- last post: 01.01.0001 12:00 AM PDT
It is hinted in the book that the Covenant learn the coordinates of earth in the same manner in which they found out the coordinates of Reach: research that they did in order to try and find more Forerunner artifacts. In Fall or Reach, Master Chief encounters Hunters that are scanning some rock that has the coordinates of the Halo complex; and the Covenant know exactly where the Forerunner artifact on Reach is; there is most likely a Forerunner artifact on Earth, and the Covenant just didn't know about the fact that Earth is our home planet.
Another possibility is the fact that the Admiral (I can't remember his name) is hiding something; it could be that his "project" inadvertantly led to the Covies finding out where Earth is. But I really think that it is the former, that the Covies were searching for another Forerunner coordinate artifact, and will find another mapstone to the location of another Halo complex.