In 2010 May 23-24, SDO observed the launch of two successive coronal mass
ejections (CMEs), which were subsequently tracked by the SECCHI suite onboard
STEREO. Using the COR2 coronagraphs and the heliospheric imagers (HIs), the
initial direction of both CMEs is determined to be slightly west of the
Sun-Earth line. We derive the CME kinematics, including the evolution of the
CME expansion until 0.4 AU. We find that, during the interaction, the second
CME decelerates from a speed above 500 km/s to 380 km/s the speed of the
leading edge of the first CME. STEREO observes a complex structure composed of
two different bright tracks in HI2-A but only one bright track in HI2-B. In
situ measurements from Wind show an "isolated" ICME, with the geometry of a
flux rope preceded by a shock. Measurements in the sheath are consistent with
draping around the transient. By combining remote-sensing and in situ
measurements, we determine that this event shows a clear instance of deflection
of two CMEs after their collision, and we estimate the deflection of the first
CME to be about 10 degrees towards the Sun-Earth line. The arrival time,
arrival speed and radius at Earth of the first CME are best predicted from
remote-sensing observations taken before the collision of the CMEs. Due to the
over-expansion of the CME after the collision, there are few, if any, signs of
interaction in in situ measurements. This study illustrates that complex
interactions during the Sun-to-Earth propagation may not be revealed by in situ
measurements alone.
