  We present a strong lensing analysis of the galaxy cluster Abell 370
(z=0.375) based on the recent multicolor ACS images obtained as part of the
Early Release Observation (ERO) that followed the Hubble Service Mission #4.
Back in 1987, the giant gravitational arc (z=0.725) in Abell 370 was one of the
first pieces of evidence that massive clusters are dense enough to act as
strong gravitational lenses. The new observations reveal in detail its disklike
morphology, and we show that it can be interpreted as a complex five-image
configuration, with a total magnification factor of 32+/-4. Moreover, the high
resolution multicolor information allowed us to identify 10 multiply imaged
background galaxies. We derive a mean Einstein radius of RE=39+/-2" for a
source redshift at z=2, corresponding to a mass of M(<RE) = 2.82+/-0.15 1e14
Msol and M(<250 kpc)=3.8+/-0.2 1e14 Msol, in good agreement with Subaru
weak-lensing measurements. The typical mass model error is smaller than 5%, a
factor 3 of improvement compared to the previous lensing analysis. Abell 370
mass distribution is confirmed to be bi-modal with very small offset between
the dark matter, the X-ray gas and the stellar mass. Combining this information
with the velocity distribution reveals that Abell 370 is likely the merging of
two equally massive clusters along the line of sight, explaining the very high
mass density necessary to efficiently produce strong lensing. These new
observations stress the importance of multicolor imaging for the identification
of multiple images which is key to determining an accurate mass model. The very
large Einstein radius makes Abell 370 one of the best clusters to search for
high redshift galaxies through strong magnification in the central region.
