Starscape: The Silver Bullet — Brad Aiken

It is the year 2497, and Earth is about to come under attack by its former inhabitants. For nearly five centuries humans have peacefully coexisted with the Neanderthal descendants who they banished to the planet Teconea. But now, the Neanderthals want their home world back. They have amassed a vast armada, well concealed in the ion storms of the Orion system until Danny Stryker, a Federation trader, stumbles upon their secret.
Humankind's best hope of survival against the powerful Teconean Empire, the Federation Defense System, is nearly complete. A potent new energy generator in the shape of an obelisk is the last key element needed to complete the FDS. Stryker discovers that the gilded obelisk is lost in time. The only hope for survival is to send someone back to nineteenth century California to retrieve it before the Teconean invasion is launched.

An aficionado of the Old West, Stryker eagerly volunteers to make the trip. Fitting into the nineteenth century has its challenges, and to make matters worse, Teconean spies have infiltrated deep into the ranks of the Space Corps. The race through time and space is on.

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