The fragments of Comet Shoemaker-Levy 9 take aim at Jupiter in this artist's concept. The first of 21 fragments hit the planet on July 16, 1994 (one of the impacts is depicted in this illustration). Each impact produced a powerful fireball and left a dark scar in Jupiter's atmosphere. The scars lasted for months. The impacts spurred Congress to require NASA to start looking for comets and asteroids that could potentially hit Earth. [NASA/GSFC]