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.
In the ending of Halo 2, Spartan 117 gets onboard of this Forerunner Ship, in his mission to kill Prophet of Truth.
Unknowingly Spartan 117 exits Slipstream space, near Earth.
Images
http://img146.echo.cx/img146/4969/sightsthumb9mq.jpg 
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