Complex sentence: At the time, the mission was one of the most complex ever undertaken, involving five lengthy periods of extra-vehicular activity, and its resounding success was an enormous boon for NASA, as well as for the astronomers who now had a fully capable space telescope.
Split into simpler sentences: At the time, the mission was one of the most complex, with five long EVAs. Its success was a boon for NASA, as well as for the astronomers with a more capable space telescope.