From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
The Mirror Spaceship appeared in "Honest to Goodness? There Lies Namek, Planet of Hope". When Bulma, Gohan, and Kuririn, set out on their journey to Planet Namek, they were suddenly attacked and captured by an invisible spaceship. They found out that the only passengers on the spaceship were children led by a boy named Zeshin. Their homeworld had been attacked by a battalion of Freeza's Force led by Kewie and they were the only survivors.