The Siege of Earth (The Ember War Saga);

Only an ember of humanity survived the first Xaros invasion. Now, the Xaros return to deliver the final blow to Earth and her defenders.
A moon carrying an armada of Xaros drones unleashes its deadly cargo on the solar system. Fortress Mars stands between the enemy and an ill-defended Earth. While the battle rages over the red planet, the Breitenfeld must launch a desperate mission to Pluto to cut off enemy reinforcements.
Earth stands on the brink of ruin, and humanity needs every hero it can find to turn back the tide of destruction.
The Ember War enters its final stages in THE SIEGE OF EARTH.