Silence in Solitude - Book II in the Roads of Heaven - Melissa Scott

In Five-Twelfths of Heaven, Silence Leigh discovered that she was not only unusual, as a female pilot, but that impossible thing, a female magus. Her unique abilities make her the only person capable of reaching Earth, humanity's original home, now sealed behind a mysterious barrier - but first she must learn to use her new-found talents. As the Hegemon's men close in on her and her husbands and teacher, she must make a dangerous bargain: undertake an impossible rescue mission in exchange for a vital map. If she succeeds, she may be able to save Earth. If she fails...