  Carbon monoxide is an excellent tracer of the physical conditions of gas in
molecular outflows from young stars. To understand the outflow mechanism we
need to investigate the origin of the molecular emission and the structure and
interaction of the outflowing molecular gas. Deriving the physical parameters
of the gas will help us to trace and understand the various gas components in
the flow. We observed CO(12-11) line emission at various positions along the
L1157 bipolar outflow with GREAT aboard SOFIA. Comparing these new data with
CO(2-1), we find basically constant line ratios along the outflow and even at
the position of the source. These line ratios lead us to estimates of 10^5 to
10^6 cm^-3 for the gas density and 60 to 100 K for the gas temperature of the
outflowing gas. The constrained density and temperature values indicate that we
are mostly tracing a low-velocity gas component everywhere along the outflow,
which is intermediate between the already known cold gas component, which gets
entrained into the flow, and the hot gas, which gets shocked in the outflow.
