By Ben Bova

A wave of lethal gamma radiation is expanding from the core of the Milky Way galaxy at the speed of light, killing everything in its path.   The countdown to when the death wave will reach Earth and the rest of the solar system is at two thousand years.  Humans were helped by Predeccessors, but a policy of exiling discontented young people to the outer planets has led to a deep divide between the new worlds and the homeworld, and those tensions are about to explode into open war.