  We present a spectroscopic survey of the giant stellar stream found in the
halo of the Andromeda galaxy. Taken with the DEIMOS multi-object spectrograph
on the Keck2 telescope, these data display a narrow velocity dispersion of
11+/-3 km/s, with a steady radial velocity gradient of 245 km/s over the 125
kpc radial extent of the stream studied so far. This implies that the Andromeda
galaxy possesses a substantial dark matter halo. We fit the orbit of the stream
in different galaxy potential models. In a simple model with a composite bulge,
disk and halo, where the halo follows a ``universal'' profile that is
compressed by the formation of the baryonic components, we find that the
kinematics of the stream require a total mass inside 125 kpc of
7.5^(+2.5)_(-1.3) x 10^{11} M_Sun, or more than 5.4 x 10^{11} M_Sun at the 99%
confidence level. This is the first galaxy in which it has been possible to
measure the halo mass distribution by such direct dynamical means over such a
large distance range. The resulting orbit shows that if M32 or NGC 205 are
connected with the stream, they must either trail or lag the densest region of
the stream by more than 100 kpc. Furthermore, according to the best-fit orbit,
the stream passes very close to M31, causing its demise as a coherent structure
and producing a fan of stars that will pollute the inner halo, thereby
confusing efforts to measure the properties of genuine halo populations. Our
data show that several recently identified planetary nebulae, which have been
proposed as evidence for the existence of a new companion of M31, are likely
members of the Andromeda Stream.
