40% accuracy.
