We report on VLBI observations of supernova 1986J in the spiral galaxy NGC 891 at two new epochs , 1990 July and 1999 February , t = 7.4 and 15.9 yr after the explosion , and on a comprehensive analysis of these and earlier observations from t \sim 4 yr after the explosion date , which we estimate to be 1983.2 \pm 1.1 .
The source is a shell or composite , and continues to show a complex morphology with large brightness modulations along the ridge and with protrusions .
The supernova is moderately to strongly decelerated .
The average outer radius expands as t ^ { 0.71 \pm 0.11 } , and the expansion velocity has slowed to 6000 km s ^ { -1 } at t = 15.9 yr from an extrapolated 20,000 km s ^ { -1 } at t = 0.25 yr .
The structure changes significantly with time , showing that the evolution is not self-similar .
The shell structure is best visible at the latest epoch , when the protrusions have diminished somewhat in prominence and a new , compact component has appeared .
The radio spectrum shows a clear inversion above 10 GHz .
This might be related to a pulsar nebula becoming visible through the debris of the explosion .
The radio flux density between 1.5 and 23 GHz decreases strongly with time , with the flux density \propto t ^ { -2.94 \pm 0.24 } between t \sim 15 to 19 yr .
This decrease is much more rapid than that found in earlier measurements up to t \sim 6 yr .