Complex sentence: Vova, a pig, become too fat to fly on a rocket, and Belka (a dog) flew instead of him, but Belka lost control of rocket and flew away from the circus.
Split into simpler sentences: Vova, a pig, became too fat to fly on a rocket, and Belka, a dog, flew instead of him. But Belka lost control of the rocket and flew away from the circus.