Fortress Sol - Stephen Baxter

When Rab was a baby, his mother made a decision which would change his life. She feared he would be sent to work in the hellish mines of Mercury, to eke out his life until he was worn out, all in the name of maintaning the defense of the Solar Sytem. But when her desperate attempt to flee with her 2 year old failed, she took a desperate step to save him. She cut off his hand.

Decades later, Rab has been spared the physical hardships he can no longer endure, and is now based on the Mask, the all-encompassing structure which hides the Solar Sytem from alien eyes. And it is during his watch that a spaceship arrives, one which has travelled for a hundred years from a long-forgotten colony planet.

If they pierce the Mask, everything humanity has created will be left open to the alien threat. But this strange ship, bearing an offshoot of the species, may bring something else with them. Hope.