Complex sentence: The Vice Admiral arms a "Nova thermonuclear mine", a weapon that would destroy the planet while Cortana, still aboard "Ascendant Justice", learns that the Covenant already know the location of Earth and are preparing an invasion fleet.
Split into simpler sentences: The Vice Admiral arms a "Nova thermonuclear mine", a weapon that would destroy the planet. Meanwhile, Cortana, still aboard "Ascendant Justice" learns that the Covenant already know the location of Earth and are preparing an invasion fleet.