  Twenty years after the explosion of SN 1987A, we are now able to observe the
three-dimensional spatially resolved inner ejecta. Detailed mapping of newly
synthesised material and its radioactive decay daughter products sheds light on
the explosion mechanism. This may reveal the geometry of the explosion and its
connection to the equatorial ring and the outer rings around SN 1987A. We have
used integral field spectroscopy to image the supernova ejecta and the
equatorial ring in the emission lines of [Si I]+[Fe II] and He I. The spectral
information can be mapped into a radial velocity image revealing the expansion
of the ejecta both as projected onto the sky and perpendicular to the sky
plane. The inner ejecta are spatially resolved in a North-South direction and
are clearly asymmetric. We argue that the bulk of the ejecta is situated in the
same plane as defined by the equatorial ring and does not form a bipolar
structure as has been suggested. The exact shape of the ejecta is modelled and
we find that an elongated triaxial ellipsoid fits the observations best. From
our spectral analyses of the ejecta spectrum we find that most of the He I, [Si
I] and [Fe I-II] emission originates in the core material which has undergone
explosive nucleosynthesis. The He I emission may be the result of alpha-rich
freeze-out if the positron energy is deposited locally. Our observations
clearly indicate a non-symmetric explosion mechanism for SN 1987A. The
elongation and velocity asymmetries point towards a large-scale spatial
non-spherical distribution as predicted in recent explosion models. The
orientation of the ejecta in the plane of the equatorial ring argues against a
jet-induced explosion through the poles due to stellar rotation.
