The Weight of the Stars by K. Ancrum
Ryann Bird dreams of travelling across the stars. But she settles for acting out and skipping school. Until she meets Alexandria, a furious loner. After a horrific accident leaves Alexandria with a broken arm, the girls are brought together despite themselves - and Ryann learns Alexandria's secret: Her mother is an astronaut on a one-way trip past the edge of the solar system. Every night, Alexandria waits for radio signals from her mother. And now Ryann lifts Alexandria onto the roof day after day, until the silence between them grows into friendship . . . . and eventually something more.