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They had called in some elites or humans and had them pick them up and drop them off at earth.

I don't believe any covenant ship would be that much faster than the forerunner dreadnought. It was even stated in Ghosts of Onyx that it took the dreadnought only a few hours to travel to earth.--Jugus 05:34, 15 May 2009 (UTC)
Slipspace, man, they used slipspace jump.