Complex sentence: The first observation of a major impact event occurred in 1994: the collision of the comet Shoemaker-Levy 9 with Jupiter; to date, no such events have been observed on Earth.
Split into simpler sentences: The first observation of a major impact event occurred in 1994: the collision of the comet Shoemaker-Levy 9 with Jupiter. To date, no such events have been observed on Earth.