  The SuperNova / Acceleration Probe (SNAP) is a space-based experiment to
measure the expansion history of the Universe and study both its dark energy
and the dark matter. The experiment is motivated by the startling discovery
that the expansion of the Universe is accelerating. A 0.7 square-degree imager
comprised of 36 large format fully-depleted n-type CCD's sharing a focal plane
with 36 HgCdTe detectors forms the heart of SNAP, allowing discovery and
lightcurve measurements simultaneously for many supernovae. The imager and a
high-efficiency low-resolution integral field spectrograph are coupled to a 2-m
three mirror anastigmat wide-field telescope, which will be placed in a
high-earth orbit. The SNAP mission can obtain high-signal-to-noise calibrated
light-curves and spectra for over 2000 Type Ia supernovae at redshifts between
z=0.1 and 1.7. The resulting data set can not only determine the amount of dark
energy with high precision, but test the nature of the dark energy by examining
its equation of state. In particular, dark energy due to a cosmological
constant can be differentiated from alternatives such as "quintessence", by
measuring the dark energy's equation of state to an accuracy of +/-0.05, and by
studying its time dependence.
