  The use of Type Ia supernovae as calibrated standard candles is one of the
most powerful tools to study the expansion history of the universe and thereby
its energy components. While the analysis of some ~50 supernovae at redshifts
around z~0.5 have provided strong evidence for an energy component with
negative pressure, ``dark energy'', more data is needed to enable an accurate
estimate of the amount and nature of this energy. This might be accomplished by
a dedicated space telescope, the SuperNova / Acceleration Probe (2000; SNAP),
which aims at collecting a large number of supernovae with z<2.
  In this paper we assess the ability of the SNAP mission to determine various
properties of the ``dark energy.'' To exemplify, we expect SNAP, if operated
for three years to study Type Ia supernovae, to be able to determine the
parameters in a linear equation of state w(z)=w0 + w1 z to within a statistical
uncertainty of +-0.04 for w0 and +0.15,-0.17 for w1 assuming that the universe
is known to be flat and an independent high precision (sigma_{Omega_m}=0.015)
measurement of the mass density Omega_m, is used to constrain the fit. An
additional improvement can be obtained if a large number of low-z, as well as
high-z, supernovae are included in the sample.
