Stephen P. Synnott and Bradford A. Smith said ( IAUC 4806 ) its discovery on July 7 , 1989 , speaking only of â 17 frames taken over 21 daysâ , which gives a discovery date of sometime before June 16 . Stephen P. Synnott and Bradford A. Smith announced its discovery on July 7 , 1989 , speaking only of â 17 frames taken over 21 daysâ , which gives a discovery date of sometime before June 16 .