Under Falling Skies by Kate MacLeod

Scout Shannon's whole family died the day the Space Farers dropped an asteroid on their domed city. Now she lives alone, out in the wild with only her dogs for company. She prefers it that way.

But Scout finds herself at a crossroads. One road leads back to a quiet life snug under the protective dome of a city. The other road leads to a life in the rebellion, a life of adventure and excitement but also danger. Dare she try to find the rebels hiding in the hills?

Then a chance encounter with a stranger from the other side of the galaxy threatens to derail what remains of Scout's life. The entire galaxy awaits her, if she survives the next four days.

"Under Falling Skies", a young adult science fiction novel, set on a remote planet with a distinctly Old West feel. For fans of gunslinging women and young girl assassins. And dogs.