  We used low resolution spectroscopy from VLT/FORS1, and high resolution
spectra from VLT/UVES, to estimate the physical conditions in the ORs, using
nebular analysis for emission lines such as [O II], [O III], [N II], and [S
II]. We also measured the velocity at two positions of the ORs to test a
geometrical model for the rings. Additionally, we used data from the HST
science archives to check the evolution of the ORs of SN 1987A for a period
that covers almost 11 years. We measured the flux in four different regions,
two for each outer ring. We chose regions away from the two bright foreground
stars, and as far as possible from the ER, and we created light curves for the
emission lines of [O III], H{\alpha}, and [N II]. The profiles of the
lightcurves display a declining behaviour, which is consistent with the initial
supernova flash powering of the ORs. The electron density of the emitting gas
in the ORs, as estimated through nebular analysis from forbidden [O II] and [S
II] lines, is <= 3*10^3 cm^-3, has not changed over the last ~ 15 years, and
also the [N II] temperature remains fairly constant at ~ 1.2*10^4 K. We find no
obvious difference in density and temperature for the two ORs. The highest
density, as measured from the decay of H{\alpha}, could, however, be ~ 5*10^3
cm^-3, and since the decay is somewhat faster for the southern outer ring than
for the northern, the highest density in the ORs may be found in the southern
outer ring. For an assumed distance of 50 kpc to the supernova, the distance
between the supernova and the closest parts of the ORs could be as short as ~
1.7*10^18 cm. Interaction between the supernova ejecta and the outer rings
could therefore start in less than ~ 20 years. We do not expect the ORs to show
the same optical display as the equatorial ring when this happens.
