Summary
There is a Forerunner ship used to power High Charity. When the Flood attack the city, the Prophet of Truth uncouples the ship from High Charity's power grid, and uses it to travel to Earth. The Forerunner ship travels vertically, like a rocket. It is also capable of slipstream travel. It is also possible that this ship could be the "Ark" However this is unlikely for the Prophets would have surely used the Ark to activate the remaining Installations as soon as they went on standby after Installation 05 was deactivated.
At the end of Halo 2 Master Chief gets onboard this Forerunner Ship, in his mission to kill the Prophet of Truth. When Cortana attempts to slow its launch sequence, she describes being fought by a presence, this suggests an enemy AI inside the ship.