  On 2017-09-10, solar energetic particles (SEPs) originating from the active
region 12673 were registered as a ground level enhancement (GLE) at Earth and
the biggest GLE on the surface of Mars as observed by the Radiation Assessment
Detector (RAD) since the landing of the Curiosity rover in August 2012. Based
on multi-point coronagraph images, we identify the initial 3D kinematics of an
extremely fast CME and its shock front as well as another 2 CMEs launched hours
earlier (with moderate speeds) using the Graduated Cylindrical Shell (GCS)
model. These three CMEs interacted as they propagated outwards into the
heliosphere and merged into a complex interplanetary CME (ICME). The arrival of
the shock and ICME at Mars caused a very significant Forbush Decrease (FD) seen
by RAD only a few hours later than that at Earth which is about 0.5 AU closer
to the Sun. We investigate the propagation of the three CMEs and the consequent
ICME together with the shock using the Drag Based Model (DBM) and the WSA-ENLIL
plus cone model constrained by the in-situ SEP and FD/shock onset timing. The
synergistic modeling of the ICME and SEP arrivals at Earth and Mars suggests
that in order to better predict potentially hazardous space weather impacts at
Earth and other heliospheric locations for human exploration missions, it is
essential to analyze 1) the CME kinematics, especially during their
interactions and 2) the spatially and temporally varying heliospheric
conditions, such as the evolution and propagation of the stream interaction
regions.
