Complex sentence: Kal-El's rocket was set on a course for the planet Earth In the 20th century moments before Krypton's explosion, but it crashed into an asteroid and remained there for approximately one thousand years.
Split into simpler sentences: Kal-El's rocket was set on a course for the planet Earth, in the 20th century moments before Krypton was destroy. But the rocket crashed into an asteroid and remained there for approximately one thousand years.