Ancestor Across Time, Book 2, The Keltecian Threat

Scott Kennedy thought his time travel actions had eliminated the Keltecian threat once and for all. But these lizard-like creatures were resilient and patient, lying dormant in their Hyper-Sleep Pods at Lagrange Point L2 in the solar system. They awaited an assault vessel from Keltius, in route with reinforcements to obliterate all humankind and take Earth for their own. The only chance Earth has in defeating The Keltecian Threat is with the help of Earth's mentors from the planet Gliese 12...