The evolution of the shape and size of the ejecta of SN 1987A is analyzed over a period of \sim 8 years based on HST images and spectra taken between 1278 and 4336 days after the supernova outburst .
We combine both proprietary and archival HST data obtained with the FOC , WFPC2 and STIS .
The low resolution near-UV prism FOC spectrum obtained at day 3043 has not been described previously .
Although the FWHM of the ejecta grew linearly over the time span studied , the appearance of the SN envelope also changed markedly with wavelength .
At visible wavelengths ( \lambda \simeq 5000 Å ) the ejecta became progressively more elongated , reaching an ellipticity of \epsilon \simeq 0.25 by day 4000 .
In the near-UV ( \lambda \simeq 2500 Å ) , the ejecta remained closely circular ( \epsilon \leq 0.1 ) and \sim 50 % larger in angular extent than in the visible .
The FOC prism observations show that the large extent of the SN envelope is confined to a grouping of resonance lines spanning Mg i \lambda 2852 , Mg ii \lambda \lambda 2795,2802Å and several Fe ii multiplets – thereby confirming that the larger size of the debris in the near-UV is due to scattering in these optically thick transitions compared to the optically thin forbidden and semi-forbidden transitions that dominate the visible spectrum .
The available data are not of sufficient quality to detect the slight deviation from linear expansion expected for the outermost regions of the near-UV images as predicted by Chugai et al .
( 1997 ) .