 The first impact occurred at 20 : 13 UTC on July 16 , 1994 , when fragment A of the nucleus entered Jupiter 's southern hemisphere at a speed of about 60 km / s . Instruments on Galileo detected a fireball that reached a peak temperature of about 24 @,@ 000 K , compared to the typical Jovian cloudtop temperature of about 130 K , before expanding and cooling rapidly to about 1500 K after 40 s . The plume from the fireball quickly reached a height of over 3 @,@ 000 km . A few minutes after the impact fireball was detected , Galileo measured renewed heating , probably due to ejected material falling back onto the planet . Earth @-@ based observers detected the fireball rising over the limb of the planet shortly after the initial impact . 
