Among various factors affecting the space weather effects of a coronal mass
ejection (CME), its propagation trajectory in the interplanetary space is an
important one determining whether and when the CME will hit the Earth. Many
direct observations have revealed that a CME may not propagate along a straight
trajectory in the corona, but whether or not a CME also experiences a deflected
propagation in the interplanetary space is a question, which has never been
fully answered. Here by investigating the propagation process of an isolated
CME from the corona to interplanetary space during 2008 September 12 -- 19, we
present solid evidence that the CME was deflected not only in the corona but
also in the interplanetary space. The deflection angle in the interplanetary
space is more than 20 degrees toward the west, resulting a significant change
in the probability the CME encounters the Earth. A further modeling and
simulation-based analysis suggest that the cause of the deflection in the
interplanetary space is the interaction between the CME and the solar wind,
which is different from that happening in the corona.
