- You may be looking for the novel of the same name.
Zeta Major was, according to Lord Ferdinand du Vindice, a "trapped asteroid". Tegan discovered documents of the Zeta Project which described it as "the outer planet in the Zeta System. A captured asteroid."
It was also headquarters for the Zeta Project and, according to documents found by Ferdinand, "engineered as an experimental complex to facilitate conversion". Ferdinand therefore saw it as a legitimate military target and all but destroyed it, with Nyssa and Tegan in tow. (PROSE: Zeta Major)