Using in situ measurements and remote-sensing observations, we study a
coronal mass ejection (CME) that left the Sun on 9 July 2013 and impacted both
Mercury and Earth while the planets were in radial alignment (within
$3^\circ$). The CME had an initial speed as measured by coronagraphs of 580
$\pm$ 20 km s$^{-1}$, an inferred speed at Mercury of 580 $\pm$ 30 km s$^{-1}$
and a measured maximum speed at Earth of 530 km s$^{-1}$, indicating that it
did not decelerate substantially in the inner heliosphere. The magnetic field
measurements made by MESSENGER and {\it Wind} reveal a very similar magnetic
ejecta at both planets. We consider the CME expansion as measured by the ejecta
duration and the decrease of the magnetic field strength between Mercury and
Earth and the velocity profile measured {\it in situ} by {\it Wind}. The
long-duration magnetic ejecta (20 and 42 hours at Mercury and Earth,
respectively) is found to be associated with a relatively slowly expanding
ejecta at 1 AU, revealing that the large size of the ejecta is due to the CME
itself or its expansion in the corona or innermost heliosphere, and not due to
a rapid expansion between Mercury at 0.45 AU and Earth at 1 AU. We also find
evidence that the CME sheath is composed of compressed material accumulated
before the shock formed, as well as more recently shocked material.
