The Osiris Invasion: Book Two of Seeds of a Fallen Empire

In The Osiris Invasion, the Earth of the future is attacked by an alien force known as the Orians who come from the Orion Stars. The Earth has suffered a second Dark Ages in the 22nd and 23rd century and recovered enough that they are planning to send an exploration mission to alpha centauri.
However, an alien starship crashes on Earth, pursued by another group of aliens called the Orians, and the Earth of the future must deal with this alien invasion…