Trying to claim the Covenant have any chance in space is laughable. Even low-end calculations for the Imperium puts lances spitting out gigajoules of energy, and Covenant shields go down to megajoules. Void Shields on even the smallest vessels, meanwhile, take hours of lance bombardment and still keep kicking. An Imperium vessel could feasibly tank hundreds of Covenant vessels firing on it indefinitely, and swat them at its leisure.