From Halopedia, the Halo wiki
When the High Prophet of Regret's Fleet of Sacred Consecration exited slipspace in the Sol system and passed near Io before the Battle of Earth, Io Station's Probe Charlie Three-One-Niner detected the fleet and reported it to a naval crewman aboard the station. At first the crewman believed that the detection was a faulty reading, but the station's computer confirmed the probe was functioning properly. The crewman then had the computer alert the UNSC Home Fleet that the Covenant was in the Sol system en route to Earth, as the Fleet of Sacred Consecration passed over Io Station. Cortana on Cairo Station received the warning and informed Fleet Admiral Lord Terrence Hood. The station contacted John-117 on the Forerunner Dreadnought before the SPARTAN-II contacted Cairo Station later in the battle.
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