The Key of Vaga was a structure designed collaboratively by the Vagans and the Doctor. It allowed for an inter-dimensional craft — also designed by the partnership — to return to Vaga whenever the occupants wanted.
On arriving on Vaga, the Fifth Doctor and Adric deactivated the Key of Vaga momentarily; the arrival of the Vagan craft alongside the Doctor's TARDIS could have destroyed the planet. When it did arrive, it hovered above the Key and after leaving the planet, supposedly for good, the Key of Vaga became obsolete. (PROSE: The Key of Vaga)