  We report here on 5 GHz global very-long-baseline interferometry (VLBI)
observations of SN 1986J, 16 yr after its explosion. We obtained a
high-resolution image of the supernova, which shows a distorted shell of radio
emission, indicative of a deformation of the shock front. The angular size of
the shell is $\sim4.7 {\rm mas}$, corresponding to a linear size of $\sim6.8
\times 10^{17} {\rm cm}$ for a distance of 9.6 Mpc to NGC 891. The average
speed of the shell has decreased from $\sim$7400 \kms in 1988.74 down to about
$6300 {\rm km s^{-1}}$ in 1999.14, indicative of a mild deceleration in the
expansion of SN 1986J. Assuming a standard density profile for the progenitor
wind ($\rho_{\rm cs} \propto r^{-s}, s=2$), the swept-up mass by the shock
front is $\sim$2.2\Ms. This large swept-up mass, coupled with the mild
deceleration suffered by the supernova, suggests that the mass of the
hydrogen-rich envelope ejected at explosion was $\ga12$ \Ms. Thus, the
supernova progenitor must have kept intact most of its hydrogen-rich envelope
by the time of explosion, which favours a single, massive star progenitor
scenario. We find a flux density for SN 1986J of $\sim$7.2 mJy at the observing
frequency of 5 GHz, which results in a radio luminosity of $\sim1.4 \times
10^{37} {\rm erg s^{-1}}$ for the frequency range $10^7$--$10^{10}$ Hz ($\alpha
=-0.69; S_\nu \propto \nu^{\alpha})$. We detect four bright knots that
delineate the shell structure, and an absolute minimum of emission, which we
tentatively identify with the centre of the supernova explosion. If this is the
case, SN 1986J has then suffered an asymmetric expansion. We suggest that this
asymmetry is due to the collision of the supernova ejecta with an anisotropic,
clumpy (or filamentary) medium.
